| USING THE IJG JPEG LIBRARY |
| |
| This file was part of the Independent JPEG Group's software: |
| Copyright (C) 1994-2013, Thomas G. Lane, Guido Vollbeding. |
| libjpeg-turbo Modifications: |
| Copyright (C) 2010, 2014-2018, 2020, 2022, 2024, D. R. Commander. |
| Copyright (C) 2015, Google, Inc. |
| For conditions of distribution and use, see the accompanying README.ijg file. |
| |
| |
| This file describes how to use the IJG JPEG library within an application |
| program. Read it if you want to write a program that uses the library. |
| |
| The file example.txt provides heavily commented skeleton code for calling the |
| JPEG library. Also see jpeglib.h (the include file to be used by application |
| programs) for full details about data structures and function parameter lists. |
| The library source code, of course, is the ultimate reference. |
| |
| Note that there have been *major* changes from the application interface |
| presented by IJG version 4 and earlier versions. The old design had several |
| inherent limitations, and it had accumulated a lot of cruft as we added |
| features while trying to minimize application-interface changes. We have |
| sacrificed backward compatibility in the version 5 rewrite, but we think the |
| improvements justify this. |
| |
| |
| TABLE OF CONTENTS |
| ----------------- |
| |
| Overview: |
| Functions provided by the library |
| Outline of typical usage |
| Basic library usage: |
| Data formats |
| Compression details |
| Decompression details |
| Partial image decompression |
| Mechanics of usage: include files, linking, etc |
| Advanced features: |
| Compression parameter selection |
| Decompression parameter selection |
| Special color spaces |
| Error handling |
| Compressed data handling (source and destination managers) |
| I/O suspension |
| Progressive JPEG support |
| Buffered-image mode |
| Abbreviated datastreams and multiple images |
| Special markers |
| ICC profiles |
| Raw (downsampled) image data |
| Really raw data: DCT coefficients |
| Progress monitoring |
| Memory management |
| Memory usage |
| Library compile-time options |
| Portability considerations |
| |
| You should read at least the overview and basic usage sections before trying |
| to program with the library. The sections on advanced features can be read |
| if and when you need them. |
| |
| |
| OVERVIEW |
| ======== |
| |
| Functions provided by the library |
| --------------------------------- |
| |
| The IJG JPEG library provides C code to read and write JPEG-compressed image |
| files. The surrounding application program receives or supplies image data a |
| scanline at a time, using a straightforward uncompressed image format. All |
| details of color conversion and other preprocessing/postprocessing can be |
| handled by the library. |
| |
| The library includes a substantial amount of code that is not covered by the |
| JPEG standard but is necessary for typical applications of JPEG. These |
| functions preprocess the image before JPEG compression or postprocess it after |
| decompression. They include colorspace conversion, downsampling/upsampling, |
| and color quantization. The application indirectly selects use of this code |
| by specifying the format in which it wishes to supply or receive image data. |
| For example, if colormapped output is requested, then the decompression |
| library automatically invokes color quantization. |
| |
| A wide range of quality vs. speed tradeoffs are possible in JPEG processing, |
| and even more so in decompression postprocessing. The decompression library |
| provides multiple implementations that cover most of the useful tradeoffs, |
| ranging from very-high-quality down to fast-preview operation. On the |
| compression side we have generally not provided low-quality choices, since |
| compression is normally less time-critical. It should be understood that the |
| low-quality modes may not meet the JPEG standard's accuracy requirements; |
| nonetheless, they are useful for viewers. |
| |
| A word about functions *not* provided by the library. We handle a subset of |
| the ISO JPEG standard; most baseline, extended-sequential, and progressive |
| JPEG processes are supported. (Our subset includes all features now in common |
| use.) Unsupported ISO options include: |
| * Hierarchical storage |
| * Lossless JPEG |
| * DNL marker |
| * Nonintegral subsampling ratios |
| We support both 8- and 12-bit data precision, but this is a compile-time |
| choice rather than a run-time choice; hence it is difficult to use both |
| precisions in a single application. |
| |
| By itself, the library handles only interchange JPEG datastreams --- in |
| particular the widely used JFIF file format. The library can be used by |
| surrounding code to process interchange or abbreviated JPEG datastreams that |
| are embedded in more complex file formats. (For example, this library is |
| used by the free LIBTIFF library to support JPEG compression in TIFF.) |
| |
| |
| Outline of typical usage |
| ------------------------ |
| |
| The rough outline of a JPEG compression operation is: |
| |
| Allocate and initialize a JPEG compression object |
| Specify the destination for the compressed data (eg, a file) |
| Set parameters for compression, including image size & colorspace |
| jpeg_start_compress(...); |
| while (scan lines remain to be written) |
| jpeg_write_scanlines(...); |
| jpeg_finish_compress(...); |
| Release the JPEG compression object |
| |
| A JPEG compression object holds parameters and working state for the JPEG |
| library. We make creation/destruction of the object separate from starting |
| or finishing compression of an image; the same object can be re-used for a |
| series of image compression operations. This makes it easy to re-use the |
| same parameter settings for a sequence of images. Re-use of a JPEG object |
| also has important implications for processing abbreviated JPEG datastreams, |
| as discussed later. |
| |
| The image data to be compressed is supplied to jpeg_write_scanlines() from |
| in-memory buffers. If the application is doing file-to-file compression, |
| reading image data from the source file is the application's responsibility. |
| The library emits compressed data by calling a "data destination manager", |
| which typically will write the data into a file; but the application can |
| provide its own destination manager to do something else. |
| |
| Similarly, the rough outline of a JPEG decompression operation is: |
| |
| Allocate and initialize a JPEG decompression object |
| Specify the source of the compressed data (eg, a file) |
| Call jpeg_read_header() to obtain image info |
| Set parameters for decompression |
| jpeg_start_decompress(...); |
| while (scan lines remain to be read) |
| jpeg_read_scanlines(...); |
| jpeg_finish_decompress(...); |
| Release the JPEG decompression object |
| |
| This is comparable to the compression outline except that reading the |
| datastream header is a separate step. This is helpful because information |
| about the image's size, colorspace, etc is available when the application |
| selects decompression parameters. For example, the application can choose an |
| output scaling ratio that will fit the image into the available screen size. |
| |
| The decompression library obtains compressed data by calling a data source |
| manager, which typically will read the data from a file; but other behaviors |
| can be obtained with a custom source manager. Decompressed data is delivered |
| into in-memory buffers passed to jpeg_read_scanlines(). |
| |
| It is possible to abort an incomplete compression or decompression operation |
| by calling jpeg_abort(); or, if you do not need to retain the JPEG object, |
| simply release it by calling jpeg_destroy(). |
| |
| JPEG compression and decompression objects are two separate struct types. |
| However, they share some common fields, and certain routines such as |
| jpeg_destroy() can work on either type of object. |
| |
| The JPEG library has no static variables: all state is in the compression |
| or decompression object. Therefore it is possible to process multiple |
| compression and decompression operations concurrently, using multiple JPEG |
| objects. |
| |
| Both compression and decompression can be done in an incremental memory-to- |
| memory fashion, if suitable source/destination managers are used. See the |
| section on "I/O suspension" for more details. |
| |
| |
| BASIC LIBRARY USAGE |
| =================== |
| |
| Data formats |
| ------------ |
| |
| Before diving into procedural details, it is helpful to understand the |
| image data format that the JPEG library expects or returns. |
| |
| The standard input image format is a rectangular array of pixels, with each |
| pixel having the same number of "component" or "sample" values (color |
| channels). You must specify how many components there are and the colorspace |
| interpretation of the components. Most applications will use RGB data |
| (three components per pixel) or grayscale data (one component per pixel). |
| PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE. |
| A remarkable number of people manage to miss this, only to find that their |
| programs don't work with grayscale JPEG files. |
| |
| There is no provision for colormapped input. JPEG files are always full-color |
| or full grayscale (or sometimes another colorspace such as CMYK). You can |
| feed in a colormapped image by expanding it to full-color format. However |
| JPEG often doesn't work very well with source data that has been colormapped, |
| because of dithering noise. This is discussed in more detail in the JPEG FAQ |
| and the other references mentioned in the README.ijg file. |
| |
| Pixels are stored by scanlines, with each scanline running from left to |
| right. The component values for each pixel are adjacent in the row; for |
| example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an |
| array of data type JSAMPLE --- which is typically "unsigned char", unless |
| you've changed jmorecfg.h. (You can also change the RGB pixel layout, say |
| to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in |
| that file before doing so.) |
| |
| A 2-D array of pixels is formed by making a list of pointers to the starts of |
| scanlines; so the scanlines need not be physically adjacent in memory. Even |
| if you process just one scanline at a time, you must make a one-element |
| pointer array to conform to this structure. Pointers to JSAMPLE rows are of |
| type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY. |
| |
| The library accepts or supplies one or more complete scanlines per call. |
| It is not possible to process part of a row at a time. Scanlines are always |
| processed top-to-bottom. You can process an entire image in one call if you |
| have it all in memory, but usually it's simplest to process one scanline at |
| a time. |
| |
| For best results, source data values should have the precision specified by |
| BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress |
| data that's only 6 bits/channel, you should left-justify each value in a |
| byte before passing it to the compressor. If you need to compress data |
| that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12. |
| (See "Library compile-time options", later.) |
| |
| |
| The data format returned by the decompressor is the same in all details, |
| except that colormapped output is supported. (Again, a JPEG file is never |
| colormapped. But you can ask the decompressor to perform on-the-fly color |
| quantization to deliver colormapped output.) If you request colormapped |
| output then the returned data array contains a single JSAMPLE per pixel; |
| its value is an index into a color map. The color map is represented as |
| a 2-D JSAMPARRAY in which each row holds the values of one color component, |
| that is, colormap[i][j] is the value of the i'th color component for pixel |
| value (map index) j. Note that since the colormap indexes are stored in |
| JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE |
| (ie, at most 256 colors for an 8-bit JPEG library). |
| |
| |
| Compression details |
| ------------------- |
| |
| Here we revisit the JPEG compression outline given in the overview. |
| |
| 1. Allocate and initialize a JPEG compression object. |
| |
| A JPEG compression object is a "struct jpeg_compress_struct". (It also has |
| a bunch of subsidiary structures which are allocated via malloc(), but the |
| application doesn't control those directly.) This struct can be just a local |
| variable in the calling routine, if a single routine is going to execute the |
| whole JPEG compression sequence. Otherwise it can be static or allocated |
| from malloc(). |
| |
| You will also need a structure representing a JPEG error handler. The part |
| of this that the library cares about is a "struct jpeg_error_mgr". If you |
| are providing your own error handler, you'll typically want to embed the |
| jpeg_error_mgr struct in a larger structure; this is discussed later under |
| "Error handling". For now we'll assume you are just using the default error |
| handler. The default error handler will print JPEG error/warning messages |
| on stderr, and it will call exit() if a fatal error occurs. |
| |
| You must initialize the error handler structure, store a pointer to it into |
| the JPEG object's "err" field, and then call jpeg_create_compress() to |
| initialize the rest of the JPEG object. |
| |
| Typical code for this step, if you are using the default error handler, is |
| |
| struct jpeg_compress_struct cinfo; |
| struct jpeg_error_mgr jerr; |
| ... |
| cinfo.err = jpeg_std_error(&jerr); |
| jpeg_create_compress(&cinfo); |
| |
| jpeg_create_compress allocates a small amount of memory, so it could fail |
| if you are out of memory. In that case it will exit via the error handler; |
| that's why the error handler must be initialized first. |
| |
| |
| 2. Specify the destination for the compressed data (eg, a file). |
| |
| As previously mentioned, the JPEG library delivers compressed data to a |
| "data destination" module. The library includes one data destination |
| module which knows how to write to a stdio stream. You can use your own |
| destination module if you want to do something else, as discussed later. |
| |
| If you use the standard destination module, you must open the target stdio |
| stream beforehand. Typical code for this step looks like: |
| |
| FILE *outfile; |
| ... |
| if ((outfile = fopen(filename, "wb")) == NULL) { |
| fprintf(stderr, "can't open %s\n", filename); |
| exit(1); |
| } |
| jpeg_stdio_dest(&cinfo, outfile); |
| |
| where the last line invokes the standard destination module. |
| |
| WARNING: it is critical that the binary compressed data be delivered to the |
| output file unchanged. On non-Unix systems the stdio library may perform |
| newline translation or otherwise corrupt binary data. To suppress this |
| behavior, you may need to use a "b" option to fopen (as shown above), or use |
| setmode() or another routine to put the stdio stream in binary mode. See |
| cjpeg.c and djpeg.c for code that has been found to work on many systems. |
| |
| You can select the data destination after setting other parameters (step 3), |
| if that's more convenient. You may not change the destination between |
| calling jpeg_start_compress() and jpeg_finish_compress(). |
| |
| |
| 3. Set parameters for compression, including image size & colorspace. |
| |
| You must supply information about the source image by setting the following |
| fields in the JPEG object (cinfo structure): |
| |
| image_width Width of image, in pixels |
| image_height Height of image, in pixels |
| input_components Number of color channels (samples per pixel) |
| in_color_space Color space of source image |
| |
| The image dimensions are, hopefully, obvious. JPEG supports image dimensions |
| of 1 to 64K pixels in either direction. The input color space is typically |
| RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special |
| color spaces", later, for more info.) The in_color_space field must be |
| assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or |
| JCS_GRAYSCALE. |
| |
| JPEG has a large number of compression parameters that determine how the |
| image is encoded. Most applications don't need or want to know about all |
| these parameters. You can set all the parameters to reasonable defaults by |
| calling jpeg_set_defaults(); then, if there are particular values you want |
| to change, you can do so after that. The "Compression parameter selection" |
| section tells about all the parameters. |
| |
| You must set in_color_space correctly before calling jpeg_set_defaults(), |
| because the defaults depend on the source image colorspace. However the |
| other three source image parameters need not be valid until you call |
| jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more |
| than once, if that happens to be convenient. |
| |
| Typical code for a 24-bit RGB source image is |
| |
| cinfo.image_width = Width; /* image width and height, in pixels */ |
| cinfo.image_height = Height; |
| cinfo.input_components = 3; /* # of color components per pixel */ |
| cinfo.in_color_space = JCS_RGB; /* colorspace of input image */ |
| |
| jpeg_set_defaults(&cinfo); |
| /* Make optional parameter settings here */ |
| |
| |
| 4. jpeg_start_compress(...); |
| |
| After you have established the data destination and set all the necessary |
| source image info and other parameters, call jpeg_start_compress() to begin |
| a compression cycle. This will initialize internal state, allocate working |
| storage, and emit the first few bytes of the JPEG datastream header. |
| |
| Typical code: |
| |
| jpeg_start_compress(&cinfo, TRUE); |
| |
| The "TRUE" parameter ensures that a complete JPEG interchange datastream |
| will be written. This is appropriate in most cases. If you think you might |
| want to use an abbreviated datastream, read the section on abbreviated |
| datastreams, below. |
| |
| Once you have called jpeg_start_compress(), you may not alter any JPEG |
| parameters or other fields of the JPEG object until you have completed |
| the compression cycle. |
| |
| |
| 5. while (scan lines remain to be written) |
| jpeg_write_scanlines(...); |
| |
| Now write all the required image data by calling jpeg_write_scanlines() |
| one or more times. You can pass one or more scanlines in each call, up |
| to the total image height. In most applications it is convenient to pass |
| just one or a few scanlines at a time. The expected format for the passed |
| data is discussed under "Data formats", above. |
| |
| Image data should be written in top-to-bottom scanline order. |
| Rec. ITU-T T.81 | ISO/IEC 10918-1 says, "Applications determine which edges of |
| a source image are defined as top, bottom, left, and right." However, if you |
| want your files to be compatible with everyone else's, then top-to-bottom order |
| must be used. If the source data must be read in bottom-to-top order, then you |
| can use the JPEG library's virtual array mechanism to invert the data |
| efficiently. Examples of this can be found in the sample application cjpeg. |
| |
| The library maintains a count of the number of scanlines written so far |
| in the next_scanline field of the JPEG object. Usually you can just use |
| this variable as the loop counter, so that the loop test looks like |
| "while (cinfo.next_scanline < cinfo.image_height)". |
| |
| Code for this step depends heavily on the way that you store the source data. |
| example.txt shows the following code for the case of a full-size 2-D source |
| array containing 3-byte RGB pixels: |
| |
| JSAMPROW row_pointer[1]; /* pointer to a single row */ |
| int row_stride; /* physical row width in buffer */ |
| |
| row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */ |
| |
| while (cinfo.next_scanline < cinfo.image_height) { |
| row_pointer[0] = &image_buffer[cinfo.next_scanline * row_stride]; |
| jpeg_write_scanlines(&cinfo, row_pointer, 1); |
| } |
| |
| jpeg_write_scanlines() returns the number of scanlines actually written. |
| This will normally be equal to the number passed in, so you can usually |
| ignore the return value. It is different in just two cases: |
| * If you try to write more scanlines than the declared image height, |
| the additional scanlines are ignored. |
| * If you use a suspending data destination manager, output buffer overrun |
| will cause the compressor to return before accepting all the passed lines. |
| This feature is discussed under "I/O suspension", below. The normal |
| stdio destination manager will NOT cause this to happen. |
| In any case, the return value is the same as the change in the value of |
| next_scanline. |
| |
| |
| 6. jpeg_finish_compress(...); |
| |
| After all the image data has been written, call jpeg_finish_compress() to |
| complete the compression cycle. This step is ESSENTIAL to ensure that the |
| last bufferload of data is written to the data destination. |
| jpeg_finish_compress() also releases working memory associated with the JPEG |
| object. |
| |
| Typical code: |
| |
| jpeg_finish_compress(&cinfo); |
| |
| If using the stdio destination manager, don't forget to close the output |
| stdio stream (if necessary) afterwards. |
| |
| If you have requested a multi-pass operating mode, such as Huffman code |
| optimization, jpeg_finish_compress() will perform the additional passes using |
| data buffered by the first pass. In this case jpeg_finish_compress() may take |
| quite a while to complete. With the default compression parameters, this will |
| not happen. |
| |
| It is an error to call jpeg_finish_compress() before writing the necessary |
| total number of scanlines. If you wish to abort compression, call |
| jpeg_abort() as discussed below. |
| |
| After completing a compression cycle, you may dispose of the JPEG object |
| as discussed next, or you may use it to compress another image. In that case |
| return to step 2, 3, or 4 as appropriate. If you do not change the |
| destination manager, the new datastream will be written to the same target. |
| If you do not change any JPEG parameters, the new datastream will be written |
| with the same parameters as before. Note that you can change the input image |
| dimensions freely between cycles, but if you change the input colorspace, you |
| should call jpeg_set_defaults() to adjust for the new colorspace; and then |
| you'll need to repeat all of step 3. |
| |
| |
| 7. Release the JPEG compression object. |
| |
| When you are done with a JPEG compression object, destroy it by calling |
| jpeg_destroy_compress(). This will free all subsidiary memory (regardless of |
| the previous state of the object). Or you can call jpeg_destroy(), which |
| works for either compression or decompression objects --- this may be more |
| convenient if you are sharing code between compression and decompression |
| cases. (Actually, these routines are equivalent except for the declared type |
| of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy() |
| should be passed a j_common_ptr.) |
| |
| If you allocated the jpeg_compress_struct structure from malloc(), freeing |
| it is your responsibility --- jpeg_destroy() won't. Ditto for the error |
| handler structure. |
| |
| Typical code: |
| |
| jpeg_destroy_compress(&cinfo); |
| |
| |
| 8. Aborting. |
| |
| If you decide to abort a compression cycle before finishing, you can clean up |
| in either of two ways: |
| |
| * If you don't need the JPEG object any more, just call |
| jpeg_destroy_compress() or jpeg_destroy() to release memory. This is |
| legitimate at any point after calling jpeg_create_compress() --- in fact, |
| it's safe even if jpeg_create_compress() fails. |
| |
| * If you want to re-use the JPEG object, call jpeg_abort_compress(), or call |
| jpeg_abort() which works on both compression and decompression objects. |
| This will return the object to an idle state, releasing any working memory. |
| jpeg_abort() is allowed at any time after successful object creation. |
| |
| Note that cleaning up the data destination, if required, is your |
| responsibility; neither of these routines will call term_destination(). |
| (See "Compressed data handling", below, for more about that.) |
| |
| jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG |
| object that has reported an error by calling error_exit (see "Error handling" |
| for more info). The internal state of such an object is likely to be out of |
| whack. Either of these two routines will return the object to a known state. |
| |
| |
| Decompression details |
| --------------------- |
| |
| Here we revisit the JPEG decompression outline given in the overview. |
| |
| 1. Allocate and initialize a JPEG decompression object. |
| |
| This is just like initialization for compression, as discussed above, |
| except that the object is a "struct jpeg_decompress_struct" and you |
| call jpeg_create_decompress(). Error handling is exactly the same. |
| |
| Typical code: |
| |
| struct jpeg_decompress_struct cinfo; |
| struct jpeg_error_mgr jerr; |
| ... |
| cinfo.err = jpeg_std_error(&jerr); |
| jpeg_create_decompress(&cinfo); |
| |
| (Both here and in the IJG code, we usually use variable name "cinfo" for |
| both compression and decompression objects.) |
| |
| |
| 2. Specify the source of the compressed data (eg, a file). |
| |
| As previously mentioned, the JPEG library reads compressed data from a "data |
| source" module. The library includes one data source module which knows how |
| to read from a stdio stream. You can use your own source module if you want |
| to do something else, as discussed later. |
| |
| If you use the standard source module, you must open the source stdio stream |
| beforehand. Typical code for this step looks like: |
| |
| FILE *infile; |
| ... |
| if ((infile = fopen(filename, "rb")) == NULL) { |
| fprintf(stderr, "can't open %s\n", filename); |
| exit(1); |
| } |
| jpeg_stdio_src(&cinfo, infile); |
| |
| where the last line invokes the standard source module. |
| |
| WARNING: it is critical that the binary compressed data be read unchanged. |
| On non-Unix systems the stdio library may perform newline translation or |
| otherwise corrupt binary data. To suppress this behavior, you may need to use |
| a "b" option to fopen (as shown above), or use setmode() or another routine to |
| put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that |
| has been found to work on many systems. |
| |
| You may not change the data source between calling jpeg_read_header() and |
| jpeg_finish_decompress(). If you wish to read a series of JPEG images from |
| a single source file, you should repeat the jpeg_read_header() to |
| jpeg_finish_decompress() sequence without reinitializing either the JPEG |
| object or the data source module; this prevents buffered input data from |
| being discarded. |
| |
| |
| 3. Call jpeg_read_header() to obtain image info. |
| |
| Typical code for this step is just |
| |
| jpeg_read_header(&cinfo, TRUE); |
| |
| This will read the source datastream header markers, up to the beginning |
| of the compressed data proper. On return, the image dimensions and other |
| info have been stored in the JPEG object. The application may wish to |
| consult this information before selecting decompression parameters. |
| |
| More complex code is necessary if |
| * A suspending data source is used --- in that case jpeg_read_header() |
| may return before it has read all the header data. See "I/O suspension", |
| below. The normal stdio source manager will NOT cause this to happen. |
| * Abbreviated JPEG files are to be processed --- see the section on |
| abbreviated datastreams. Standard applications that deal only in |
| interchange JPEG files need not be concerned with this case either. |
| |
| It is permissible to stop at this point if you just wanted to find out the |
| image dimensions and other header info for a JPEG file. In that case, |
| call jpeg_destroy() when you are done with the JPEG object, or call |
| jpeg_abort() to return it to an idle state before selecting a new data |
| source and reading another header. |
| |
| |
| 4. Set parameters for decompression. |
| |
| jpeg_read_header() sets appropriate default decompression parameters based on |
| the properties of the image (in particular, its colorspace). However, you |
| may well want to alter these defaults before beginning the decompression. |
| For example, the default is to produce full color output from a color file. |
| If you want colormapped output you must ask for it. Other options allow the |
| returned image to be scaled and allow various speed/quality tradeoffs to be |
| selected. "Decompression parameter selection", below, gives details. |
| |
| If the defaults are appropriate, nothing need be done at this step. |
| |
| Note that all default values are set by each call to jpeg_read_header(). |
| If you reuse a decompression object, you cannot expect your parameter |
| settings to be preserved across cycles, as you can for compression. |
| You must set desired parameter values each time. |
| |
| |
| 5. jpeg_start_decompress(...); |
| |
| Once the parameter values are satisfactory, call jpeg_start_decompress() to |
| begin decompression. This will initialize internal state, allocate working |
| memory, and prepare for returning data. |
| |
| Typical code is just |
| |
| jpeg_start_decompress(&cinfo); |
| |
| If you have requested a multi-pass operating mode, such as 2-pass color |
| quantization, jpeg_start_decompress() will do everything needed before data |
| output can begin. In this case jpeg_start_decompress() may take quite a while |
| to complete. With a single-scan (non progressive) JPEG file and default |
| decompression parameters, this will not happen; jpeg_start_decompress() will |
| return quickly. |
| |
| After this call, the final output image dimensions, including any requested |
| scaling, are available in the JPEG object; so is the selected colormap, if |
| colormapped output has been requested. Useful fields include |
| |
| output_width image width and height, as scaled |
| output_height |
| out_color_components # of color components in out_color_space |
| output_components # of color components returned per pixel |
| colormap the selected colormap, if any |
| actual_number_of_colors number of entries in colormap |
| |
| output_components is 1 (a colormap index) when quantizing colors; otherwise it |
| equals out_color_components. It is the number of JSAMPLE values that will be |
| emitted per pixel in the output arrays. |
| |
| Typically you will need to allocate data buffers to hold the incoming image. |
| You will need output_width * output_components JSAMPLEs per scanline in your |
| output buffer, and a total of output_height scanlines will be returned. |
| |
| Note: if you are using the JPEG library's internal memory manager to allocate |
| data buffers (as djpeg does), then the manager's protocol requires that you |
| request large buffers *before* calling jpeg_start_decompress(). This is a |
| little tricky since the output_XXX fields are not normally valid then. You |
| can make them valid by calling jpeg_calc_output_dimensions() after setting the |
| relevant parameters (scaling, output color space, and quantization flag). |
| |
| |
| 6. while (scan lines remain to be read) |
| jpeg_read_scanlines(...); |
| |
| Now you can read the decompressed image data by calling jpeg_read_scanlines() |
| one or more times. At each call, you pass in the maximum number of scanlines |
| to be read (ie, the height of your working buffer); jpeg_read_scanlines() |
| will return up to that many lines. The return value is the number of lines |
| actually read. The format of the returned data is discussed under "Data |
| formats", above. Don't forget that grayscale and color JPEGs will return |
| different data formats! |
| |
| Image data is returned in top-to-bottom scanline order. If you must write |
| out the image in bottom-to-top order, you can use the JPEG library's virtual |
| array mechanism to invert the data efficiently. Examples of this can be |
| found in the sample application djpeg. |
| |
| The library maintains a count of the number of scanlines returned so far |
| in the output_scanline field of the JPEG object. Usually you can just use |
| this variable as the loop counter, so that the loop test looks like |
| "while (cinfo.output_scanline < cinfo.output_height)". (Note that the test |
| should NOT be against image_height, unless you never use scaling. The |
| image_height field is the height of the original unscaled image.) |
| The return value always equals the change in the value of output_scanline. |
| |
| If you don't use a suspending data source, it is safe to assume that |
| jpeg_read_scanlines() reads at least one scanline per call, until the |
| bottom of the image has been reached. |
| |
| If you use a buffer larger than one scanline, it is NOT safe to assume that |
| jpeg_read_scanlines() fills it. (The current implementation returns only a |
| few scanlines per call, no matter how large a buffer you pass.) So you must |
| always provide a loop that calls jpeg_read_scanlines() repeatedly until the |
| whole image has been read. |
| |
| |
| 7. jpeg_finish_decompress(...); |
| |
| After all the image data has been read, call jpeg_finish_decompress() to |
| complete the decompression cycle. This causes working memory associated |
| with the JPEG object to be released. |
| |
| Typical code: |
| |
| jpeg_finish_decompress(&cinfo); |
| |
| If using the stdio source manager, don't forget to close the source stdio |
| stream if necessary. |
| |
| It is an error to call jpeg_finish_decompress() before reading the correct |
| total number of scanlines. If you wish to abort decompression, call |
| jpeg_abort() as discussed below. |
| |
| After completing a decompression cycle, you may dispose of the JPEG object as |
| discussed next, or you may use it to decompress another image. In that case |
| return to step 2 or 3 as appropriate. If you do not change the source |
| manager, the next image will be read from the same source. |
| |
| |
| 8. Release the JPEG decompression object. |
| |
| When you are done with a JPEG decompression object, destroy it by calling |
| jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of |
| destroying compression objects applies here too. |
| |
| Typical code: |
| |
| jpeg_destroy_decompress(&cinfo); |
| |
| |
| 9. Aborting. |
| |
| You can abort a decompression cycle by calling jpeg_destroy_decompress() or |
| jpeg_destroy() if you don't need the JPEG object any more, or |
| jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object. |
| The previous discussion of aborting compression cycles applies here too. |
| |
| |
| Partial image decompression |
| --------------------------- |
| |
| Partial image decompression is convenient for performance-critical applications |
| that wish to view only a portion of a large JPEG image without decompressing |
| the whole thing. It it also useful in memory-constrained environments (such as |
| on mobile devices.) This library provides the following functions to support |
| partial image decompression: |
| |
| 1. Skipping rows when decompressing |
| |
| jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines); |
| |
| This function provides application programmers with the ability to skip over |
| multiple rows in the JPEG image. |
| |
| Suspending data sources are not supported by this function. Calling |
| jpeg_skip_scanlines() with a suspending data source will result in undefined |
| behavior. Two-pass color quantization is also not supported by this function. |
| Calling jpeg_skip_scanlines() with two-pass color quantization enabled will |
| result in an error. |
| |
| jpeg_skip_scanlines() will not allow skipping past the bottom of the image. If |
| the value of num_lines is large enough to skip past the bottom of the image, |
| then the function will skip to the end of the image instead. |
| |
| If the value of num_lines is valid, then jpeg_skip_scanlines() will always |
| skip all of the input rows requested. There is no need to inspect the return |
| value of the function in that case. |
| |
| Best results will be achieved by calling jpeg_skip_scanlines() for large chunks |
| of rows. The function should be viewed as a way to quickly jump to a |
| particular vertical offset in the JPEG image in order to decode a subset of the |
| image. Used in this manner, it will provide significant performance |
| improvements. |
| |
| Calling jpeg_skip_scanlines() for small values of num_lines has several |
| potential drawbacks: |
| 1) JPEG decompression occurs in blocks, so if jpeg_skip_scanlines() is |
| called from the middle of a decompression block, then it is likely that |
| much of the decompression work has already been done for the first |
| couple of rows that need to be skipped. |
| 2) When this function returns, it must leave the decompressor in a state |
| such that it is ready to read the next line. This may involve |
| decompressing a block that must be partially skipped. |
| These issues are especially tricky for cases in which upsampling requires |
| context rows. In the worst case, jpeg_skip_scanlines() will perform similarly |
| to jpeg_read_scanlines() (since it will actually call jpeg_read_scanlines().) |
| |
| 2. Decompressing partial scanlines |
| |
| jpeg_crop_scanline (j_decompress_ptr cinfo, JDIMENSION *xoffset, |
| JDIMENSION *width) |
| |
| This function provides application programmers with the ability to decompress |
| only a portion of each row in the JPEG image. It must be called after |
| jpeg_start_decompress() and before any calls to jpeg_read_scanlines() or |
| jpeg_skip_scanlines(). |
| |
| If xoffset and width do not form a valid subset of the image row, then this |
| function will generate an error. Note that if the output image is scaled, then |
| xoffset and width are relative to the scaled image dimensions. |
| |
| xoffset and width are passed by reference because xoffset must fall on an iMCU |
| boundary. If it doesn't, then it will be moved left to the nearest iMCU |
| boundary, and width will be increased accordingly. If the calling program does |
| not like the adjusted values of xoffset and width, then it can call |
| jpeg_crop_scanline() again with new values (for instance, if it wants to move |
| xoffset to the nearest iMCU boundary to the right instead of to the left.) |
| |
| After calling this function, cinfo->output_width will be set to the adjusted |
| width. This value should be used when allocating an output buffer to pass to |
| jpeg_read_scanlines(). |
| |
| The output image from a partial-width decompression will be identical to the |
| corresponding image region from a full decode, with one exception: The "fancy" |
| (smooth) h2v2 (4:2:0) and h2v1 (4:2:2) upsampling algorithms fill in the |
| missing chroma components by averaging the chroma components from neighboring |
| pixels, except on the right and left edges of the image (where there are no |
| neighboring pixels.) When performing a partial-width decompression, these |
| "fancy" upsampling algorithms may treat the left and right edges of the partial |
| image region as if they are the left and right edges of the image, meaning that |
| the upsampling algorithm may be simplified. The result is that the pixels on |
| the left or right edge of the partial image may not be exactly identical to the |
| corresponding pixels in the original image. |
| |
| |
| Mechanics of usage: include files, linking, etc |
| ----------------------------------------------- |
| |
| Applications using the JPEG library should include the header file jpeglib.h |
| to obtain declarations of data types and routines. Before including |
| jpeglib.h, include system headers that define at least the typedefs FILE and |
| size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on |
| older Unix systems, you may need <sys/types.h> to define size_t. |
| |
| If the application needs to refer to individual JPEG library error codes, also |
| include jerror.h to define those symbols. |
| |
| jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are |
| installing the JPEG header files in a system directory, you will want to |
| install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h. |
| |
| The most convenient way to include the JPEG code into your executable program |
| is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix |
| machines) and reference it at your link step. If you use only half of the |
| library (only compression or only decompression), only that much code will be |
| included from the library, unless your linker is hopelessly brain-damaged. |
| The supplied build system builds libjpeg.a automatically. |
| |
| It may be worth pointing out that the core JPEG library does not actually |
| require the stdio library: only the default source/destination managers and |
| error handler need it. You can use the library in a stdio-less environment |
| if you replace those modules and use jmemnobs.c (or another memory manager of |
| your own devising). More info about the minimum system library requirements |
| may be found in jinclude.h. |
| |
| |
| ADVANCED FEATURES |
| ================= |
| |
| Compression parameter selection |
| ------------------------------- |
| |
| This section describes all the optional parameters you can set for JPEG |
| compression, as well as the "helper" routines provided to assist in this |
| task. Proper setting of some parameters requires detailed understanding |
| of the JPEG standard; if you don't know what a parameter is for, it's best |
| not to mess with it! See REFERENCES in the README.ijg file for pointers to |
| more info about JPEG. |
| |
| It's a good idea to call jpeg_set_defaults() first, even if you plan to set |
| all the parameters; that way your code is more likely to work with future JPEG |
| libraries that have additional parameters. For the same reason, we recommend |
| you use a helper routine where one is provided, in preference to twiddling |
| cinfo fields directly. |
| |
| The helper routines are: |
| |
| jpeg_set_defaults (j_compress_ptr cinfo) |
| This routine sets all JPEG parameters to reasonable defaults, using |
| only the input image's color space (field in_color_space, which must |
| already be set in cinfo). Many applications will only need to use |
| this routine and perhaps jpeg_set_quality(). |
| |
| jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace) |
| Sets the JPEG file's colorspace (field jpeg_color_space) as specified, |
| and sets other color-space-dependent parameters appropriately. See |
| "Special color spaces", below, before using this. A large number of |
| parameters, including all per-component parameters, are set by this |
| routine; if you want to twiddle individual parameters you should call |
| jpeg_set_colorspace() before rather than after. |
| |
| jpeg_default_colorspace (j_compress_ptr cinfo) |
| Selects an appropriate JPEG colorspace based on cinfo->in_color_space, |
| and calls jpeg_set_colorspace(). This is actually a subroutine of |
| jpeg_set_defaults(). It's broken out in case you want to change |
| just the colorspace-dependent JPEG parameters. |
| |
| jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline) |
| Constructs JPEG quantization tables appropriate for the indicated |
| quality setting. The quality value is expressed on the 0..100 scale |
| recommended by IJG (cjpeg's "-quality" switch uses this routine). |
| Note that the exact mapping from quality values to tables may change |
| in future IJG releases as more is learned about DCT quantization. |
| If the force_baseline parameter is TRUE, then the quantization table |
| entries are constrained to the range 1..255 for full JPEG baseline |
| compatibility. In the current implementation, this only makes a |
| difference for quality settings below 25, and it effectively prevents |
| very small/low quality files from being generated. The IJG decoder |
| is capable of reading the non-baseline files generated at low quality |
| settings when force_baseline is FALSE, but other decoders may not be. |
| |
| jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor, |
| boolean force_baseline) |
| Same as jpeg_set_quality() except that the generated tables are the |
| sample tables given in Annex K (Clause K.1) of |
| Rec. ITU-T T.81 (1992) | ISO/IEC 10918-1:1994, multiplied by the |
| specified scale factor (which is expressed as a percentage; thus |
| scale_factor = 100 reproduces the spec's tables). Note that larger |
| scale factors give lower quality. This entry point is useful for |
| conforming to the Adobe PostScript DCT conventions, but we do not |
| recommend linear scaling as a user-visible quality scale otherwise. |
| force_baseline again constrains the computed table entries to 1..255. |
| |
| int jpeg_quality_scaling (int quality) |
| Converts a value on the IJG-recommended quality scale to a linear |
| scaling percentage. Note that this routine may change or go away |
| in future releases --- IJG may choose to adopt a scaling method that |
| can't be expressed as a simple scalar multiplier, in which case the |
| premise of this routine collapses. Caveat user. |
| |
| jpeg_default_qtables (j_compress_ptr cinfo, boolean force_baseline) |
| [libjpeg v7+ API/ABI emulation only] |
| Set default quantization tables with linear q_scale_factor[] values |
| (see below). |
| |
| jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl, |
| const unsigned int *basic_table, |
| int scale_factor, boolean force_baseline) |
| Allows an arbitrary quantization table to be created. which_tbl |
| indicates which table slot to fill. basic_table points to an array |
| of 64 unsigned ints given in normal array order. These values are |
| multiplied by scale_factor/100 and then clamped to the range 1..65535 |
| (or to 1..255 if force_baseline is TRUE). |
| CAUTION: prior to library version 6a, jpeg_add_quant_table expected |
| the basic table to be given in JPEG zigzag order. If you need to |
| write code that works with either older or newer versions of this |
| routine, you must check the library version number. Something like |
| "#if JPEG_LIB_VERSION >= 61" is the right test. |
| |
| jpeg_simple_progression (j_compress_ptr cinfo) |
| Generates a default scan script for writing a progressive-JPEG file. |
| This is the recommended method of creating a progressive file, |
| unless you want to make a custom scan sequence. You must ensure that |
| the JPEG color space is set correctly before calling this routine. |
| |
| |
| Compression parameters (cinfo fields) include: |
| |
| boolean arith_code |
| If TRUE, use arithmetic coding. |
| If FALSE, use Huffman coding. |
| |
| J_DCT_METHOD dct_method |
| Selects the algorithm used for the DCT step. Choices are: |
| JDCT_ISLOW: accurate integer method |
| JDCT_IFAST: less accurate integer method [legacy feature] |
| JDCT_FLOAT: floating-point method [legacy feature] |
| JDCT_DEFAULT: default method (normally JDCT_ISLOW) |
| JDCT_FASTEST: fastest method (normally JDCT_IFAST) |
| When the Independent JPEG Group's software was first released in 1991, |
| the compression time for a 1-megapixel JPEG image on a mainstream PC |
| was measured in minutes. Thus, JDCT_IFAST provided noticeable |
| performance benefits. On modern CPUs running libjpeg-turbo, however, |
| the compression time for a 1-megapixel JPEG image is measured in |
| milliseconds, and thus the performance benefits of JDCT_IFAST are much |
| less noticeable. On modern x86/x86-64 CPUs that support AVX2 |
| instructions, JDCT_IFAST and JDCT_ISLOW have similar performance. On |
| other types of CPUs, JDCT_IFAST is generally about 5-15% faster than |
| JDCT_ISLOW. |
| |
| For quality levels of 90 and below, there should be little or no |
| perceptible quality difference between the two algorithms. For quality |
| levels above 90, however, the difference between JDCT_IFAST and |
| JDCT_ISLOW becomes more pronounced. With quality=97, for instance, |
| JDCT_IFAST incurs generally about a 1-3 dB loss in PSNR relative to |
| JDCT_ISLOW, but this can be larger for some images. Do not use |
| JDCT_IFAST with quality levels above 97. The algorithm often |
| degenerates at quality=98 and above and can actually produce a more |
| lossy image than if lower quality levels had been used. Also, in |
| libjpeg-turbo, JDCT_IFAST is not fully accelerated for quality levels |
| above 97, so it will be slower than JDCT_ISLOW. |
| |
| JDCT_FLOAT does not produce significantly more accurate results than |
| JDCT_ISLOW, and it is much slower. JDCT_FLOAT may also give different |
| results on different machines due to varying roundoff behavior, whereas |
| the integer methods should give the same results on all machines. |
| |
| J_COLOR_SPACE jpeg_color_space |
| int num_components |
| The JPEG color space and corresponding number of components; see |
| "Special color spaces", below, for more info. We recommend using |
| jpeg_set_color_space() if you want to change these. |
| |
| boolean optimize_coding |
| TRUE causes the compressor to compute optimal Huffman coding tables |
| for the image. This requires an extra pass over the data and |
| therefore costs a good deal of space and time. The default is |
| FALSE, which tells the compressor to use the supplied or default |
| Huffman tables. In most cases optimal tables save only a few percent |
| of file size compared to the default tables. Note that when this is |
| TRUE, you need not supply Huffman tables at all, and any you do |
| supply will be overwritten. |
| |
| unsigned int restart_interval |
| int restart_in_rows |
| To emit restart markers in the JPEG file, set one of these nonzero. |
| Set restart_interval to specify the exact interval in MCUs. Set |
| restart_in_rows to specify the interval in MCU rows. (If |
| restart_in_rows is not 0, then restart_interval is set after the image |
| width in MCUs is computed.) Defaults are zero (no restarts). One |
| restart marker per MCU row is often a good choice. NOTE: the overhead |
| of restart markers is higher in grayscale JPEG files than in color |
| files, and MUCH higher in progressive JPEGs. If you use restarts, you |
| may want to use larger intervals in those cases. |
| |
| const jpeg_scan_info *scan_info |
| int num_scans |
| By default, scan_info is NULL; this causes the compressor to write a |
| single-scan sequential JPEG file. If not NULL, scan_info points to |
| an array of scan definition records of length num_scans. The |
| compressor will then write a JPEG file having one scan for each scan |
| definition record. This is used to generate noninterleaved or |
| progressive JPEG files. The library checks that the scan array |
| defines a valid JPEG scan sequence. (jpeg_simple_progression creates |
| a suitable scan definition array for progressive JPEG.) This is |
| discussed further under "Progressive JPEG support". |
| |
| int smoothing_factor |
| If non-zero, the input image is smoothed; the value should be 1 for |
| minimal smoothing to 100 for maximum smoothing. Consult jcsample.c |
| for details of the smoothing algorithm. The default is zero. |
| |
| boolean write_JFIF_header |
| If TRUE, a JFIF APP0 marker is emitted. jpeg_set_defaults() and |
| jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space |
| (ie, YCbCr or grayscale) is selected, otherwise FALSE. |
| |
| UINT8 JFIF_major_version |
| UINT8 JFIF_minor_version |
| The version number to be written into the JFIF marker. |
| jpeg_set_defaults() initializes the version to 1.01 (major=minor=1). |
| You should set it to 1.02 (major=1, minor=2) if you plan to write |
| any JFIF 1.02 extension markers. |
| |
| UINT8 density_unit |
| UINT16 X_density |
| UINT16 Y_density |
| The resolution information to be written into the JFIF marker; |
| not used otherwise. density_unit may be 0 for unknown, |
| 1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1 |
| indicating square pixels of unknown size. |
| |
| boolean write_Adobe_marker |
| If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and |
| jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK, |
| or YCCK is selected, otherwise FALSE. It is generally a bad idea |
| to set both write_JFIF_header and write_Adobe_marker. In fact, |
| you probably shouldn't change the default settings at all --- the |
| default behavior ensures that the JPEG file's color space can be |
| recognized by the decoder. |
| |
| JQUANT_TBL *quant_tbl_ptrs[NUM_QUANT_TBLS] |
| Pointers to coefficient quantization tables, one per table slot, |
| or NULL if no table is defined for a slot. Usually these should |
| be set via one of the above helper routines; jpeg_add_quant_table() |
| is general enough to define any quantization table. The other |
| routines will set up table slot 0 for luminance quality and table |
| slot 1 for chrominance. |
| |
| int q_scale_factor[NUM_QUANT_TBLS] |
| [libjpeg v7+ API/ABI emulation only] |
| Linear quantization scaling factors (0-100, default 100) |
| for use with jpeg_default_qtables(). |
| See rdswitch.c and cjpeg.c for an example of usage. |
| Note that the q_scale_factor[] values use "linear" scales, so JPEG |
| quality levels chosen by the user must be converted to these scales |
| using jpeg_quality_scaling(). Here is an example that corresponds to |
| cjpeg -quality 90,70: |
| |
| jpeg_set_defaults(cinfo); |
| |
| /* Set luminance quality 90. */ |
| cinfo->q_scale_factor[0] = jpeg_quality_scaling(90); |
| /* Set chrominance quality 70. */ |
| cinfo->q_scale_factor[1] = jpeg_quality_scaling(70); |
| |
| jpeg_default_qtables(cinfo, force_baseline); |
| |
| CAUTION: Setting separate quality levels for chrominance and luminance |
| is mainly only useful if chrominance subsampling is disabled. 2x2 |
| chrominance subsampling (AKA "4:2:0") is the default, but you can |
| explicitly disable subsampling as follows: |
| |
| cinfo->comp_info[0].v_samp_factor = 1; |
| cinfo->comp_info[0].h_samp_factor = 1; |
| |
| JHUFF_TBL *dc_huff_tbl_ptrs[NUM_HUFF_TBLS] |
| JHUFF_TBL *ac_huff_tbl_ptrs[NUM_HUFF_TBLS] |
| Pointers to Huffman coding tables, one per table slot, or NULL if |
| no table is defined for a slot. Slots 0 and 1 are filled with the |
| JPEG sample tables by jpeg_set_defaults(). If you need to allocate |
| more table structures, jpeg_alloc_huff_table() may be used. |
| Note that optimal Huffman tables can be computed for an image |
| by setting optimize_coding, as discussed above; there's seldom |
| any need to mess with providing your own Huffman tables. |
| |
| |
| [libjpeg v7+ API/ABI emulation only] |
| The actual dimensions of the JPEG image that will be written to the file are |
| given by the following fields. These are computed from the input image |
| dimensions and the compression parameters by jpeg_start_compress(). You can |
| also call jpeg_calc_jpeg_dimensions() to obtain the values that will result |
| from the current parameter settings. This can be useful if you are trying |
| to pick a scaling ratio that will get close to a desired target size. |
| |
| JDIMENSION jpeg_width Actual dimensions of output image. |
| JDIMENSION jpeg_height |
| |
| |
| Per-component parameters are stored in the struct cinfo.comp_info[i] for |
| component number i. Note that components here refer to components of the |
| JPEG color space, *not* the source image color space. A suitably large |
| comp_info[] array is allocated by jpeg_set_defaults(); if you choose not |
| to use that routine, it's up to you to allocate the array. |
| |
| int component_id |
| The one-byte identifier code to be recorded in the JPEG file for |
| this component. For the standard color spaces, we recommend you |
| leave the default values alone. |
| |
| int h_samp_factor |
| int v_samp_factor |
| Horizontal and vertical sampling factors for the component; must |
| be 1..4 according to the JPEG standard. Note that larger sampling |
| factors indicate a higher-resolution component; many people find |
| this behavior quite unintuitive. The default values are 2,2 for |
| luminance components and 1,1 for chrominance components, except |
| for grayscale where 1,1 is used. |
| |
| int quant_tbl_no |
| Quantization table number for component. The default value is |
| 0 for luminance components and 1 for chrominance components. |
| |
| int dc_tbl_no |
| int ac_tbl_no |
| DC and AC entropy coding table numbers. The default values are |
| 0 for luminance components and 1 for chrominance components. |
| |
| int component_index |
| Must equal the component's index in comp_info[]. (Beginning in |
| release v6, the compressor library will fill this in automatically; |
| you don't have to.) |
| |
| |
| Decompression parameter selection |
| --------------------------------- |
| |
| Decompression parameter selection is somewhat simpler than compression |
| parameter selection, since all of the JPEG internal parameters are |
| recorded in the source file and need not be supplied by the application. |
| (Unless you are working with abbreviated files, in which case see |
| "Abbreviated datastreams", below.) Decompression parameters control |
| the postprocessing done on the image to deliver it in a format suitable |
| for the application's use. Many of the parameters control speed/quality |
| tradeoffs, in which faster decompression may be obtained at the price of |
| a poorer-quality image. The defaults select the highest quality (slowest) |
| processing. |
| |
| The following fields in the JPEG object are set by jpeg_read_header() and |
| may be useful to the application in choosing decompression parameters: |
| |
| JDIMENSION image_width Width and height of image |
| JDIMENSION image_height |
| int num_components Number of color components |
| J_COLOR_SPACE jpeg_color_space Colorspace of image |
| boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen |
| UINT8 JFIF_major_version Version information from JFIF marker |
| UINT8 JFIF_minor_version |
| UINT8 density_unit Resolution data from JFIF marker |
| UINT16 X_density |
| UINT16 Y_density |
| boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen |
| UINT8 Adobe_transform Color transform code from Adobe marker |
| |
| The JPEG color space, unfortunately, is something of a guess since the JPEG |
| standard proper does not provide a way to record it. In practice most files |
| adhere to the JFIF or Adobe conventions, and the decoder will recognize these |
| correctly. See "Special color spaces", below, for more info. |
| |
| |
| The decompression parameters that determine the basic properties of the |
| returned image are: |
| |
| J_COLOR_SPACE out_color_space |
| Output color space. jpeg_read_header() sets an appropriate default |
| based on jpeg_color_space; typically it will be RGB or grayscale. |
| The application can change this field to request output in a different |
| colorspace. For example, set it to JCS_GRAYSCALE to get grayscale |
| output from a color file. (This is useful for previewing: grayscale |
| output is faster than full color since the color components need not |
| be processed.) Note that not all possible color space transforms are |
| currently implemented; you may need to extend jdcolor.c if you want an |
| unusual conversion. |
| |
| unsigned int scale_num, scale_denom |
| Scale the image by the fraction scale_num/scale_denom. Default is |
| 1/1, or no scaling. Currently, the only supported scaling ratios |
| are M/8 with all M from 1 to 16, or any reduced fraction thereof (such |
| as 1/2, 3/4, etc.) (The library design allows for arbitrary |
| scaling ratios but this is not likely to be implemented any time soon.) |
| Smaller scaling ratios permit significantly faster decoding since |
| fewer pixels need be processed and a simpler IDCT method can be used. |
| |
| boolean quantize_colors |
| If set TRUE, colormapped output will be delivered. Default is FALSE, |
| meaning that full-color output will be delivered. |
| |
| The next three parameters are relevant only if quantize_colors is TRUE. |
| |
| int desired_number_of_colors |
| Maximum number of colors to use in generating a library-supplied color |
| map (the actual number of colors is returned in a different field). |
| Default 256. Ignored when the application supplies its own color map. |
| |
| boolean two_pass_quantize |
| If TRUE, an extra pass over the image is made to select a custom color |
| map for the image. This usually looks a lot better than the one-size- |
| fits-all colormap that is used otherwise. Default is TRUE. Ignored |
| when the application supplies its own color map. |
| |
| J_DITHER_MODE dither_mode |
| Selects color dithering method. Supported values are: |
| JDITHER_NONE no dithering: fast, very low quality |
| JDITHER_ORDERED ordered dither: moderate speed and quality |
| JDITHER_FS Floyd-Steinberg dither: slow, high quality |
| Default is JDITHER_FS. (At present, ordered dither is implemented |
| only in the single-pass, standard-colormap case. If you ask for |
| ordered dither when two_pass_quantize is TRUE or when you supply |
| an external color map, you'll get F-S dithering.) |
| |
| When quantize_colors is TRUE, the target color map is described by the next |
| two fields. colormap is set to NULL by jpeg_read_header(). The application |
| can supply a color map by setting colormap non-NULL and setting |
| actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress() |
| selects a suitable color map and sets these two fields itself. |
| [Implementation restriction: at present, an externally supplied colormap is |
| only accepted for 3-component output color spaces.] |
| |
| JSAMPARRAY colormap |
| The color map, represented as a 2-D pixel array of out_color_components |
| rows and actual_number_of_colors columns. Ignored if not quantizing. |
| CAUTION: if the JPEG library creates its own colormap, the storage |
| pointed to by this field is released by jpeg_finish_decompress(). |
| Copy the colormap somewhere else first, if you want to save it. |
| |
| int actual_number_of_colors |
| The number of colors in the color map. |
| |
| Additional decompression parameters that the application may set include: |
| |
| J_DCT_METHOD dct_method |
| Selects the algorithm used for the DCT step. Choices are: |
| JDCT_ISLOW: accurate integer method |
| JDCT_IFAST: less accurate integer method [legacy feature] |
| JDCT_FLOAT: floating-point method [legacy feature] |
| JDCT_DEFAULT: default method (normally JDCT_ISLOW) |
| JDCT_FASTEST: fastest method (normally JDCT_IFAST) |
| When the Independent JPEG Group's software was first released in 1991, |
| the decompression time for a 1-megapixel JPEG image on a mainstream PC |
| was measured in minutes. Thus, JDCT_IFAST provided noticeable |
| performance benefits. On modern CPUs running libjpeg-turbo, however, |
| the decompression time for a 1-megapixel JPEG image is measured in |
| milliseconds, and thus the performance benefits of JDCT_IFAST are much |
| less noticeable. On modern x86/x86-64 CPUs that support AVX2 |
| instructions, JDCT_IFAST and JDCT_ISLOW have similar performance. On |
| other types of CPUs, JDCT_IFAST is generally about 5-15% faster than |
| JDCT_ISLOW. |
| |
| If the JPEG image was compressed using a quality level of 85 or below, |
| then there should be little or no perceptible quality difference |
| between the two algorithms. When decompressing images that were |
| compressed using quality levels above 85, however, the difference |
| between JDCT_IFAST and JDCT_ISLOW becomes more pronounced. With images |
| compressed using quality=97, for instance, JDCT_IFAST incurs generally |
| about a 4-6 dB loss in PSNR relative to JDCT_ISLOW, but this can be |
| larger for some images. If you can avoid it, do not use JDCT_IFAST |
| when decompressing images that were compressed using quality levels |
| above 97. The algorithm often degenerates for such images and can |
| actually produce a more lossy output image than if the JPEG image had |
| been compressed using lower quality levels. |
| |
| JDCT_FLOAT does not produce significantly more accurate results than |
| JDCT_ISLOW, and it is much slower. JDCT_FLOAT may also give different |
| results on different machines due to varying roundoff behavior, whereas |
| the integer methods should give the same results on all machines. |
| |
| boolean do_fancy_upsampling |
| If TRUE, do careful upsampling of chroma components. If FALSE, |
| a faster but sloppier method is used. Default is TRUE. The visual |
| impact of the sloppier method is often very small. |
| |
| boolean do_block_smoothing |
| If TRUE, interblock smoothing is applied in early stages of decoding |
| progressive JPEG files; if FALSE, not. Default is TRUE. Early |
| progression stages look "fuzzy" with smoothing, "blocky" without. |
| In any case, block smoothing ceases to be applied after the first few |
| AC coefficients are known to full accuracy, so it is relevant only |
| when using buffered-image mode for progressive images. |
| |
| boolean enable_1pass_quant |
| boolean enable_external_quant |
| boolean enable_2pass_quant |
| These are significant only in buffered-image mode, which is |
| described in its own section below. |
| |
| |
| The output image dimensions are given by the following fields. These are |
| computed from the source image dimensions and the decompression parameters |
| by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions() |
| to obtain the values that will result from the current parameter settings. |
| This can be useful if you are trying to pick a scaling ratio that will get |
| close to a desired target size. It's also important if you are using the |
| JPEG library's memory manager to allocate output buffer space, because you |
| are supposed to request such buffers *before* jpeg_start_decompress(). |
| |
| JDIMENSION output_width Actual dimensions of output image. |
| JDIMENSION output_height |
| int out_color_components Number of color components in out_color_space. |
| int output_components Number of color components returned. |
| int rec_outbuf_height Recommended height of scanline buffer. |
| |
| When quantizing colors, output_components is 1, indicating a single color map |
| index per pixel. Otherwise it equals out_color_components. The output arrays |
| are required to be output_width * output_components JSAMPLEs wide. |
| |
| rec_outbuf_height is the recommended minimum height (in scanlines) of the |
| buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the |
| library will still work, but time will be wasted due to unnecessary data |
| copying. In high-quality modes, rec_outbuf_height is always 1, but some |
| faster, lower-quality modes set it to larger values (typically 2 to 4). |
| If you are going to ask for a high-speed processing mode, you may as well |
| go to the trouble of honoring rec_outbuf_height so as to avoid data copying. |
| (An output buffer larger than rec_outbuf_height lines is OK, but won't |
| provide any material speed improvement over that height.) |
| |
| |
| Special color spaces |
| -------------------- |
| |
| The JPEG standard itself is "color blind" and doesn't specify any particular |
| color space. It is customary to convert color data to a luminance/chrominance |
| color space before compressing, since this permits greater compression. The |
| existing de-facto JPEG file format standards specify YCbCr or grayscale data |
| (JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special |
| applications such as multispectral images, other color spaces can be used, |
| but it must be understood that such files will be unportable. |
| |
| The JPEG library can handle the most common colorspace conversions (namely |
| RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown |
| color space, passing it through without conversion. If you deal extensively |
| with an unusual color space, you can easily extend the library to understand |
| additional color spaces and perform appropriate conversions. |
| |
| For compression, the source data's color space is specified by field |
| in_color_space. This is transformed to the JPEG file's color space given |
| by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color |
| space depending on in_color_space, but you can override this by calling |
| jpeg_set_colorspace(). Of course you must select a supported transformation. |
| jccolor.c currently supports the following transformations: |
| RGB => YCbCr |
| RGB => GRAYSCALE |
| YCbCr => GRAYSCALE |
| CMYK => YCCK |
| plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB, |
| YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN. |
| |
| The de-facto file format standards (JFIF and Adobe) specify APPn markers that |
| indicate the color space of the JPEG file. It is important to ensure that |
| these are written correctly, or omitted if the JPEG file's color space is not |
| one of the ones supported by the de-facto standards. jpeg_set_colorspace() |
| will set the compression parameters to include or omit the APPn markers |
| properly, so long as it is told the truth about the JPEG color space. |
| For example, if you are writing some random 3-component color space without |
| conversion, don't try to fake out the library by setting in_color_space and |
| jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an |
| APPn marker of your own devising to identify the colorspace --- see "Special |
| markers", below. |
| |
| When told that the color space is UNKNOWN, the library will default to using |
| luminance-quality compression parameters for all color components. You may |
| well want to change these parameters. See the source code for |
| jpeg_set_colorspace(), in jcparam.c, for details. |
| |
| For decompression, the JPEG file's color space is given in jpeg_color_space, |
| and this is transformed to the output color space out_color_space. |
| jpeg_read_header's setting of jpeg_color_space can be relied on if the file |
| conforms to JFIF or Adobe conventions, but otherwise it is no better than a |
| guess. If you know the JPEG file's color space for certain, you can override |
| jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also |
| selects a default output color space based on (its guess of) jpeg_color_space; |
| set out_color_space to override this. Again, you must select a supported |
| transformation. jdcolor.c currently supports |
| YCbCr => RGB |
| YCbCr => GRAYSCALE |
| RGB => GRAYSCALE |
| GRAYSCALE => RGB |
| YCCK => CMYK |
| as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an |
| application can force grayscale JPEGs to look like color JPEGs if it only |
| wants to handle one case.) |
| |
| The two-pass color quantizer, jquant2.c, is specialized to handle RGB data |
| (it weights distances appropriately for RGB colors). You'll need to modify |
| the code if you want to use it for non-RGB output color spaces. Note that |
| jquant2.c is used to map to an application-supplied colormap as well as for |
| the normal two-pass colormap selection process. |
| |
| CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG |
| files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect. |
| This is arguably a bug in Photoshop, but if you need to work with Photoshop |
| CMYK files, you will have to deal with it in your application. We cannot |
| "fix" this in the library by inverting the data during the CMYK<=>YCCK |
| transform, because that would break other applications, notably Ghostscript. |
| Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK |
| data in the same inverted-YCCK representation used in bare JPEG files, but |
| the surrounding PostScript code performs an inversion using the PS image |
| operator. I am told that Photoshop 3.0 will write uninverted YCCK in |
| EPS/JPEG files, and will omit the PS-level inversion. (But the data |
| polarity used in bare JPEG files will not change in 3.0.) In either case, |
| the JPEG library must not invert the data itself, or else Ghostscript would |
| read these EPS files incorrectly. |
| |
| |
| Error handling |
| -------------- |
| |
| When the default error handler is used, any error detected inside the JPEG |
| routines will cause a message to be printed on stderr, followed by exit(). |
| You can supply your own error handling routines to override this behavior |
| and to control the treatment of nonfatal warnings and trace/debug messages. |
| The file example.txt illustrates the most common case, which is to have the |
| application regain control after an error rather than exiting. |
| |
| The JPEG library never writes any message directly; it always goes through |
| the error handling routines. Three classes of messages are recognized: |
| * Fatal errors: the library cannot continue. |
| * Warnings: the library can continue, but the data is corrupt, and a |
| damaged output image is likely to result. |
| * Trace/informational messages. These come with a trace level indicating |
| the importance of the message; you can control the verbosity of the |
| program by adjusting the maximum trace level that will be displayed. |
| |
| You may, if you wish, simply replace the entire JPEG error handling module |
| (jerror.c) with your own code. However, you can avoid code duplication by |
| only replacing some of the routines depending on the behavior you need. |
| This is accomplished by calling jpeg_std_error() as usual, but then overriding |
| some of the method pointers in the jpeg_error_mgr struct, as illustrated by |
| example.txt. |
| |
| All of the error handling routines will receive a pointer to the JPEG object |
| (a j_common_ptr which points to either a jpeg_compress_struct or a |
| jpeg_decompress_struct; if you need to tell which, test the is_decompressor |
| field). This struct includes a pointer to the error manager struct in its |
| "err" field. Frequently, custom error handler routines will need to access |
| additional data which is not known to the JPEG library or the standard error |
| handler. The most convenient way to do this is to embed either the JPEG |
| object or the jpeg_error_mgr struct in a larger structure that contains |
| additional fields; then casting the passed pointer provides access to the |
| additional fields. Again, see example.txt for one way to do it. (Beginning |
| with IJG version 6b, there is also a void pointer "client_data" in each |
| JPEG object, which the application can also use to find related data. |
| The library does not touch client_data at all.) |
| |
| The individual methods that you might wish to override are: |
| |
| error_exit (j_common_ptr cinfo) |
| Receives control for a fatal error. Information sufficient to |
| generate the error message has been stored in cinfo->err; call |
| output_message to display it. Control must NOT return to the caller; |
| generally this routine will exit() or longjmp() somewhere. |
| Typically you would override this routine to get rid of the exit() |
| default behavior. Note that if you continue processing, you should |
| clean up the JPEG object with jpeg_abort() or jpeg_destroy(). |
| |
| output_message (j_common_ptr cinfo) |
| Actual output of any JPEG message. Override this to send messages |
| somewhere other than stderr. Note that this method does not know |
| how to generate a message, only where to send it. |
| |
| format_message (j_common_ptr cinfo, char *buffer) |
| Constructs a readable error message string based on the error info |
| stored in cinfo->err. This method is called by output_message. Few |
| applications should need to override this method. One possible |
| reason for doing so is to implement dynamic switching of error message |
| language. |
| |
| emit_message (j_common_ptr cinfo, int msg_level) |
| Decide whether or not to emit a warning or trace message; if so, |
| calls output_message. The main reason for overriding this method |
| would be to abort on warnings. msg_level is -1 for warnings, |
| 0 and up for trace messages. |
| |
| Only error_exit() and emit_message() are called from the rest of the JPEG |
| library; the other two are internal to the error handler. |
| |
| The actual message texts are stored in an array of strings which is pointed to |
| by the field err->jpeg_message_table. The messages are numbered from 0 to |
| err->last_jpeg_message, and it is these code numbers that are used in the |
| JPEG library code. You could replace the message texts (for instance, with |
| messages in French or German) by changing the message table pointer. See |
| jerror.h for the default texts. CAUTION: this table will almost certainly |
| change or grow from one library version to the next. |
| |
| It may be useful for an application to add its own message texts that are |
| handled by the same mechanism. The error handler supports a second "add-on" |
| message table for this purpose. To define an addon table, set the pointer |
| err->addon_message_table and the message numbers err->first_addon_message and |
| err->last_addon_message. If you number the addon messages beginning at 1000 |
| or so, you won't have to worry about conflicts with the library's built-in |
| messages. See the sample applications cjpeg/djpeg for an example of using |
| addon messages (the addon messages are defined in cderror.h). |
| |
| Actual invocation of the error handler is done via macros defined in jerror.h: |
| ERREXITn(...) for fatal errors |
| WARNMSn(...) for corrupt-data warnings |
| TRACEMSn(...) for trace and informational messages. |
| These macros store the message code and any additional parameters into the |
| error handler struct, then invoke the error_exit() or emit_message() method. |
| The variants of each macro are for varying numbers of additional parameters. |
| The additional parameters are inserted into the generated message using |
| standard printf() format codes. |
| |
| See jerror.h and jerror.c for further details. |
| |
| |
| Compressed data handling (source and destination managers) |
| ---------------------------------------------------------- |
| |
| The JPEG compression library sends its compressed data to a "destination |
| manager" module. The default destination manager just writes the data to a |
| memory buffer or to a stdio stream, but you can provide your own manager to |
| do something else. Similarly, the decompression library calls a "source |
| manager" to obtain the compressed data; you can provide your own source |
| manager if you want the data to come from somewhere other than a memory |
| buffer or a stdio stream. |
| |
| In both cases, compressed data is processed a bufferload at a time: the |
| destination or source manager provides a work buffer, and the library invokes |
| the manager only when the buffer is filled or emptied. (You could define a |
| one-character buffer to force the manager to be invoked for each byte, but |
| that would be rather inefficient.) The buffer's size and location are |
| controlled by the manager, not by the library. For example, the memory |
| source manager just makes the buffer pointer and length point to the original |
| data in memory. In this case the buffer-reload procedure will be invoked |
| only if the decompressor ran off the end of the datastream, which would |
| indicate an erroneous datastream. |
| |
| The work buffer is defined as an array of datatype JOCTET, which is generally |
| "char" or "unsigned char". On a machine where char is not exactly 8 bits |
| wide, you must define JOCTET as a wider data type and then modify the data |
| source and destination modules to transcribe the work arrays into 8-bit units |
| on external storage. |
| |
| A data destination manager struct contains a pointer and count defining the |
| next byte to write in the work buffer and the remaining free space: |
| |
| JOCTET *next_output_byte; /* => next byte to write in buffer */ |
| size_t free_in_buffer; /* # of byte spaces remaining in buffer */ |
| |
| The library increments the pointer and decrements the count until the buffer |
| is filled. The manager's empty_output_buffer method must reset the pointer |
| and count. The manager is expected to remember the buffer's starting address |
| and total size in private fields not visible to the library. |
| |
| A data destination manager provides three methods: |
| |
| init_destination (j_compress_ptr cinfo) |
| Initialize destination. This is called by jpeg_start_compress() |
| before any data is actually written. It must initialize |
| next_output_byte and free_in_buffer. free_in_buffer must be |
| initialized to a positive value. |
| |
| empty_output_buffer (j_compress_ptr cinfo) |
| This is called whenever the buffer has filled (free_in_buffer |
| reaches zero). In typical applications, it should write out the |
| *entire* buffer (use the saved start address and buffer length; |
| ignore the current state of next_output_byte and free_in_buffer). |
| Then reset the pointer & count to the start of the buffer, and |
| return TRUE indicating that the buffer has been dumped. |
| free_in_buffer must be set to a positive value when TRUE is |
| returned. A FALSE return should only be used when I/O suspension is |
| desired (this operating mode is discussed in the next section). |
| |
| term_destination (j_compress_ptr cinfo) |
| Terminate destination --- called by jpeg_finish_compress() after all |
| data has been written. In most applications, this must flush any |
| data remaining in the buffer. Use either next_output_byte or |
| free_in_buffer to determine how much data is in the buffer. |
| |
| term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you |
| want the destination manager to be cleaned up during an abort, you must do it |
| yourself. |
| |
| You will also need code to create a jpeg_destination_mgr struct, fill in its |
| method pointers, and insert a pointer to the struct into the "dest" field of |
| the JPEG compression object. This can be done in-line in your setup code if |
| you like, but it's probably cleaner to provide a separate routine similar to |
| the jpeg_stdio_dest() or jpeg_mem_dest() routines of the supplied destination |
| managers. |
| |
| Decompression source managers follow a parallel design, but with some |
| additional frammishes. The source manager struct contains a pointer and count |
| defining the next byte to read from the work buffer and the number of bytes |
| remaining: |
| |
| const JOCTET *next_input_byte; /* => next byte to read from buffer */ |
| size_t bytes_in_buffer; /* # of bytes remaining in buffer */ |
| |
| The library increments the pointer and decrements the count until the buffer |
| is emptied. The manager's fill_input_buffer method must reset the pointer and |
| count. In most applications, the manager must remember the buffer's starting |
| address and total size in private fields not visible to the library. |
| |
| A data source manager provides five methods: |
| |
| init_source (j_decompress_ptr cinfo) |
| Initialize source. This is called by jpeg_read_header() before any |
| data is actually read. Unlike init_destination(), it may leave |
| bytes_in_buffer set to 0 (in which case a fill_input_buffer() call |
| will occur immediately). |
| |
| fill_input_buffer (j_decompress_ptr cinfo) |
| This is called whenever bytes_in_buffer has reached zero and more |
| data is wanted. In typical applications, it should read fresh data |
| into the buffer (ignoring the current state of next_input_byte and |
| bytes_in_buffer), reset the pointer & count to the start of the |
| buffer, and return TRUE indicating that the buffer has been reloaded. |
| It is not necessary to fill the buffer entirely, only to obtain at |
| least one more byte. bytes_in_buffer MUST be set to a positive value |
| if TRUE is returned. A FALSE return should only be used when I/O |
| suspension is desired (this mode is discussed in the next section). |
| |
| skip_input_data (j_decompress_ptr cinfo, long num_bytes) |
| Skip num_bytes worth of data. The buffer pointer and count should |
| be advanced over num_bytes input bytes, refilling the buffer as |
| needed. This is used to skip over a potentially large amount of |
| uninteresting data (such as an APPn marker). In some applications |
| it may be possible to optimize away the reading of the skipped data, |
| but it's not clear that being smart is worth much trouble; large |
| skips are uncommon. bytes_in_buffer may be zero on return. |
| A zero or negative skip count should be treated as a no-op. |
| |
| resync_to_restart (j_decompress_ptr cinfo, int desired) |
| This routine is called only when the decompressor has failed to find |
| a restart (RSTn) marker where one is expected. Its mission is to |
| find a suitable point for resuming decompression. For most |
| applications, we recommend that you just use the default resync |
| procedure, jpeg_resync_to_restart(). However, if you are able to back |
| up in the input data stream, or if you have a-priori knowledge about |
| the likely location of restart markers, you may be able to do better. |
| Read the read_restart_marker() and jpeg_resync_to_restart() routines |
| in jdmarker.c if you think you'd like to implement your own resync |
| procedure. |
| |
| term_source (j_decompress_ptr cinfo) |
| Terminate source --- called by jpeg_finish_decompress() after all |
| data has been read. Often a no-op. |
| |
| For both fill_input_buffer() and skip_input_data(), there is no such thing |
| as an EOF return. If the end of the file has been reached, the routine has |
| a choice of exiting via ERREXIT() or inserting fake data into the buffer. |
| In most cases, generating a warning message and inserting a fake EOI marker |
| is the best course of action --- this will allow the decompressor to output |
| however much of the image is there. In pathological cases, the decompressor |
| may swallow the EOI and again demand data ... just keep feeding it fake EOIs. |
| jdatasrc.c illustrates the recommended error recovery behavior. |
| |
| term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want |
| the source manager to be cleaned up during an abort, you must do it yourself. |
| |
| You will also need code to create a jpeg_source_mgr struct, fill in its method |
| pointers, and insert a pointer to the struct into the "src" field of the JPEG |
| decompression object. This can be done in-line in your setup code if you |
| like, but it's probably cleaner to provide a separate routine similar to the |
| jpeg_stdio_src() or jpeg_mem_src() routines of the supplied source managers. |
| |
| For more information, consult the memory and stdio source and destination |
| managers in jdatasrc.c and jdatadst.c. |
| |
| |
| I/O suspension |
| -------------- |
| |
| Some applications need to use the JPEG library as an incremental memory-to- |
| memory filter: when the compressed data buffer is filled or emptied, they want |
| control to return to the outer loop, rather than expecting that the buffer can |
| be emptied or reloaded within the data source/destination manager subroutine. |
| The library supports this need by providing an "I/O suspension" mode, which we |
| describe in this section. |
| |
| The I/O suspension mode is not a panacea: nothing is guaranteed about the |
| maximum amount of time spent in any one call to the library, so it will not |
| eliminate response-time problems in single-threaded applications. If you |
| need guaranteed response time, we suggest you "bite the bullet" and implement |
| a real multi-tasking capability. |
| |
| To use I/O suspension, cooperation is needed between the calling application |
| and the data source or destination manager; you will always need a custom |
| source/destination manager. (Please read the previous section if you haven't |
| already.) The basic idea is that the empty_output_buffer() or |
| fill_input_buffer() routine is a no-op, merely returning FALSE to indicate |
| that it has done nothing. Upon seeing this, the JPEG library suspends |
| operation and returns to its caller. The surrounding application is |
| responsible for emptying or refilling the work buffer before calling the |
| JPEG library again. |
| |
| Compression suspension: |
| |
| For compression suspension, use an empty_output_buffer() routine that returns |
| FALSE; typically it will not do anything else. This will cause the |
| compressor to return to the caller of jpeg_write_scanlines(), with the return |
| value indicating that not all the supplied scanlines have been accepted. |
| The application must make more room in the output buffer, adjust the output |
| buffer pointer/count appropriately, and then call jpeg_write_scanlines() |
| again, pointing to the first unconsumed scanline. |
| |
| When forced to suspend, the compressor will backtrack to a convenient stopping |
| point (usually the start of the current MCU); it will regenerate some output |
| data when restarted. Therefore, although empty_output_buffer() is only |
| called when the buffer is filled, you should NOT write out the entire buffer |
| after a suspension. Write only the data up to the current position of |
| next_output_byte/free_in_buffer. The data beyond that point will be |
| regenerated after resumption. |
| |
| Because of the backtracking behavior, a good-size output buffer is essential |
| for efficiency; you don't want the compressor to suspend often. (In fact, an |
| overly small buffer could lead to infinite looping, if a single MCU required |
| more data than would fit in the buffer.) We recommend a buffer of at least |
| several Kbytes. You may want to insert explicit code to ensure that you don't |
| call jpeg_write_scanlines() unless there is a reasonable amount of space in |
| the output buffer; in other words, flush the buffer before trying to compress |
| more data. |
| |
| The compressor does not allow suspension while it is trying to write JPEG |
| markers at the beginning and end of the file. This means that: |
| * At the beginning of a compression operation, there must be enough free |
| space in the output buffer to hold the header markers (typically 600 or |
| so bytes). The recommended buffer size is bigger than this anyway, so |
| this is not a problem as long as you start with an empty buffer. However, |
| this restriction might catch you if you insert large special markers, such |
| as a JFIF thumbnail image, without flushing the buffer afterwards. |
| * When you call jpeg_finish_compress(), there must be enough space in the |
| output buffer to emit any buffered data and the final EOI marker. In the |
| current implementation, half a dozen bytes should suffice for this, but |
| for safety's sake we recommend ensuring that at least 100 bytes are free |
| before calling jpeg_finish_compress(). |
| |
| A more significant restriction is that jpeg_finish_compress() cannot suspend. |
| This means you cannot use suspension with multi-pass operating modes, namely |
| Huffman code optimization and multiple-scan output. Those modes write the |
| whole file during jpeg_finish_compress(), which will certainly result in |
| buffer overrun. (Note that this restriction applies only to compression, |
| not decompression. The decompressor supports input suspension in all of its |
| operating modes.) |
| |
| Decompression suspension: |
| |
| For decompression suspension, use a fill_input_buffer() routine that simply |
| returns FALSE (except perhaps during error recovery, as discussed below). |
| This will cause the decompressor to return to its caller with an indication |
| that suspension has occurred. This can happen at four places: |
| * jpeg_read_header(): will return JPEG_SUSPENDED. |
| * jpeg_start_decompress(): will return FALSE, rather than its usual TRUE. |
| * jpeg_read_scanlines(): will return the number of scanlines already |
| completed (possibly 0). |
| * jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE. |
| The surrounding application must recognize these cases, load more data into |
| the input buffer, and repeat the call. In the case of jpeg_read_scanlines(), |
| increment the passed pointers past any scanlines successfully read. |
| |
| Just as with compression, the decompressor will typically backtrack to a |
| convenient restart point before suspending. When fill_input_buffer() is |
| called, next_input_byte/bytes_in_buffer point to the current restart point, |
| which is where the decompressor will backtrack to if FALSE is returned. |
| The data beyond that position must NOT be discarded if you suspend; it needs |
| to be re-read upon resumption. In most implementations, you'll need to shift |
| this data down to the start of your work buffer and then load more data after |
| it. Again, this behavior means that a several-Kbyte work buffer is essential |
| for decent performance; furthermore, you should load a reasonable amount of |
| new data before resuming decompression. (If you loaded, say, only one new |
| byte each time around, you could waste a LOT of cycles.) |
| |
| The skip_input_data() source manager routine requires special care in a |
| suspension scenario. This routine is NOT granted the ability to suspend the |
| decompressor; it can decrement bytes_in_buffer to zero, but no more. If the |
| requested skip distance exceeds the amount of data currently in the input |
| buffer, then skip_input_data() must set bytes_in_buffer to zero and record the |
| additional skip distance somewhere else. The decompressor will immediately |
| call fill_input_buffer(), which should return FALSE, which will cause a |
| suspension return. The surrounding application must then arrange to discard |
| the recorded number of bytes before it resumes loading the input buffer. |
| (Yes, this design is rather baroque, but it avoids complexity in the far more |
| common case where a non-suspending source manager is used.) |
| |
| If the input data has been exhausted, we recommend that you emit a warning |
| and insert dummy EOI markers just as a non-suspending data source manager |
| would do. This can be handled either in the surrounding application logic or |
| within fill_input_buffer(); the latter is probably more efficient. If |
| fill_input_buffer() knows that no more data is available, it can set the |
| pointer/count to point to a dummy EOI marker and then return TRUE just as |
| though it had read more data in a non-suspending situation. |
| |
| The decompressor does not attempt to suspend within standard JPEG markers; |
| instead it will backtrack to the start of the marker and reprocess the whole |
| marker next time. Hence the input buffer must be large enough to hold the |
| longest standard marker in the file. Standard JPEG markers should normally |
| not exceed a few hundred bytes each (DHT tables are typically the longest). |
| We recommend at least a 2K buffer for performance reasons, which is much |
| larger than any correct marker is likely to be. For robustness against |
| damaged marker length counts, you may wish to insert a test in your |
| application for the case that the input buffer is completely full and yet |
| the decoder has suspended without consuming any data --- otherwise, if this |
| situation did occur, it would lead to an endless loop. (The library can't |
| provide this test since it has no idea whether "the buffer is full", or |
| even whether there is a fixed-size input buffer.) |
| |
| The input buffer would need to be 64K to allow for arbitrary COM or APPn |
| markers, but these are handled specially: they are either saved into allocated |
| memory, or skipped over by calling skip_input_data(). In the former case, |
| suspension is handled correctly, and in the latter case, the problem of |
| buffer overrun is placed on skip_input_data's shoulders, as explained above. |
| Note that if you provide your own marker handling routine for large markers, |
| you should consider how to deal with buffer overflow. |
| |
| Multiple-buffer management: |
| |
| In some applications it is desirable to store the compressed data in a linked |
| list of buffer areas, so as to avoid data copying. This can be handled by |
| having empty_output_buffer() or fill_input_buffer() set the pointer and count |
| to reference the next available buffer; FALSE is returned only if no more |
| buffers are available. Although seemingly straightforward, there is a |
| pitfall in this approach: the backtrack that occurs when FALSE is returned |
| could back up into an earlier buffer. For example, when fill_input_buffer() |
| is called, the current pointer & count indicate the backtrack restart point. |
| Since fill_input_buffer() will set the pointer and count to refer to a new |
| buffer, the restart position must be saved somewhere else. Suppose a second |
| call to fill_input_buffer() occurs in the same library call, and no |
| additional input data is available, so fill_input_buffer must return FALSE. |
| If the JPEG library has not moved the pointer/count forward in the current |
| buffer, then *the correct restart point is the saved position in the prior |
| buffer*. Prior buffers may be discarded only after the library establishes |
| a restart point within a later buffer. Similar remarks apply for output into |
| a chain of buffers. |
| |
| The library will never attempt to backtrack over a skip_input_data() call, |
| so any skipped data can be permanently discarded. You still have to deal |
| with the case of skipping not-yet-received data, however. |
| |
| It's much simpler to use only a single buffer; when fill_input_buffer() is |
| called, move any unconsumed data (beyond the current pointer/count) down to |
| the beginning of this buffer and then load new data into the remaining buffer |
| space. This approach requires a little more data copying but is far easier |
| to get right. |
| |
| |
| Progressive JPEG support |
| ------------------------ |
| |
| Progressive JPEG rearranges the stored data into a series of scans of |
| increasing quality. In situations where a JPEG file is transmitted across a |
| slow communications link, a decoder can generate a low-quality image very |
| quickly from the first scan, then gradually improve the displayed quality as |
| more scans are received. The final image after all scans are complete is |
| identical to that of a regular (sequential) JPEG file of the same quality |
| setting. Progressive JPEG files are often slightly smaller than equivalent |
| sequential JPEG files, but the possibility of incremental display is the main |
| reason for using progressive JPEG. |
| |
| The IJG encoder library generates progressive JPEG files when given a |
| suitable "scan script" defining how to divide the data into scans. |
| Creation of progressive JPEG files is otherwise transparent to the encoder. |
| Progressive JPEG files can also be read transparently by the decoder library. |
| If the decoding application simply uses the library as defined above, it |
| will receive a final decoded image without any indication that the file was |
| progressive. Of course, this approach does not allow incremental display. |
| To perform incremental display, an application needs to use the decoder |
| library's "buffered-image" mode, in which it receives a decoded image |
| multiple times. |
| |
| Each displayed scan requires about as much work to decode as a full JPEG |
| image of the same size, so the decoder must be fairly fast in relation to the |
| data transmission rate in order to make incremental display useful. However, |
| it is possible to skip displaying the image and simply add the incoming bits |
| to the decoder's coefficient buffer. This is fast because only Huffman |
| decoding need be done, not IDCT, upsampling, colorspace conversion, etc. |
| The IJG decoder library allows the application to switch dynamically between |
| displaying the image and simply absorbing the incoming bits. A properly |
| coded application can automatically adapt the number of display passes to |
| suit the time available as the image is received. Also, a final |
| higher-quality display cycle can be performed from the buffered data after |
| the end of the file is reached. |
| |
| Progressive compression: |
| |
| To create a progressive JPEG file (or a multiple-scan sequential JPEG file), |
| set the scan_info cinfo field to point to an array of scan descriptors, and |
| perform compression as usual. Instead of constructing your own scan list, |
| you can call the jpeg_simple_progression() helper routine to create a |
| recommended progression sequence; this method should be used by all |
| applications that don't want to get involved in the nitty-gritty of |
| progressive scan sequence design. (If you want to provide user control of |
| scan sequences, you may wish to borrow the scan script reading code found |
| in rdswitch.c, so that you can read scan script files just like cjpeg's.) |
| When scan_info is not NULL, the compression library will store DCT'd data |
| into a buffer array as jpeg_write_scanlines() is called, and will emit all |
| the requested scans during jpeg_finish_compress(). This implies that |
| multiple-scan output cannot be created with a suspending data destination |
| manager, since jpeg_finish_compress() does not support suspension. We |
| should also note that the compressor currently forces Huffman optimization |
| mode when creating a progressive JPEG file, because the default Huffman |
| tables are unsuitable for progressive files. |
| |
| Progressive decompression: |
| |
| When buffered-image mode is not used, the decoder library will read all of |
| a multi-scan file during jpeg_start_decompress(), so that it can provide a |
| final decoded image. (Here "multi-scan" means either progressive or |
| multi-scan sequential.) This makes multi-scan files transparent to the |
| decoding application. However, existing applications that used suspending |
| input with version 5 of the IJG library will need to be modified to check |
| for a suspension return from jpeg_start_decompress(). |
| |
| To perform incremental display, an application must use the library's |
| buffered-image mode. This is described in the next section. |
| |
| |
| Buffered-image mode |
| ------------------- |
| |
| In buffered-image mode, the library stores the partially decoded image in a |
| coefficient buffer, from which it can be read out as many times as desired. |
| This mode is typically used for incremental display of progressive JPEG files, |
| but it can be used with any JPEG file. Each scan of a progressive JPEG file |
| adds more data (more detail) to the buffered image. The application can |
| display in lockstep with the source file (one display pass per input scan), |
| or it can allow input processing to outrun display processing. By making |
| input and display processing run independently, it is possible for the |
| application to adapt progressive display to a wide range of data transmission |
| rates. |
| |
| The basic control flow for buffered-image decoding is |
| |
| jpeg_create_decompress() |
| set data source |
| jpeg_read_header() |
| set overall decompression parameters |
| cinfo.buffered_image = TRUE; /* select buffered-image mode */ |
| jpeg_start_decompress() |
| for (each output pass) { |
| adjust output decompression parameters if required |
| jpeg_start_output() /* start a new output pass */ |
| for (all scanlines in image) { |
| jpeg_read_scanlines() |
| display scanlines |
| } |
| jpeg_finish_output() /* terminate output pass */ |
| } |
| jpeg_finish_decompress() |
| jpeg_destroy_decompress() |
| |
| This differs from ordinary unbuffered decoding in that there is an additional |
| level of looping. The application can choose how many output passes to make |
| and how to display each pass. |
| |
| The simplest approach to displaying progressive images is to do one display |
| pass for each scan appearing in the input file. In this case the outer loop |
| condition is typically |
| while (!jpeg_input_complete(&cinfo)) |
| and the start-output call should read |
| jpeg_start_output(&cinfo, cinfo.input_scan_number); |
| The second parameter to jpeg_start_output() indicates which scan of the input |
| file is to be displayed; the scans are numbered starting at 1 for this |
| purpose. (You can use a loop counter starting at 1 if you like, but using |
| the library's input scan counter is easier.) The library automatically reads |
| data as necessary to complete each requested scan, and jpeg_finish_output() |
| advances to the next scan or end-of-image marker (hence input_scan_number |
| will be incremented by the time control arrives back at jpeg_start_output()). |
| With this technique, data is read from the input file only as needed, and |
| input and output processing run in lockstep. |
| |
| After reading the final scan and reaching the end of the input file, the |
| buffered image remains available; it can be read additional times by |
| repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output() |
| sequence. For example, a useful technique is to use fast one-pass color |
| quantization for display passes made while the image is arriving, followed by |
| a final display pass using two-pass quantization for highest quality. This |
| is done by changing the library parameters before the final output pass. |
| Changing parameters between passes is discussed in detail below. |
| |
| In general the last scan of a progressive file cannot be recognized as such |
| until after it is read, so a post-input display pass is the best approach if |
| you want special processing in the final pass. |
| |
| When done with the image, be sure to call jpeg_finish_decompress() to release |
| the buffered image (or just use jpeg_destroy_decompress()). |
| |
| If input data arrives faster than it can be displayed, the application can |
| cause the library to decode input data in advance of what's needed to produce |
| output. This is done by calling the routine jpeg_consume_input(). |
| The return value is one of the following: |
| JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan) |
| JPEG_REACHED_EOI: reached the EOI marker (end of image) |
| JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data |
| JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan |
| JPEG_SUSPENDED: suspended before completing any of the above |
| (JPEG_SUSPENDED can occur only if a suspending data source is used.) This |
| routine can be called at any time after initializing the JPEG object. It |
| reads some additional data and returns when one of the indicated significant |
| events occurs. (If called after the EOI marker is reached, it will |
| immediately return JPEG_REACHED_EOI without attempting to read more data.) |
| |
| The library's output processing will automatically call jpeg_consume_input() |
| whenever the output processing overtakes the input; thus, simple lockstep |
| display requires no direct calls to jpeg_consume_input(). But by adding |
| calls to jpeg_consume_input(), you can absorb data in advance of what is |
| being displayed. This has two benefits: |
| * You can limit buildup of unprocessed data in your input buffer. |
| * You can eliminate extra display passes by paying attention to the |
| state of the library's input processing. |
| |
| The first of these benefits only requires interspersing calls to |
| jpeg_consume_input() with your display operations and any other processing |
| you may be doing. To avoid wasting cycles due to backtracking, it's best to |
| call jpeg_consume_input() only after a hundred or so new bytes have arrived. |
| This is discussed further under "I/O suspension", above. (Note: the JPEG |
| library currently is not thread-safe. You must not call jpeg_consume_input() |
| from one thread of control if a different library routine is working on the |
| same JPEG object in another thread.) |
| |
| When input arrives fast enough that more than one new scan is available |
| before you start a new output pass, you may as well skip the output pass |
| corresponding to the completed scan. This occurs for free if you pass |
| cinfo.input_scan_number as the target scan number to jpeg_start_output(). |
| The input_scan_number field is simply the index of the scan currently being |
| consumed by the input processor. You can ensure that this is up-to-date by |
| emptying the input buffer just before calling jpeg_start_output(): call |
| jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or |
| JPEG_REACHED_EOI. |
| |
| The target scan number passed to jpeg_start_output() is saved in the |
| cinfo.output_scan_number field. The library's output processing calls |
| jpeg_consume_input() whenever the current input scan number and row within |
| that scan is less than or equal to the current output scan number and row. |
| Thus, input processing can "get ahead" of the output processing but is not |
| allowed to "fall behind". You can achieve several different effects by |
| manipulating this interlock rule. For example, if you pass a target scan |
| number greater than the current input scan number, the output processor will |
| wait until that scan starts to arrive before producing any output. (To avoid |
| an infinite loop, the target scan number is automatically reset to the last |
| scan number when the end of image is reached. Thus, if you specify a large |
| target scan number, the library will just absorb the entire input file and |
| then perform an output pass. This is effectively the same as what |
| jpeg_start_decompress() does when you don't select buffered-image mode.) |
| When you pass a target scan number equal to the current input scan number, |
| the image is displayed no faster than the current input scan arrives. The |
| final possibility is to pass a target scan number less than the current input |
| scan number; this disables the input/output interlock and causes the output |
| processor to simply display whatever it finds in the image buffer, without |
| waiting for input. (However, the library will not accept a target scan |
| number less than one, so you can't avoid waiting for the first scan.) |
| |
| When data is arriving faster than the output display processing can advance |
| through the image, jpeg_consume_input() will store data into the buffered |
| image beyond the point at which the output processing is reading data out |
| again. If the input arrives fast enough, it may "wrap around" the buffer to |
| the point where the input is more than one whole scan ahead of the output. |
| If the output processing simply proceeds through its display pass without |
| paying attention to the input, the effect seen on-screen is that the lower |
| part of the image is one or more scans better in quality than the upper part. |
| Then, when the next output scan is started, you have a choice of what target |
| scan number to use. The recommended choice is to use the current input scan |
| number at that time, which implies that you've skipped the output scans |
| corresponding to the input scans that were completed while you processed the |
| previous output scan. In this way, the decoder automatically adapts its |
| speed to the arriving data, by skipping output scans as necessary to keep up |
| with the arriving data. |
| |
| When using this strategy, you'll want to be sure that you perform a final |
| output pass after receiving all the data; otherwise your last display may not |
| be full quality across the whole screen. So the right outer loop logic is |
| something like this: |
| do { |
| absorb any waiting input by calling jpeg_consume_input() |
| final_pass = jpeg_input_complete(&cinfo); |
| adjust output decompression parameters if required |
| jpeg_start_output(&cinfo, cinfo.input_scan_number); |
| ... |
| jpeg_finish_output() |
| } while (!final_pass); |
| rather than quitting as soon as jpeg_input_complete() returns TRUE. This |
| arrangement makes it simple to use higher-quality decoding parameters |
| for the final pass. But if you don't want to use special parameters for |
| the final pass, the right loop logic is like this: |
| for (;;) { |
| absorb any waiting input by calling jpeg_consume_input() |
| jpeg_start_output(&cinfo, cinfo.input_scan_number); |
| ... |
| jpeg_finish_output() |
| if (jpeg_input_complete(&cinfo) && |
| cinfo.input_scan_number == cinfo.output_scan_number) |
| break; |
| } |
| In this case you don't need to know in advance whether an output pass is to |
| be the last one, so it's not necessary to have reached EOF before starting |
| the final output pass; rather, what you want to test is whether the output |
| pass was performed in sync with the final input scan. This form of the loop |
| will avoid an extra output pass whenever the decoder is able (or nearly able) |
| to keep up with the incoming data. |
| |
| When the data transmission speed is high, you might begin a display pass, |
| then find that much or all of the file has arrived before you can complete |
| the pass. (You can detect this by noting the JPEG_REACHED_EOI return code |
| from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().) |
| In this situation you may wish to abort the current display pass and start a |
| new one using the newly arrived information. To do so, just call |
| jpeg_finish_output() and then start a new pass with jpeg_start_output(). |
| |
| A variant strategy is to abort and restart display if more than one complete |
| scan arrives during an output pass; this can be detected by noting |
| JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This |
| idea should be employed with caution, however, since the display process |
| might never get to the bottom of the image before being aborted, resulting |
| in the lower part of the screen being several passes worse than the upper. |
| In most cases it's probably best to abort an output pass only if the whole |
| file has arrived and you want to begin the final output pass immediately. |
| |
| When receiving data across a communication link, we recommend always using |
| the current input scan number for the output target scan number; if a |
| higher-quality final pass is to be done, it should be started (aborting any |
| incomplete output pass) as soon as the end of file is received. However, |
| many other strategies are possible. For example, the application can examine |
| the parameters of the current input scan and decide whether to display it or |
| not. If the scan contains only chroma data, one might choose not to use it |
| as the target scan, expecting that the scan will be small and will arrive |
| quickly. To skip to the next scan, call jpeg_consume_input() until it |
| returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher |
| number as the target scan for jpeg_start_output(); but that method doesn't |
| let you inspect the next scan's parameters before deciding to display it. |
| |
| |
| In buffered-image mode, jpeg_start_decompress() never performs input and |
| thus never suspends. An application that uses input suspension with |
| buffered-image mode must be prepared for suspension returns from these |
| routines: |
| * jpeg_start_output() performs input only if you request 2-pass quantization |
| and the target scan isn't fully read yet. (This is discussed below.) |
| * jpeg_read_scanlines(), as always, returns the number of scanlines that it |
| was able to produce before suspending. |
| * jpeg_finish_output() will read any markers following the target scan, |
| up to the end of the file or the SOS marker that begins another scan. |
| (But it reads no input if jpeg_consume_input() has already reached the |
| end of the file or a SOS marker beyond the target output scan.) |
| * jpeg_finish_decompress() will read until the end of file, and thus can |
| suspend if the end hasn't already been reached (as can be tested by |
| calling jpeg_input_complete()). |
| jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress() |
| all return TRUE if they completed their tasks, FALSE if they had to suspend. |
| In the event of a FALSE return, the application must load more input data |
| and repeat the call. Applications that use non-suspending data sources need |
| not check the return values of these three routines. |
| |
| |
| It is possible to change decoding parameters between output passes in the |
| buffered-image mode. The decoder library currently supports only very |
| limited changes of parameters. ONLY THE FOLLOWING parameter changes are |
| allowed after jpeg_start_decompress() is called: |
| * dct_method can be changed before each call to jpeg_start_output(). |
| For example, one could use a fast DCT method for early scans, changing |
| to a higher quality method for the final scan. |
| * dither_mode can be changed before each call to jpeg_start_output(); |
| of course this has no impact if not using color quantization. Typically |
| one would use ordered dither for initial passes, then switch to |
| Floyd-Steinberg dither for the final pass. Caution: changing dither mode |
| can cause more memory to be allocated by the library. Although the amount |
| of memory involved is not large (a scanline or so), it may cause the |
| initial max_memory_to_use specification to be exceeded, which in the worst |
| case would result in an out-of-memory failure. |
| * do_block_smoothing can be changed before each call to jpeg_start_output(). |
| This setting is relevant only when decoding a progressive JPEG image. |
| During the first DC-only scan, block smoothing provides a very "fuzzy" look |
| instead of the very "blocky" look seen without it; which is better seems a |
| matter of personal taste. But block smoothing is nearly always a win |
| during later stages, especially when decoding a successive-approximation |
| image: smoothing helps to hide the slight blockiness that otherwise shows |
| up on smooth gradients until the lowest coefficient bits are sent. |
| * Color quantization mode can be changed under the rules described below. |
| You *cannot* change between full-color and quantized output (because that |
| would alter the required I/O buffer sizes), but you can change which |
| quantization method is used. |
| |
| When generating color-quantized output, changing quantization method is a |
| very useful way of switching between high-speed and high-quality display. |
| The library allows you to change among its three quantization methods: |
| 1. Single-pass quantization to a fixed color cube. |
| Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL. |
| 2. Single-pass quantization to an application-supplied colormap. |
| Selected by setting cinfo.colormap to point to the colormap (the value of |
| two_pass_quantize is ignored); also set cinfo.actual_number_of_colors. |
| 3. Two-pass quantization to a colormap chosen specifically for the image. |
| Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL. |
| (This is the default setting selected by jpeg_read_header, but it is |
| probably NOT what you want for the first pass of progressive display!) |
| These methods offer successively better quality and lesser speed. However, |
| only the first method is available for quantizing in non-RGB color spaces. |
| |
| IMPORTANT: because the different quantizer methods have very different |
| working-storage requirements, the library requires you to indicate which |
| one(s) you intend to use before you call jpeg_start_decompress(). (If we did |
| not require this, the max_memory_to_use setting would be a complete fiction.) |
| You do this by setting one or more of these three cinfo fields to TRUE: |
| enable_1pass_quant Fixed color cube colormap |
| enable_external_quant Externally-supplied colormap |
| enable_2pass_quant Two-pass custom colormap |
| All three are initialized FALSE by jpeg_read_header(). But |
| jpeg_start_decompress() automatically sets TRUE the one selected by the |
| current two_pass_quantize and colormap settings, so you only need to set the |
| enable flags for any other quantization methods you plan to change to later. |
| |
| After setting the enable flags correctly at jpeg_start_decompress() time, you |
| can change to any enabled quantization method by setting two_pass_quantize |
| and colormap properly just before calling jpeg_start_output(). The following |
| special rules apply: |
| 1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass |
| or 2-pass mode from a different mode, or when you want the 2-pass |
| quantizer to be re-run to generate a new colormap. |
| 2. To switch to an external colormap, or to change to a different external |
| colormap than was used on the prior pass, you must call |
| jpeg_new_colormap() after setting cinfo.colormap. |
| NOTE: if you want to use the same colormap as was used in the prior pass, |
| you should not do either of these things. This will save some nontrivial |
| switchover costs. |
| (These requirements exist because cinfo.colormap will always be non-NULL |
| after completing a prior output pass, since both the 1-pass and 2-pass |
| quantizers set it to point to their output colormaps. Thus you have to |
| do one of these two things to notify the library that something has changed. |
| Yup, it's a bit klugy, but it's necessary to do it this way for backwards |
| compatibility.) |
| |
| Note that in buffered-image mode, the library generates any requested colormap |
| during jpeg_start_output(), not during jpeg_start_decompress(). |
| |
| When using two-pass quantization, jpeg_start_output() makes a pass over the |
| buffered image to determine the optimum color map; it therefore may take a |
| significant amount of time, whereas ordinarily it does little work. The |
| progress monitor hook is called during this pass, if defined. It is also |
| important to realize that if the specified target scan number is greater than |
| or equal to the current input scan number, jpeg_start_output() will attempt |
| to consume input as it makes this pass. If you use a suspending data source, |
| you need to check for a FALSE return from jpeg_start_output() under these |
| conditions. The combination of 2-pass quantization and a not-yet-fully-read |
| target scan is the only case in which jpeg_start_output() will consume input. |
| |
| |
| Application authors who support buffered-image mode may be tempted to use it |
| for all JPEG images, even single-scan ones. This will work, but it is |
| inefficient: there is no need to create an image-sized coefficient buffer for |
| single-scan images. Requesting buffered-image mode for such an image wastes |
| memory. Worse, it can cost time on large images, since the buffered data has |
| to be swapped out or written to a temporary file. If you are concerned about |
| maximum performance on baseline JPEG files, you should use buffered-image |
| mode only when the incoming file actually has multiple scans. This can be |
| tested by calling jpeg_has_multiple_scans(), which will return a correct |
| result at any time after jpeg_read_header() completes. |
| |
| It is also worth noting that when you use jpeg_consume_input() to let input |
| processing get ahead of output processing, the resulting pattern of access to |
| the coefficient buffer is quite nonsequential. It's best to use the memory |
| manager jmemnobs.c if you can (ie, if you have enough real or virtual main |
| memory). If not, at least make sure that max_memory_to_use is set as high as |
| possible. If the JPEG memory manager has to use a temporary file, you will |
| probably see a lot of disk traffic and poor performance. (This could be |
| improved with additional work on the memory manager, but we haven't gotten |
| around to it yet.) |
| |
| In some applications it may be convenient to use jpeg_consume_input() for all |
| input processing, including reading the initial markers; that is, you may |
| wish to call jpeg_consume_input() instead of jpeg_read_header() during |
| startup. This works, but note that you must check for JPEG_REACHED_SOS and |
| JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes. |
| Once the first SOS marker has been reached, you must call |
| jpeg_start_decompress() before jpeg_consume_input() will consume more input; |
| it'll just keep returning JPEG_REACHED_SOS until you do. If you read a |
| tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI |
| without ever returning JPEG_REACHED_SOS; be sure to check for this case. |
| If this happens, the decompressor will not read any more input until you call |
| jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not |
| using buffered-image mode, but in that case it's basically a no-op after the |
| initial markers have been read: it will just return JPEG_SUSPENDED. |
| |
| |
| Abbreviated datastreams and multiple images |
| ------------------------------------------- |
| |
| A JPEG compression or decompression object can be reused to process multiple |
| images. This saves a small amount of time per image by eliminating the |
| "create" and "destroy" operations, but that isn't the real purpose of the |
| feature. Rather, reuse of an object provides support for abbreviated JPEG |
| datastreams. Object reuse can also simplify processing a series of images in |
| a single input or output file. This section explains these features. |
| |
| A JPEG file normally contains several hundred bytes worth of quantization |
| and Huffman tables. In a situation where many images will be stored or |
| transmitted with identical tables, this may represent an annoying overhead. |
| The JPEG standard therefore permits tables to be omitted. The standard |
| defines three classes of JPEG datastreams: |
| * "Interchange" datastreams contain an image and all tables needed to decode |
| the image. These are the usual kind of JPEG file. |
| * "Abbreviated image" datastreams contain an image, but are missing some or |
| all of the tables needed to decode that image. |
| * "Abbreviated table specification" (henceforth "tables-only") datastreams |
| contain only table specifications. |
| To decode an abbreviated image, it is necessary to load the missing table(s) |
| into the decoder beforehand. This can be accomplished by reading a separate |
| tables-only file. A variant scheme uses a series of images in which the first |
| image is an interchange (complete) datastream, while subsequent ones are |
| abbreviated and rely on the tables loaded by the first image. It is assumed |
| that once the decoder has read a table, it will remember that table until a |
| new definition for the same table number is encountered. |
| |
| It is the application designer's responsibility to figure out how to associate |
| the correct tables with an abbreviated image. While abbreviated datastreams |
| can be useful in a closed environment, their use is strongly discouraged in |
| any situation where data exchange with other applications might be needed. |
| Caveat designer. |
| |
| The JPEG library provides support for reading and writing any combination of |
| tables-only datastreams and abbreviated images. In both compression and |
| decompression objects, a quantization or Huffman table will be retained for |
| the lifetime of the object, unless it is overwritten by a new table definition. |
| |
| |
| To create abbreviated image datastreams, it is only necessary to tell the |
| compressor not to emit some or all of the tables it is using. Each |
| quantization and Huffman table struct contains a boolean field "sent_table", |
| which normally is initialized to FALSE. For each table used by the image, the |
| header-writing process emits the table and sets sent_table = TRUE unless it is |
| already TRUE. (In normal usage, this prevents outputting the same table |
| definition multiple times, as would otherwise occur because the chroma |
| components typically share tables.) Thus, setting this field to TRUE before |
| calling jpeg_start_compress() will prevent the table from being written at |
| all. |
| |
| If you want to create a "pure" abbreviated image file containing no tables, |
| just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the |
| tables. If you want to emit some but not all tables, you'll need to set the |
| individual sent_table fields directly. |
| |
| To create an abbreviated image, you must also call jpeg_start_compress() |
| with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress() |
| will force all the sent_table fields to FALSE. (This is a safety feature to |
| prevent abbreviated images from being created accidentally.) |
| |
| To create a tables-only file, perform the same parameter setup that you |
| normally would, but instead of calling jpeg_start_compress() and so on, call |
| jpeg_write_tables(&cinfo). This will write an abbreviated datastream |
| containing only SOI, DQT and/or DHT markers, and EOI. All the quantization |
| and Huffman tables that are currently defined in the compression object will |
| be emitted unless their sent_tables flag is already TRUE, and then all the |
| sent_tables flags will be set TRUE. |
| |
| A sure-fire way to create matching tables-only and abbreviated image files |
| is to proceed as follows: |
| |
| create JPEG compression object |
| set JPEG parameters |
| set destination to tables-only file |
| jpeg_write_tables(&cinfo); |
| set destination to image file |
| jpeg_start_compress(&cinfo, FALSE); |
| write data... |
| jpeg_finish_compress(&cinfo); |
| |
| Since the JPEG parameters are not altered between writing the table file and |
| the abbreviated image file, the same tables are sure to be used. Of course, |
| you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence |
| many times to produce many abbreviated image files matching the table file. |
| |
| You cannot suppress output of the computed Huffman tables when Huffman |
| optimization is selected. (If you could, there'd be no way to decode the |
| image...) Generally, you don't want to set optimize_coding = TRUE when |
| you are trying to produce abbreviated files. |
| |
| In some cases you might want to compress an image using tables which are |
| not stored in the application, but are defined in an interchange or |
| tables-only file readable by the application. This can be done by setting up |
| a JPEG decompression object to read the specification file, then copying the |
| tables into your compression object. See jpeg_copy_critical_parameters() |
| for an example of copying quantization tables. |
| |
| |
| To read abbreviated image files, you simply need to load the proper tables |
| into the decompression object before trying to read the abbreviated image. |
| If the proper tables are stored in the application program, you can just |
| allocate the table structs and fill in their contents directly. For example, |
| to load a fixed quantization table into table slot "n": |
| |
| if (cinfo.quant_tbl_ptrs[n] == NULL) |
| cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo); |
| quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */ |
| for (i = 0; i < 64; i++) { |
| /* Qtable[] is desired quantization table, in natural array order */ |
| quant_ptr->quantval[i] = Qtable[i]; |
| } |
| |
| Code to load a fixed Huffman table is typically (for AC table "n"): |
| |
| if (cinfo.ac_huff_tbl_ptrs[n] == NULL) |
| cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo); |
| huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */ |
| for (i = 1; i <= 16; i++) { |
| /* counts[i] is number of Huffman codes of length i bits, i=1..16 */ |
| huff_ptr->bits[i] = counts[i]; |
| } |
| for (i = 0; i < 256; i++) { |
| /* symbols[] is the list of Huffman symbols, in code-length order */ |
| huff_ptr->huffval[i] = symbols[i]; |
| } |
| |
| (Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a |
| constant JQUANT_TBL object is not safe. If the incoming file happened to |
| contain a quantization table definition, your master table would get |
| overwritten! Instead allocate a working table copy and copy the master table |
| into it, as illustrated above. Ditto for Huffman tables, of course.) |
| |
| You might want to read the tables from a tables-only file, rather than |
| hard-wiring them into your application. The jpeg_read_header() call is |
| sufficient to read a tables-only file. You must pass a second parameter of |
| FALSE to indicate that you do not require an image to be present. Thus, the |
| typical scenario is |
| |
| create JPEG decompression object |
| set source to tables-only file |
| jpeg_read_header(&cinfo, FALSE); |
| set source to abbreviated image file |
| jpeg_read_header(&cinfo, TRUE); |
| set decompression parameters |
| jpeg_start_decompress(&cinfo); |
| read data... |
| jpeg_finish_decompress(&cinfo); |
| |
| In some cases, you may want to read a file without knowing whether it contains |
| an image or just tables. In that case, pass FALSE and check the return value |
| from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found, |
| JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value, |
| JPEG_SUSPENDED, is possible when using a suspending data source manager.) |
| Note that jpeg_read_header() will not complain if you read an abbreviated |
| image for which you haven't loaded the missing tables; the missing-table check |
| occurs later, in jpeg_start_decompress(). |
| |
| |
| It is possible to read a series of images from a single source file by |
| repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence, |
| without releasing/recreating the JPEG object or the data source module. |
| (If you did reinitialize, any partial bufferload left in the data source |
| buffer at the end of one image would be discarded, causing you to lose the |
| start of the next image.) When you use this method, stored tables are |
| automatically carried forward, so some of the images can be abbreviated images |
| that depend on tables from earlier images. |
| |
| If you intend to write a series of images into a single destination file, |
| you might want to make a specialized data destination module that doesn't |
| flush the output buffer at term_destination() time. This would speed things |
| up by some trifling amount. Of course, you'd need to remember to flush the |
| buffer after the last image. You can make the later images be abbreviated |
| ones by passing FALSE to jpeg_start_compress(). |
| |
| |
| Special markers |
| --------------- |
| |
| Some applications may need to insert or extract special data in the JPEG |
| datastream. The JPEG standard provides marker types "COM" (comment) and |
| "APP0" through "APP15" (application) to hold application-specific data. |
| Unfortunately, the use of these markers is not specified by the standard. |
| COM markers are fairly widely used to hold user-supplied text. The JFIF file |
| format spec uses APP0 markers with specified initial strings to hold certain |
| data. Adobe applications use APP14 markers beginning with the string "Adobe" |
| for miscellaneous data. Other APPn markers are rarely seen, but might |
| contain almost anything. |
| |
| If you wish to store user-supplied text, we recommend you use COM markers |
| and place readable 7-bit ASCII text in them. Newline conventions are not |
| standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR |
| (Mac style). A robust COM reader should be able to cope with random binary |
| garbage, including nulls, since some applications generate COM markers |
| containing non-ASCII junk. (But yours should not be one of them.) |
| |
| For program-supplied data, use an APPn marker, and be sure to begin it with an |
| identifying string so that you can tell whether the marker is actually yours. |
| It's probably best to avoid using APP0 or APP14 for any private markers. |
| (NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you |
| not use APP8 markers for any private purposes, either.) |
| |
| Keep in mind that at most 65533 bytes can be put into one marker, but you |
| can have as many markers as you like. |
| |
| By default, the IJG compression library will write a JFIF APP0 marker if the |
| selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if |
| the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but |
| we don't recommend it. The decompression library will recognize JFIF and |
| Adobe markers and will set the JPEG colorspace properly when one is found. |
| |
| |
| You can write special markers immediately following the datastream header by |
| calling jpeg_write_marker() after jpeg_start_compress() and before the first |
| call to jpeg_write_scanlines(). When you do this, the markers appear after |
| the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before |
| all else. Specify the marker type parameter as "JPEG_COM" for COM or |
| "JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write |
| any marker type, but we don't recommend writing any other kinds of marker.) |
| For example, to write a user comment string pointed to by comment_text: |
| jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text)); |
| |
| If it's not convenient to store all the marker data in memory at once, |
| you can instead call jpeg_write_m_header() followed by multiple calls to |
| jpeg_write_m_byte(). If you do it this way, it's your responsibility to |
| call jpeg_write_m_byte() exactly the number of times given in the length |
| parameter to jpeg_write_m_header(). (This method lets you empty the |
| output buffer partway through a marker, which might be important when |
| using a suspending data destination module. In any case, if you are using |
| a suspending destination, you should flush its buffer after inserting |
| any special markers. See "I/O suspension".) |
| |
| Or, if you prefer to synthesize the marker byte sequence yourself, |
| you can just cram it straight into the data destination module. |
| |
| If you are writing JFIF 1.02 extension markers (thumbnail images), don't |
| forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the |
| correct JFIF version number in the JFIF header marker. The library's default |
| is to write version 1.01, but that's wrong if you insert any 1.02 extension |
| markers. (We could probably get away with just defaulting to 1.02, but there |
| used to be broken decoders that would complain about unknown minor version |
| numbers. To reduce compatibility risks it's safest not to write 1.02 unless |
| you are actually using 1.02 extensions.) |
| |
| |
| When reading, two methods of handling special markers are available: |
| 1. You can ask the library to save the contents of COM and/or APPn markers |
| into memory, and then examine them at your leisure afterwards. |
| 2. You can supply your own routine to process COM and/or APPn markers |
| on-the-fly as they are read. |
| The first method is simpler to use, especially if you are using a suspending |
| data source; writing a marker processor that copes with input suspension is |
| not easy (consider what happens if the marker is longer than your available |
| input buffer). However, the second method conserves memory since the marker |
| data need not be kept around after it's been processed. |
| |
| For either method, you'd normally set up marker handling after creating a |
| decompression object and before calling jpeg_read_header(), because the |
| markers of interest will typically be near the head of the file and so will |
| be scanned by jpeg_read_header. Once you've established a marker handling |
| method, it will be used for the life of that decompression object |
| (potentially many datastreams), unless you change it. Marker handling is |
| determined separately for COM markers and for each APPn marker code. |
| |
| |
| To save the contents of special markers in memory, call |
| jpeg_save_markers(cinfo, marker_code, length_limit) |
| where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n. |
| (To arrange to save all the special marker types, you need to call this |
| routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer |
| than length_limit data bytes, only length_limit bytes will be saved; this |
| parameter allows you to avoid chewing up memory when you only need to see the |
| first few bytes of a potentially large marker. If you want to save all the |
| data, set length_limit to 0xFFFF; that is enough since marker lengths are only |
| 16 bits. As a special case, setting length_limit to 0 prevents that marker |
| type from being saved at all. (That is the default behavior, in fact.) |
| |
| After jpeg_read_header() completes, you can examine the special markers by |
| following the cinfo->marker_list pointer chain. All the special markers in |
| the file appear in this list, in order of their occurrence in the file (but |
| omitting any markers of types you didn't ask for). Both the original data |
| length and the saved data length are recorded for each list entry; the latter |
| will not exceed length_limit for the particular marker type. Note that these |
| lengths exclude the marker length word, whereas the stored representation |
| within the JPEG file includes it. (Hence the maximum data length is really |
| only 65533.) |
| |
| It is possible that additional special markers appear in the file beyond the |
| SOS marker at which jpeg_read_header stops; if so, the marker list will be |
| extended during reading of the rest of the file. This is not expected to be |
| common, however. If you are short on memory you may want to reset the length |
| limit to zero for all marker types after finishing jpeg_read_header, to |
| ensure that the max_memory_to_use setting cannot be exceeded due to addition |
| of later markers. |
| |
| The marker list remains stored until you call jpeg_finish_decompress or |
| jpeg_abort, at which point the memory is freed and the list is set to empty. |
| (jpeg_destroy also releases the storage, of course.) |
| |
| Note that the library is internally interested in APP0 and APP14 markers; |
| if you try to set a small nonzero length limit on these types, the library |
| will silently force the length up to the minimum it wants. (But you can set |
| a zero length limit to prevent them from being saved at all.) Also, in a |
| 16-bit environment, the maximum length limit may be constrained to less than |
| 65533 by malloc() limitations. It is therefore best not to assume that the |
| effective length limit is exactly what you set it to be. |
| |
| |
| If you want to supply your own marker-reading routine, you do it by calling |
| jpeg_set_marker_processor(). A marker processor routine must have the |
| signature |
| boolean jpeg_marker_parser_method (j_decompress_ptr cinfo) |
| Although the marker code is not explicitly passed, the routine can find it |
| in cinfo->unread_marker. At the time of call, the marker proper has been |
| read from the data source module. The processor routine is responsible for |
| reading the marker length word and the remaining parameter bytes, if any. |
| Return TRUE to indicate success. (FALSE should be returned only if you are |
| using a suspending data source and it tells you to suspend. See the standard |
| marker processors in jdmarker.c for appropriate coding methods if you need to |
| use a suspending data source.) |
| |
| If you override the default APP0 or APP14 processors, it is up to you to |
| recognize JFIF and Adobe markers if you want colorspace recognition to occur |
| properly. We recommend copying and extending the default processors if you |
| want to do that. (A better idea is to save these marker types for later |
| examination by calling jpeg_save_markers(); that method doesn't interfere |
| with the library's own processing of these markers.) |
| |
| jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive |
| --- if you call one it overrides any previous call to the other, for the |
| particular marker type specified. |
| |
| A simple example of an external COM processor can be found in djpeg.c. |
| Also, see jpegtran.c for an example of using jpeg_save_markers. |
| |
| |
| ICC profiles |
| ------------ |
| |
| Two functions are provided for writing and reading International Color |
| Consortium (ICC) device profiles embedded in JFIF JPEG image files: |
| |
| void jpeg_write_icc_profile (j_compress_ptr cinfo, |
| const JOCTET *icc_data_ptr, |
| unsigned int icc_data_len); |
| boolean jpeg_read_icc_profile (j_decompress_ptr cinfo, |
| JOCTET **icc_data_ptr, |
| unsigned int *icc_data_len); |
| |
| The ICC has defined a standard for including such data in JPEG "APP2" markers. |
| The aforementioned functions do not know anything about the internal structure |
| of the ICC profile data; they just know how to embed the profile data into a |
| JPEG file while writing it, or to extract the profile data from a JPEG file |
| while reading it. |
| |
| jpeg_write_icc_profile() must be called after calling jpeg_start_compress() and |
| before the first call to jpeg_write_scanlines() or jpeg_write_raw_data(). This |
| ordering ensures that the APP2 marker(s) will appear after the SOI and JFIF or |
| Adobe markers, but before all other data. |
| |
| jpeg_read_icc_profile() returns TRUE if an ICC profile was found and FALSE |
| otherwise. If an ICC profile was found, then the function will allocate a |
| memory region containing the profile and will return a pointer to that memory |
| region in *icc_data_ptr, as well as the length of the region in *icc_data_len. |
| This memory region is allocated by the library using malloc() and must be freed |
| by the caller using free() when the memory region is no longer needed. Callers |
| wishing to use jpeg_read_icc_profile() must call |
| |
| jpeg_save_markers(cinfo, JPEG_APP0 + 2, 0xFFFF); |
| |
| prior to calling jpeg_read_header(). jpeg_read_icc_profile() can be called at |
| any point between jpeg_read_header() and jpeg_finish_decompress(). |
| |
| |
| Raw (downsampled) image data |
| ---------------------------- |
| |
| Some applications need to supply already-downsampled image data to the JPEG |
| compressor, or to receive raw downsampled data from the decompressor. The |
| library supports this requirement by allowing the application to write or |
| read raw data, bypassing the normal preprocessing or postprocessing steps. |
| The interface is different from the standard one and is somewhat harder to |
| use. If your interest is merely in bypassing color conversion, we recommend |
| that you use the standard interface and simply set jpeg_color_space = |
| in_color_space (or jpeg_color_space = out_color_space for decompression). |
| The mechanism described in this section is necessary only to supply or |
| receive downsampled image data, in which not all components have the same |
| dimensions. |
| |
| |
| To compress raw data, you must supply the data in the colorspace to be used |
| in the JPEG file (please read the earlier section on Special color spaces) |
| and downsampled to the sampling factors specified in the JPEG parameters. |
| You must supply the data in the format used internally by the JPEG library, |
| namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional |
| arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one |
| color component. This structure is necessary since the components are of |
| different sizes. If the image dimensions are not a multiple of the MCU size, |
| you must also pad the data correctly (usually, this is done by replicating |
| the last column and/or row). The data must be padded to a multiple of a DCT |
| block in each component: that is, each downsampled row must contain a |
| multiple of 8 valid samples, and there must be a multiple of 8 sample rows |
| for each component. (For applications such as conversion of digital TV |
| images, the standard image size is usually a multiple of the DCT block size, |
| so that no padding need actually be done.) |
| |
| The procedure for compression of raw data is basically the same as normal |
| compression, except that you call jpeg_write_raw_data() in place of |
| jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do |
| the following: |
| * Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().) |
| This notifies the library that you will be supplying raw data. |
| * Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace() |
| call is a good idea. Note that since color conversion is bypassed, |
| in_color_space is ignored, except that jpeg_set_defaults() uses it to |
| choose the default jpeg_color_space setting. |
| * Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and |
| cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the |
| dimensions of the data you are supplying, it's wise to set them |
| explicitly, rather than assuming the library's defaults are what you want. |
| |
| To pass raw data to the library, call jpeg_write_raw_data() in place of |
| jpeg_write_scanlines(). The two routines work similarly except that |
| jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY. |
| The scanlines count passed to and returned from jpeg_write_raw_data is |
| measured in terms of the component with the largest v_samp_factor. |
| |
| jpeg_write_raw_data() processes one MCU row per call, which is to say |
| v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines |
| value must be at least max_v_samp_factor*DCTSIZE, and the return value will |
| be exactly that amount (or possibly some multiple of that amount, in future |
| library versions). This is true even on the last call at the bottom of the |
| image; don't forget to pad your data as necessary. |
| |
| The required dimensions of the supplied data can be computed for each |
| component as |
| cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row |
| cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image |
| after jpeg_start_compress() has initialized those fields. If the valid data |
| is smaller than this, it must be padded appropriately. For some sampling |
| factors and image sizes, additional dummy DCT blocks are inserted to make |
| the image a multiple of the MCU dimensions. The library creates such dummy |
| blocks itself; it does not read them from your supplied data. Therefore you |
| need never pad by more than DCTSIZE samples. An example may help here. |
| Assume 2h2v downsampling of YCbCr data, that is |
| cinfo->comp_info[0].h_samp_factor = 2 for Y |
| cinfo->comp_info[0].v_samp_factor = 2 |
| cinfo->comp_info[1].h_samp_factor = 1 for Cb |
| cinfo->comp_info[1].v_samp_factor = 1 |
| cinfo->comp_info[2].h_samp_factor = 1 for Cr |
| cinfo->comp_info[2].v_samp_factor = 1 |
| and suppose that the nominal image dimensions (cinfo->image_width and |
| cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will |
| compute downsampled_width = 101 and width_in_blocks = 13 for Y, |
| downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same |
| for the height fields). You must pad the Y data to at least 13*8 = 104 |
| columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The |
| MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16 |
| scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual |
| sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed, |
| so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row |
| of Y data is dummy, so it doesn't matter what you pass for it in the data |
| arrays, but the scanlines count must total up to 112 so that all of the Cb |
| and Cr data gets passed. |
| |
| Output suspension is supported with raw-data compression: if the data |
| destination module suspends, jpeg_write_raw_data() will return 0. |
| In this case the same data rows must be passed again on the next call. |
| |
| |
| Decompression with raw data output implies bypassing all postprocessing: |
| you cannot ask for rescaling or color quantization, for instance. More |
| seriously, you must deal with the color space and sampling factors present in |
| the incoming file. If your application only handles, say, 2h1v YCbCr data, |
| you must check for and fail on other color spaces or other sampling factors. |
| The library will not convert to a different color space for you. |
| |
| To obtain raw data output, set cinfo->raw_data_out = TRUE before |
| jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to |
| verify that the color space and sampling factors are ones you can handle. |
| Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The |
| decompression process is otherwise the same as usual. |
| |
| jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a |
| buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is |
| the same as for raw-data compression). The buffer you pass must be large |
| enough to hold the actual data plus padding to DCT-block boundaries. As with |
| compression, any entirely dummy DCT blocks are not processed so you need not |
| allocate space for them, but the total scanline count includes them. The |
| above example of computing buffer dimensions for raw-data compression is |
| equally valid for decompression. |
| |
| Input suspension is supported with raw-data decompression: if the data source |
| module suspends, jpeg_read_raw_data() will return 0. You can also use |
| buffered-image mode to read raw data in multiple passes. |
| |
| |
| Really raw data: DCT coefficients |
| --------------------------------- |
| |
| It is possible to read or write the contents of a JPEG file as raw DCT |
| coefficients. This facility is mainly intended for use in lossless |
| transcoding between different JPEG file formats. Other possible applications |
| include lossless cropping of a JPEG image, lossless reassembly of a |
| multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc. |
| |
| To read the contents of a JPEG file as DCT coefficients, open the file and do |
| jpeg_read_header() as usual. But instead of calling jpeg_start_decompress() |
| and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the |
| entire image into a set of virtual coefficient-block arrays, one array per |
| component. The return value is a pointer to an array of virtual-array |
| descriptors. Each virtual array can be accessed directly using the JPEG |
| memory manager's access_virt_barray method (see Memory management, below, |
| and also read structure.txt's discussion of virtual array handling). Or, |
| for simple transcoding to a different JPEG file format, the array list can |
| just be handed directly to jpeg_write_coefficients(). |
| |
| Each block in the block arrays contains quantized coefficient values in |
| normal array order (not JPEG zigzag order). The block arrays contain only |
| DCT blocks containing real data; any entirely-dummy blocks added to fill out |
| interleaved MCUs at the right or bottom edges of the image are discarded |
| during reading and are not stored in the block arrays. (The size of each |
| block array can be determined from the width_in_blocks and height_in_blocks |
| fields of the component's comp_info entry.) This is also the data format |
| expected by jpeg_write_coefficients(). |
| |
| When you are done using the virtual arrays, call jpeg_finish_decompress() |
| to release the array storage and return the decompression object to an idle |
| state; or just call jpeg_destroy() if you don't need to reuse the object. |
| |
| If you use a suspending data source, jpeg_read_coefficients() will return |
| NULL if it is forced to suspend; a non-NULL return value indicates successful |
| completion. You need not test for a NULL return value when using a |
| non-suspending data source. |
| |
| It is also possible to call jpeg_read_coefficients() to obtain access to the |
| decoder's coefficient arrays during a normal decode cycle in buffered-image |
| mode. This frammish might be useful for progressively displaying an incoming |
| image and then re-encoding it without loss. To do this, decode in buffered- |
| image mode as discussed previously, then call jpeg_read_coefficients() after |
| the last jpeg_finish_output() call. The arrays will be available for your use |
| until you call jpeg_finish_decompress(). |
| |
| |
| To write the contents of a JPEG file as DCT coefficients, you must provide |
| the DCT coefficients stored in virtual block arrays. You can either pass |
| block arrays read from an input JPEG file by jpeg_read_coefficients(), or |
| allocate virtual arrays from the JPEG compression object and fill them |
| yourself. In either case, jpeg_write_coefficients() is substituted for |
| jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is |
| * Create compression object |
| * Set all compression parameters as necessary |
| * Request virtual arrays if needed |
| * jpeg_write_coefficients() |
| * jpeg_finish_compress() |
| * Destroy or re-use compression object |
| jpeg_write_coefficients() is passed a pointer to an array of virtual block |
| array descriptors; the number of arrays is equal to cinfo.num_components. |
| |
| The virtual arrays need only have been requested, not realized, before |
| jpeg_write_coefficients() is called. A side-effect of |
| jpeg_write_coefficients() is to realize any virtual arrays that have been |
| requested from the compression object's memory manager. Thus, when obtaining |
| the virtual arrays from the compression object, you should fill the arrays |
| after calling jpeg_write_coefficients(). The data is actually written out |
| when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes |
| the file header. |
| |
| When writing raw DCT coefficients, it is crucial that the JPEG quantization |
| tables and sampling factors match the way the data was encoded, or the |
| resulting file will be invalid. For transcoding from an existing JPEG file, |
| we recommend using jpeg_copy_critical_parameters(). This routine initializes |
| all the compression parameters to default values (like jpeg_set_defaults()), |
| then copies the critical information from a source decompression object. |
| The decompression object should have just been used to read the entire |
| JPEG input file --- that is, it should be awaiting jpeg_finish_decompress(). |
| |
| jpeg_write_coefficients() marks all tables stored in the compression object |
| as needing to be written to the output file (thus, it acts like |
| jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid |
| emitting abbreviated JPEG files by accident. If you really want to emit an |
| abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables' |
| individual sent_table flags, between calling jpeg_write_coefficients() and |
| jpeg_finish_compress(). |
| |
| |
| Progress monitoring |
| ------------------- |
| |
| Some applications may need to regain control from the JPEG library every so |
| often. The typical use of this feature is to produce a percent-done bar or |
| other progress display. (For a simple example, see cjpeg.c or djpeg.c.) |
| Although you do get control back frequently during the data-transferring pass |
| (the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes |
| will occur inside jpeg_finish_compress or jpeg_start_decompress; those |
| routines may take a long time to execute, and you don't get control back |
| until they are done. |
| |
| You can define a progress-monitor routine which will be called periodically |
| by the library. No guarantees are made about how often this call will occur, |
| so we don't recommend you use it for mouse tracking or anything like that. |
| At present, a call will occur once per MCU row, scanline, or sample row |
| group, whichever unit is convenient for the current processing mode; so the |
| wider the image, the longer the time between calls. During the data |
| transferring pass, only one call occurs per call of jpeg_read_scanlines or |
| jpeg_write_scanlines, so don't pass a large number of scanlines at once if |
| you want fine resolution in the progress count. (If you really need to use |
| the callback mechanism for time-critical tasks like mouse tracking, you could |
| insert additional calls inside some of the library's inner loops.) |
| |
| To establish a progress-monitor callback, create a struct jpeg_progress_mgr, |
| fill in its progress_monitor field with a pointer to your callback routine, |
| and set cinfo->progress to point to the struct. The callback will be called |
| whenever cinfo->progress is non-NULL. (This pointer is set to NULL by |
| jpeg_create_compress or jpeg_create_decompress; the library will not change |
| it thereafter. So if you allocate dynamic storage for the progress struct, |
| make sure it will live as long as the JPEG object does. Allocating from the |
| JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You |
| can use the same callback routine for both compression and decompression. |
| |
| The jpeg_progress_mgr struct contains four fields which are set by the library: |
| long pass_counter; /* work units completed in this pass */ |
| long pass_limit; /* total number of work units in this pass */ |
| int completed_passes; /* passes completed so far */ |
| int total_passes; /* total number of passes expected */ |
| During any one pass, pass_counter increases from 0 up to (not including) |
| pass_limit; the step size is usually but not necessarily 1. The pass_limit |
| value may change from one pass to another. The expected total number of |
| passes is in total_passes, and the number of passes already completed is in |
| completed_passes. Thus the fraction of work completed may be estimated as |
| completed_passes + (pass_counter/pass_limit) |
| -------------------------------------------- |
| total_passes |
| ignoring the fact that the passes may not be equal amounts of work. |
| |
| When decompressing, pass_limit can even change within a pass, because it |
| depends on the number of scans in the JPEG file, which isn't always known in |
| advance. The computed fraction-of-work-done may jump suddenly (if the library |
| discovers it has overestimated the number of scans) or even decrease (in the |
| opposite case). It is not wise to put great faith in the work estimate. |
| |
| When using the decompressor's buffered-image mode, the progress monitor work |
| estimate is likely to be completely unhelpful, because the library has no way |
| to know how many output passes will be demanded of it. Currently, the library |
| sets total_passes based on the assumption that there will be one more output |
| pass if the input file end hasn't yet been read (jpeg_input_complete() isn't |
| TRUE), but no more output passes if the file end has been reached when the |
| output pass is started. This means that total_passes will rise as additional |
| output passes are requested. If you have a way of determining the input file |
| size, estimating progress based on the fraction of the file that's been read |
| will probably be more useful than using the library's value. |
| |
| |
| Memory management |
| ----------------- |
| |
| This section covers some key facts about the JPEG library's built-in memory |
| manager. For more info, please read structure.txt's section about the memory |
| manager, and consult the source code if necessary. |
| |
| All memory and temporary file allocation within the library is done via the |
| memory manager. If necessary, you can replace the "back end" of the memory |
| manager to control allocation yourself (for example, if you don't want the |
| library to use malloc() and free() for some reason). |
| |
| Some data is allocated "permanently" and will not be freed until the JPEG |
| object is destroyed. Most data is allocated "per image" and is freed by |
| jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the |
| memory manager yourself to allocate structures that will automatically be |
| freed at these times. Typical code for this is |
| ptr = (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE, size); |
| Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object. |
| Use alloc_large instead of alloc_small for anything bigger than a few Kbytes. |
| There are also alloc_sarray and alloc_barray routines that automatically |
| build 2-D sample or block arrays. |
| |
| The library's minimum space requirements to process an image depend on the |
| image's width, but not on its height, because the library ordinarily works |
| with "strip" buffers that are as wide as the image but just a few rows high. |
| Some operating modes (eg, two-pass color quantization) require full-image |
| buffers. Such buffers are treated as "virtual arrays": only the current strip |
| need be in memory, and the rest can be swapped out to a temporary file. |
| |
| When using temporary files, the library will make the in-memory buffers for |
| its virtual arrays just big enough to stay within a "maximum memory" setting. |
| Your application can set this limit by setting cinfo->mem->max_memory_to_use |
| after creating the JPEG object. (Of course, there is still a minimum size for |
| the buffers, so the max-memory setting is effective only if it is bigger than |
| the minimum space needed.) If you allocate any large structures yourself, you |
| must allocate them before jpeg_start_compress() or jpeg_start_decompress() in |
| order to have them counted against the max memory limit. Also keep in mind |
| that space allocated with alloc_small() is ignored, on the assumption that |
| it's too small to be worth worrying about; so a reasonable safety margin |
| should be left when setting max_memory_to_use. |
| |
| NOTE: Unless you develop your own memory manager back end, then temporary files |
| will never be used. The back end provided in libjpeg-turbo (jmemnobs.c) simply |
| malloc()s and free()s virtual arrays, and an error occurs if the required |
| memory exceeds the limit specified in cinfo->mem->max_memory_to_use. |
| |
| |
| Memory usage |
| ------------ |
| |
| Working memory requirements while performing compression or decompression |
| depend on image dimensions, image characteristics (such as colorspace and |
| JPEG process), and operating mode (application-selected options). |
| |
| As of v6b, the decompressor requires: |
| 1. About 24K in more-or-less-fixed-size data. This varies a bit depending |
| on operating mode and image characteristics (particularly color vs. |
| grayscale), but it doesn't depend on image dimensions. |
| 2. Strip buffers (of size proportional to the image width) for IDCT and |
| upsampling results. The worst case for commonly used sampling factors |
| is about 34 bytes * width in pixels for a color image. A grayscale image |
| only needs about 8 bytes per pixel column. |
| 3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG |
| file (including progressive JPEGs), or whenever you select buffered-image |
| mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's |
| 3 bytes per pixel for a color image. Worst case (1x1 sampling) requires |
| 6 bytes/pixel. For grayscale, figure 2 bytes/pixel. |
| 4. To perform 2-pass color quantization, the decompressor also needs a |
| 128K color lookup table and a full-image pixel buffer (3 bytes/pixel). |
| This does not count any memory allocated by the application, such as a |
| buffer to hold the final output image. |
| |
| The above figures are valid for 8-bit JPEG data precision and a machine with |
| 32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and |
| quantization pixel buffer. The "fixed-size" data will be somewhat smaller |
| with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual |
| color spaces will require different amounts of space. |
| |
| The full-image coefficient and pixel buffers, if needed at all, do not |
| have to be fully RAM resident; you can have the library use temporary |
| files instead when the total memory usage would exceed a limit you set. |
| (But if your OS supports virtual memory, it's probably better to just use |
| jmemnobs and let the OS do the swapping.) |
| |
| The compressor's memory requirements are similar, except that it has no need |
| for color quantization. Also, it needs a full-image DCT coefficient buffer |
| if Huffman-table optimization is asked for, even if progressive mode is not |
| requested. |
| |
| If you need more detailed information about memory usage in a particular |
| situation, you can enable the MEM_STATS code in jmemmgr.c. |
| |
| |
| Library compile-time options |
| ---------------------------- |
| |
| A number of compile-time options are available by modifying jmorecfg.h. |
| |
| The JPEG standard provides for both the baseline 8-bit DCT process and |
| a 12-bit DCT process. The IJG code supports 12-bit lossy JPEG if you define |
| BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be |
| larger than a char, so it affects the surrounding application's image data. |
| The sample applications cjpeg and djpeg can support 12-bit mode only for PPM |
| and GIF file formats; you must disable the other file formats to compile a |
| 12-bit cjpeg or djpeg. At present, a 12-bit library can handle *only* 12-bit |
| images, not both precisions. |
| |
| Note that a 12-bit library always compresses in Huffman optimization mode, |
| in order to generate valid Huffman tables. This is necessary because our |
| default Huffman tables only cover 8-bit data. If you need to output 12-bit |
| files in one pass, you'll have to supply suitable default Huffman tables. |
| You may also want to supply your own DCT quantization tables; the existing |
| quality-scaling code has been developed for 8-bit use, and probably doesn't |
| generate especially good tables for 12-bit. |
| |
| The maximum number of components (color channels) in the image is determined |
| by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we |
| expect that few applications will need more than four or so. |
| |
| On machines with unusual data type sizes, you may be able to improve |
| performance or reduce memory space by tweaking the various typedefs in |
| jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s |
| is quite slow; consider trading memory for speed by making JCOEF, INT16, and |
| UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int. |
| You probably don't want to make JSAMPLE be int unless you have lots of memory |
| to burn. |
| |
| You can reduce the size of the library by compiling out various optional |
| functions. To do this, undefine xxx_SUPPORTED symbols as necessary. |
| |
| You can also save a few K by not having text error messages in the library; |
| the standard error message table occupies about 5Kb. This is particularly |
| reasonable for embedded applications where there's no good way to display |
| a message anyway. To do this, remove the creation of the message table |
| (jpeg_std_message_table[]) from jerror.c, and alter format_message to do |
| something reasonable without it. You could output the numeric value of the |
| message code number, for example. If you do this, you can also save a couple |
| more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing; |
| you don't need trace capability anyway, right? |
| |
| |
| Portability considerations |
| -------------------------- |
| |
| The JPEG library has been written to be extremely portable; the sample |
| applications cjpeg and djpeg are slightly less so. This section summarizes |
| the design goals in this area. (If you encounter any bugs that cause the |
| library to be less portable than is claimed here, we'd appreciate hearing |
| about them.) |
| |
| The code works fine on ANSI C and C++ compilers, using any of the popular |
| system include file setups, and some not-so-popular ones too. |
| |
| The code is not dependent on the exact sizes of the C data types. As |
| distributed, we make the assumptions that |
| char is at least 8 bits wide |
| short is at least 16 bits wide |
| int is at least 16 bits wide |
| long is at least 32 bits wide |
| (These are the minimum requirements of the ANSI C standard.) Wider types will |
| work fine, although memory may be used inefficiently if char is much larger |
| than 8 bits or short is much bigger than 16 bits. The code should work |
| equally well with 16- or 32-bit ints. |
| |
| In a system where these assumptions are not met, you may be able to make the |
| code work by modifying the typedefs in jmorecfg.h. However, you will probably |
| have difficulty if int is less than 16 bits wide, since references to plain |
| int abound in the code. |
| |
| char can be either signed or unsigned, although the code runs faster if an |
| unsigned char type is available. If char is wider than 8 bits, you will need |
| to redefine JOCTET and/or provide custom data source/destination managers so |
| that JOCTET represents exactly 8 bits of data on external storage. |
| |
| The JPEG library proper does not assume ASCII representation of characters. |
| But some of the image file I/O modules in cjpeg/djpeg do have ASCII |
| dependencies in file-header manipulation; so does cjpeg's select_file_type() |
| routine. |
| |
| The JPEG library does not rely heavily on the C library. In particular, C |
| stdio is used only by the data source/destination modules and the error |
| handler, all of which are application-replaceable. (cjpeg/djpeg are more |
| heavily dependent on stdio.) malloc and free are called only from the memory |
| manager "back end" module, so you can use a different memory allocator by |
| replacing that one file. |
| |
| More info about porting the code may be gleaned by reading jconfig.txt, |
| jmorecfg.h, and jinclude.h. |