| libpng.txt - a description on how to use and modify libpng |
| |
| libpng 1.0 beta 2 - version 0.86 |
| For conditions of distribution and use, see copyright notice in png.h |
| Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc. |
| January 10, 1996 |
| Updated/rewritten per request in the libpng FAQ |
| Copyright (c) 1995 Frank J. T. Wojcik |
| December 18, 1995 |
| |
| I. Introduction |
| |
| This file describes how to use and modify the PNG reference library |
| (known as libpng) for your own use. There are five sections to this |
| file: introduction, structures, reading, writing, and modification and |
| configuration notes for various special platforms. Other then this |
| file, example.c is a good starting point for using the library, as |
| it is heavily commented and should include everything most people |
| will need. |
| |
| Libpng was written as a companion to the PNG specification, as a |
| way to reduce the amount of time and effort it takes to support |
| the PNG file format in application programs. Most users will not |
| have to modify the library significantly; advanced users may want |
| to modify it more. The library was coded for both kind of users. |
| All attempts were made to make it as complete as possible, while |
| keeping the code easy to understand. Currently, this library |
| only supports C. Support for other languages is being considered. |
| |
| Libpng has been designed to handle multiple sessions at one time, |
| to be easily modifiable, to be portable to the vast majority of |
| machines (ANSI, K&R, 16 bit, 32 bit) available, and to be easy to |
| use. The ultimate goal of libpng is to promote the acceptance of |
| the PNG file format in whatever way possible. While there is still |
| work to be done (see the todo.txt file), libpng should cover the |
| majority of the needs of it's users. |
| |
| Libpng uses zlib for its compression and decompression of PNG files. |
| The zlib compression utility is a general purpose utility that is |
| useful for more then PNG files, and can be used without libpng. |
| See the documentation delivered with zlib for more details. |
| |
| |
| |
| II. Structures |
| |
| There are two main structures that are important to libpng, png_struct |
| and png_info. The first, png_struct, is an internal structure that |
| will not, for the most part, be used by the general user except as |
| the first variable passed to every png function call. |
| |
| The png_info structure is designed to provide information about the |
| png file. All of its fields are intended to be examined or modified |
| by the user. See png.h for a good description of the png_info fields. |
| png.h is also an invaluable reference for programming with libpng. |
| |
| And while I'm on the topic, make sure you include the png header file: |
| |
| #include <png.h> |
| |
| |
| |
| III. Reading |
| |
| Checking PNG files: |
| |
| Libpng provides a simple check to see if a file is a PNG file. To |
| use it, pass in the first 1 to 8 bytes of the file, and it will return |
| true or false (1 or 0) depending on whether the bytes could be part |
| of a PNG file. Of course, the more bytes you pass in, the greater |
| the accuracy of the prediction. If you pass in more then eight bytes, |
| libpng will only look at the first eight bytes. |
| |
| fread(header, 1, number, fp); |
| is_png = png_check_sig(header, number); |
| |
| Reading PNG files: |
| |
| We'll now walk you through the possible functions to call when reading |
| in a PNG file, briefly explaining the syntax and purpose of each one. |
| See example.c and png.h for more detail. While Progressive reading |
| is covered in the next section, you will still need some of the |
| functions discussed in this section to read a PNG file. |
| |
| The first thing you need to do while reading a PNG file, aside from |
| the standard I/O initialization, is to allocate and initialize |
| png_struct and png_info. As these are both large, you may not want to |
| store these on the stack, unless you have stack space to spare. Of |
| course, you will want to check if malloc returns NULL. |
| |
| png_structp png_ptr = malloc(sizeof (png_struct)); |
| if (!png_ptr) |
| return; |
| png_infop info_ptr = malloc(sizeof (png_info)); |
| if (!info_ptr) |
| { |
| free(png_ptr); |
| return; |
| } |
| |
| You may also want to do any i/o initialization here, before |
| you get into libpng, so if it doesn't work, you don't have |
| much to undo. |
| |
| FILE *fp = fopen(file_name, "rb"); |
| if (!fp) |
| { |
| free(png_ptr); |
| free(info_ptr); |
| return; |
| } |
| |
| If you are not using the standard i/o functions, you will need |
| to replace them with custom functions. See the discussion under |
| Customizing libpng. |
| |
| After you have these structures, you will need to set up the |
| error handling. When libpng encounters an error, it expects to |
| longjmp back to your routine. Therefore, you will need to call |
| setjmp and pass the jmpbuf field of your png_struct. If you |
| read the file from different routines, you will need to update |
| the jmpbuf field every time you enter a new routine that will |
| call a png_ function. See your documentation of setjmp/longjmp |
| for your compiler for more information on setjmp/longjmp. See |
| the discussion on libpng error handling in the Customizing Libpng |
| section below for more information on the libpng error handling. |
| If an error occurs, and libpng longjmp's back to your setjmp, |
| you will want to call png_read_destroy() to free any memory. |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_read_destroy(png_ptr, info_ptr, (png_info *)0); |
| /* free pointers before returning, if necessary */ |
| free(png_ptr); |
| free(info_ptr); |
| fclose(fp); |
| return; |
| } |
| |
| Next, you will need to call png_info_init() and png_read_init(). |
| These functions make sure all the fields are initialized to useful |
| values, and, in the case of png_read_init(), and allocate any memory |
| needed for internal uses. You must call png_info_init() first, as |
| png_read_init() could do a longjmp, and if the info is not initialized, |
| the png_read_destroy() could try to png_free() random addresses, which |
| would be bad. |
| |
| png_info_init(info_ptr); |
| png_read_init(png_ptr); |
| |
| Now you need to set up the input code. The default for libpng is |
| to use the C function fread(). If you use this, you will need to |
| pass a valid FILE * in the function png_init_io(). Be sure that |
| the file is opened in binary mode. If you wish to handle reading |
| data in another way, see the discussion on png i/o handling in the |
| Customizing Libpng section below. |
| |
| png_init_io(png_ptr, fp); |
| |
| You are now ready to read all the file information up to the actual |
| image data. You do this with a call to png_read_info(). |
| |
| png_read_info(png_ptr, info_ptr); |
| |
| The png_info structure is now filled in with all the data necessary |
| to read the file. Some of the more important parts of the png_info are: |
| |
| width - holds the width of the file |
| height - holds the height of the file |
| bit_depth - holds the bit depth of one of the image channels |
| color_type - describes the channels and what they mean |
| (see the PNG_COLOR_TYPE_ macros for more information) |
| channels - number of channels of info for the color type |
| pixel_depth - bits per pixel, the result of multiplying the bit_depth |
| times the channels |
| rowbytes - number of bytes needed to hold a row |
| interlace_type - currently 0 for none, 1 for interlaced |
| valid - this details which optional chunks were found in the |
| file to see if a chunk was present, AND valid with the |
| appropriate PNG_INFO_<chunk name> define. |
| |
| These are also important, but their validity depends on whether a |
| corresponding chunk exists. Use valid (see above) to ensure that what |
| you're doing with these values makes sense. |
| |
| palette - the palette for the file |
| num_palette - number of entries in the palette |
| gamma - the gamma the file is written at |
| sig_bit - the number of significant bits |
| for the gray, red, green, and blue channels, whichever are |
| appropriate for the given color type. |
| sig_bit_number - number of channels |
| trans_values - transparent pixel for non-paletted images |
| trans - array of transparent entries for paletted images |
| number_trans - number of transparent entries |
| hist - histogram of palette |
| text - text comments in the file. |
| num_text - number of comments |
| |
| for more information, see the png_info definition in png.h and the |
| PNG specification for chunk contents. Be careful with trusting |
| rowbytes, as some of the transformations could increase the space |
| needed to hold a row (expand, rgbx, xrgb, graph_to_rgb, etc.). |
| See png_update_info(), below. |
| |
| A quick word about text and num_text. PNG stores comments in |
| keyword/text pairs, one pair per chunk. While there are |
| suggested keywords, there is no requirement to restrict the use |
| to these strings. There is a requirement to have at least one |
| character for a keyword. It is strongly suggested that keywords |
| be sensible to humans (that's the point), so don't use abbreviations. |
| See the png specification for more details. There is no requirement |
| to have text after the keyword. |
| |
| Keywords are restricted to 80 characters without leading or trailing |
| spaces, but spaces are allowed within the keyword It is possible to |
| have the same keyword any number of times. The text field is an |
| array of png_text structures, each holding pointer to a keyword |
| and a pointer to a text string. Only the text string may be null. |
| The keyword/text pairs are put into the array in the order that |
| they are received. However, some or all of the text chunks may be |
| after the image, so to make sure you have read all the text chunks, |
| don't mess with these until after you read the stuff after the image. |
| This will be mentioned again below in the discussion that goes with |
| png_read_end(). |
| |
| After you've read the file information, you can set up the library to |
| handle any special transformations of the image data. The various |
| ways to transform the data will be described in the order that they |
| occur. This is important, as some of these change the color type |
| and bit depth of the data, and some others only work on certain |
| color types and bit depths. Even though each transformation should |
| check to see if it has data that it can do somthing with, you should |
| make sure to only enable a transformation if it will be valid for |
| the data. For example, don't swap red and blue on grayscale data. |
| |
| This transforms bit depths of less than 8 to 8 bits, changes paletted |
| images to rgb, and adds an alpha channel if there is transparency |
| information in a tRNS chunk. This is probably most useful on grayscale |
| images with bit depths of 2 or 4 and tRNS chunks. |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_PALETTE && |
| info_ptr->bit_depth < 8) |
| png_set_expand(png_ptr); |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY && |
| info_ptr->bit_depth < 8) |
| png_set_expand(png_ptr); |
| |
| if (info_ptr->valid & PNG_INFO_tRNS) |
| png_set_expand(png_ptr); |
| |
| This handles alpha and transparency by replacing it with a background |
| value. If there was a valid one in the file, you can use it if you |
| want. However, you can replace it with your own if you want also. If |
| there wasn't one in the file, you must supply a color. If libpng is |
| doing gamma correction, you will need to tell libpng where the |
| background came from so it can do the appropriate gamma correction. |
| If you are modifying the color data with png_set_expand(), you must |
| indicate whether the background needs to be expanded. See the |
| function definition in png.h for more details. |
| |
| png_color_16 my_background; |
| |
| if (info_ptr->valid & PNG_INFO_bKGD) |
| png_set_backgrond(png_ptr, &(info_ptr->background), |
| PNG_BACKGROUND_GAMMA_FILE, 1, 1.0); |
| else |
| png_set_background(png_ptr, &my_background, |
| PNG_BACKGROUND_GAMMA_SCREEN, 0, 1.0); |
| |
| This handles gamma transformations of the data. Pass both the file |
| gamma and the desired screen gamma. If the file does not have a |
| gamma value, you can pass one anyway if you wish. Note that file |
| gammas are inverted from screen gammas. See the discussions on |
| gamma in the PNG specification for more information. It is strongly |
| recommended that viewers support gamma correction. |
| |
| if (info_ptr->valid & PNG_INFO_gAMA) |
| png_set_gamma(png_ptr, screen_gamma, info_ptr->gamma); |
| else |
| png_set_gamma(png_ptr, screen_gamma, 0.45); |
| |
| PNG can have files with 16 bits per channel. If you only can handle |
| 8 bits per channel, this will strip the pixels down to 8 bit. |
| |
| if (info_ptr->bit_depth == 16) |
| png_set_strip_16(png_ptr); |
| |
| If you need to reduce an rgb file to a paletted file (perhaps because |
| a paletted file has more entries then will fit on your screen) |
| png_set_dither() will do that. Note that this is a simple match |
| dither, that merely finds the closest color available. This should |
| work fairly well with optimized palettes, and fairly badly with linear |
| color cubes. If you pass a palette that is larger then |
| maximum_colors, the file will reduce the number of colors in the |
| palette so it will fit into maximum_colors. If there is a histogram, |
| it will use it to make intelligent choices when reducing the palette. |
| If there is no histogram, it may not do as good a job. |
| |
| This function will be rewritten and/or replaced in libpng 0.9, which |
| will have full two pass dithering with optimized palettes. |
| |
| if (info_ptr->color_type & PNG_COLOR_MASK_COLOR) |
| { |
| if (info_ptr->valid & PNG_INFO_PLTE) |
| png_set_dither(png_ptr, info_ptr->palette, |
| info_ptr->num_palette, max_screen_colors, |
| info_ptr->histogram); |
| else |
| { |
| png_color std_color_cube[MAX_SCREEN_COLORS] = |
| { ... colors ... }; |
| |
| png_set_dither(png_ptr, std_color_cube, MAX_SCREEN_COLORS, |
| MAX_SCREEN_COLORS, NULL); |
| } |
| } |
| |
| PNG files describe monocrome as black is zero and white is one. The |
| following code will reverse this (make black be one and white be zero): |
| |
| if (info_ptr->bit_depth == 1 && |
| info_ptr->color_type == PNG_COLOR_GRAY) |
| png_set_invert(png_ptr); |
| |
| PNG files have possible bit depths of 1, 2, 4, 8, and 16. However, |
| they also provide a way to describe the true bit depth of the image. |
| It is then required that values be "scaled" or "shifted" up to the bit |
| depth used in the file. See the PNG specification for details. This |
| code reduces the pixels back down to the true bit depth: |
| |
| if (info_ptr->valid & PNG_INFO_sBIT) |
| png_set_shift(png_ptr, &(info_ptr->sig_bit)); |
| |
| PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| they can, resulting in, for example, 8 pixels per byte for 1 bit |
| files. This code expands to 1 pixel per byte without changing the |
| values of the pixels: |
| |
| if (info_ptr->bit_depth < 8) |
| png_set_packing(png_ptr); |
| |
| PNG files store 3 color pixels in red, green, blue order. This code |
| changes the storage of the pixels to blue, green, red: |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_RGB || |
| info_ptr->color_type == PNG_COLOR_TYPE_RGB_ALPHA) |
| png_set_bgr(png_ptr); |
| |
| For some uses, you may want a gray-scale image to be represented as |
| rgb. This code will do that conversion: |
| |
| if (info_ptr->color_type == PNG_COLOR_TYPE_GRAY || |
| info_ptr->color_type == PNG_COLOR_TYPE_GRAY_ALPHA) |
| png_set_gray_to_rgb(png_ptr); |
| |
| PNG files store 16 bit pixels in network byte order (big-endian, |
| ie. most significant bits first). This code chages the storage to the |
| other way (little-endian, ie. least significant bits first, eg. the |
| way PCs store them): |
| |
| if (info_ptr->bit_depth == 16) |
| png_set_swap(png_ptr); |
| |
| PNG files store rgb pixels packed into 3 bytes. This code packs them |
| into 4 bytes: |
| |
| if (info_ptr->bit_depth == 8 && |
| info_ptr->color_type == PNG_COLOR_TYPE_RGB) |
| png_set_filler(png_ptr, filler_byte, PNG_FILLER_BEFORE); |
| |
| where filler_byte is the number to fill with, and the location is |
| either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether |
| you want the filler before the rgb or after. |
| |
| The last thing to handle is interlacing; this is covered in detail below, |
| but you must call the function here. |
| |
| if (info_ptr->interlace_type) |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| After setting the transformations, you can update your palette by |
| calling png_start_read_image(). This function is provided for those |
| who need an updated palette before they read the image data. If you |
| don't call this function, the library will automatically call it |
| before it reads the first row. |
| |
| png_start_read_image(png_ptr); |
| |
| libpng can update your png_info structure to reflect any |
| transformations you've requested with this call. This is most useful |
| to update the info structures rowbytes field, so you can use it to |
| allocate your image memory. This function calls |
| png_start_read_image(), so you don't have to call both of them. |
| |
| png_read_update_info(png_ptr, info_ptr); |
| |
| After you call png_read_update_info(), you can allocate any |
| memory you need to hold the image. As the actual allocation |
| varies among applications, no example will be given. If you |
| are allocating one large chunk, you may find it useful to |
| build an array of pointers to each row, as it will be needed |
| for some of the functions below. |
| |
| After you've allocated memory, you can read the image data. |
| The simplest way to do this is in one function call. If you are |
| allocating enough memory to hold the whole image, you can just |
| call png_read_image() and libpng will read in all the image data |
| and put it in the memory area supplied. You will need to pass in |
| an array of pointers to each row. |
| |
| This function automatically handles interlacing, so you don't need |
| to call png_set_interlace_handling() or call this function multiple |
| times, or any of that other stuff necessary with png_read_rows(). |
| |
| png_read_image(png_ptr, row_pointers); |
| |
| where row_pointers is: |
| |
| png_bytep row_pointers[height]; |
| |
| You can point to void or char or whatever you use for pixels. |
| |
| If you don't want to read the whole image in at once, you can |
| use png_read_rows() instead. If there is no interlacing (check |
| info_ptr->interlace_type), this is simple: |
| |
| png_read_rows(png_ptr, row_pointers, NULL, number_of_rows); |
| |
| row_pointers is the same as in the png_read_image() call. |
| |
| If you are just calling one row at a time, you can do this for |
| row_pointers: |
| |
| png_bytep row_pointers = row; |
| |
| png_read_rows(png_ptr, &row_pointers, NULL, 1); |
| |
| If the file is interlaced (info_ptr->interlace_type != 0), things get |
| a good deal harder. The only currently (as of 12/95) defined |
| interlacing scheme for PNG files (info_ptr->interlace_type == 1) is a |
| complicated interlace scheme, known as Adam7, that breaks down an |
| image into seven smaller images of varying size. libpng will fill out |
| those images or it will give them to you "as is". If you want to fill |
| them out, there are two ways to do that. The one mentioned in the PNG |
| specification is to expand each pixel to cover those pixels that have |
| not been read yet. This results in a blocky image for the first pass, |
| which gradually smoothes out as more pixels are read. The other |
| method is the "sparkle" method, where pixels are draw only in their |
| final locations, with the rest of the image remaining whatever colors |
| they were initialized to before the start of the read. The first |
| method usually looks better, but tends to be slower, as there are more |
| pixels to put in the rows. |
| |
| If you don't want libpng to handle the interlacing details, just |
| call png_read_rows() the correct number of times to read in all |
| seven images. See the PNG specification for more details on the |
| interlacing scheme. |
| |
| If you want libpng to expand the images, call this above: |
| |
| if (info_ptr->interlace_type) |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| This will return the number of passes needed. Currently, this |
| is seven, but may change if another interlace type is added. |
| This function can be called even if the file is not interlaced, |
| when it will return one. |
| |
| If you are not going to display the image after each pass, but are |
| going to wait until the entire image is read in, use the sparkle |
| effect. This effect is faster and the end result of either method |
| is exactly the same. If you are planning on displaying the image |
| after each pass, the rectangle effect is generally considered the |
| better looking one. |
| |
| If you only want the "sparkle" effect, just call png_read_rows() as |
| normal, with the third parameter NULL. Make sure you make pass over |
| the image number_passes times, and you don't change the data in the |
| rows between calls. You can change the locations of the data, just |
| not the data. Each pass only writes the pixels appropriate for that |
| pass, and assumes the data from previous passes is still valid. |
| |
| png_read_rows(png_ptr, row_pointers, NULL, number_of_rows); |
| |
| If you only want the first effect (the rectangles), do the same as |
| before except pass the row buffer in the third parameter, and leave |
| the second parameter NULL. |
| |
| png_read_rows(png_ptr, NULL, row_pointers, number_of_rows); |
| |
| After you are finished reading the image, you can finish reading |
| the file. If you are interested in comments or time, you should |
| pass the png_info pointer from the png_read_info() call. If you |
| are not interested, you can pass NULL. |
| |
| png_read_end(png_ptr, info_ptr); |
| |
| When you are done, you can free all memory used by libpng like this: |
| |
| png_read_destroy(png_ptr, info_ptr, (png_info *)0); |
| |
| After that, you can discard the structures, or reuse them another |
| read or write. For a more compact example of reading a PNG image, |
| see the file example.c. |
| |
| |
| Reading PNG files progressively: |
| |
| The progressive reader is slightly different then the non-progressive |
| reader. Instead of calling png_read_info(), png_read_rows(), and |
| png_read_end(), you make one call to png_process_data(), which calls |
| callbacks when it has the info, a row, or the end of the image. You |
| set up these callbacks with png_set_progressive_read_fn(). You don't |
| have to worry about the input/output functions of libpng, as you are |
| giving the library the data directly in png_process_data(). I will |
| assume that you have read the second on reading PNG files above, |
| so I will only highlight the differences (although I will show |
| all of the code). |
| |
| png_structp png_ptr; |
| png_infop info_ptr; |
| |
| int |
| initialize_png_reader() |
| { |
| png_ptr = malloc(sizeof (png_struct)); |
| if (!png_ptr) |
| return -1; |
| info_ptr = malloc(sizeof (png_info)); |
| if (!info_ptr) |
| { |
| free(png_ptr); |
| return -1; |
| } |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_read_destroy(png_ptr, info_ptr, (png_info *)0); |
| /* free pointers before returning, if necessary */ |
| free(png_ptr); |
| free(info_ptr); |
| return -1; |
| } |
| |
| png_info_init(info_ptr); |
| png_read_init(png_ptr); |
| |
| /* this one's new. You will need to provide all three |
| function callbacks, even if you aren't using them all. |
| You can use any void pointer as the user_ptr, and |
| retrieve the pointer from inside the callbacks using |
| the function png_get_progressive_ptr(png_ptr); */ |
| png_set_progressive_read_fn(png_ptr, user_ptr, |
| info_callback, row_callback, end_callback); |
| |
| return 0; |
| } |
| |
| int |
| process_data(png_bytep buffer, png_uint_32 length) |
| { |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_read_destroy(png_ptr, info_ptr, (png_info *)0); |
| free(png_ptr); |
| free(info_ptr); |
| return -1; |
| } |
| |
| /* this one's new also. Simply give it a chunk of data |
| from the file stream (in order, of course). On Segmented |
| machines, don't give it any more then 64K. The library |
| seems to run fine with sizes of 4K, although you can give |
| it much less if necessary (I assume you can give it chunks |
| of 1 byte, but I haven't tried less then 256 bytes yet). |
| When this function returns, you may want to display any |
| rows that were generated in the row callback. */ |
| png_process_data(png_ptr, info_ptr, buffer, length); |
| return 0; |
| } |
| |
| info_callback(png_structp png_ptr, png_infop info) |
| { |
| do any setup here, including setting any of the transformations |
| mentioned in the Reading PNG files section. For now, you _must_ |
| call either png_start_read_image() or png_read_update_info() |
| after all the transformations are set (even if you don't set |
| any). You may start getting rows before png_process_data() |
| returns, so this is your last chance to prepare for that. |
| } |
| |
| row_callback(png_structp png_ptr, png_bytep new_row, |
| png_uint_32 row_num, int pass) |
| { |
| this function is called for every row in the image. If the |
| image is interlacing, and you turned on the interlace handler, |
| this function will be called for every row in every pass. |
| Some of these rows will not be changed from the previous pass. |
| When the row is not changed, the new_row variable will be NULL. |
| The rows and passes are called in order, so you don't really |
| need the row_num and pass, but I'm supplying them because it |
| may make your life easier. |
| |
| For the non-NULL rows of interlaced images, you must call |
| png_progressive_combine_row() passing in the row and the |
| old row. You can call this function for NULL rows (it will |
| just return) and for non-interlaced images (it just does the |
| memcpy for you) if it will make the code easier. Thus, you |
| can just do this for all cases: |
| |
| png_progressive_combine_row(png_ptr, old_row, new_row); |
| |
| where old_row is what was displayed for previous rows. Note |
| that the first pass (pass == 0 really) will completely cover |
| the old row, so the rows do not have to be initialized. After |
| the first pass (and only for interlaced images), you will have |
| to pass the current row, and the function will combine the |
| old row and the new row. |
| } |
| |
| end_callback(png_structp png_ptr, png_infop info) |
| { |
| this function is called when the whole image has been read, |
| including any chunks after the image (up to and including |
| the IEND). You will usually have the same info chunk as you |
| had in the header, although some data may have been added |
| to the comments and time fields. |
| |
| Most people won't do much here, perhaps setting a flag that |
| marks the image as finished. |
| } |
| |
| |
| IV. Writing |
| |
| Much of this is very similar to reading. However, everything of |
| importance is repeated here, so you don't have to constantly look |
| back up in the reading section to understand writing. |
| |
| The first thing you need to do while writing a PNG file is to allocate |
| and initialize png_struct and png_info. As these are both large, you |
| may not want to store these on the stack, unless you have stack space |
| to spare. |
| |
| png_structp png_ptr = malloc(sizeof (png_struct)); |
| if (!png_ptr) |
| return; |
| png_infop info_ptr = malloc(sizeof (png_info)); |
| if (!info_ptr) |
| { |
| free(png_ptr); |
| return; |
| } |
| |
| You may also want to do any i/o initialization here, before |
| you get into libpng, so if it doesn't work, you don't have |
| much to undo. |
| |
| FILE *fp = fopen(file_name, "wb"); |
| if (!fp) |
| { |
| free(png_ptr); |
| free(info_ptr); |
| return; |
| } |
| |
| After you have these structures, you will need to set up the |
| error handling. When libpng encounters an error, it expects to |
| longjmp back to your routine. Therefore, you will need to call |
| setjmp and pass the jmpbuf field of your png_struct. If you |
| write the file from different routines, you will need to update |
| the jmpbuf field every time you enter a new routine that will |
| call a png_ function. See your documentation of setjmp/longjmp |
| for your compiler for more information on setjmp/longjmp. See |
| the discussion on libpng error handling in the Customizing Libpng |
| section below for more information on the libpng error handling. |
| |
| if (setjmp(png_ptr->jmpbuf)) |
| { |
| png_write_destroy(png_ptr); |
| /* free pointers before returning. Make sure you clean up |
| anything else you've done. */ |
| free(png_ptr); |
| free(info_ptr); |
| fclose(fp); |
| return; |
| } |
| |
| Next, you will need to call png_info_init() and png_write_init(). |
| These functions make sure all the fields are initialized to useful |
| values, and, in the case of png_write_init(), allocate any memory |
| needed for internal uses. Do png_info_init() first, so if |
| png_write_init() longjmps, you know info_ptr is valid, so you |
| don't free random memory pointers, which would be bad. |
| |
| png_info_init(info_ptr); |
| png_write_init(png_ptr); |
| |
| Now you need to set up the input code. The default for libpng is |
| to use the C function fwrite(). If you use this, you will need to |
| pass a valid FILE * in the function png_init_io(). Be sure that |
| the file is opened in binary mode. If you wish to handle writing |
| data in another way, see the discussion on png i/o handling in the |
| Customizing Libpng section below. |
| |
| png_init_io(png_ptr, fp); |
| |
| You now have the option of modifying how the compression library |
| will run. The following functions are mainly for testing, but |
| may be useful in certain special cases, like if you need to |
| write png files extremely fast and are willing to give up some |
| compression, or if you want to get the maximum possible compression |
| at the expense of slower writing. If you have no special needs |
| in this area, let the library do what it wants, as it has been |
| carefully tuned to deliver the best speed/compression ratio. |
| See the compression library for more details. |
| |
| /* turn on or off filtering (1 or 0) */ |
| png_set_filtering(png_ptr, 1); |
| |
| /* compression level (0 - none, 6 - default, 9 - maximum) */ |
| png_set_compression_level(png_ptr, Z_DEFAULT_COMPRESSION); |
| png_set_compression_mem_level(png_ptr, 8); |
| png_set_compression_strategy(png_ptr, Z_DEFAULT_STRATEGY); |
| png_set_compression_window_bits(png_ptr, 15); |
| png_set_compression_method(png_ptr, 8); |
| |
| You now need to fill in the png_info structure with all the data |
| you wish to write before the actual image. Note that the only thing |
| you are allowed to write after the image is the text chunks and the |
| time chunk. See png_write_end() for more information on that. If you |
| wish to write them before the image, fill them in now. If you want to |
| wait until after the data, don't fill them until png_write_end(). For |
| all the fields in png_info, see png.h. For explanations of what the |
| fields contain, see the PNG specification. |
| |
| Some of the more important parts of the png_info are: |
| |
| width - holds the width of the file |
| height - holds the height of the file |
| bit_depth - holds the bit depth of one of the image channels |
| color_type - describes the channels and what they mean |
| see the PNG_COLOR_TYPE_ defines for more information |
| interlace_type - currently 0 for none, 1 for interlaced |
| valid - this describes which optional chunks to write to the |
| file. Note that if you are writing a |
| PNG_COLOR_TYPE_PALETTE file, the PLTE chunk is not |
| optional, but must still be marked for writing. To |
| mark chunks for writing, OR valid with the appropriate |
| PNG_INFO_<chunk name> define. |
| palette - the palette for the file |
| num_palette - number of entries in the palette |
| gamma - the gamma the file is written at |
| sig_bit - the number of significant bits |
| for the gray, red, green, and blue channels, whichever are |
| appropriate for the given color type. |
| sig_bit_number - number of channels |
| trans_values - transparent pixel for non-paletted images |
| trans - array of transparent entries for paletted images |
| number_trans - number of transparent entries |
| hist - histogram of palette |
| text - text comments in the file. |
| num_text - number of comments |
| |
| A quick word about text and num_text. text is an array of png_text |
| structures. num_text is the number of valid structures in the array. |
| If you want, you can use max_text to hold the size of the array, but |
| libpng ignores it for writing (it does use it for reading). Each |
| png_text structure holds a keyword-text value, and a compression type. |
| The compression types have the same valid numbers as the compression |
| types of the image data. Currently, the only valid number is zero. |
| However, you can store text either compressed or uncompressed, unlike |
| images which always have to be compressed. So if you don't want the |
| text compressed, set the compression type to -1. Until text gets |
| arount 1000 bytes, it is not worth compressing it. |
| |
| The keyword-text pairs work like this. Keywords should be short |
| simple descriptions of what the comment is about. Some typical |
| keywords are found in the PNG specification, as is some recomendations |
| on keywords. You can repeat keywords in a file. You can even write |
| some text before the image and some after. For example, you may want |
| to put a description of the image before the image, but leave the |
| disclaimer until after, so viewers working over modem connections |
| don't have to wait for the disclaimer to go over the modem before |
| they start seeing the image. Finally, keywords should be full |
| words, not abbreviations. Keywords can not contain NUL characters, |
| and should not contain control characters. Text in general should |
| not contain control characters. The keyword must be present, but |
| you can leave off the text string on non-compressed pairs. |
| Compressed pairs must have a text string, as only the text string |
| is compressed anyway, so the compression would be meaningless. |
| |
| PNG supports modification time via the png_time structure. Two |
| conversion routines are proved, png_convert_from_time_t() for |
| time_t and png_convert_from_struct_tm() for struct tm. The |
| time_t routine uses gmtime(). You don't have to use either of |
| these, but if you wish to fill in the png_time structure directly, |
| you should provide the time in universal time (GMT) if possible |
| instead of your local time. Note that the year number is the full |
| year (ie 1996, rather than 96) |
| |
| It is possible to have libpng flush any pending output, either manually, |
| or automatically after a certain number of lines have been written. To |
| flush the output stream a single time call: |
| |
| png_write_flush(png_ptr); |
| |
| and to have libpng flush the output stream periodically after a certain |
| number of scanlines have been written, call: |
| |
| png_set_flush(png_ptr, nrows); |
| |
| Note that the distance between rows is from the last time png_write_flush |
| was called, or the first row of the image if it has never been called. |
| So if you write 50 lines, and then png_set_flush 25, it will flush the |
| output on the next scanline, and on line 75, unless png_write_flush is |
| called earlier. If nrows is too small (less than about 10 lines) the |
| image compression may decrease dramatically (although this may be |
| acceptable for real-time applications). Infrequent flushing will only |
| degrade the compression performance by a few percent over images that |
| do not use flushing. |
| |
| You are now ready to write all the file information up to the actual |
| image data. You do this with a call to png_write_info(). |
| |
| png_write_info(png_ptr, info_ptr); |
| |
| After you've read the file information, you can set up the library to |
| handle any special transformations of the image data. The various |
| ways to transform the data will be described in the order that they |
| occur. This is important, as some of these change the color type |
| and bit depth of the data, and some others only work on certain |
| color types and bit depths. Even though each transformation should |
| check to see if it has data that it can do somthing with, you should |
| make sure to only enable a transformation if it will be valid for |
| the data. For example, don't swap red and blue on grayscale data. |
| |
| PNG files store rgb pixels packed into 3 bytes. This code tells |
| the library to use 4 bytes per pixel |
| |
| png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE); |
| |
| where the 0 is not used for writing, and the location is either |
| PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether you |
| want the filler before the rgb or after. |
| |
| PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as |
| they can, resulting in, for example, 8 pixels per byte for 1 bit files. |
| If the data is supplied at 1 pixel per byte, use this code, which will |
| correctly pack the values: |
| |
| png_set_packing(png_ptr); |
| |
| PNG files reduce possible bit depths to 1, 2, 4, 8, and 16. If your |
| data is of another bit depth, but is packed into the bytes correctly, |
| this will scale the values to appear to be the correct bit depth. |
| Make sure you write a sBIT chunk when you do this, so others, if |
| they want, can reduce the values down to their true depth. |
| |
| /* do this before png_write_info() */ |
| info_ptr->valid |= PNG_INFO_sBIT; |
| |
| /* note that you can cheat and set all the values of |
| sig_bit to true_bit_depth if you want */ |
| if (info_ptr->color_type & PNG_COLOR_MASK_COLOR) |
| { |
| info_ptr->sig_bit.red = true_bit_depth; |
| info_ptr->sig_bit.green = true_bit_depth; |
| info_ptr->sig_bit.blue = true_bit_depth; |
| } |
| else |
| { |
| info_ptr->sig_bit.gray = true_bit_depth; |
| } |
| |
| if (info_ptr->color_type & PNG_COLOR_MASK_ALPHA) |
| { |
| info_ptr->sig_bit.alpha = true_bit_depth; |
| } |
| |
| png_set_shift(png_ptr, &(info_ptr->sig_bit)); |
| |
| PNG files store 16 bit pixels in network byte order (big-endian, |
| ie. most significant bits first). This code would be used to supply |
| them the other way (little-endian, ie. least significant bits first, |
| eg. the way PCs store them): |
| |
| png_set_swap(png_ptr); |
| |
| PNG files store 3 color pixels in red, green, blue order. This code |
| would be used to supply the pixels as blue, green, red: |
| |
| png_set_bgr(png_ptr); |
| |
| PNG files describe monochrome as black being zero and white being |
| one. This code would be used to supply the pixels with this reversed |
| (black being one and white being zero): |
| |
| png_set_invert(png_ptr); |
| |
| That's it for the transformations. Now you can write the image data. |
| The simplest way to do this is in one function call. If have the |
| whole image in memory, you can just call png_write_image() and libpng |
| will write the image. You will need to pass in an array of pointers to |
| each row. This function automatically handles interlacing, so you don't |
| need to call png_set_interlace_handling() or call this function multiple |
| times, or any of that other stuff necessary with png_write_rows(). |
| |
| png_write_image(png_ptr, row_pointers); |
| |
| where row_pointers is: |
| |
| png_bytef *row_pointers[height]; |
| |
| You can point to void or char or whatever you use for pixels. |
| |
| If you can't want to write the whole image at once, you can |
| use png_write_rows() instead. If the file is not interlaced, |
| this is simple: |
| |
| png_write_rows(png_ptr, row_pointers, number_of_rows); |
| |
| row_pointers is the same as in the png_write_image() call. |
| |
| If you are just calling one row at a time, you can do this for |
| row_pointers: |
| |
| png_bytep row_pointers = row; |
| |
| png_write_rows(png_ptr, &row_pointers, 1); |
| |
| When the file is interlaced, things can get a good deal more |
| complicated. The only currently (as of 12/95) defined interlacing |
| scheme for PNG files is a compilcated interlace scheme, known as |
| Adam7, that breaks down an image into seven smaller images of varying |
| size. libpng will build these images for you, or you can do them |
| yourself. If you want to build them yourself, see the PNG |
| specification for details of which pixels to write when. |
| |
| If you don't want libpng to handle the interlacing details, just |
| call png_write_rows() the correct number of times to write all |
| seven sub-images. |
| |
| If you want libpng to build the sub-images, call this before you start |
| writing any rows: |
| |
| number_passes = png_set_interlace_handling(png_ptr); |
| |
| This will return the number of passes needed. Currently, this |
| is seven, but may change if another interlace type is added. |
| |
| Then write the image number_passes times. |
| |
| png_write_rows(png_ptr, row_pointers, number_of_rows); |
| |
| As some of these rows are not used, and thus return immediately, |
| you may want to read about interlacing in the PNG specification, |
| and only update the rows that are actually used. |
| |
| After you are finished writing the image, you should finish writing |
| the file. If you are interested in writing comments or time, you should |
| pass the an appropriately filled png_info pointer. If you |
| are not interested, you can pass NULL. Be careful that you don't |
| write the same text or time chunks here as you did in png_write_info(). |
| |
| png_write_end(png_ptr, info_ptr); |
| |
| When you are done, you can free all memory used by libpng like this: |
| |
| png_write_destroy(png_ptr); |
| |
| Any data you allocated for png_info, you must free yourself. |
| |
| After that, you can discard the structures, or reuse them another |
| read or write. For a more compact example of writing a PNG image, |
| see the file example.c. |
| |
| |
| V. Modifying/Customizing libpng: |
| |
| There are two issues here. The first is changing how libpng does |
| standard things like memory allocation, input/output, and error handling. |
| The second deals with more complicated things like adding new chunks, |
| adding new transformations, and generally changing how libpng works. |
| |
| All of the memory allocation, input/output, and error handling in libpng |
| goes through callbacks which are user setable. The default routines |
| are in pngerror.c, pngmem.c, and pngio.c. To change these functions, |
| call the approprate _fn function. |
| |
| Memory allocation is done through the functions png_large_malloc(), |
| png_malloc(), png_realloc(), png_large_free(), and png_free(). |
| These currently just call the standard C functions. The large |
| functions must handle exactly 64K, but they don't have to handle |
| more then that. If your pointers can't access more then 64K at a |
| time, you will want to set MAXSEG_64K in zlib.h. Since it is unlikely |
| that the method of handling memory allocation on a platform will |
| change between applications, these functions must be modified in the |
| library at compile time. |
| |
| Input/Output in libpng is done throught png_read() and png_write(), which |
| currently just call fread() and fwrite(). The FILE * is stored in |
| png_struct, and is initialized via png_init_io(). If you wish to change |
| the method of I/O, the library supplies callbacks that you can set through |
| the function png_set_read_fn() and png_set_write_fn() at run time. These |
| functions also provide a void pointer that can be retrieved via the function |
| png_get_io_ptr(). For example: |
| |
| png_set_read_fn(png_structp png_ptr, voidp io_ptr, |
| png_rw_ptr read_data_fn) |
| |
| png_set_write_fn(png_structp png_ptr, voidp io_ptr, |
| png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn); |
| |
| voidp io_ptr = png_get_io_ptr(png_ptr); |
| |
| The replacement I/O functions should have prototypes as follows: |
| |
| void user_read_data(png_structp png_ptr, png_bytep data, |
| png_uint_32 length); |
| void user_write_data(png_structp png_ptr, png_bytep data, |
| png_uint_32 length); |
| void user_flush_data(png_structp png_ptr); |
| |
| Note that you can pass NULL for the flush function if you are not doing |
| flushing of the output data. |
| |
| Error handling in libpng is done through png_error() and png_warning(). |
| Errors handled through png_error() are fatal, meaning that png_error() |
| should never return to it's caller. Currently, this is handled via |
| setjmp() and longjmp(), but you could change this to do things like |
| exit() if you should wish. On non-fatal errors, png_warning() is called |
| to print a warning message, and then control returns to the calling code. |
| By default png_error() and png_warning() print a message on stderr. If |
| you wish to change the behavior of the error functions, you will need to |
| set up your own message callbacks. You do this like the I/O callbacks above. |
| |
| png_set_message_fn(png_structp png_ptr, png_voidp msg_ptr, |
| png_msg_ptr error_fn, png_msg_ptr warning_fn); |
| |
| png_voidp msg_ptr = png_get_msg_ptr(png_ptr); |
| |
| The replacement message functions should have parameters as follows: |
| |
| void user_error_fn(png_struct png_ptr, png_const_charp error_msg); |
| void user_warning_fn(png_struct png_ptr, png_const_charp warning_msg); |
| |
| The motivation behind using setjmp() and longjmp() is the C++ throw and |
| catch exception handling methods. This makes the code much easier to write, |
| as there is no need to check every return code of every function call. |
| However, there are some uncertainties about the status of local variables |
| after a longjmp, so the user may want to be careful about doing anything after |
| setjmp returns non zero besides returning itself. Consult your compiler |
| documentation for more details. |
| |
| If you need to read or write custom chunks, you will need to get deeper |
| into the libpng code. First, read the PNG specification, and have |
| a first level of understanding of how it works. Pay particular |
| attention to the sections that describe chunk names, and look |
| at how other chunks were designed, so you can do things similar. |
| Second, check out the sections of libpng that read and write chunks. |
| Try to find a chunk that is similar to yours, and copy off of it. |
| More details can be found in the comments inside the code. |
| |
| If you wish to write your own transformation for the data, look |
| through the part of the code that does the transformations, and check |
| out some of the more simple ones to get an idea of how they work. Try |
| to find a similar transformation to the one you want to add, and copy |
| off of it. More details can be found in the comments inside the code |
| itself. |
| |
| Configuring for 16 bit platforms: |
| |
| You will probably need to change the png_large_malloc() and |
| png_large_free() routines in pngmem.c, as these are requred |
| to allocate 64K. Also, you will want to look into zconf.h to tell |
| zlib (and thus libpng) that it cannot allocate more then 64K at a |
| time. Even if you can, the memory won't be accessable. So limit zlib |
| and libpng to 64K by defining MAXSEG_64K. |
| |
| Configuring for Medium Model: |
| |
| Libpng's support for medium model has been tested on most of the popular |
| complers. Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets |
| defined, and FAR gets defined to far in pngconf.h, and you should be |
| all set. Everything in the library (except for zlib's structure) is |
| expecting far data. You must use the typedefs with the p or pp on |
| the end for pointers (or at least look at them and be careful). Make |
| note that the row's of data are defined as png_bytepp which is a |
| unsigned char far * far * |
| |
| Configuring for gui/windowing platforms: |
| |
| You will need to change the error message display in png_error() and |
| png_warning() to display a message instead of fprinting it to stderr. |
| You may want to write a single function to do this and call it something |
| like png_message(). On some compliers, you may have to change the |
| memory allocators (png_malloc, etc.). |
| |
| |
| Configuring for compiler xxx: |
| |
| All includes for libpng are in pngconf.h. If you need to add/change/delete |
| an include, this is the place to do it. The includes that are not |
| needed outside libpng are protected by the PNG_INTERNAL definition, |
| which is only defined for those routines inside libpng itself. The |
| files in libpng proper only include png.h. |
| |
| Removing unwanted object code: |
| |
| There are a bunch of #define's in pngconf.h that control what parts of |
| libpng are compiled. All the defines end in _SUPPORT. If you are |
| never going to use an ability, you can change the #define to #undef and |
| save yourself code and data space. All the reading and writing |
| specific code are in seperate files, so the linker should only grab |
| the files it needs. However, if you want to make sure, or if you |
| are building a stand alone library, all the reading files start with |
| pngr and all the writing files start with pngw. The files that |
| don't match either (like png.c, pngtrans.c, etc.) are used for |
| both reading and writing, and always need to be included. The |
| progressive reader is in pngpread.c |
| |