| .TH JPEGTRAN 1 "30 August 2024" |
| .SH NAME |
| jpegtran \- lossless transformation of JPEG files |
| .SH SYNOPSIS |
| .B jpegtran |
| [ |
| .I options |
| ] |
| [ |
| .I filename |
| ] |
| .LP |
| .SH DESCRIPTION |
| .LP |
| .B jpegtran |
| performs various useful transformations of JPEG files. |
| It can translate the coded representation from one variant of JPEG to another, |
| for example from baseline JPEG to progressive JPEG or vice versa. It can also |
| perform some rearrangements of the image data, for example turning an image |
| from landscape to portrait format by rotation. |
| .PP |
| For EXIF files and JPEG files containing Exif data, you may prefer to use |
| .B exiftran |
| instead. |
| .PP |
| .B jpegtran |
| works by rearranging the compressed data (DCT coefficients), without |
| ever fully decoding the image. Therefore, its transformations are lossless: |
| there is no image degradation at all, which would not be true if you used |
| .B djpeg |
| followed by |
| .B cjpeg |
| to accomplish the same conversion. But by the same token, |
| .B jpegtran |
| cannot perform lossy operations such as changing the image quality. However, |
| while the image data is losslessly transformed, metadata can be removed. See |
| the |
| .B \-copy |
| option for specifics. |
| .PP |
| .B jpegtran |
| reads the named JPEG/JFIF file, or the standard input if no file is |
| named, and produces a JPEG/JFIF file on the standard output. |
| .SH OPTIONS |
| All switch names may be abbreviated; for example, |
| .B \-optimize |
| may be written |
| .B \-opt |
| or |
| .BR \-o . |
| Upper and lower case are equivalent. |
| British spellings are also accepted (e.g., |
| .BR \-optimise ), |
| though for brevity these are not mentioned below. |
| .PP |
| To specify the coded JPEG representation used in the output file, |
| .B jpegtran |
| accepts a subset of the switches recognized by |
| .BR cjpeg : |
| .TP |
| .B \-optimize |
| Perform optimization of entropy encoding parameters. |
| .TP |
| .B \-progressive |
| Create progressive JPEG file. |
| .TP |
| .BI \-restart " N" |
| Emit a JPEG restart marker every N MCU rows, or every N MCUs if "B" is attached |
| to the number. |
| .TP |
| .B \-arithmetic |
| Use arithmetic coding. |
| .TP |
| .BI \-scans " file" |
| Use the scan script given in the specified text file. |
| .PP |
| See |
| .BR cjpeg (1) |
| for more details about these switches. |
| If you specify none of these switches, you get a plain baseline-JPEG output |
| file. The quality setting and so forth are determined by the input file. |
| .PP |
| The image can be losslessly transformed by giving one of these switches: |
| .TP |
| .B \-flip horizontal |
| Mirror image horizontally (left-right). |
| .TP |
| .B \-flip vertical |
| Mirror image vertically (top-bottom). |
| .TP |
| .B \-rotate 90 |
| Rotate image 90 degrees clockwise. |
| .TP |
| .B \-rotate 180 |
| Rotate image 180 degrees. |
| .TP |
| .B \-rotate 270 |
| Rotate image 270 degrees clockwise (or 90 ccw). |
| .TP |
| .B \-transpose |
| Transpose image (across UL-to-LR axis). |
| .TP |
| .B \-transverse |
| Transverse transpose (across UR-to-LL axis). |
| .PP |
| The transpose transformation has no restrictions regarding image dimensions. |
| The other transformations operate rather oddly if the image dimensions are not |
| a multiple of the iMCU size (usually 8 or 16 pixels), because they can only |
| transform complete blocks of DCT coefficient data in the desired way. |
| .PP |
| .BR jpegtran 's |
| default behavior when transforming an odd-size image is designed |
| to preserve exact reversibility and mathematical consistency of the |
| transformation set. As stated, transpose is able to flip the entire image |
| area. Horizontal mirroring leaves any partial iMCU column at the right edge |
| untouched, but is able to flip all rows of the image. Similarly, vertical |
| mirroring leaves any partial iMCU row at the bottom edge untouched, but is |
| able to flip all columns. The other transforms can be built up as sequences |
| of transpose and flip operations; for consistency, their actions on edge |
| pixels are defined to be the same as the end result of the corresponding |
| transpose-and-flip sequence. |
| .PP |
| For practical use, you may prefer to discard any untransformable edge pixels |
| rather than having a strange-looking strip along the right and/or bottom edges |
| of a transformed image. To do this, add the |
| .B \-trim |
| switch: |
| .TP |
| .B \-trim |
| Drop non-transformable edge blocks. |
| .IP |
| Obviously, a transformation with |
| .B \-trim |
| is not reversible, so strictly speaking |
| .B jpegtran |
| with this switch is not lossless. Also, the expected mathematical |
| equivalences between the transformations no longer hold. For example, |
| .B \-rot 270 -trim |
| trims only the bottom edge, but |
| .B \-rot 90 -trim |
| followed by |
| .B \-rot 180 -trim |
| trims both edges. |
| .TP |
| .B \-perfect |
| If you are only interested in perfect transformations, add the |
| .B \-perfect |
| switch. This causes |
| .B jpegtran |
| to fail with an error if the transformation is not perfect. |
| .IP |
| For example, you may want to do |
| .IP |
| .B (jpegtran \-rot 90 -perfect |
| .I foo.jpg |
| .B || djpeg |
| .I foo.jpg |
| .B | pnmflip \-r90 | cjpeg) |
| .IP |
| to do a perfect rotation, if available, or an approximated one if not. |
| .PP |
| This version of \fBjpegtran\fR also offers a lossless crop option, which |
| discards data outside of a given image region but losslessly preserves what is |
| inside. Like the rotate and flip transforms, lossless crop is restricted by |
| the current JPEG format; the upper left corner of the selected region must fall |
| on an iMCU boundary. If it doesn't, then it is silently moved up and/or left |
| to the nearest iMCU boundary (the lower right corner is unchanged.) Thus, the |
| output image covers at least the requested region, but it may cover more. The |
| adjustment of the region dimensions may be optionally disabled by attaching an |
| 'f' character ("force") to the width or height number. |
| |
| The image can be losslessly cropped by giving the switch: |
| .TP |
| .B \-crop WxH+X+Y |
| Crop the image to a rectangular region of width W and height H, starting at |
| point X,Y. The lossless crop feature discards data outside of a given image |
| region but losslessly preserves what is inside. Like the rotate and flip |
| transforms, lossless crop is restricted by the current JPEG format; the upper |
| left corner of the selected region must fall on an iMCU boundary. If it |
| doesn't, then it is silently moved up and/or left to the nearest iMCU boundary |
| (the lower right corner is unchanged.) |
| .PP |
| If W or H is larger than the width/height of the input image, then the output |
| image is expanded in size, and the expanded region is filled in with zeros |
| (neutral gray). Attaching an 'f' character ("flatten") to the width number |
| will cause each block in the expanded region to be filled in with the DC |
| coefficient of the nearest block in the input image rather than grayed out. |
| Attaching an 'r' character ("reflect") to the width number will cause the |
| expanded region to be filled in with repeated reflections of the input image |
| rather than grayed out. |
| .PP |
| A complementary lossless wipe option is provided to discard (gray out) data |
| inside a given image region while losslessly preserving what is outside: |
| .TP |
| .B \-wipe WxH+X+Y |
| Wipe (gray out) a rectangular region of width W and height H from the input |
| image, starting at point X,Y. |
| .PP |
| Attaching an 'f' character ("flatten") to the width number will cause the |
| region to be filled with the average of adjacent blocks rather than grayed out. |
| If the wipe region and the region outside the wipe region, when adjusted to the |
| nearest iMCU boundary, form two horizontally adjacent rectangles, then |
| attaching an 'r' character ("reflect") to the width number will cause the wipe |
| region to be filled with repeated reflections of the outside region rather than |
| grayed out. |
| .PP |
| A lossless drop option is also provided, which allows another JPEG image to be |
| inserted ("dropped") into the input image data at a given position, replacing |
| the existing image data at that position: |
| .TP |
| .B \-drop +X+Y filename |
| Drop (insert) another image at point X,Y |
| .PP |
| Both the input image and the drop image must have the same subsampling level. |
| It is best if they also have the same quantization (quality.) Otherwise, the |
| quantization of the output image will be adapted to accommodate the higher of |
| the input image quality and the drop image quality. The trim option can be |
| used with the drop option to requantize the drop image to match the input |
| image. Note that a grayscale image can be dropped into a full-color image or |
| vice versa, as long as the full-color image has no vertical subsampling. If |
| the input image is grayscale and the drop image is full-color, then the |
| chrominance channels from the drop image will be discarded. |
| .PP |
| Other not-strictly-lossless transformation switches are: |
| .TP |
| .B \-grayscale |
| Force grayscale output. |
| .IP |
| This option discards the chrominance channels if the input image is YCbCr |
| (ie, a standard color JPEG), resulting in a grayscale JPEG file. The |
| luminance channel is preserved exactly, so this is a better method of reducing |
| to grayscale than decompression, conversion, and recompression. This switch |
| is particularly handy for fixing a monochrome picture that was mistakenly |
| encoded as a color JPEG. (In such a case, the space savings from getting rid |
| of the near-empty chroma channels won't be large; but the decoding time for |
| a grayscale JPEG is substantially less than that for a color JPEG.) |
| .PP |
| .B jpegtran |
| also recognizes these switches that control what to do with "extra" markers, |
| such as comment blocks: |
| .TP |
| .B \-copy none |
| Copy no extra markers from source file. This setting suppresses all |
| comments and other metadata in the source file. |
| .TP |
| .B \-copy comments |
| Copy only comment markers. This setting copies comments from the source file |
| but discards any other metadata. |
| .TP |
| .B \-copy icc |
| Copy only ICC profile markers. This setting copies the ICC profile from the |
| source file but discards any other metadata. |
| .TP |
| .B \-copy all |
| Copy all extra markers. This setting preserves miscellaneous markers |
| found in the source file, such as JFIF thumbnails, Exif data, and Photoshop |
| settings. In some files, these extra markers can be sizable. Note that this |
| option will copy thumbnails as-is; they will not be transformed. |
| .PP |
| The default behavior is \fB-copy comments\fR. (Note: in IJG releases v6 and |
| v6a, \fBjpegtran\fR always did the equivalent of \fB-copy none\fR.) |
| .PP |
| Additional switches recognized by jpegtran are: |
| .TP |
| .BI \-icc " file" |
| Embed ICC color management profile contained in the specified file. Note that |
| this will cause \fBjpegtran\fR to ignore any APP2 markers in the input file, |
| even if \fB-copy all\fR or \fB-copy icc\fR is specified. |
| .TP |
| .BI \-maxmemory " N" |
| Set limit for amount of memory to use in processing large images. Value is |
| in thousands of bytes, or millions of bytes if "M" is attached to the |
| number. For example, |
| .B \-max 4m |
| selects 4000000 bytes. If more space is needed, an error will occur. |
| .TP |
| .BI \-maxscans " N" |
| Abort if the input image contains more than |
| .I N |
| scans. This feature demonstrates a method by which applications can guard |
| against denial-of-service attacks instigated by specially-crafted malformed |
| JPEG images containing numerous scans with missing image data or image data |
| consisting only of "EOB runs" (a feature of progressive JPEG images that allows |
| potentially hundreds of thousands of adjoining zero-value pixels to be |
| represented using only a few bytes.) Attempting to transform such malformed |
| JPEG images can cause excessive CPU activity, since the decompressor must fully |
| process each scan (even if the scan is corrupt) before it can proceed to the |
| next scan. |
| .TP |
| .BI \-outfile " name" |
| Send output image to the named file, not to standard output. |
| .TP |
| .BI \-report |
| Report transformation progress. |
| .TP |
| .BI \-strict |
| Treat all warnings as fatal. This feature also demonstrates a method by which |
| applications can guard against attacks instigated by specially-crafted |
| malformed JPEG images. Enabling this option will cause the decompressor to |
| abort if the input image contains incomplete or corrupt image data. |
| .TP |
| .B \-verbose |
| Enable debug printout. More |
| .BR \-v 's |
| give more output. Also, version information is printed at startup. |
| .TP |
| .B \-debug |
| Same as |
| .BR \-verbose . |
| .TP |
| .B \-version |
| Print version information and exit. |
| .SH EXAMPLES |
| .LP |
| This example converts a baseline JPEG file to progressive form: |
| .IP |
| .B jpegtran \-progressive |
| .I foo.jpg |
| .B > |
| .I fooprog.jpg |
| .PP |
| This example rotates an image 90 degrees clockwise, discarding any |
| unrotatable edge pixels: |
| .IP |
| .B jpegtran \-rot 90 -trim |
| .I foo.jpg |
| .B > |
| .I foo90.jpg |
| .SH ENVIRONMENT |
| .TP |
| .B JPEGMEM |
| If this environment variable is set, its value is the default memory limit. |
| The value is specified as described for the |
| .B \-maxmemory |
| switch. |
| .B JPEGMEM |
| overrides the default value specified when the program was compiled, and |
| itself is overridden by an explicit |
| .BR \-maxmemory . |
| .SH SEE ALSO |
| .BR cjpeg (1), |
| .BR djpeg (1), |
| .BR rdjpgcom (1), |
| .BR wrjpgcom (1) |
| .br |
| Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
| Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
| .SH AUTHOR |
| Independent JPEG Group |
| .PP |
| This file was modified by The libjpeg-turbo Project to include only information |
| relevant to libjpeg-turbo and to wordsmith certain sections. |
| .SH BUGS |
| The transform options can't transform odd-size images perfectly. Use |
| .B \-trim |
| or |
| .B \-perfect |
| if you don't like the results. |
| .PP |
| The entire image is read into memory and then written out again, even in |
| cases where this isn't really necessary. Expect swapping on large images, |
| especially when using the more complex transform options. |