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293 lines
9.3 KiB
293 lines
9.3 KiB
.TH JPEGTRAN 1 "13 September 2013" |
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.SH NAME |
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jpegtran \- lossless transformation of JPEG files |
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.SH SYNOPSIS |
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.B jpegtran |
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[ |
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.I options |
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] |
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[ |
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.I filename |
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] |
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.LP |
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.SH DESCRIPTION |
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.LP |
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.B jpegtran |
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performs various useful transformations of JPEG files. |
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It can translate the coded representation from one variant of JPEG to another, |
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for example from baseline JPEG to progressive JPEG or vice versa. It can also |
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perform some rearrangements of the image data, for example turning an image |
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from landscape to portrait format by rotation. |
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.PP |
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.B jpegtran |
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works by rearranging the compressed data (DCT coefficients), without |
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ever fully decoding the image. Therefore, its transformations are lossless: |
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there is no image degradation at all, which would not be true if you used |
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.B djpeg |
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followed by |
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.B cjpeg |
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to accomplish the same conversion. But by the same token, |
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.B jpegtran |
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cannot perform lossy operations such as changing the image quality. |
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.PP |
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.B jpegtran |
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reads the named JPEG/JFIF file, or the standard input if no file is |
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named, and produces a JPEG/JFIF file on the standard output. |
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.SH OPTIONS |
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All switch names may be abbreviated; for example, |
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.B \-optimize |
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may be written |
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.B \-opt |
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or |
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.BR \-o . |
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Upper and lower case are equivalent. |
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British spellings are also accepted (e.g., |
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.BR \-optimise ), |
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though for brevity these are not mentioned below. |
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.PP |
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To specify the coded JPEG representation used in the output file, |
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.B jpegtran |
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accepts a subset of the switches recognized by |
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.BR cjpeg : |
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.TP |
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.B \-optimize |
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Perform optimization of entropy encoding parameters. |
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.TP |
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.B \-progressive |
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Create progressive JPEG file. |
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.TP |
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.BI \-restart " N" |
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Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is |
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attached to the number. |
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.TP |
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.B \-arithmetic |
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Use arithmetic coding. |
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.TP |
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.BI \-scans " file" |
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Use the scan script given in the specified text file. |
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.PP |
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See |
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.BR cjpeg (1) |
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for more details about these switches. |
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If you specify none of these switches, you get a plain baseline-JPEG output |
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file. The quality setting and so forth are determined by the input file. |
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.PP |
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The image can be losslessly transformed by giving one of these switches: |
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.TP |
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.B \-flip horizontal |
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Mirror image horizontally (left-right). |
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.TP |
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.B \-flip vertical |
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Mirror image vertically (top-bottom). |
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.TP |
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.B \-rotate 90 |
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Rotate image 90 degrees clockwise. |
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.TP |
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.B \-rotate 180 |
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Rotate image 180 degrees. |
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.TP |
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.B \-rotate 270 |
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Rotate image 270 degrees clockwise (or 90 ccw). |
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.TP |
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.B \-transpose |
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Transpose image (across UL-to-LR axis). |
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.TP |
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.B \-transverse |
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Transverse transpose (across UR-to-LL axis). |
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.IP |
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The transpose transformation has no restrictions regarding image dimensions. |
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The other transformations operate rather oddly if the image dimensions are not |
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a multiple of the iMCU size (usually 8 or 16 pixels), because they can only |
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transform complete blocks of DCT coefficient data in the desired way. |
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.IP |
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.BR jpegtran 's |
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default behavior when transforming an odd-size image is designed |
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to preserve exact reversibility and mathematical consistency of the |
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transformation set. As stated, transpose is able to flip the entire image |
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area. Horizontal mirroring leaves any partial iMCU column at the right edge |
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untouched, but is able to flip all rows of the image. Similarly, vertical |
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mirroring leaves any partial iMCU row at the bottom edge untouched, but is |
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able to flip all columns. The other transforms can be built up as sequences |
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of transpose and flip operations; for consistency, their actions on edge |
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pixels are defined to be the same as the end result of the corresponding |
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transpose-and-flip sequence. |
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.IP |
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For practical use, you may prefer to discard any untransformable edge pixels |
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rather than having a strange-looking strip along the right and/or bottom edges |
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of a transformed image. To do this, add the |
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.B \-trim |
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switch: |
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.TP |
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.B \-trim |
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Drop non-transformable edge blocks. |
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.IP |
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Obviously, a transformation with |
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.B \-trim |
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is not reversible, so strictly speaking |
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.B jpegtran |
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with this switch is not lossless. Also, the expected mathematical |
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equivalences between the transformations no longer hold. For example, |
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.B \-rot 270 -trim |
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trims only the bottom edge, but |
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.B \-rot 90 -trim |
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followed by |
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.B \-rot 180 -trim |
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trims both edges. |
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.IP |
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If you are only interested in perfect transformation, add the |
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.B \-perfect |
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switch: |
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.TP |
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.B \-perfect |
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Fails with an error if the transformation is not perfect. |
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.IP |
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For example you may want to do |
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.IP |
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.B (jpegtran \-rot 90 -perfect |
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.I foo.jpg |
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.B || djpeg |
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.I foo.jpg |
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.B | pnmflip \-r90 | cjpeg) |
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.IP |
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to do a perfect rotation if available or an approximated one if not. |
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.PP |
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We also offer a lossless-crop option, which discards data outside a given |
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image region but losslessly preserves what is inside. Like the rotate and |
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flip transforms, lossless crop is restricted by the current JPEG format: the |
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upper left corner of the selected region must fall on an iMCU boundary. If |
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this does not hold for the given crop parameters, we silently move the upper |
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left corner up and/or left to make it so, simultaneously increasing the |
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region dimensions to keep the lower right crop corner unchanged. (Thus, the |
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output image covers at least the requested region, but may cover more.) |
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The adjustment of the region dimensions may be optionally disabled. |
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The image can be losslessly cropped by giving the switch: |
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.TP |
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.B \-crop WxH+X+Y |
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Crop to a rectangular subarea of width W, height H starting at point X,Y. |
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.PP |
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A complementary lossless-wipe option is provided to discard (gray out) data |
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inside a given image region while losslessly preserving what is outside: |
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.TP |
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.B \-wipe WxH+X+Y |
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Wipe (gray out) a rectangular subarea of width W, height H starting at point |
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X,Y. |
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.PP |
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Other not-strictly-lossless transformation switches are: |
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.TP |
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.B \-grayscale |
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Force grayscale output. |
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.IP |
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This option discards the chrominance channels if the input image is YCbCr |
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(ie, a standard color JPEG), resulting in a grayscale JPEG file. The |
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luminance channel is preserved exactly, so this is a better method of reducing |
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to grayscale than decompression, conversion, and recompression. This switch |
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is particularly handy for fixing a monochrome picture that was mistakenly |
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encoded as a color JPEG. (In such a case, the space savings from getting rid |
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of the near-empty chroma channels won't be large; but the decoding time for |
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a grayscale JPEG is substantially less than that for a color JPEG.) |
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.TP |
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.BI \-scale " M/N" |
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Scale the output image by a factor M/N. |
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.IP |
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Currently supported scale factors are M/N with all M from 1 to 16, where N is |
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the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted, |
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then M specifies the DCT scaled size to be applied on the given input. For |
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baseline JPEG this is equivalent to M/8 scaling, since the source DCT size |
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for baseline JPEG is 8. |
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.B Caution: |
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An implementation of the JPEG SmartScale extension is required for this |
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feature. SmartScale enabled JPEG is not yet widely implemented, so many |
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decoders will be unable to view a SmartScale extended JPEG file at all. |
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.PP |
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.B jpegtran |
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also recognizes these switches that control what to do with "extra" markers, |
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such as comment blocks: |
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.TP |
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.B \-copy none |
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Copy no extra markers from source file. This setting suppresses all |
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comments and other excess baggage present in the source file. |
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.TP |
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.B \-copy comments |
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Copy only comment markers. This setting copies comments from the source file, |
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but discards any other inessential (for image display) data. |
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.TP |
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.B \-copy all |
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Copy all extra markers. This setting preserves miscellaneous markers |
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found in the source file, such as JFIF thumbnails, Exif data, and Photoshop |
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settings. In some files these extra markers can be sizable. |
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.IP |
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The default behavior is |
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.BR "\-copy comments" . |
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(Note: in IJG releases v6 and v6a, |
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.B jpegtran |
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always did the equivalent of |
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.BR "\-copy none" .) |
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.PP |
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Additional switches recognized by jpegtran are: |
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.TP |
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.BI \-maxmemory " N" |
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Set limit for amount of memory to use in processing large images. Value is |
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in thousands of bytes, or millions of bytes if "M" is attached to the |
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number. For example, |
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.B \-max 4m |
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selects 4000000 bytes. If more space is needed, temporary files will be used. |
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.TP |
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.BI \-outfile " name" |
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Send output image to the named file, not to standard output. |
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.TP |
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.B \-verbose |
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Enable debug printout. More |
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.BR \-v 's |
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give more output. Also, version information is printed at startup. |
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.TP |
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.B \-debug |
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Same as |
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.BR \-verbose . |
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.SH EXAMPLES |
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.LP |
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This example converts a baseline JPEG file to progressive form: |
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.IP |
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.B jpegtran \-progressive |
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.I foo.jpg |
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.B > |
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.I fooprog.jpg |
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.PP |
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This example rotates an image 90 degrees clockwise, discarding any |
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unrotatable edge pixels: |
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.IP |
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.B jpegtran \-rot 90 -trim |
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.I foo.jpg |
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.B > |
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.I foo90.jpg |
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.SH ENVIRONMENT |
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.TP |
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.B JPEGMEM |
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If this environment variable is set, its value is the default memory limit. |
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The value is specified as described for the |
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.B \-maxmemory |
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switch. |
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.B JPEGMEM |
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overrides the default value specified when the program was compiled, and |
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itself is overridden by an explicit |
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.BR \-maxmemory . |
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.SH SEE ALSO |
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.BR cjpeg (1), |
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.BR djpeg (1), |
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.BR rdjpgcom (1), |
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.BR wrjpgcom (1) |
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.br |
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard", |
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44. |
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.SH AUTHOR |
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Independent JPEG Group |
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.SH BUGS |
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The transform options can't transform odd-size images perfectly. Use |
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.B \-trim |
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or |
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.B \-perfect |
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if you don't like the results. |
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.PP |
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The entire image is read into memory and then written out again, even in |
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cases where this isn't really necessary. Expect swapping on large images, |
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especially when using the more complex transform options.
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