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.
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 djpeg followed by cjpeg to accomplish the same conversion. But by the same token, jpegtran cannot perform lossy operations such as changing the image quality.
To specify the coded JPEG representation used in the output file, jpegtran accepts a subset of the switches recognized by cjpeg:
See 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.
The image can be losslessly transformed by giving one of these switches:
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.
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.
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 -trim switch:
Obviously, a transformation with -trim is not reversible, so strictly speaking jpegtran with this switch is not lossless. Also, the expected mathematical equivalences between the transformations no longer hold. For example, -rot 270 -trim trims only the bottom edge, but -rot 90 -trim followed by -rot 180 -trim trims both edges.
Another not-strictly-lossless transformation switch is:
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.)
jpegtran also recognizes these switches that control what to do with "extra" markers, such as comment blocks:
The default behavior is -copy comments. (Note: in IJG releases v6 and v6a, jpegtran always did the equivalent of -copy none.)
Additional switches recognized by jpegtran are:
This example converts a baseline JPEG file to progressive form:
This example rotates an image 90 degrees clockwise, discarding any unrotatable edge pixels:
The transform options can't transform odd-size images perfectly. Use -trim if you don't like the results without it.
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.