Libjpeg
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libjpeg is a library of computer programs created by the Independent JPEG Group which contain functions to manipulate JPEG images. It is written entirely in the C programming language. The current version of the library is 6b, although it has not been updated since 1998.
A common error that users of the library sometimes stumble upon is "JPEG parameter struct mismatch: library thinks size is X, caller expects Y" where X and Y are some numbers in the range 400-500. The reason for the error is that the user is using a version of the header file, jpeglib.h, that is different from the version of the installed binaries. This occurs when two different versions of libjpeg have been installed.
The jpegtran command-line program is useful to optimize the compression of a JPEG file, convert between progressive and non-progressive JPEG formats, eliminate non-standard application-specific data inserted by some image programs, or to perform certain transformations on a file — such as grayscaling, or rotating and flipping (within certain limits) — all done "losslessly" (i.e. without decompressing and recompressing the data, and so causing a reduction of image quality due to generation loss).