- Remove the section in libjpeg.txt that advised against building
libjpeg as a shared library. We obviously do not follow that advice,
and libjpeg-turbo does guarantee backward ABI compatibility in our
libjpeg API library, even though libjpeg did not and does not.
(Future expansion of our libjpeg API library, if necessary, will be
accomplished using get/set functions that store the new parameters
in the opaque master structs. Refer to
db2986c96f.)
- Unmention install.txt, which was never relevant to libjpeg-turbo and
was removed in v1.3 (6f96153c67).
- Remove extraneous spaces.
- Document the fact that TWO_FILE_COMMANDLINE must be defined in order
to use the two-file interface with cjpeg, djpeg, jpegtran, and
wrjpgcom. libjpeg-turbo never enables that interface by default.
- Refer to the "slow" [I]DCT algorithms as "accurate" instead, since
they are not slow under libjpeg-turbo.
- Adjust documentation claims to reflect the fact that the "slow" and
"fast" algorithms produce about the same performance on AVX2-equipped
CPUs (because of the dual-lane nature of AVX2, it was not possible to
accelerate the "fast" algorithm beyond what was achievable with SSE2.)
Also adjust the claims to reflect the fact that the "fast" algorithm
tends to be ~5-15% faster than the "slow" algorithm on
non-AVX2-equipped CPUs, regardless of the use of the libjpeg-turbo
SIMD extensions.
- Indicate the legacy status of the "fast" and float algorithms in the
documentation and cjpeg/djpeg usage info.
- Remove obsolete paragraph in the djpeg man page that suggested that
the float algorithm could be faster than the "fast" algorithm on some
CPUs.
- Restore GIF read/compressed GIF write support from jpeg-6a and
jpeg-9d.
- Integrate jpegtran -wipe and -drop options from jpeg-9a and jpeg-9d.
- Integrate jpegtran -crop extension (for expanding the image size) from
jpeg-9a and jpeg-9d.
- Integrate other minor code tweaks from jpeg-9*
libjpeg-turbo never included that code, because it requires an external
library (the Utah Raster Toolkit.) The RLE image format was supplanted
by GIF in the late 1980s, so it is rarely seen these days. (It had a
lousy Weissman score, anyhow.)
This re-introduces a feature of the obsolete system-specific libjpeg
memory managers-- namely the ability to limit the amount of main memory
used by the library during decompression or multi-pass compression.
This is mainly beneficial for two reasons:
- Works around a 2 GB limit in libFuzzer
- Allows security-sensitive applications to set a memory limit for the
JPEG decoder so as to work around the progressive JPEG exploit
(LJT-01-004) described here:
http://www.libjpeg-turbo.org/pmwiki/uploads/About/TwoIssueswiththeJPEGStandard.pdf
This commit also removes obsolete documentation regarding the MS-DOS
memory manager (which itself was removed long ago) and changes the
documentation of the -maxmemory switch and JPEGMEM environment variable
to reflect the fact that backing stores are never used in libjpeg-turbo.
Inspired by:
066fee2e7dCloses#143
Quality values > 95 are not useless. They just may not provide as good
of a size vs. perceptual quality tradeoff as lower quality values. This
also displays the default quality value in the cjpeg usage.
Closes#39
