Color quantization is a legacy feature that serves little or no purpose
with lossless JPEG images. 9f756bc67a
eliminated interaction issues between the lossless decompressor and the
color quantizers related to out-of-range 12-bit samples, but referring
to #701, other interaction issues apparently still exist. Such issues
are likely, given the fact that the color quantizers were not designed
with lossless decompression in mind.
This commit reverts 9f756bc67a, since the
issues it fixed are no longer relevant because of this commit and
2192560d74.
Fixed#672Fixes#673Fixes#674Fixes#676Fixes#677Fixes#678Fixes#679Fixes#681Fixes#683Fixes#701
12-bit is the only data precision for which the range of the sample data
type exceeds the valid sample range, so it is possible to craft a 12-bit
lossless JPEG image that contains out-of-range 12-bit samples.
Attempting to decompress such an image using color quantization or merged
upsampling (NOTE: libjpeg-turbo cannot generate YCbCr or subsampled
lossless JPEG images, but it can decompress them) caused segfaults or
buffer overruns when those algorithms attempted to use the out-of-range
sample values as array indices. This commit modifies the lossless
decompressor so that it range-limits the output of the scaler when using
12-bit samples.
Fixes#670Fixes#672Fixes#673Fixes#674Fixes#675Fixes#676Fixes#677Fixes#678Fixes#679Fixes#681Fixes#683
- Rename jpeg_simple_lossless() to jpeg_enable_lossless() and modify the
function so that it stores the lossless parameters directly in the Ss
and Al fields of jpeg_compress_struct rather than using a scan script.
- Move the cjpeg -lossless switch into "Switches for advanced users".
- Document the libjpeg API and run-time features that are unavailable in
lossless mode, and ensure that all parameters, functions, and switches
related to unavailable features are ignored or generate errors in
lossless mode.
- Defer any action that depends on whether lossless mode is enabled
until jpeg_start_compress()/jpeg_start_decompress() is called.
- Document the purpose of the point transform value.
- "Codec" stands for coder/decoder, so it is a bit awkward to say
"lossless compression codec" and "lossless decompression codec".
Use "lossless compressor" and "lossless decompressor" instead.
- Restore backward API/ABI compatibility with libjpeg v6b:
* Move the new 'lossless' field from the exposed jpeg_compress_struct
and jpeg_decompress_struct structures into the opaque
jpeg_comp_master and jpeg_decomp_master structures, and allocate the
master structures in the body of jpeg_create_compress() and
jpeg_create_decompress().
* Remove the new 'process' field from jpeg_compress_struct and
jpeg_decompress_struct and replace it with the old
'progressive_mode' field and the new 'lossless' field.
* Remove the new 'data_unit' field from jpeg_compress_struct and
jpeg_decompress_struct and replace it with a locally-computed
data unit variable.
* Restore the names of macros and fields that refer to DCT blocks, and
document that they have a different meaning in lossless mode. (Most
of them aren't very meaningful in lossless mode anyhow.)
* Remove the new alloc_darray() method from jpeg_memory_mgr and
replace it with an internal macro that wraps the alloc_sarray()
method.
* Move the JDIFF* data types from jpeglib.h and jmorecfg.h into
jpegint.h.
* Remove the new 'codec' field from jpeg_compress_struct and
jpeg_decompress_struct and instead reuse the existing internal
coefficient control, forward/inverse DCT, and entropy
encoding/decoding structures for lossless compression/decompression.
* Repurpose existing error codes rather than introducing new ones.
(The new JERR_BAD_RESTART and JWRN_MUST_DOWNSCALE codes remain,
although JWRN_MUST_DOWNSCALE will probably be removed in
libjpeg-turbo, since we have a different way of handling multiple
data precisions.)
- Automatically enable lossless mode when a scan script with parameters
that are only valid for lossless mode is detected, and document the
use of scan scripts to generate lossless JPEG images.
- Move the sequential and shared Huffman routines back into jchuff.c and
jdhuff.c, and document that those routines are shared with jclhuff.c
and jdlhuff.c as well as with jcphuff.c and jdphuff.c.
- Move MAX_DIFF_BITS from jchuff.h into jclhuff.c, the only place where
it is used.
- Move the predictor and scaler code into jclossls.c and jdlossls.c.
- Streamline register usage in the [un]differencers (inspired by similar
optimizations in the color [de]converters.)
- Restructure the logic in a few places to reduce duplicated code.
- Ensure that all lossless-specific code is guarded by
C_LOSSLESS_SUPPORTED or D_LOSSLESS_SUPPORTED and that the library can
be built successfully if either or both of those macros is undefined.
- Remove all short forms of external names introduced by the lossless
JPEG patch. (These will not be needed by libjpeg-turbo, so there is
no use cleaning them up.)
- Various wordsmithing, formatting, and punctuation tweaks
- Eliminate various compiler warnings.
