Commit b56e8b28ba544c439265da17d955ec6ab5aa9277
2022-11-08T15:01:18
Clean up the lossless JPEG feature - 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.