Commit 7bb958b732e6b4f261595e2d1527d46964fe3aed
2024-03-04T12:12:03
12-bit: Don't gen opt Huff tbls if tbls supplied (regression introduced by e8b40f3c2ba187ba95c13c3e8ce21c8534256df7) The documented behavior of the libjpeg API is to compute optimal Huffman tables when generating 12-bit lossy Huffman-coded JPEG images, unless the calling application supplies its own Huffman tables. However, e8b40f3c2ba187ba95c13c3e8ce21c8534256df7 and 96bc40c1b36775afdbad4ae05a6b3f48e2eebeb9 modified jinit_c_master_control() so that it always set cinfo->optimize_coding to TRUE when generarating 12-bit lossy Huffman-coded JPEG images, which prevented calling applications from supplying custom Huffman tables for such images. This commit modifies jinit_c_master_control() so that it only overrides cinfo->optimize_coding when generating 12-bit lossy Huffman-coded JPEG images if all Huffman table slots are empty or all slots contain default Huffman tables. Determining whether the latter is true requires using memcmp() to compare the allocated Huffman tables with the default Huffman tables, because: - The documented behavior of jpeg_set_defaults() is to initialize any empty Huffman table slot with the default Huffman table corresponding to that slot, regardless of the data precision. There is also no requirement that the data precision be specified prior to calling jpeg_set_defaults(). Thus, there is no reliable way to prevent jpeg_set_defaults() from initializing empty Huffman table slots with default Huffman tables, which are useless for 12-bit data precision. - There is no requirement that custom Huffman tables be defined prior to calling jpeg_set_defaults(). A calling application could call jpeg_set_defaults() and modify the values in the default Huffman tables rather than allocating new tables. Thus, there is no reliable way to detect whether the allocated Huffman tables contain default values without comparing the tables with the default Huffman tables. Fortunately, comparing the allocated Huffman tables with the default Huffman tables is the last stop on the logic train, so it won't happen unless cinfo->data_precision == 12, cinfo->arith_code == FALSE, cinfo->optimize_coding == FALSE, and one or more Huffman tables are allocated. (If the compressor object is reused, this ensures that the full comparison will be performed at most once.) Custom Huffman tables will be flagged as non-default when the first non-default value is encountered, and the worst case (comparing 400 bytes) is very fast on modern CPUs anyhow. Fixes #751