8ff6ca6e73
libjpeg-turbo turns on dithering by default. When we decode to RGBA, it uses Floyd-Steinberg dithering - consistent with the libjpeg6b gold standard, When we decode to 565, it uses ordered dithering. Ordered dithering for 565 is not part of the 6b standard (libjpeg6b doesn't even support 565) and is causing us a number of issues: (1) Ordered dithering + nearest neighbor sampling is causing checkerboard visual artifacts in some outputs. (2) The ordered dither function in libjpeg-turbo actually behaves differently depending on the alignment of the memory that it decodes into. This means that two image decodes that should be identical may look different just because they decode into different memory blocks. This was causing some diffs on Gold with the scanline_subset test that were causing me some confusion. (3) Maybe not the best evidence, but visually I can't tell a difference with and without dithering (except when nearest neighbor scaling causes the checkerboard artifact). (4) Turning off dithering should be a more significant performance improvement than you might expect. libjpeg-turbo has SIMD color conversions to 565, but when dithering is on, it defaults to scalar code. This CL should make every jpeg decode to 565 on Gold look slightly different. Yay! BUG=skia: Review URL: https://codereview.chromium.org/1349563007 |
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Skia is a complete 2D graphic library for drawing Text, Geometries, and Images. See full details, and build instructions, at https://skia.org.