tree dbc9c6b38ca6be76f0011623f3a529a60ce1d570
parent f77c47b78217883e6f074dd7e3e5bed5f82e144d
author lsalzman <lsalzman@mozilla.com> 1470422925 -0700
committer Commit bot <commit-bot@chromium.org> 1470422925 -0700

SkBlendARGB32 and S32[A]_Blend_BlitRow32 are currently formulated as: SkAlphaMulQ(src, src_scale) + SkAlphaMulQ(dst, dst_scale), which boils down to ((src*src_scale)>>8) + ((dst*dst_scale)>>8). In particular, note that the intermediate precision is discarded before the two parts are added together, causing the final result to possibly inaccurate.

In Firefox, we use SkCanvas::saveLayer in combination with a backdrop that initializes the layer to the background. When this is blended back onto background using transparency, where the source and destination pixel colors are the same, the resulting color after the blend is not preserved due to the lost precision mentioned above. In cases where this operation is repeatedly performed, this causes substantially noticeable differences in color as evidenced in this downstream Firefox bug report:  https://bugzilla.mozilla.org/show_bug.cgi?id=1200684

In the test-case in the downstream report, essentially it does blend(src=0xFF2E3338, dst=0xFF2E3338, scale=217), which gives the result 0xFF2E3237, while we would expect to get back 0xFF2E3338.

This problem goes away if the blend is instead reformulated to effectively do (src*src_scale + dst*dst_scale)>>8, which keeps the intermediate precision during the addition before shifting it off.

This modifies the blending operations thusly. The performance should remain mostly unchanged, or possibly improve slightly, so there should be no real downside to doing this, with the benefit of making the results more accurate. Without this, it is currently unsafe for Firefox to blend a layer back onto itself that was initialized with a copy of its background.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2097883002
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

[mtklein adds...]
No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2097883002