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
and handled that itself internally, rather than calling SkXfermode. This CL removes
the kModulate_Mode alias from its list.
git-svn-id: http://skia.googlecode.com/svn/trunk@7509 2bbb7eff-a529-9590-31e7-b0007b416f81
element. This filter has two inputs, since normal blending can't be used. The
GPU side uses two filter stages to accomplish this: one to sample the
background, and one to sample the foreground and blend it.
Review URL: https://codereview.appspot.com/6463081/
git-svn-id: http://skia.googlecode.com/svn/trunk@5231 2bbb7eff-a529-9590-31e7-b0007b416f81
