40254c2c2d
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
37 lines
1.1 KiB
C++
37 lines
1.1 KiB
C++
/*
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* Copyright 2015 Google Inc.
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*
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* Use of this source code is governed by a BSD-style license that can be
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* found in the LICENSE file.
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*/
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#include "gm.h"
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DEF_SIMPLE_GM(blend, canvas, 300, 100) {
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SkPaint p;
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// All three of these blocks should be the same color.
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canvas->save();
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canvas->scale(100,100);
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p.setColor(SK_ColorRED);
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canvas->drawRect(SkRect::MakeXYWH(0,0,1,1), p);
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p.setColor(0xFC208000);
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canvas->drawRect(SkRect::MakeXYWH(0,0,1,1), p);
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p.setColor(SK_ColorRED);
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canvas->drawRect(SkRect::MakeXYWH(1,0,1,1), p);
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canvas->saveLayer(nullptr, nullptr);
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p.setColor(0xFC208000);
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canvas->drawRect(SkRect::MakeXYWH(1,0,1,1), p);
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canvas->restore();
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p.setColor(SK_ColorRED);
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canvas->drawRect(SkRect::MakeXYWH(2,0,1,1), p);
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canvas->saveLayerAlpha(nullptr, 0xFC);
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p.setColor(sk_tool_utils::color_to_565(0xFF208000));
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canvas->drawRect(SkRect::MakeXYWH(2,0,1,1), p);
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canvas->restore();
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canvas->restore();
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}
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