Go back to using dual source blending for lcd src-over even with non-opaque color

This is change is currently still safe since earlier in Skia we are still requiring
the dst to be opaque. The change is a workaround to spots where trying to read the
dst to do in shader blending is failing for some reason. This also should give back
a little performance since doing dual source blending should be better than shader
blends.

Bug: chromium:732341
Change-Id: I795f8a520f87f3fbf5d63a9509fbd9f394ea2b29
Reviewed-on: https://skia-review.googlesource.com/19703
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>

2 files changed, 10 insertions, 7 deletions

diff --git a/src/gpu/effects/GrPorterDuffXferProcessor.cpp b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
index 254b44b055..1d34b1ffe4 100644
--- a/src/gpu/effects/GrPorterDuffXferProcessor.cpp
+++ b/src/gpu/effects/GrPorterDuffXferProcessor.cpp
@@ -817,8 +817,8 @@ static inline GrXPFactory::AnalysisProperties analysis_properties(
             // and DstATop would also work). We also fall into the dst read case for src-over if we
             // do not have dual source blending.
             if (SkBlendMode::kSrcOver != mode ||
-                !color.isOpaque() ||
-                !caps.shaderCaps()->dualSourceBlendingSupport()) {
+                /*!color.isOpaque() ||*/ // See comment in MakeSrcOverXferProcessor about isOpaque.
+                (formula.hasSecondaryOutput() && !caps.shaderCaps()->dualSourceBlendingSupport())) {
                 props |= AnalysisProperties::kReadsDstInShader;
             }
         }
@@ -903,8 +903,10 @@ sk_sp<const GrXferProcessor> GrPorterDuffXPFactory::MakeSrcOverXferProcessor(
 
     // Currently up the stack Skia is requiring that the dst is opaque or that the client has said
     // the opaqueness doesn't matter. Thus for src-over we don't need to worry about the src color
-    // being opaque or not. For now we disable the check for opaqueness, but in the future we should
-    // pass down the knowledge about dst opaqueness and make the correct decision here.
+    // being opaque or not. This allows us to use faster code paths as well as avoid various bugs
+    // that occur with dst reads in the shader blending. For now we disable the check for
+    // opaqueness, but in the future we should pass down the knowledge about dst opaqueness and make
+    // the correct decision here.
     //
     // This also fixes a chrome bug on macs where we are getting random fuzziness when doing
     // blending in the shader for non opaque sources.
@@ -919,7 +921,8 @@ sk_sp<const GrXferProcessor> GrPorterDuffXPFactory::MakeSrcOverXferProcessor(
 
     BlendFormula blendFormula;
     blendFormula = get_lcd_blend_formula(SkBlendMode::kSrcOver);
-    if (!color.isOpaque() ||
+    // See comment above regarding why the opaque check is commented out here.
+    if (/*!color.isOpaque() ||*/
         (blendFormula.hasSecondaryOutput() && !caps.shaderCaps()->dualSourceBlendingSupport())) {
         return sk_sp<GrXferProcessor>(
                 new ShaderPDXferProcessor(hasMixedSamples, SkBlendMode::kSrcOver, coverage));
diff --git a/tests/GrPorterDuffTest.cpp b/tests/GrPorterDuffTest.cpp
index 78497d2b31..8a6e58e5dd 100644
--- a/tests/GrPorterDuffTest.cpp
+++ b/tests/GrPorterDuffTest.cpp
@@ -1032,8 +1032,8 @@ static void test_lcd_coverage_fallback_case(skiatest::Reporter* reporter, const
     // Test with non-opaque alpha
     color = GrColorPackRGBA(123, 45, 67, 221);
     coverage = GrProcessorAnalysisCoverage::kLCD;
-    TEST_ASSERT(GrXPFactory::GetAnalysisProperties(xpf, color, coverage, caps) &
-                GrXPFactory::AnalysisProperties::kRequiresDstTexture);
+    TEST_ASSERT(!(GrXPFactory::GetAnalysisProperties(xpf, color, coverage, caps) &
+                GrXPFactory::AnalysisProperties::kRequiresDstTexture));
     sk_sp<const GrXferProcessor> xp(
             GrXPFactory::MakeXferProcessor(xpf, color, coverage, false, caps));
     if (!xp) {