Skip paint chunks with effectively invisible opacity.

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
index 83cf5e45e90c8d5f3da455493a94a93311c6a8d0..121f0009d0ec6eb8cde0fa83bcb1e970ed92fe44 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -499,12 +499,30 @@ void PaintArtifactCompositor::LayerizeGroup(
   // due to copying the chunk list. Subtotal: O((qd + p)d) = O(qd^2 + pd)
   // Assuming p > d, the total complexity would be O(pqd + qd^2 + pd) = O(pqd)
   while (chunk_it != paint_artifact.PaintChunks().end()) {
-    // Look at the effect node of the next chunk. There are 3 possible cases:
+    // Look at the effect node of the next chunk. If the chunk has
+    // effectively-invisible opacity we skip it. Otherwise, there are 3 possible
+    // cases:
     // A. The next chunk belongs to the current group but no subgroup.
     // B. The next chunk does not belong to the current group.
     // C. The next chunk belongs to some subgroup of the current group.
     const EffectPaintPropertyNode* chunk_effect =
         chunk_it->properties.property_tree_state.Effect();
+
+    // TODO(wkorman): Do we need to check blend mode, mask, or filter to skip

[wkorman, 2017/04/24 19:49:38]

Seeking your thought on this comment as I haven't thought deeply about it and
plan to review once I see CQ dry run results. If we pass all tests we're
probably ok as is.

+    // only when appropriate? See also PaintLayer::IsTransparent() which was
+    // previously incorporated via PaintLayer::PaintsWithTransparency().
+
+    // Skip paint chunks that are effectively invisible due to opacity. The
+    // lower bound of visibility is considered to be 0.0004f < 1/2048. With
+    // 10-bit color channels (only available on the newest Macs as of 2016;
+    // otherwise it's 8-bit), we see that an alpha of 1/2048 or less leads to a
+    // color output of less than 0.5 in all channels, hence not visible.
+    static const float kMinimumVisibleOpacity = 0.0004f;
+    if (chunk_effect->Opacity() < kMinimumVisibleOpacity) {
+      chunk_it++;
+      continue;
+    }
+
     if (chunk_effect == &current_group) {
       // Case A: The next chunk belongs to the current group but no subgroup.
       bool is_foreign =