Fold alpha to the draw in savelayer-draw-restore patterns with non-opaque draw

src/core/SkRecordOpts.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkRecordOpts.h"
 
 #include "SkRecordPattern.h"
@@ skipping 88 matching lines @@
 
         SkPaint* drawPaint = pattern->second<SkPaint>();
         if (drawPaint == NULL) {
             // We can just give the draw the SaveLayer's paint.
             // TODO(mtklein): figure out how to do this clearly
             return false;
         }
 
         const uint32_t layerColor = layerPaint->getColor();
         const uint32_t  drawColor =  drawPaint->getColor();
-        if (!IsOnlyAlpha(layerColor)  || !IsOpaque(drawColor) ||
-            HasAnyEffect(*layerPaint) || CantFoldAlpha(*drawPaint)) {
-            // Too fancy for us.  Actually, as long as layerColor is just an alpha
-            // we can blend it into drawColor's alpha; drawColor doesn't strictly have to be opaque.
+        if (!IsOnlyAlpha(layerColor) || HasAnyEffect(*layerPaint) || CantFoldAlpha(*drawPaint)) {
+            // Too fancy for us.
             return false;
         }
 
-        drawPaint->setColor(SkColorSetA(drawColor, SkColorGetA(layerColor)));
+        drawPaint->setAlpha(SkMulDiv255Round(SkColorGetA(drawColor), SkColorGetA(layerColor)));
         return KillSaveLayerAndRestore(record, begin);
     }
 
     static bool KillSaveLayerAndRestore(SkRecord* record, unsigned saveLayerIndex) {
         record->replace<NoOp>(saveLayerIndex);    // SaveLayer
         record->replace<NoOp>(saveLayerIndex+2);  // Restore
         return true;
     }
 
     static bool HasAnyEffect(const SkPaint& paint) {
@@ skipping 11 matching lines @@
     // path effect, mask filter and/or rasterizer.
     // TODO: most likely the looper and only some xfer modes are the hard
     // constraints
     static bool CantFoldAlpha(const SkPaint& paint) {
         return paint.getXfermode()    ||
                paint.getColorFilter() ||
                paint.getLooper()      ||
                paint.getImageFilter();
     }
 
-    static bool IsOpaque(SkColor color) {
-        return SkColorGetA(color) == SK_AlphaOPAQUE;
-    }
     static bool IsOnlyAlpha(SkColor color) {
         return SK_ColorTRANSPARENT == SkColorSetA(color, SK_AlphaTRANSPARENT);
     }
 };
 void SkRecordNoopSaveLayerDrawRestores(SkRecord* record) {
     SaveLayerDrawRestoreNooper pass;
     apply(&pass, record);
 }