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

src/core/SkRecordOpts.cpp

Index: src/core/SkRecordOpts.cpp
diff --git a/src/core/SkRecordOpts.cpp b/src/core/SkRecordOpts.cpp
index 4a51beea001df2d138dcc218fff401892859ae0c..17595c01c55b5cef09a6509163346f2ebb0996a2 100644
--- a/src/core/SkRecordOpts.cpp
+++ b/src/core/SkRecordOpts.cpp
@@ -111,14 +111,14 @@ struct SaveLayerDrawRestoreNooper {
 
         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->setColor(SkColorSetA(drawColor,
+                                        SkFloatToIntRound(SkColorGetA(drawColor) *

[mtklein, 2015/01/13 18:48:20]

Let's use SkMulDiv255Round?

[Kimmo Kinnunen, 2015/01/14 13:07:51]

Done.

+                                                          SkColorGetA(layerColor) / 255.0f)));
         return KillSaveLayerAndRestore(record, begin);
     }
 
@@ -150,9 +150,6 @@ struct SaveLayerDrawRestoreNooper {
                paint.getImageFilter();
     }
 
-    static bool IsOpaque(SkColor color) {
-        return SkColorGetA(color) == SK_AlphaOPAQUE;
-    }
     static bool IsOnlyAlpha(SkColor color) {
         return SK_ColorTRANSPARENT == SkColorSetA(color, SK_AlphaTRANSPARENT);
     }