Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/skia/OpaqueRegionSkia.cpp

 /*
  * Copyright (c) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 23 matching lines @@
 
 #include "platform/graphics/GraphicsContext.h"
 
 #include "SkColorFilter.h"
 #include "SkShader.h"
 
 namespace blink {
 
 OpaqueRegionSkia::OpaqueRegionSkia()
     : m_opaqueRect(SkRect::MakeEmpty())
+    , m_treatOverwriteAsOpaque(false)
 {
 }
 
 void OpaqueRegionSkia::reset()
 {
     ASSERT(m_canvasLayerStack.isEmpty());
     m_opaqueRect = SkRect::MakeEmpty();
 }
 
 IntRect OpaqueRegionSkia::asRect() const
@@ skipping 35 matching lines @@
     case SkXfermode::kSrcIn_Mode: // source * dest
     case SkXfermode::kDstIn_Mode: // dest * source
     case SkXfermode::kSrcOut_Mode: // source * (1-dest)
     case SkXfermode::kDstOut_Mode: // dest * (1-source)
     case SkXfermode::kSrcATop_Mode: // dest
     case SkXfermode::kXor_Mode: // source + dest - 2*(source*dest)
         return false;
     }
 }
 
+static inline bool xfermodeIsOverwrite(const SkPaint& paint)
+{
+    SkXfermode* xfermode = paint.getXfermode();
+    if (!xfermode)
+        return false; // default to kSrcOver_Mode
+    SkXfermode::Mode mode;
+    if (!xfermode->asMode(&mode))
+        return false;
+    switch (mode) {
+    case SkXfermode::kSrc_Mode:
+    case SkXfermode::kClear_Mode:
+        return true;
+    default:
+        return false;
+    }
+}
+
 // Returns true if the xfermode will keep the dst opaque, assuming the dst is already opaque.
 static inline bool xfermodePreservesOpaque(const SkPaint& paint, bool srcIsOpaque)
 {
     SkXfermode* xfermode = paint.getXfermode();
     if (!xfermode)
         return true; // default to kSrcOver_Mode
     SkXfermode::Mode mode;
     if (!xfermode->asMode(&mode))
         return false;
 
@@ skipping 186 matching lines @@
     if (!canvasTransform.mapRect(&targetRect))
         fillsBounds = false;
 
     // Apply the current clip.
     SkRect deviceClipRect;
     if (!getDeviceClipAsRect(context, deviceClipRect))
         fillsBounds = false;
     else if (!targetRect.intersect(deviceClipRect))
         return;
 
+    if (m_treatOverwriteAsOpaque && fillsBounds && xfermodeIsOverwrite(paint)) {
+        markRectAsOpaque(targetRect);
+        return;
+    }
+
     bool drawsOpaque = paintIsOpaque(paint, drawType, sourceBitmap);
     bool xfersOpaque = xfermodeIsOpaque(paint, drawsOpaque);
-    bool preservesOpaque = xfermodePreservesOpaque(paint, drawsOpaque);
 
-    if (fillsBounds && xfersOpaque)
+    if (fillsBounds && xfersOpaque) {
         markRectAsOpaque(targetRect);
-    else if (!preservesOpaque)
-        markRectAsNonOpaque(targetRect);
+    } else if (!m_treatOverwriteAsOpaque) {
+        bool preservesOpaque = xfermodePreservesOpaque(paint, drawsOpaque);
+        if (!preservesOpaque)

[Stephen White, 2014/07/25 19:29:06]

Nit: unless I'm missing something, this could all be inline:

if (!m_treatOverwriteAsOpaque && !xfermodePreservesOpaque(paint, drawsOpaque)) {
    markRectAsNonOpaque(targetRect);
}

+            markRectAsNonOpaque(targetRect);
+    }
 }
 
 void OpaqueRegionSkia::didDrawUnbounded(const GraphicsContext* context, const SkPaint& paint, DrawType drawType)
 {
     ASSERT(!context->paintingDisabled());
     bool drawsOpaque = paintIsOpaque(paint, drawType, 0);
     bool preservesOpaque = xfermodePreservesOpaque(paint, drawsOpaque);
 
     if (preservesOpaque)
         return;
@@ skipping 116 matching lines @@
     opaqueRect.setEmpty();
 }
 
 SkRect& OpaqueRegionSkia::currentTrackingOpaqueRect()
 {
     // If we are drawing into a canvas layer, then track the opaque rect in that layer.
     return m_canvasLayerStack.isEmpty() ? m_opaqueRect : m_canvasLayerStack.last().opaqueRect;
 }
 
 } // namespace blink