Bug 884226 - Add 'willReadFrequently' getContext() option for 2D canvas r=ehsan,gwright

content/canvas/src/CanvasRenderingContext2D.cpp

@@ -1055,6 +1055,26 @@ CanvasRenderingContext2D::Render(gfxContext *ctx, GraphicsFilter aFilter, uint32
   return rv;
 }
 
+NS_IMETHODIMP
+CanvasRenderingContext2D::SetContextOptions(JSContext* aCx, JS::Handle<JS::Value> aOptions)
+{
+  if (aOptions.isNullOrUndefined()) {
+    return NS_OK;
+  }
+
+  ContextAttributes2D attributes;
+  NS_ENSURE_TRUE(attributes.Init(aCx, aOptions), NS_ERROR_UNEXPECTED);
+
+#ifdef USE_SKIA_GPU
+  if (Preferences::GetBool("gfx.canvas.willReadFrequently.enable", false)) {
+    // Use software when there is going to be a lot of readback
+    mForceSoftware = attributes.mWillReadFrequently;
+  }
+#endif
+
+  return NS_OK;
+}
+
 void
 CanvasRenderingContext2D::GetImageBuffer(uint8_t** aImageBuffer,
                                          int32_t* aFormat)

content/canvas/src/CanvasRenderingContext2D.h

@@ -413,6 +413,7 @@ public:
   // this rect is in canvas device space
   void Redraw(const mozilla::gfx::Rect &r);
   NS_IMETHOD Redraw(const gfxRect &r) MOZ_OVERRIDE { Redraw(ToRect(r)); return NS_OK; }
+  NS_IMETHOD SetContextOptions(JSContext* aCx, JS::Handle<JS::Value> aOptions) MOZ_OVERRIDE;
 
   // this rect is in mTarget's current user space
   void RedrawUser(const gfxRect &r);

dom/webidl/CanvasRenderingContext2D.webidl

@@ -15,6 +15,11 @@ interface HitRegionOptions;
 
 enum CanvasWindingRule { "nonzero", "evenodd" };
 
+dictionary ContextAttributes2D {
+  // whether or not we're planning to do a lot of readback operations
+  boolean willReadFrequently = false;
+};
+
 interface CanvasRenderingContext2D {
 
   // back-reference to the canvas.  Might be null if we're not