diff --git a/content/canvas/public/nsICanvasRenderingContextInternal.h b/content/canvas/public/nsICanvasRenderingContextInternal.h
index 3f496534b65..a1a1fc28e72 100644
--- a/content/canvas/public/nsICanvasRenderingContextInternal.h
+++ b/content/canvas/public/nsICanvasRenderingContextInternal.h
@@ -97,6 +97,7 @@ public:
   // dst alpha is always 1.0.  If this is never called, the context
   // defaults to false (not opaque).
   NS_IMETHOD SetIsOpaque(bool isOpaque) = 0;
+  virtual bool GetIsOpaque() = 0;
 
   // Invalidate this context and release any held resources, in preperation
   // for possibly reinitializing with SetDimensions/InitializeWithSurface.
diff --git a/content/canvas/src/CanvasRenderingContext2D.cpp b/content/canvas/src/CanvasRenderingContext2D.cpp
index ee62d8db550..2631b6b01f2 100755
--- a/content/canvas/src/CanvasRenderingContext2D.cpp
+++ b/content/canvas/src/CanvasRenderingContext2D.cpp
@@ -988,6 +988,10 @@ CanvasRenderingContext2D::SetIsOpaque(bool isOpaque)
     ClearTarget();
   }
 
+  if (mOpaque) {
+    EnsureTarget();
+  }
+
   return NS_OK;
 }
 
@@ -1062,6 +1066,10 @@ CanvasRenderingContext2D::SetContextOptions(JSContext* aCx, JS::Handle<JS::Value
     mForceSoftware = attributes.mWillReadFrequently;
   }
 
+  if (!attributes.mAlpha) {
+    SetIsOpaque(true);
+  }
+
   return NS_OK;
 }
 
@@ -3807,6 +3815,30 @@ CanvasRenderingContext2D::GetImageDataArray(JSContext* aCx,
   // inherited from Thebes canvas and is no longer true
   uint8_t* dst = data + dstWriteRect.y * (aWidth * 4) + dstWriteRect.x * 4;
 
+  if (mOpaque) {
+    for (int32_t j = 0; j < dstWriteRect.height; ++j) {
+      for (int32_t i = 0; i < dstWriteRect.width; ++i) {
+        // XXX Is there some useful swizzle MMX we can use here?
+#if MOZ_LITTLE_ENDIAN
+        uint8_t b = *src++;
+        uint8_t g = *src++;
+        uint8_t r = *src++;
+        src++;
+#else
+        src++;
+        uint8_t r = *src++;
+        uint8_t g = *src++;
+        uint8_t b = *src++;
+#endif
+        *dst++ = r;
+        *dst++ = g;
+        *dst++ = b;
+        *dst++ = 255;
+      }
+      src += srcStride - (dstWriteRect.width * 4);
+      dst += (aWidth * 4) - (dstWriteRect.width * 4);
+    }
+  } else
   for (int32_t j = 0; j < dstWriteRect.height; ++j) {
     for (int32_t i = 0; i < dstWriteRect.width; ++i) {
       // XXX Is there some useful swizzle MMX we can use here?
diff --git a/content/canvas/src/CanvasRenderingContext2D.h b/content/canvas/src/CanvasRenderingContext2D.h
index c3fb0aa16f9..fcec5b21a61 100644
--- a/content/canvas/src/CanvasRenderingContext2D.h
+++ b/content/canvas/src/CanvasRenderingContext2D.h
@@ -408,6 +408,7 @@ public:
   { EnsureTarget(); return mTarget->Snapshot(); }
 
   NS_IMETHOD SetIsOpaque(bool isOpaque) MOZ_OVERRIDE;
+  bool GetIsOpaque() MOZ_OVERRIDE { return mOpaque; }
   NS_IMETHOD Reset() MOZ_OVERRIDE;
   already_AddRefed<CanvasLayer> GetCanvasLayer(nsDisplayListBuilder* aBuilder,
                                                CanvasLayer *aOldLayer,
diff --git a/content/canvas/src/WebGLContext.h b/content/canvas/src/WebGLContext.h
index 8933bfe01d0..2c56966b44d 100644
--- a/content/canvas/src/WebGLContext.h
+++ b/content/canvas/src/WebGLContext.h
@@ -178,6 +178,7 @@ public:
     mozilla::TemporaryRef<mozilla::gfx::SourceSurface> GetSurfaceSnapshot() MOZ_OVERRIDE;
 
     NS_IMETHOD SetIsOpaque(bool b) MOZ_OVERRIDE { return NS_OK; };
+    bool GetIsOpaque() MOZ_OVERRIDE { return false; }
     NS_IMETHOD SetContextOptions(JSContext* aCx,
                                  JS::Handle<JS::Value> aOptions) MOZ_OVERRIDE;
 
diff --git a/content/canvas/test/test_canvas.html b/content/canvas/test/test_canvas.html
index c73a4ac2003..7142bfaf796 100644
--- a/content/canvas/test/test_canvas.html
+++ b/content/canvas/test/test_canvas.html
@@ -21577,6 +21577,24 @@ function test_linedash() {
 }
 </script>
 
+<p>Canvas test: test_opaque</p>
+<canvas id="c688" width="150" height="50"></canvas>
+<script type="text/javascript">
+
+function test_opaque() {
+  var c = document.getElementById("c688");
+  var ctx = c.getContext("2d", {alpha: false});
+  ctx.fillStyle = "green";
+  ctx.fillRect(0,0,10,10);
+  ctx.fillStyle = "rgba(255,0,0,.5)";
+  ctx.fillRect(10,0,10,10);
+
+  isPixel(ctx, 20, 20, 0, 0, 0, 255, 0);
+  isPixel(ctx, 5, 5, 0, 128, 0, 255, 0);
+  isPixel(ctx, 15, 5, 128, 0, 0, 255, 0);
+}
+</script>
+
 <script>
 
 function asyncTestsDone() {
@@ -24873,6 +24891,12 @@ function runTests() {
   throw e;
   ok(false, "unexpected exception thrown in: test_linedash");
  }
+try {
+  test_opaque();
+ } catch(e) {
+  throw e;
+  ok(false, "unexpected exception thrown in: test_opaque");
+ }
  try {
   // run this test last since it replaces the getContext method
   test_type_replace();
diff --git a/content/html/content/src/HTMLCanvasElement.cpp b/content/html/content/src/HTMLCanvasElement.cpp
index d330c884321..bcb1a314172 100644
--- a/content/html/content/src/HTMLCanvasElement.cpp
+++ b/content/html/content/src/HTMLCanvasElement.cpp
@@ -781,7 +781,7 @@ HTMLCanvasElement::UpdateContext(JSContext* aCx, JS::Handle<JS::Value> aNewConte
 
   nsIntSize sz = GetWidthHeight();
 
-  nsresult rv = mCurrentContext->SetIsOpaque(GetIsOpaque());
+  nsresult rv = mCurrentContext->SetIsOpaque(HasAttr(kNameSpaceID_None, nsGkAtoms::moz_opaque));
   if (NS_FAILED(rv)) {
     mCurrentContext = nullptr;
     mCurrentContextId.Truncate();
@@ -903,6 +903,10 @@ HTMLCanvasElement::GetContextAtIndex(int32_t index)
 bool
 HTMLCanvasElement::GetIsOpaque()
 {
+  if (mCurrentContext) {
+    return mCurrentContext->GetIsOpaque();
+  }
+
   return HasAttr(kNameSpaceID_None, nsGkAtoms::moz_opaque);
 }
 
diff --git a/dom/webidl/CanvasRenderingContext2D.webidl b/dom/webidl/CanvasRenderingContext2D.webidl
index 18598cee182..6b1b375c562 100644
--- a/dom/webidl/CanvasRenderingContext2D.webidl
+++ b/dom/webidl/CanvasRenderingContext2D.webidl
@@ -16,6 +16,8 @@ enum CanvasWindingRule { "nonzero", "evenodd" };
 dictionary ContextAttributes2D {
   // whether or not we're planning to do a lot of readback operations
   boolean willReadFrequently = false;
+  // signal if the canvas contains an alpha channel
+  boolean alpha = true;
 };
 
 dictionary HitRegionOptions {