2D Canvas doesn't blend video with the destination buffer.

content/browser/renderer_host/render_widget_host_view_browsertest.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/command_line.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "content/browser/gpu/compositor_util.h"
 #include "content/browser/gpu/gpu_data_manager_impl.h"
@@ skipping 473 matching lines @@
     if (!result) {
       quit_callback.Run();
       return;
     }
 
     media::SkCanvasVideoRenderer video_renderer;
 
     SkBitmap bitmap;
     bitmap.allocN32Pixels(video_frame->visible_rect().width(),
                           video_frame->visible_rect().height());
     bitmap.eraseColor(SK_ColorTRANSPARENT);

[danakj, 2014/08/06 18:31:48]

btw can we remove this then? 

[dshwang, 2014/08/06 19:22:01]

we can remove this if we use kSrc_Mode, instead of kSrcOver_Mode.
But IMO current code is better, because it's easier to understand.

The logic flow is
1. create and clear SkBitmap
2. create SkCanvas based on SkBitmap
3. draw video_frame on SkCanvas <- 2d graphics folks usually expects video_frame
would be composited by SrcOver.

[danakj, 2014/08/06 19:23:41]

It's a waste of time to touch all the pixels twice tho, just put a comment like
you did in cc?

Maybe a test doesn't matter but I think test speed should matter a bit. Up to
you/aelias though.

[dshwang, 2014/08/07 12:47:53]

I apply your optimization to new patch set. Thank you for discussing.

     SkCanvas canvas(bitmap);
 
     video_renderer.Paint(video_frame.get(),
                          &canvas,
                          video_frame->visible_rect(),
-                         0xff);
+                         0xff,
+                         SkXfermode::kSrcOver_Mode);
 
     CopyFromCompositingSurfaceCallback(quit_callback,
                                        result,
                                        bitmap);
   }
 
   void SetExpectedCopyFromCompositingSurfaceResult(bool result,
                                                    const SkBitmap& bitmap) {
     expected_copy_from_compositing_surface_result_ = result;
     expected_copy_from_compositing_surface_bitmap_ = bitmap;
@@ skipping 343 matching lines @@
                         CompositingRenderWidgetHostViewBrowserTestTabCapture,
                         testing::ValuesIn(kAllCompositingModes));
 INSTANTIATE_TEST_CASE_P(GLAndSoftwareCompositing,
                         CompositingRenderWidgetHostViewTabCaptureHighDPI,
                         testing::ValuesIn(kAllCompositingModes));
 
 #endif  // !defined(OS_ANDROID) && !defined(OS_IOS)
 
 }  // namespace
 }  // namespace content