Clear DesktopFrame in DxgiFrame to avoid legacy image

webrtc/modules/desktop_capture/win/dxgi_frame.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 22 matching lines @@
   if (resolution_change_detector_.IsChanged(size)) {
     // Once the output size changed, recreate the SharedDesktopFrame.
     frame_.reset();
     resolution_change_detector_.Reset();
   }
 
   if (!frame_) {
     std::unique_ptr<DesktopFrame> frame;
     if (factory_) {
       frame = SharedMemoryDesktopFrame::Create(size, factory_);
+      // DirectX capturer won't paint each pixel in the frame due to its one
+      // capturer per monitor design. So once the new frame is created from
+      // shared memory, we should clear it to avoid the legacy image to be
+      // remained on it. See http://crbug.com/708766.
+      memset(frame->data(), 0, frame->stride() * frame->size().height());

[Sergey Ulanov, 2017/04/20 22:01:10]

I don't think it matters if it's a sharedmemory or not. There is nothing in
BasicDesktopFrame that would zero the buffer. The frame needs to be cleared out
in both cases.

Also this is safe only when stride == width*kBytesPerPixel. It's always the case
for SharedMemoryDesktopFrame and BasicDesktopFrame, but may change, so maybe add
a DCHECK?

[Hzj_jie, 2017/04/20 22:48:58]

Sorry, I always under the impression that we are using default initialization in
BasicDesktopFrame, but indeed not.
https://cs.chromium.org/chromium/src/third_party/webrtc/modules/desktop_captu...

I have updated the code.

     } else {
       frame.reset(new BasicDesktopFrame(size));
     }
     if (!frame) {
       return false;
     }
 
     frame_ = SharedDesktopFrame::Wrap(std::move(frame));
   }
 
   return !!frame_;
 }
 
 SharedDesktopFrame* DxgiFrame::frame() const {
   RTC_DCHECK(frame_);
   return frame_.get();
 }
 
 DxgiFrame::Context* DxgiFrame::context() {
   RTC_DCHECK(frame_);
   return &context_;
 }
 
 }  // namespace webrtc