Fullscreen tab capture: Prefer standard resolutions

content/browser/media/capture/web_contents_video_capture_device.cc

 // Copyright (c) 2012 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.
 //
 // Implementation notes: This needs to work on a variety of hardware
 // configurations where the speed of the CPU and GPU greatly affect overall
 // performance. Spanning several threads, the process of capturing has been
 // split up into four conceptual stages:
 //
 //   1. Reserve Buffer: Before a frame can be captured, a slot in the client's
@@ skipping 624 matching lines @@
 }
 
 gfx::Size WebContentsCaptureMachine::ComputeOptimalViewSize() const {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 
   // TODO(miu): Propagate capture frame size changes as new "preferred size"
   // updates, rather than just using the max frame size.
   // http://crbug.com/350491
   gfx::Size optimal_size = oracle_proxy_->max_frame_size();
 
+  // If the max frame size is close to a common video aspect ratio, compute a
+  // standard resolution for that aspect ratio.  For example, given 1365x768,
+  // which is very close to 16:9, the optimal size would be 1280x720.  The
+  // purpose of this logic is to prevent scaling quality issues caused by "one
+  // pixel stretching" and/or odd-to-even dimension scaling, and to improve the
+  // performance of consumers of the captured video.
+  const auto HasIntendedAspectRatio = [](const gfx::Size& size, int x, int y) {

[Kevin M, 2015/07/30 00:23:06]

Can you factor this out into a standalone fn that takes gfx::Size and write some
unit tests for it?

[miu, 2015/07/30 20:22:53]

Done.  Didn't factor anything out, but did write unit tests.

+    const int a = y * size.width();
+    const int b = x * size.height();
+    const int percentage_diff = 100 * std::abs((a - b)) / b;
+    return percentage_diff <= 1;  // Effectively, anything strictly <2%.
+  };
+  const auto RoundToExactAspectRatio = [](const gfx::Size& size, int x, int y) {
+    const int adjusted_height =

[Kevin M, 2015/07/30 00:23:06]

Can you add some comments or pick a more descriptive name for x & y to
communicate the requirements for even multiples?

[miu, 2015/07/30 20:22:53]

Done.

+        std::max(size.height() - (size.height() % y), y);
+    DCHECK_EQ((adjusted_height * x) % y, 0);
+    return gfx::Size(adjusted_height * x / y, adjusted_height);
+  };
+  if (HasIntendedAspectRatio(optimal_size, 16, 9))
+    optimal_size = RoundToExactAspectRatio(optimal_size, 160, 90);
+  else if (HasIntendedAspectRatio(optimal_size, 4, 3))
+    optimal_size = RoundToExactAspectRatio(optimal_size, 64, 48);
+  // Else, do not make an adjustment.
+
   // If the ratio between physical and logical pixels is greater than 1:1,
   // shrink |optimal_size| by that amount.  Then, when external code resizes the
   // render widget to the "preferred size," the widget will be physically
   // rendered at the exact capture size, thereby eliminating unnecessary scaling
   // operations in the graphics pipeline.
   RenderWidgetHost* const rwh = tracker_->GetTargetRenderWidgetHost();
   RenderWidgetHostView* const rwhv = rwh ? rwh->GetView() : NULL;
   if (rwhv) {
     const gfx::NativeView view = rwhv->GetNativeView();
     const float scale = ui::GetScaleFactorForNativeView(view);
@@ skipping 144 matching lines @@
     scoped_ptr<Client> client) {
   DVLOG(1) << "Allocating " << params.requested_format.frame_size.ToString();
   core_->AllocateAndStart(params, client.Pass());
 }
 
 void WebContentsVideoCaptureDevice::StopAndDeAllocate() {
   core_->StopAndDeAllocate();
 }
 
 }  // namespace content