Implement all resolution change policies for desktop and tab capture.

content/browser/media/capture/capture_resolution_evaluator.h

+// Copyright 2015 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.
+
+#ifndef CONTENT_BROWSER_MEDIA_CAPTURE_CAPTURE_RESOLUTION_EVALUATOR_H_
+#define CONTENT_BROWSER_MEDIA_CAPTURE_CAPTURE_RESOLUTION_EVALUATOR_H_
+
+#include "content/common/content_export.h"
+#include "media/base/video_capture_types.h"
+#include "ui/gfx/geometry/size.h"
+
+namespace content {
+
+// Encapsulates the logic that determines the capture frame resolution based on:
+//   1. The configured maximum frame resolution and resolution change policy.
+//   2. The resolution of the source content.
+//   3. The current capabilities of the end-to-end system, in terms of data
+//      volume.
+class CONTENT_EXPORT CaptureResolutionEvaluator {

[Wez, 2015/05/13 19:05:08]

nit: Evaluator doesn't seem the right name for this; CaptureResolutionChooser?
Or ..Selector or ..Calculator?

[miu, 2015/05/14 01:21:32]

Done.  I like "chooser," since an upcoming change will introduce logic to "snap
to" one of a specific set of resolutions.

+ public:
+  CaptureResolutionEvaluator(
+      const gfx::Size& max_frame_size,
+      media::ResolutionChangePolicy resolution_change_policy);

[Wez, 2015/05/13 19:05:09]

Suggest briefly explaining what resolution_change_policy can be.

[miu, 2015/05/14 01:21:32]

Done.

+  ~CaptureResolutionEvaluator();
+
+  // Returns the current capture frame resolution to use.
+  gfx::Size capture_size() const {
+    return capture_size_;
+  }
+
+  // Updates the capture size based on a change in the resolution of the source
+  // content.
+  void UpdateForNewSourceSize(const gfx::Size& source_size);

[Wez, 2015/05/13 19:05:08]

nit: I'd suggest naming these SetSourceSize() and SetMaxPixelsPerFrame()

[miu, 2015/05/14 01:21:32]

Done.

+
+  // Updates the capture size to one that satisfies the current capabilities of
+  // the system, in terms of |num_pixels| per frame.

[Wez, 2015/05/13 19:05:09]

I think it'd be clearer to split what the function does from _why_ it does it,
e.g:

"Updates the capture size to not exceed |num_pixels| per frame. This is useful
in limiting capture resource consumption (CPU, memory bandwidth), which is
typically proportional to pixels-per-frame."

[miu, 2015/05/14 01:21:32]

Done.  I liked your wording, so I plagiarized it.  ;-)

+  void UpdateForNewCapabilityLimit(int num_pixels);
+
+ private:
+  // Called after any update that requires |capture_size_| be re-computed.
+  void UpdateCaptureSize();

[Wez, 2015/05/13 19:05:09]

If you update the others to Set... then suggest this become
RefreshCaptureSize(), or RecalculateCaptureSize().

[miu, 2015/05/14 01:21:32]

Done.

+
+  // Hard constraints.

[Wez, 2015/05/13 19:05:08]

Where does |min_frame_size_| come from - it doesn't seem to be a parameter?

[miu, 2015/05/14 01:21:32]

Done.

+  const gfx::Size max_frame_size_;
+  const gfx::Size min_frame_size_;
+
+  // Selects the set of heuristics to use.

[Wez, 2015/05/13 19:05:08]

nit: Selects -> Specifies

[miu, 2015/05/14 01:21:32]

Done.

+  const media::ResolutionChangePolicy resolution_change_policy_;
+
+  // The capture frame resolution to use, ignoring the limitations imposed by
+  // the capability metric.

[Wez, 2015/05/13 19:05:09]

This isn't very clear; naively I'd assume this must be the same as the source
size but I think it takes into account the resolution policy and max/min
constraints, but not the pixel per frame limit?

In which case consider calling this constrained_size_?

[miu, 2015/05/14 01:21:32]

Done.  Agreed, I like your naming better.

+  gfx::Size ideal_capture_size_;
+
+  // The last computed capture frame resolution.
+  gfx::Size capture_size_;

[Wez, 2015/05/13 19:05:08]

nit: "The current computed capture frame resolution."

[miu, 2015/05/14 01:21:32]

Done.

+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_MEDIA_CAPTURE_RESOLUTION_EVALUATOR_H_