improvements of the FPS counter in the HUDLayer

cc/frame_rate_counter.h

 // Copyright 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.
 
 #ifndef CCFrameRateCounter_h
 #define CCFrameRateCounter_h
 
 #include "base/basictypes.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/time.h"
 
 namespace cc {
 
 // This class maintains a history of timestamps, and provides functionality to
 // intelligently compute average frames per second (and standard deviation).
 class FrameRateCounter {
 public:
     static scoped_ptr<FrameRateCounter> create();
 
     void markBeginningOfFrame(base::TimeTicks timestamp);
     void markEndOfFrame();
     int currentFrameNumber() const { return m_currentFrameNumber; }
     void getAverageFPSAndStandardDeviation(double& averageFPS, double& standardDeviation) const;
     int timeStampHistorySize() const { return kTimeStampHistorySize; }
 
     // n = 0 returns the oldest frame retained in the history,
     // while n = timeStampHistorySize() - 1 returns the timestamp most recent frame.
-    base::TimeTicks timeStampOfRecentFrame(int n);
+    base::TimeTicks timeStampOfRecentFrame(int n) const;
 
     // This is a heuristic that can be used to ignore frames in a reasonable way. Returns
     // true if the given frame interval is too fast or too slow, based on constant thresholds.
     bool isBadFrameInterval(base::TimeDelta intervalBetweenConsecutiveFrames) const;
 
     int droppedFrameCount() const { return m_droppedFrameCount; }
 
 private:
     FrameRateCounter();
 
     base::TimeDelta frameInterval(int frameNumber) const;
     int frameIndex(int frameNumber) const;
     bool isBadFrame(int frameNumber) const;
 
     // Two thresholds (measured in seconds) that describe what is considered to be a "no-op frame" that should not be counted.
     // - if the frame is too fast, then given our compositor implementation, the frame probably was a no-op and did not draw.
     // - if the frame is too slow, then there is probably not animating content, so we should not pollute the average.
     static const double kFrameTooFast;
     static const double kFrameTooSlow;
 
     // If a frame takes longer than this threshold (measured in seconds) then we
     // (naively) assume that it missed a screen refresh; that is, we dropped a frame.
     // FIXME: Determine this threshold based on monitor refresh rate, crbug.com/138642.
     static const double kDroppedFrameTime;
 
-    static const int kTimeStampHistorySize = 120;
+    static const int kTimeStampHistorySize = 170;

[egraether, 2012/10/26 00:06:52]

I increased the number of samples in the graph so the text fits above the graph.

 
     int m_currentFrameNumber;
     base::TimeTicks m_timeStampHistory[kTimeStampHistorySize];
 
     int m_droppedFrameCount;
 
     DISALLOW_COPY_AND_ASSIGN(FrameRateCounter);
 };
 
 }  // namespace cc
 
 #endif