improving UI of FPS counter

cc/frame_rate_counter.cc

 // 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.
 
 #include "cc/frame_rate_counter.h"
 
 #include <cmath>
 
 #include "base/metrics/histogram.h"
 #include "cc/proxy.h"
 
 namespace cc {
 
 const double FrameRateCounter::kFrameTooFast = 1.0 / 70.0; // measured in seconds
-const double FrameRateCounter::kFrameTooSlow = 1.0 / 12.0;
+const double FrameRateCounter::kFrameTooSlow = 1.0;

[egraether, 2012/11/12 23:44:06]

Reduced the kFrameTooSlow value so that only time deltas of above one second get
sorted out. This change happened due to feedback complaining that really slow
framerates, e.g. during lags, are not showing up.

[nduca, 2012/11/13 00:51:46]

How about .25 second? I'm thinking about pauses as you scroll down...

 const double FrameRateCounter::kDroppedFrameTime = 1.0 / 50.0;
 
 // safeMod works on -1, returning m-1 in that case.
 static inline int safeMod(int number, int modulus)
 {
     return (number + modulus) % modulus;
 }
 
 // static
 scoped_ptr<FrameRateCounter> FrameRateCounter::create(bool hasImplThread) {
@@ skipping 48 matching lines @@
     bool intervalTooFast = schedulerAllowsDoubleFrames ? delta < kFrameTooFast : delta <= 0.0;
     bool intervalTooSlow = delta > kFrameTooSlow;
     return intervalTooFast || intervalTooSlow;
 }
 
 bool FrameRateCounter::isBadFrame(int frameNumber) const
 {
     return isBadFrameInterval(frameInterval(frameNumber));
 }
 
-void FrameRateCounter::getAverageFPSAndStandardDeviation(double& averageFPS, double& standardDeviation) const
+double FrameRateCounter::getAverageFPS() const

[egraether, 2012/11/12 23:44:06]

Method returns now the average only. Standard deviation was removed due to wrong
calculation.

 {
     int frameNumber = m_currentFrameNumber - 1;
     int frameCount = 0;
-    double fpsVarianceNumerator = 0;
-
-    averageFPS = 0;
-    standardDeviation = 0;
+    double frameDeltas = 0;
+    double averageFPS = 0;
 
     // Walk backwards through the samples looking for a run of good frame
-    // timings from which to compute the mean and standard deviation.
+    // timings from which to compute the mean.
     //
     // Slow frames occur just because the user is inactive, and should be
     // ignored. Fast frames are ignored if the scheduler is in single-thread
     // mode in order to represent the true frame rate in spite of the fact that
     // the first few swapbuffers happen instantly which skews the statistics
     // too much for short lived animations.
     //
-    // isBadFrame encapsulates the frame too slow/frame too fast logic.
+    // isBadFrameInterval encapsulates the frame too slow/frame too fast logic.
 
-    // Go through all available historical data.
-    while (frameIndex(frameNumber) != frameIndex(m_currentFrameNumber) && frameNumber >= 0) {
+    while (frameIndex(frameNumber) != frameIndex(m_currentFrameNumber) && frameNumber >= 0 && frameDeltas < 1.0) {

[egraether, 2012/11/12 23:44:06]

I added a check for a maximum time of 1 second, so that the average is not too
widespread.

         base::TimeDelta delta = frameInterval(frameNumber);
 
         if (!isBadFrameInterval(delta)) {
             frameCount++;
-            double x = 1.0 / delta.InSecondsF();

[egraether, 2012/11/12 23:44:06]

This average calculation is wrong, because x is already a speed value and
therefore an average. Averaging averages leads to a wrong result. The error was
in most cases really small though.

-            double deltaFromAverage = x - averageFPS;
-            // Change with caution - numerics. http://en.wikipedia.org/wiki/Standard_deviation
-            averageFPS += deltaFromAverage / frameCount;
-            fpsVarianceNumerator += deltaFromAverage * (x - averageFPS);

[egraether, 2012/11/12 23:44:06]

The standard deviation calculation was thereon wrong as well. It is no longer
used, so I removed it completely.

+            frameDeltas += delta.InSecondsF();
         } else if (frameCount)
-            // We've gathered a run of good samples, so stop.
             break;
+
         frameNumber--;
     }
 
     if (frameCount)
-        standardDeviation = sqrt(fpsVarianceNumerator / frameCount);
+        averageFPS = frameCount / frameDeltas;

[egraether, 2012/11/12 23:44:06]

The average needs to be calculated by total count divided by total time.

[nduca, 2012/11/13 00:51:46]

frameDeltas is an oddly named var. Can you make better?

+
+    return averageFPS;
 }
 
 base::TimeTicks FrameRateCounter::timeStampOfRecentFrame(int n) const
 {
     DCHECK(n >= 0);
     DCHECK(n < kTimeStampHistorySize);
     int desiredIndex = (frameIndex(m_currentFrameNumber) + n) % kTimeStampHistorySize;
     return m_timeStampHistory[desiredIndex];
 }
 
 }  // namespace cc