improving UI of FPS counter

cc/frame_rate_counter.cc

Index: cc/frame_rate_counter.cc
diff --git a/cc/frame_rate_counter.cc b/cc/frame_rate_counter.cc
index d84b74436a08dad69bac4df8e5c8f1e1d01c2c23..4b1153e877780f817b204d181a25d86255180074 100644
--- a/cc/frame_rate_counter.cc
+++ b/cc/frame_rate_counter.cc
@@ -12,7 +12,7 @@
 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.
@@ -81,17 +81,15 @@ 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
@@ -99,27 +97,24 @@ void FrameRateCounter::getAverageFPSAndStandardDeviation(double& averageFPS, dou
     // 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