Add a mechanism to provide non-linear scaling of fling acceleration.

ui/base/gestures/gesture_sequence.cc

Index: ui/base/gestures/gesture_sequence.cc
diff --git a/ui/base/gestures/gesture_sequence.cc b/ui/base/gestures/gesture_sequence.cc
index bc3b4f46d9b2ca7416adc10b4c66eaee37e389ff..74d735c090e700248fbd8d6aa2b1be5c4146a2c6 100644
--- a/ui/base/gestures/gesture_sequence.cc
+++ b/ui/base/gestures/gesture_sequence.cc
@@ -15,6 +15,9 @@
 #include "ui/base/gestures/gesture_util.h"
 #include "ui/gfx/rect.h"
 
+#include <stdio.h>
+
+
 namespace ui {
 
 namespace {
@@ -280,10 +283,58 @@ unsigned int ComputeTouchBitmask(const GesturePoint* points) {
   return touch_bitmask;
 }
 
+// Number of different accelerations for fling velocity adjustment.
+const unsigned kNumberOfFlingVelocityBands = 8;
+
+// Fling acceleration bands.
+static float fling_scaling_bands[kNumberOfFlingVelocityBands];
+
+// Maximum fling velocity.
+const float kMaxFlingVelocityFromFinger = 15000.0f;
+
+// True if we've configured the fling scaling bands.
+static bool did_setup_scaling = false;
+
+// Setup a default flat fling acceleration profile.
+static void SetDefaultFlingVelocityScaling() {

[sadrul, 2012/10/20 01:28:51]

I think functions in anonymous namespace don't need to be declared static

[rjkroege, 2012/10/22 15:21:34]

Done.

+  for (unsigned i = 0; i < kNumberOfFlingVelocityBands; i++) {
+    fling_scaling_bands[i] =
+        GestureConfiguration::touchscreen_fling_acceleration_adjustment();
+  }
+}
+
+// Read |kNumberOfFlingVelocityBands| comma-separated floating point
+// values from an environment variable and use that to configure the fling
+// velocity profile.
+static void ReadFlingVelocityScalingIfNecessary() {
+  if (did_setup_scaling)

[sadrul, 2012/10/20 01:28:51]

Make |did_setup_scaling| a local static variable in this function?

[rjkroege, 2012/10/22 15:21:34]

Done.

+    return;
+  did_setup_scaling = true;
+  SetDefaultFlingVelocityScaling();
+  char* pk = getenv("BANDED_FLING_VELOCITY_ADJUSTMENT");
+  if (!pk) {
+      LOG(WARNING) << "Could not read configuration var. defaulting.\n";
+      return;
+  }
+
+  char* s = pk;

[sadrul, 2012/10/20 01:28:51]

Perhaps use StringTokenizer (base/string_tokenizer.h) and base::StringToDouble
(base/string_number_conversions.h)?

[rjkroege, 2012/10/22 15:21:34]

Is this really an improvement? :-)

Done.

[rjkroege, 2012/10/22 15:21:34]

Done.

+  for (unsigned i = 0; i < kNumberOfFlingVelocityBands; i++) {
+    float f;
+    f = strtof(s, &s);
+    s++;
+    if (f > 0.f)
+      fling_scaling_bands[i] = f;
+  }
+}
+
 float CalibrateFlingVelocity(float velocity) {
-  const float velocity_scaling  =
-      GestureConfiguration::touchscreen_fling_acceleration_adjustment();
-  return velocity_scaling * velocity;
+  ReadFlingVelocityScalingIfNecessary();
+  float abs_velocity = (velocity < 0) ? velocity * -1: velocity;

[sadrul, 2012/10/20 01:28:51]

abs_velocity = fabs(velocity)?

[rjkroege, 2012/10/22 15:21:34]

Done.

+  abs_velocity = std::min(abs_velocity, kMaxFlingVelocityFromFinger - 1.f);
+  int band =
+      (abs_velocity / kMaxFlingVelocityFromFinger) *
+      kNumberOfFlingVelocityBands;
+  return fling_scaling_bands[band] * velocity;
 }
 
 }  // namespace