[Remoting Android] Use Material Design's Ripple Expansion Function for Touch Feedback

remoting/client/gl_cursor_feedback.cc

Index: remoting/client/gl_cursor_feedback.cc
diff --git a/remoting/client/gl_cursor_feedback.cc b/remoting/client/gl_cursor_feedback.cc
index 8306933013e18f2919a50b45db7ba4c878fe731a..5cb796afb4c649ffdcc898dadad26f3281f2a5e8 100644
--- a/remoting/client/gl_cursor_feedback.cc
+++ b/remoting/client/gl_cursor_feedback.cc
@@ -5,6 +5,7 @@
 #include "remoting/client/gl_cursor_feedback.h"
 
 #include <math.h>
+
 #include <array>
 
 #include "base/logging.h"
@@ -15,7 +16,32 @@
 #include "remoting/client/gl_texture_ids.h"
 
 namespace {
-const float kAnimationDurationMs = 220.f;
+
+const float kAnimationDurationMs = 300.f;
+
+// progress: [0, 1], indicating the progress of the animation.

[joedow, 2016/08/23 01:27:20]

params are referenced like this |progress|.  It would be good to have a sentence
above this stating what the function is used for.

[Yuwei, 2016/08/23 01:56:23]

Done.

+// Returns a coefficient ([0, 1]) to be multiplied with the maximum diameter to
+// get the current diameter of the animation.
+float GetExpansionCoeff(float progress) {

[joedow, 2016/08/23 01:27:20]

Coeff should be spelled out (GetExpansionCoefficient)

[Yuwei, 2016/08/23 01:56:23]

Done.

+  // Decelerating expansion. This is conforming to the material design spec.
+  // More time will be spent showing the larger circle and the animation will
+  // look more rapid given the same time duration.
+  auto get_unnormalized_coeff = [](float progress) {
+    static const float expansion_base = 400.f;

[joedow, 2016/08/23 01:27:20]

const values should use the same format as above (i.e. kExpansionBase)

[Yuwei, 2016/08/23 01:56:23]

Done.

+    return 1.f - pow(expansion_base, -progress);
+  };
+  static const float expansion_normalization = get_unnormalized_coeff(1);
+  return get_unnormalized_coeff(progress) / expansion_normalization;
+}
+
+// progress: [0, 1], indicating the progress of the animation.
+// Returns a coefficient ([0, 1]) to be multiplied with the alpha channel of the
+// texture.
+float GetAlphaCoeff(float progress) {

[joedow, 2016/08/23 01:27:20]

I don't think this needs to be a separate function, it doesn't really add
anything.

[Yuwei, 2016/08/23 01:56:23]

Done.

+  // Linear fade-out.
+  return 1.f - progress;
+}
+
 }  // namespace
 
 namespace remoting {
@@ -54,13 +80,13 @@ bool GlCursorFeedback::Draw() {
     animation_start_time_ = base::TimeTicks();
     return false;
   }
-  float diameter = progress * max_diameter_;
+  float diameter = GetExpansionCoeff(progress) * max_diameter_;
   std::array<float, 8> positions;
   FillRectangleVertexPositions(cursor_x_ - diameter / 2,
                                cursor_y_ - diameter / 2,
                                diameter, diameter, &positions);
   layer_->SetVertexPositions(positions);
-  layer_->Draw(1.f - progress);
+  layer_->Draw(GetAlphaCoeff(progress));
   return true;
 }