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

remoting/client/gl_cursor_feedback.cc

 // Copyright 2016 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 "remoting/client/gl_cursor_feedback.h"
 
 #include <math.h>
+
 #include <array>
 
 #include "base/logging.h"
 #include "remoting/client/gl_canvas.h"
 #include "remoting/client/gl_cursor_feedback_texture.h"
 #include "remoting/client/gl_math.h"
 #include "remoting/client/gl_render_layer.h"
 #include "remoting/client/gl_texture_ids.h"
 
 namespace {
-const float kAnimationDurationMs = 220.f;
+
+const float kAnimationDurationMs = 300.f;
+
+// This function is for calculating the size of the feedback animation circle at
+// the moment when the animation progress is |progress|.
+// |progress|: [0, 1], indicating the progress of the animation.
+// Returns a coefficient in [0, 1]. It will be multiplied with the maximum
+// diameter of the feedback circle to get the current diameter of the feedback
+// circle.
+float GetExpansionCoefficient(float progress) {
+  DCHECK(progress >= 0 && progress <= 1);
+
+  // 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 kExpansionBase = 400.f;
+    return 1.f - pow(kExpansionBase, -progress);
+  };
+  static const float kExpansionNormalization = get_unnormalized_coeff(1);
+  return get_unnormalized_coeff(progress) / kExpansionNormalization;
+}
+
 }  // namespace
 
 namespace remoting {
 
 GlCursorFeedback::GlCursorFeedback() {}
 
 GlCursorFeedback::~GlCursorFeedback() {}
 
 void GlCursorFeedback::SetCanvas(GlCanvas* canvas) {
   if (!canvas) {
@@ skipping 18 matching lines @@
   if (!layer_ || animation_start_time_.is_null()) {
     return false;
   }
   float progress =
       (base::TimeTicks::Now() - animation_start_time_).InMilliseconds() /
       kAnimationDurationMs;
   if (progress > 1) {
     animation_start_time_ = base::TimeTicks();
     return false;
   }
-  float diameter = progress * max_diameter_;
+  float diameter = GetExpansionCoefficient(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(1.f - progress);  // linear fade-out.

[joedow, 2016/08/23 01:58:42]

nit: move comment above the code.

[Yuwei, 2016/08/23 04:02:52]

Done.

   return true;
 }
 
 }  // namespace remoting