[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;
+const float kExpansionBase = 400.f;

[joedow, 2016/08/22 22:22:21]

Perhaps there isn't a better name, but kExpansionBase isn't very descriptive
until you read further down.  Perhaps a comment for the related consts would
help?

[Yuwei, 2016/08/22 23:46:22]

Not sure what is the best description for this constant. It's basically just a
part of the magical equation...

Just moved the const inside the function to make it look clearer.

+const float kExpansionNormalization = (1.f - pow(kExpansionBase, -1));

[joedow, 2016/08/22 22:22:22]

I think this might be clearer to create an anonymous function which is given a
progress value and returns the expansion coefficient.  You could then use that
to populate this value and the one on line 63.

[Yuwei, 2016/08/22 23:46:22]

Done.

BTW nice timestamp, 22 22:22:22 :P

+
 }  // 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_;
+  // Decelerating Expansion: (1 - 400^(-t)) / (1 - 400^(-1))

[joedow, 2016/08/22 22:22:21]

Can you update the comment a bit here?  The comment above has hard-coded values
(which would be out of date if someone changes the constant on line 20)

I think it is better to document the intention instead of the effect.  Are there
min/max requirements got |progress|.  Basically document the stuff that someone
in the future (perhaps you) would need to know.

[Yuwei, 2016/08/22 23:46:22]

Just removed the comment here. It doesn't seem useful anyway since the code is
self-explaining.
Done. (Above)

+  float expansion_coeff =
+      (1.f - pow(kExpansionBase, -progress)) / kExpansionNormalization;
+  float diameter = expansion_coeff * max_diameter_;
   std::array<float, 8> positions;
   FillRectangleVertexPositions(cursor_x_ - diameter / 2,
                                cursor_y_ - diameter / 2,
                                diameter, diameter, &positions);
   layer_->SetVertexPositions(positions);
+
+  // Linear fade-out.
   layer_->Draw(1.f - progress);
   return true;
 }
 
 }  // namespace remoting