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1 // Copyright 2017 The Chromium Authors. All rights reserved. | |
2 // Use of this source code is governed by a BSD-style license that can be | |
3 // found in the LICENSE file. | |
4 | |
5 #include "remoting/client/fling_tracker.h" | |
6 | |
7 #include <cmath> | |
8 | |
9 namespace remoting { | |
10 | |
11 namespace { | |
12 | |
13 // TODO(yuweih): May need to tweak these numbers to get better smoothness. | |
14 | |
15 // Stop flinging if the speed drops below this. | |
16 // 1px per 16ms. i.e. 1px/frame. | |
17 // TODO(yuweih): The screen unit may not be in pixel. This needs to be | |
18 // normalized with the DPI. | |
19 const float kMinTrackSpeed = 0.0625f; | |
20 | |
21 // The minimum fling duration (ms) needed to trigger the fling animation. This | |
22 // is to prevent unintentional fling with low velocity. | |
23 const float kMinFlingTime = 500.f; | |
24 | |
25 float GetExpFactor(float time_constant, float time_delta) { | |
26 return std::exp(-time_delta / time_constant); | |
27 } | |
28 | |
29 float GetDisplacement(float initial_speed_rate, float exp_factor) { | |
30 // x = v0 * (1 - e^(-t / T)) | |
31 // This comes from a solution to the linear drag equation F=-kv | |
32 return initial_speed_rate * (1.f - exp_factor); | |
33 } | |
34 | |
35 float GetSpeed(float initial_speed_rate, | |
36 float time_constant, | |
37 float exp_factor) { | |
38 // v = (1 / T) * v0 * e^(-t / T). | |
39 // Derivative of the displacement. | |
40 return (1.f / time_constant) * initial_speed_rate * exp_factor; | |
41 } | |
42 | |
43 } // namespace | |
44 | |
45 FlingTracker::FlingTracker(float time_constant) | |
46 : time_constant_(time_constant) {} | |
47 | |
48 FlingTracker::~FlingTracker() {} | |
49 | |
50 void FlingTracker::StartFling(float velocity_x, float velocity_y) { | |
51 start_time_ = base::TimeTicks::Now(); | |
52 | |
53 initial_speed_rate_ = | |
54 std::sqrt(velocity_x * velocity_x + velocity_y * velocity_y); | |
55 | |
56 if (GetSpeed(initial_speed_rate_, time_constant_, | |
57 GetExpFactor(time_constant_, kMinFlingTime)) < kMinTrackSpeed) { | |
58 StopFling(); | |
59 return; | |
60 } | |
61 | |
62 velocity_ratio_x_ = velocity_x / initial_speed_rate_; | |
63 velocity_ratio_y_ = velocity_y / initial_speed_rate_; | |
64 | |
65 previous_position_x_ = 0; | |
66 previous_position_y_ = 0; | |
67 } | |
68 | |
69 void FlingTracker::StopFling() { | |
70 initial_speed_rate_ = 0.f; | |
71 } | |
72 | |
73 bool FlingTracker::IsFlingInProgress() const { | |
74 return initial_speed_rate_ > 0; | |
75 } | |
76 | |
77 bool FlingTracker::TrackMovement(base::TimeDelta time_delta, | |
78 float* dx, | |
79 float* dy) { | |
80 if (!IsFlingInProgress()) { | |
81 return false; | |
82 } | |
83 | |
84 float time_delta_ms = time_delta.InMilliseconds(); | |
85 | |
86 float exp_factor = GetExpFactor(time_constant_, time_delta_ms); | |
87 | |
88 float speed = GetSpeed(initial_speed_rate_, time_constant_, exp_factor); | |
89 | |
90 if (speed < kMinTrackSpeed) { | |
91 StopFling(); | |
92 return false; | |
93 } | |
94 | |
95 float displacement = GetDisplacement(initial_speed_rate_, exp_factor); | |
96 | |
97 float position_x = displacement * velocity_ratio_x_; | |
98 float position_y = displacement * velocity_ratio_y_; | |
99 | |
100 *dx = position_x - previous_position_x_; | |
nicholss
2017/05/11 15:13:50
It seems to me you also do not need previous_posit
Yuwei
2017/05/11 18:26:27
I just renamed |time_delta| to |time_elapsed|. It
| |
101 *dy = position_y - previous_position_y_; | |
102 | |
103 previous_position_x_ = position_x; | |
104 previous_position_y_ = position_y; | |
105 | |
106 return true; | |
107 } | |
108 | |
109 } // namespace remoting | |
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