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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_elapsed) { |
| 26 return std::exp(-time_elapsed / 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_elapsed, |
| 78 float* dx, |
| 79 float* dy) { |
| 80 if (!IsFlingInProgress()) { |
| 81 return false; |
| 82 } |
| 83 |
| 84 float time_elapsed_ms = time_elapsed.InMilliseconds(); |
| 85 |
| 86 float exp_factor = GetExpFactor(time_constant_, time_elapsed_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_; |
| 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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