1 // Copyright (c) 2012 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.
5 #include "ui/events/gestures/velocity_calculator.h"
9 VelocityCalculator::VelocityCalculator(int buffer_size)
10 : buffer_(new Point[buffer_size]) ,
12 num_valid_entries_(0),
13 buffer_size_(buffer_size),
16 velocities_stale_(false) {
19 VelocityCalculator::~VelocityCalculator() {}
21 float VelocityCalculator::XVelocity() {
22 if (velocities_stale_)
27 float VelocityCalculator::YVelocity() {
28 if (velocities_stale_)
33 void VelocityCalculator::PointSeen(int x, int y, int64 time) {
34 buffer_[index_].x = x;
35 buffer_[index_].y = y;
36 buffer_[index_].time = time;
38 index_ = (index_ + 1) % buffer_size_;
39 if (num_valid_entries_ < buffer_size_)
42 velocities_stale_ = true;
45 float VelocityCalculator::VelocitySquared() {
46 if (velocities_stale_)
48 return x_velocity_ * x_velocity_ + y_velocity_ * y_velocity_;
51 void VelocityCalculator::UpdateVelocity() {
52 // We don't have enough data to make a good estimate of the velocity.
53 if (num_valid_entries_ < 2)
56 // Where A_i = A[i] - mean(A)
57 // x velocity = sum_i(x_i * t_i) / sum_i(t_i * t_i)
58 // This is an Ordinary Least Squares Regression.
64 for (size_t i = 0; i < num_valid_entries_; ++i) {
65 mean_x += buffer_[i].x;
66 mean_y += buffer_[i].y;
67 mean_time += buffer_[i].time;
70 // Minimize number of divides.
71 const float num_valid_entries_i = 1.0f / num_valid_entries_;
73 mean_x *= num_valid_entries_i;
74 mean_y *= num_valid_entries_i;
76 // The loss in accuracy due to rounding is insignificant compared to
77 // the error due to the resolution of the timer.
78 // Use integer division to avoid the cast to double, which would cause
79 // VelocityCalculatorTest.IsAccurateWithLargeTimes to fail.
80 mean_time /= num_valid_entries_;
82 float xt = 0; // sum_i(x_i * t_i)
83 float yt = 0; // sum_i(y_i * t_i)
84 int64 tt = 0; // sum_i(t_i * t_i)
88 for (size_t i = 0; i < num_valid_entries_; ++i) {
89 t_i = (buffer_[i].time - mean_time);
90 xt += (buffer_[i].x - mean_x) * t_i;
91 yt += (buffer_[i].y - mean_y) * t_i;
96 // Convert time from microseconds to seconds.
97 x_velocity_ = xt / (tt / 1000000.0f);
98 y_velocity_ = yt / (tt / 1000000.0f);
103 velocities_stale_ = false;
106 void VelocityCalculator::ClearHistory() {
108 num_valid_entries_ = 0;
111 velocities_stale_ = false;