1 // Copyright 2014 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/gfx/geometry/cubic_bezier.h"
10 #include "base/logging.h"
16 static const double kBezierEpsilon = 1e-7;
17 static const int MAX_STEPS = 30;
19 static double eval_bezier(double p1, double p2, double t) {
20 const double p1_times_3 = 3.0 * p1;
21 const double p2_times_3 = 3.0 * p2;
22 const double h3 = p1_times_3;
23 const double h1 = p1_times_3 - p2_times_3 + 1.0;
24 const double h2 = p2_times_3 - 6.0 * p1;
25 return t * (t * (t * h1 + h2) + h3);
28 static double eval_bezier_derivative(double p1, double p2, double t) {
29 const double h1 = 9.0 * p1 - 9.0 * p2 + 3.0;
30 const double h2 = 6.0 * p2 - 12.0 * p1;
31 const double h3 = 3.0 * p1;
32 return t * (t * h1 + h2) + h3;
35 // Finds t such that eval_bezier(x1, x2, t) = x.
36 // There is a unique solution if x1 and x2 lie within (0, 1).
37 static double bezier_interp(double x1,
45 x1 = std::min(std::max(x1, 0.0), 1.0);
46 x2 = std::min(std::max(x2, 0.0), 1.0);
47 x = std::min(std::max(x, 0.0), 1.0);
49 // We're just going to do bisection for now (for simplicity), but we could
50 // easily do some newton steps if this turns out to be a bottleneck.
53 for (int i = 0; i < MAX_STEPS; ++i, step *= 0.5) {
54 const double error = eval_bezier(x1, x2, t) - x;
55 if (std::abs(error) < kBezierEpsilon)
57 t += error > 0.0 ? -step : step;
60 // We should have terminated the above loop because we got close to x, not
61 // because we exceeded MAX_STEPS. Do a DCHECK here to confirm.
62 DCHECK_GT(kBezierEpsilon, std::abs(eval_bezier(x1, x2, t) - x));
69 CubicBezier::CubicBezier(double x1, double y1, double x2, double y2)
76 CubicBezier::~CubicBezier() {
79 double CubicBezier::Solve(double x) const {
80 return eval_bezier(y1_, y2_, bezier_interp(x1_, x2_, x));
83 double CubicBezier::Slope(double x) const {
84 double t = bezier_interp(x1_, x2_, x);
85 double dx_dt = eval_bezier_derivative(x1_, x2_, t);
86 double dy_dt = eval_bezier_derivative(y1_, y2_, t);
90 void CubicBezier::Range(double* min, double* max) const {
93 if (0 <= y1_ && y1_ < 1 && 0 <= y2_ && y2_ <= 1)
96 // Represent the function's derivative in the form at^2 + bt + c.
97 // (Technically this is (dy/dt)*(1/3), which is suitable for finding zeros
98 // but does not actually give the slope of the curve.)
99 double a = 3 * (y1_ - y2_) + 1;
100 double b = 2 * (y2_ - 2 * y1_);
103 // Check if the derivative is constant.
104 if (std::abs(a) < kBezierEpsilon &&
105 std::abs(b) < kBezierEpsilon)
108 // Zeros of the function's derivative.
112 if (std::abs(a) < kBezierEpsilon) {
113 // The function's derivative is linear.
116 // The function's derivative is a quadratic. We find the zeros of this
117 // quadratic using the quadratic formula.
118 double discriminant = b * b - 4 * a * c;
119 if (discriminant < 0)
121 double discriminant_sqrt = sqrt(discriminant);
122 t_1 = (-b + discriminant_sqrt) / (2 * a);
123 t_2 = (-b - discriminant_sqrt) / (2 * a);
129 if (0 < t_1 && t_1 < 1)
130 sol_1 = eval_bezier(y1_, y2_, t_1);
132 if (0 < t_2 && t_2 < 1)
133 sol_2 = eval_bezier(y1_, y2_, t_2);
135 *min = std::min(std::min(*min, sol_1), sol_2);
136 *max = std::max(std::max(*max, sol_1), sol_2);