1 /* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
5 // Preprocess the radii for computing the distance approximation. This should
6 // be used in the vertex shader if possible to avoid doing expensive division
7 // in the fragment shader. When dealing with a point (zero radii), approximate
8 // it as an ellipse with very small radii so that we don't need to branch.
9 vec2 inverse_radii_squared(vec2 radii) {
10 return 1.0 / max(radii * radii, 1.0e-6);
13 #ifdef WR_FRAGMENT_SHADER
15 // One iteration of Newton's method on the 2D equation of an ellipse:
17 // E(x, y) = x^2/a^2 + y^2/b^2 - 1
19 // The Jacobian of this equation is:
21 // J(E(x, y)) = [ 2*x/a^2 2*y/b^2 ]
23 // We approximate the distance with:
25 // E(x, y) / ||J(E(x, y))||
27 // See G. Taubin, "Distance Approximations for Rasterizing Implicit
28 // Curves", section 3.
30 // A scale relative to the unit scale of the ellipse may be passed in to cause
31 // the math to degenerate to length(p) when scale is 0, or otherwise give the
32 // normal distance approximation if scale is 1.
33 float distance_to_ellipse_approx(vec2 p, vec2 inv_radii_sq, float scale) {
34 vec2 p_r = p * inv_radii_sq;
35 float g = dot(p, p_r) - scale;
36 vec2 dG = (1.0 + scale) * p_r;
37 return g * inversesqrt(dot(dG, dG));
40 // Slower but more accurate version that uses the exact distance when dealing
41 // with a 0-radius point distance and otherwise uses the faster approximation
42 // when dealing with non-zero radii.
43 float distance_to_ellipse(vec2 p, vec2 radii) {
44 return distance_to_ellipse_approx(p, inverse_radii_squared(radii),
45 float(all(greaterThan(radii, vec2(0.0)))));
48 float distance_to_rounded_rect(
51 vec4 center_radius_tl,
53 vec4 center_radius_tr,
55 vec4 center_radius_br,
57 vec4 center_radius_bl,
60 // Clip against each ellipse. If the fragment is in a corner, one of the
61 // branches below will select it as the corner to calculate the distance
62 // to. We use half-space planes to detect which corner's ellipse the
63 // fragment is inside, where the plane is defined by a normal and offset.
64 // If outside any ellipse, default to a small offset so a negative distance
65 // is returned for it.
66 vec4 corner = vec4(vec2(1.0e-6), vec2(1.0));
68 // Calculate the ellipse parameters for each corner.
69 center_radius_tl.xy = center_radius_tl.xy - pos;
70 center_radius_tr.xy = (center_radius_tr.xy - pos) * vec2(-1.0, 1.0);
71 center_radius_br.xy = pos - center_radius_br.xy;
72 center_radius_bl.xy = (center_radius_bl.xy - pos) * vec2(1.0, -1.0);
74 // Evaluate each half-space plane in turn to select a corner.
75 if (dot(pos, plane_tl.xy) > plane_tl.z) {
76 corner = center_radius_tl;
78 if (dot(pos, plane_tr.xy) > plane_tr.z) {
79 corner = center_radius_tr;
81 if (dot(pos, plane_br.xy) > plane_br.z) {
82 corner = center_radius_br;
84 if (dot(pos, plane_bl.xy) > plane_bl.z) {
85 corner = center_radius_bl;
88 // Calculate the distance of the selected corner and the rectangle bounds,
89 // whichever is greater.
90 return max(distance_to_ellipse_approx(corner.xy, corner.zw, 1.0),
91 signed_distance_rect(pos, rect_bounds.xy, rect_bounds.zw));