1/*
2 * Copyright (C) 2024 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17uniform shader texture;
18uniform half thickness;
19
20/**
21 * Returns whether the current fragcoord lies on the outline, return range [0, 1].
22 * 0 = no outline, 1 = outline
23 * This assumes that the given texture has a transparent background and evaluates whether it's an
24 * outline with finite differencing.
25 */
26vec4 main(float2 fragCoord) {
27    float aN = texture.eval(fragCoord + vec2(0., thickness)).a;
28    float aS = texture.eval(fragCoord + vec2(0., -thickness)).a;
29    float dY = (aN - aS) * 0.5;
30    dY = max(dY, 0.0);
31
32    half outline = smoothstep(0.1, 1.8, dY * 5.0);
33
34    // Return the results in the R channel.
35    // Also return alpha 1 avoid visual artifacts when it is used standalone.
36    return vec4(outline, 0., 0., 1.);
37}
38