/*
 * Copyright (C) 2024 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

uniform shader texture;
uniform half thickness;

/**
 * Returns whether the current fragcoord lies on the outline, return range [0, 1].
 * 0 = no outline, 1 = outline
 * This assumes that the given texture has a transparent background and evaluates whether it's an
 * outline with finite differencing.
 */
vec4 main(float2 fragCoord) {
    float aN = texture.eval(fragCoord + vec2(0., thickness)).a;
    float aS = texture.eval(fragCoord + vec2(0., -thickness)).a;
    float dY = (aN - aS) * 0.5;
    dY = max(dY, 0.0);

    half outline = smoothstep(0.1, 1.8, dY * 5.0);

    // Return the results in the R channel.
    // Also return alpha 1 avoid visual artifacts when it is used standalone.
    return vec4(outline, 0., 0., 1.);
}