/*
 * Copyright (C) 2023 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 foreground;
uniform half imageWidth;
uniform half snowThickness;

#include "shaders/simplex2d.agsl"
#include "shaders/utils.agsl"

float random(vec2 uv) {
    return fract(sin(dot(uv, vec2(14.53898, 56.233))) * 45312.644263742);
}

vec4 main(float2 fragCoord) {
    // fragCoord should be already the adjusted UVs to have the expected rect of the image.
    vec2 uv = fragCoord / imageWidth;
    float variation = 0.3 + simplex2d(11. * uv);
    float distance = variation * snowThickness;

    float aN = foreground.eval(fragCoord + vec2(0., distance)).a;
    float aS = foreground.eval(fragCoord + vec2(0., -distance)).a;
    float dY = (aN - aS) * 0.5;
    dY = max(dY, 0.0);

    float accumulatedSnow = smoothstep(0.1, 1.8, dY * 5.0);
    vec4 color = vec4(0., 0., 0., 1.);
    color.r = accumulatedSnow;
    color.g = random(uv);
    color.b = variation;
    return color;
}