夜の雲

float gHash(vec2 p) {
    p = fract(p * vec2(123.34, 456.21));
    p += dot(p, p + 45.32);
    return fract(p.x * p.y);
}

float gNoise(vec2 p) {
    vec2 i = floor(p);
    vec2 f = fract(p);
    // 5 次の補間。3 次だと格子の継ぎ目が四角く見える。
    vec2 u = f * f * f * (f * (f * 6.0 - 15.0) + 10.0);
    float a = gHash(i);
    float b = gHash(i + vec2(1.0, 0.0));
    float c = gHash(i + vec2(0.0, 1.0));
    float d = gHash(i + vec2(1.0, 1.0));
    return mix(mix(a, b, u.x), mix(c, d, u.x), u.y);
}

float gFbm(vec2 p) {
    float v = 0.0;
    float a = 0.5;
    for (int i = 0; i < 5; i++) {
        v += a * gNoise(p);
        p = p * 2.0 + vec2(1.7, 9.2);
        a *= 0.5;
    }
    return v / 0.97;
}

vec3 gRamp(float x, vec3 a, vec3 b, vec3 c) {
    return mix(mix(a, b, smoothstep(0.0, 0.5, x)), c, smoothstep(0.5, 1.0, x));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    vec2 uv = (fragCoord * 2.0 - iResolution.xy) / iResolution.y;
    float t = iTime * 0.100;
    vec3 colorA = vec3(0.020, 0.060, 0.250);
    vec3 colorB = vec3(0.100, 0.350, 0.850);
    vec3 colorC = vec3(0.550, 0.800, 1.000);
    vec2 p = uv * 1.000 + vec2(t * 0.3, 0.0);
    float base = 0.5 + 0.5 * sin(p.y * 4.0 + gFbm(p) * 3.0 + t);
    float detail = clamp(gNoise(p * 4.0), 0.0, 1.0);
    float shade = clamp(base * (0.85 + 0.3 * detail), 0.0, 1.0);
    vec3 col = gRamp(shade, colorA, colorB, colorC);
    col = mix(col, colorC, clamp(smoothstep(0.6, 0.0, length(uv - vec2(0.0, 0.2))), 0.0, 1.0));
    fragColor = vec4(col, 1.0);
}