// A ball bouncing up and down the stairs, new colour every bounce

float hash(float n) { return fract(sin(n * 12.9898) * 43758.5453); }

vec3 hue(float h) {
    return clamp(abs(mod(h * 6.0 + vec3(0.0, 4.0, 2.0), 6.0) - 3.0) - 1.0,
                 0.0, 1.0);
}

float ballPos(float t) {
    return 12.0 * abs(sin(t * 0.5));
}

void mainImage(out vec4 fragColor, in vec2 fragCoord) {
    float i = floor(fragCoord.x / iResolution.x * 16.0);

    float cyc = iTime * 0.5 / 3.14159265;
    float bounce = floor(cyc);
    // colour changes while the ball is on the floor, faded over the bounce
    vec3 colour = mix(hue(hash(bounce + 2.0)), hue(hash(bounce + 3.0)),
                      smoothstep(0.0, 0.25, fract(cyc)));

    float d = i - ballPos(iTime);
    float v = exp(-d * d * 1.0);
    for (int j = 1; j <= 8; j++) {
        float fj = float(j);
        float dj = i - ballPos(iTime - fj * 0.12);
        v = max(v, 0.7 * exp(-fj * 0.35) * exp(-dj * dj * 1.0));
    }
    fragColor = vec4(colour * v + vec3(0.01, 0.0, 0.02), 1.0);
}