/*{
  "DESCRIPTION": "Oil slick — thin-film interference swirling across a wet surface, rainbow orders bleeding through one another.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "scale", "TYPE": "float", "DEFAULT": 2.6, "MIN": 1.0, "MAX": 6.0 },
    { "NAME": "warp",  "TYPE": "float", "DEFAULT": 3.0, "MIN": 0.5, "MAX": 6.0 },
    { "NAME": "film",  "TYPE": "float", "DEFAULT": 2.6, "MIN": 1.0, "MAX": 10.0 },
    { "NAME": "speed", "TYPE": "float", "DEFAULT": 0.5, "MIN": 0.0, "MAX": 2.0 }
  ]
}*/

float hash(vec2 p){ p = fract(p * vec2(123.34, 456.21)); p += dot(p, p + 45.32); return fract(p.x * p.y); }
float noise(vec2 p){
  vec2 i = floor(p), f = fract(p); f = f * f * (3.0 - 2.0 * f);
  float a = hash(i), b = hash(i + vec2(1.0, 0.0)), c = hash(i + vec2(0.0, 1.0)), d = hash(i + vec2(1.0, 1.0));
  return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);
}
float fbm(vec2 p){ float s = 0.0, a = 0.5; for (int i = 0; i < 4; i++){ s += a * noise(p); p = p * 2.02 + 5.0; a *= 0.5; } return s; }  // smoother thickness

// thin-film interference colour for an optical thickness d (R,G,B see different wavelengths)
vec3 thinFilm(float d){
  vec3 wl = vec3(0.62, 0.53, 0.46);                  // relative R,G,B wavelengths
  return 0.5 + 0.5 * cos(6.28318 * d / wl);
}

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE) / RENDERSIZE.y;
  vec2 p = uv * scale;
  float t = TIME * 0.08 * speed;

  // swirling film thickness via domain warp
  vec2 drift = vec2(0.3 * sin(t), 0.2 * cos(t * 1.1));
  vec2 q = vec2(fbm(p + drift), fbm(p + vec2(5.2, 1.3) - drift));
  vec2 r = vec2(fbm(p + warp * q + vec2(1.7, 9.2) + drift), fbm(p + warp * q + vec2(8.3, 2.8)));
  float thick = fbm(p + warp * r);

  float d = thick * film;                            // optical thickness -> interference order
  vec3 irid = thinFilm(d);

  // wet dark substrate; the oil rides on top with a soft sheen
  vec3 wet = vec3(0.015, 0.02, 0.03);
  float coverage = smoothstep(0.3, 0.58, thick);     // oil rides in patches over wet asphalt
  vec3 col = mix(wet, irid, coverage);
  col += irid * pow(coverage, 3.0) * 0.45;           // saturate the thick film
  col += wet * 2.0 * (1.0 - coverage) * smoothstep(0.0, 0.3, thick);   // faint reflection on bare wet

  // specular sheen from a soft overhead light on the rippled film
  float sheen = pow(max(0.0, 0.5 + 0.5 * (fbm(p * 3.0 + r) - 0.5)), 6.0);
  col += vec3(1.0) * sheen * 0.12;

  col *= 1.0 - 0.25 * dot(uv, uv);
  col = col / (1.0 + 0.2 * col);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}