/*{
  "DESCRIPTION": "Underwater caustics — animated Voronoi light-cells with bright focus nodes and thin shimmering veins on a pool floor. Seamless 14s loop.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "scale",  "TYPE": "float", "DEFAULT": 4.5, "MIN": 2.0, "MAX": 10.0 },
    { "NAME": "sharp",  "TYPE": "float", "DEFAULT": 1.5, "MIN": 0.6, "MAX": 3.0 },
    { "NAME": "tint",   "TYPE": "float", "DEFAULT": 0.55, "MIN": 0.0, "MAX": 1.0 },
    { "NAME": "bright", "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.3, "MAX": 2.2 }
  ]
}*/

vec2 hash2(vec2 p){
  p = vec2(dot(p, vec2(127.1, 311.7)), dot(p, vec2(269.5, 183.3)));
  return fract(sin(p) * 43758.5453);
}
vec3 palette(float t){
  return 0.5 + 0.5 * cos(6.28318 * (t + vec3(0.0, 0.22, 0.45) + tint));
}

// one octave of caustic light: bright focus pools at cell sites + thin veins on borders.
// Feature points orbit on circles driven by th, so the whole thing loops seamlessly.
float causticOct(vec2 p, float th){
  vec2 ip = floor(p), fp = fract(p);
  float f1 = 8.0, f2 = 8.0;
  for (int y = -1; y <= 1; y++){
    for (int x = -1; x <= 1; x++){
      vec2 g = vec2(float(x), float(y));
      vec2 o = hash2(ip + g);
      o = 0.5 + 0.5 * sin(th + 6.28318 * o);   // circular orbit -> seamless
      vec2 r = g + o - fp;
      float d = dot(r, r);
      if (d < f1){ f2 = f1; f1 = d; }
      else if (d < f2){ f2 = d; }
    }
  }
  f1 = sqrt(f1); f2 = sqrt(f2);
  float veins = smoothstep(0.11, 0.0, f2 - f1);            // thin bright cell walls
  float focus = pow(clamp(1.0 - f1 * 1.25, 0.0, 1.0), 3.0); // pooled light at centres
  return veins * 0.6 + focus * 1.0;
}

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE) / RENDERSIZE.y;

  float th = 6.28318 * fract(TIME / 14.0);
  vec2 p = uv * scale;
  p += 0.16 * vec2(sin(p.y * 2.0 + th), cos(p.x * 2.0 - th));   // shimmer warp (seamless)

  float c = causticOct(p, th) * 0.7 + causticOct(p * 2.0 + 5.0, th * 2.0) * 0.45;
  c = pow(c, sharp);

  vec3 floorCol = mix(vec3(0.0, 0.11, 0.17), vec3(0.0, 0.19, 0.25), 0.5 + 0.5 * uv.y);
  vec3 col = floorCol + c * palette(0.32 + 0.16 * c) * bright;
  col += pow(c, 2.0) * 0.3 * vec3(0.7, 0.9, 1.0);   // hot blue-white cores
  col *= 1.0 - 0.26 * dot(uv, uv);
  col = col / (1.0 + 0.3 * col);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}