/*{
  "DESCRIPTION": "Gilded foam — an Apollonian gasket of nested circles, glowing as soft golden filigree drifting in and out.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "zoom",  "TYPE": "float", "DEFAULT": 1.4, "MIN": 0.6, "MAX": 3.0 },
    { "NAME": "fold",  "TYPE": "float", "DEFAULT": 1.08, "MIN": 0.9, "MAX": 1.3 },
    { "NAME": "glow",  "TYPE": "float", "DEFAULT": 0.7, "MIN": 0.2, "MAX": 1.5 },
    { "NAME": "warm",  "TYPE": "float", "DEFAULT": 0.5, "MIN": 0.0, "MAX": 1.0 }
  ]
}*/

mat2 rot(float a){ float c = cos(a), s = sin(a); return mat2(c, -s, s, c); }

// Apollonian via fold + sphere inversion (kifs). Returns structure distance; sc = accumulated scale.
float apollonian(vec2 p, out float sc){
  float scale = 1.0;
  for (int i = 0; i < 7; i++){
    p = -1.0 + 2.0 * fract(0.5 * p + 0.5);     // fold into [-1,1] tiles
    float r2 = dot(p, p);
    float k = fold / r2;                        // inversion
    p *= k;
    scale *= k;
  }
  sc = scale;
  return 0.25 * abs(p.y) / scale;
}

// distance only (for the relief gradient)
float apoD(vec2 p){
  float scale = 1.0;
  for (int i = 0; i < 7; i++){
    p = -1.0 + 2.0 * fract(0.5 * p + 0.5);
    float r2 = dot(p, p);
    float k = fold / r2;
    p *= k; scale *= k;
  }
  return 0.25 * abs(p.y) / scale;
}
float height(vec2 p){ return 1.0 / (1.0 + apoD(p) * 45.0); }

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE) / RENDERSIZE.y;
  vec2 p = uv * zoom;
  p *= rot(TIME * 0.05);
  p += vec2(0.3 * sin(TIME * 0.07), 0.2 * cos(TIME * 0.05));   // slow drift

  float sc;
  float d = apollonian(p, sc);

  // emboss the foam: relief lighting from the height gradient -> hammered-gold dimensionality
  float e = 0.0026;
  float H = height(p);
  float Hx = height(p + vec2(e, 0.0)) - height(p - vec2(e, 0.0));
  float Hy = height(p + vec2(0.0, e)) - height(p - vec2(0.0, e));
  vec3 n = normalize(vec3(-Hx, -Hy, 0.035));
  vec3 L = normalize(vec3(0.5, 0.6, 0.7));
  float diff = 0.32 + 0.7 * max(0.0, dot(n, L));
  float spec = pow(max(0.0, dot(reflect(-L, n), vec3(0.0, 0.0, 1.0))), 18.0);

  float depth = fract(log(sc) * 0.13 + 0.04 * TIME);
  vec3 amber = vec3(0.92, 0.42, 0.08);
  vec3 pale  = vec3(1.0, 0.88, 0.55);
  vec3 gold = mix(amber, pale, depth);

  vec3 col = gold * H * diff * 1.5;             // lit gold relief
  col += pale * spec * H * glow;                // metallic glints on the ridges
  col += amber * exp(-d * 11.0) * 0.10 * glow;  // faint warm dream haze

  col *= 1.0 - 0.42 * dot(uv, uv);
  col = col / (1.0 + 0.5 * col);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}