/*{
  "DESCRIPTION": "Guilloche: an engine-turned engraving like the rosette on a banknote or a watch dial. Dozens of sinusoidally-perturbed rings weave through each other into a hypnotic moire of fine gold line-work over deep navy, slowly turning. Perturbations rotate on integer harmonics for a seamless 16s loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["guilloche", "engraving", "rosette", "moire", "line-art"],
  "INPUTS": [
    { "NAME": "speed",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "glow",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.3, "MAX": 3.0 },
    { "NAME": "hue",    "TYPE": "float", "DEFAULT": 0.0,  "MIN": 0.0, "MAX": 1.0 },
    { "NAME": "petals", "TYPE": "long",  "DEFAULT": 17,   "MIN": 5,   "MAX": 36 },
    { "NAME": "twist",  "TYPE": "float", "DEFAULT": 0.28, "MIN": 0.0, "MAX": 0.8 },
    { "NAME": "zoom",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.6, "MAX": 1.8 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
#define NR  30

const float PERIOD = 16.0;

vec3 pal(float t){ return 0.5 + 0.5 * cos(TAU * (t + vec3(0.0, 0.33, 0.66))); }

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

  float ph = fract(TIME * speed / PERIOD);
  float th = TAU * ph;

  float r = length(uv);
  float a = atan(uv.y, uv.x);

  // two interleaved families of sinusoidally-perturbed rings -> guilloche moire
  float dr = 0.019;
  float fieldA = 1e9, fieldB = 1e9;
  for (int i = 0; i < NR; i++){
    float Ri = (float(i) + 0.5) * dr;
    float pertA = 0.010 * cos(float(petals) * a + float(i) * twist + th);
    float pertB = 0.010 * cos(float(petals + 6) * a - float(i) * twist * 1.3 - th);
    fieldA = min(fieldA, abs(r - (Ri + pertA)));
    fieldB = min(fieldB, abs(r - (Ri + pertB)));
  }

  float lineA = 0.0016 / (fieldA + 0.0016);
  float lineB = 0.0016 / (fieldB + 0.0016);
  lineA = pow(lineA, 1.5);
  lineB = pow(lineB, 1.5);

  // engraving colours: gold primary, teal secondary, over deep navy
  vec3 col = vec3(0.015, 0.025, 0.06);                  // deep navy plate
  vec3 gold = mix(vec3(1.0, 0.82, 0.42), pal(hue + 0.08), 0.35);
  vec3 teal = mix(vec3(0.3, 0.85, 0.9),  pal(hue + 0.55), 0.35);
  col += gold * lineA * 1.7 * glow;
  col += teal * lineB * 1.1 * glow;
  // bright nodes where the two families cross
  col += vec3(1.0, 0.97, 0.9) * lineA * lineB * 3.0 * glow;

  // central medallion + outer fade
  col += gold * exp(-r * r * 120.0) * 0.8;
  col *= smoothstep(0.95, 0.5, r);                      // vignette the plate
  col *= 1.0 - 0.1 * dot(uv * zoom, uv * zoom);

  col = col / (1.0 + col);
  col = pow(col, vec3(0.85));
  gl_FragColor = vec4(col, 1.0);
}