/*{
  "DESCRIPTION": "Golden Angle: a sunflower head grown one seed at a time at the golden angle of 137.5 degrees. Hundreds of glowing seed-orbs spiral outward and the eye catches the Fibonacci parastichies — interlocking spiral arms — shimmering with colour. The head slowly turns for a seamless 16s loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["phyllotaxis", "golden-ratio", "spiral", "botanical", "neon"],
  "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": "seeds",  "TYPE": "long",  "DEFAULT": 600,  "MIN": 150, "MAX": 900 },
    { "NAME": "zoom",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.6, "MAX": 1.8 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
#define MAXN 900

const float PERIOD = 16.0;
const float GA = 2.399963229;            // golden angle (radians)

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 scale = 0.62 / sqrt(float(seeds));    // fit the head to the frame
  float r = length(uv);

  vec3 col = vec3(0.0);
  for (int i = 0; i < MAXN; i++){
    if (i >= seeds) break;
    float fi = float(i);
    float a  = fi * GA + th;                   // rotate the whole head (seamless)
    float rr = sqrt(fi) * scale;
    // cheap culling: skip seeds far from this pixel's radius
    if (abs(rr - r) > 0.10) continue;
    vec2 pos = rr * vec2(cos(a), sin(a));
    vec2 dv = uv - pos;
    float d2 = dot(dv, dv);
    float sz = 0.0004 + 0.0016 * rr;           // seeds grow outward
    float g = sz / (d2 + sz);
    g = g * g;
    vec3 c = pal(hue + ph + 0.45 * rr + 0.0016 * fi);
    col += c * g;
  }
  col *= glow;
  // white-hot seed centres
  col += vec3(0.9, 0.95, 1.0) * pow(max(max(col.r, col.g), col.b) * 0.5, 3.0);

  col = col / (1.0 + col);
  col = pow(col, vec3(0.82));
  col *= 1.0 - 0.16 * dot(uv * zoom, uv * zoom);
  gl_FragColor = vec4(col, 1.0);
}