/*{
  "DESCRIPTION": "Throb: a near-monochrome op-art illusion. Logarithmic spiral arms and a field of rings rushing from the centre interfere into a moire that the eye reads as motion and depth; thin chromatic-aberration fringes make it vibrate. No neon — just black, white, and a queasy shimmer. Seamless 16s loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["op-art", "illusion", "moire", "monochrome", "kinetic"],
  "INPUTS": [
    { "NAME": "speed",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 3.0 },
    { "NAME": "arms",   "TYPE": "long",  "DEFAULT": 9,    "MIN": 2,   "MAX": 24 },
    { "NAME": "twist",  "TYPE": "float", "DEFAULT": 5.0,  "MIN": 0.0, "MAX": 14.0 },
    { "NAME": "rings",  "TYPE": "float", "DEFAULT": 7.0,  "MIN": 2.0, "MAX": 18.0 },
    { "NAME": "aberr",  "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.0, "MAX": 2.0 },
    { "NAME": "zoom",   "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.4, "MAX": 2.2 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718

const float PERIOD = 16.0;

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

// the op-art field, evaluated with a per-channel zoom offset for chromatic vibration
float field(vec2 uv, float th, float sc){
  uv *= sc;
  float r = length(uv) + 1e-4;
  float a = atan(uv.y, uv.x);
  float spiral = a * float(arms) + log(r) * twist - th * 2.0;   // arms spiralling inward
  float ringf  = (1.0 / (r + 0.18)) * rings * 3.0 - th * 3.0;   // rings rushing outward
  float s1 = sin(spiral);
  float s2 = sin(ringf);
  float b1 = smoothstep(-0.08, 0.08, s1);
  float b2 = smoothstep(-0.08, 0.08, s2);
  return abs(b1 - b2);                                          // moire checker
}

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;

  uv = rot(0.25 * sin(th)) * uv;            // gentle global sway

  // breathing scale pulse (seamless), slightly different per channel -> chromatic vibration
  float pulse = 1.0 + 0.06 * sin(th);
  float ca = 0.012 * aberr;
  float vr = field(uv, th, pulse - ca);
  float vg = field(uv, th, pulse);
  float vb = field(uv, th, pulse + ca);

  vec3 col = vec3(vr, vg, vb);

  // deepen contrast and add a faint cold/warm split so the white isn't flat
  col = mix(vec3(0.02), vec3(0.97), col);
  col *= vec3(0.97, 1.0, 1.04);             // subtle tint
  // dark vignette pulls focus to the vortex
  col *= 1.0 - 0.55 * dot(uv * zoom, uv * zoom);

  gl_FragColor = vec4(col, 1.0);
}