/*{
  "DESCRIPTION": "Lissajous Ghost: a vintage vector oscilloscope. A single phosphor beam races around a harmonograph figure leaving a glowing decay-trail on the tube; faint persistence, scanlines, bloom and screen-curvature complete the analog look. The beam laps and the figure precesses on integer harmonics for a seamless 16s loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["oscilloscope", "vector", "phosphor", "harmonograph", "retro"],
  "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": "tint",    "TYPE": "float", "DEFAULT": 0.0,  "MIN": 0.0, "MAX": 1.0 },
    { "NAME": "persist", "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.2, "MAX": 2.0 },
    { "NAME": "zoom",    "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.6, "MAX": 1.8 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718
#define NSAMP 380

const float PERIOD = 16.0;

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

float segDist(vec2 p, vec2 a, vec2 b){
  vec2 pa = p - a, ba = b - a;
  float h = clamp(dot(pa, ba) / max(dot(ba, ba), 1e-6), 0.0, 1.0);
  return length(pa - ba * h);
}

// harmonograph figure: two detuned pendulums per axis (precesses with th)
vec2 curve(float s, float th){
  float x = sin(3.0 * s + 0.30 * sin(th))
          + 0.55 * sin(5.0 * s + 1.7);
  float y = cos(2.0 * s)
          + 0.55 * cos(7.0 * s + 0.30 * sin(2.0 * th) + 0.9);
  return vec2(x, y) * 0.40;
}

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

  // screen curvature (barrel) for the CRT feel
  vec2 suv = uv;
  suv *= 1.0 + 0.12 * dot(uv, uv);
  uv = suv / zoom;

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

  float span = TAU * 6.0;          // laps that fill the figure
  float beam = ph;                 // beam progress 0..1 (one full sweep per loop)

  // phosphor colour: green -> user can tint toward amber/cyan
  vec3 phos = mix(vec3(0.25, 1.0, 0.45), pal(tint + 0.15), tint * 0.8);

  vec3 col = vec3(0.0);
  vec2 prev = curve(0.0, th);
  for (int i = 1; i < NSAMP; i++){
    float u = float(i) / float(NSAMP);
    vec2 cur = curve(u * span, th);
    float d = segDist(uv, prev, cur);              // continuous thin trace
    float since = fract(beam - u);                 // time since beam passed (wraps)
    float decay = exp(-since * 5.0 / persist);     // recent = bright comet tail
    float w = 0.00004;
    float line = w / (d * d + w);
    col += phos * line * (0.06 + 2.4 * decay);
    prev = cur;
  }

  // bright beam head
  vec2 head = curve(beam * span, th);
  float dh = length(uv - head);
  col += vec3(0.9, 1.0, 0.95) * 0.0016 / (dh * dh + 0.00012);

  // faint graticule grid
  vec2 g = abs(fract(uv * 5.0) - 0.5);
  float grid = smoothstep(0.49, 0.5, max(g.x, g.y));
  col += phos * grid * 0.05;

  col *= glow;

  // scanlines + flicker (seamless) + vignette
  col *= 0.82 + 0.18 * sin(gl_FragCoord.y * 1.7);
  col *= 0.95 + 0.05 * sin(th * 3.0);
  col *= 1.0 - 0.30 * dot(suv, suv);

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