/*{
  "DESCRIPTION": "Event Horizon: a real-time gravitational lens. Photons are integrated step-by-step through curved space around a black hole, so the far side of its glowing accretion disk bends up and over the shadow into an Einstein ring. Electric-neon disk, lensed star field behind. The disk swirls and the sky turns once for a seamless 16s loop.",
  "CREDIT": "glslop agent (Claude)",
  "CATEGORIES": ["blackhole", "lensing", "relativity", "neon", "space"],
  "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": "gravity", "TYPE": "float", "DEFAULT": 1.4, "MIN": 0.4, "MAX": 2.4 },
    { "NAME": "tilt",    "TYPE": "float", "DEFAULT": 0.24,"MIN": 0.05,"MAX": 0.8 },
    { "NAME": "zoom",    "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.6, "MAX": 1.8 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718

const float PERIOD = 16.0;
const float Rs   = 0.55;     // event horizon
const float Rin  = 0.95;     // inner disk
const float Rout = 3.4;      // outer disk

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

vec3 pal(float t){
  return 0.5 + 0.5 * cos(TAU * (t + vec3(0.0, 0.35, 0.62)));
}

float hash(vec2 p){ return fract(sin(dot(p, vec2(127.1, 311.7))) * 43758.5453); }
float hash3(vec3 p){ return fract(sin(dot(p, vec3(127.1, 311.7, 74.7))) * 43758.5453); }

// starfield + faint nebula sampled by a direction
vec3 sky(vec3 dir, float th, float hue){
  dir.xz = rot(th) * dir.xz;
  vec3 c = vec3(0.0);
  // round point-stars: nearest feature point inside each direction-cell
  for (int i = 0; i < 3; i++){
    float sc = 16.0 + 22.0 * float(i);
    vec3 gp = dir * sc;
    vec3 g  = floor(gp);
    vec3 f  = fract(gp);
    float h = hash3(g);
    vec3 rnd = vec3(h, hash3(g + 1.3), hash3(g + 2.7));
    float d = length(f - rnd);
    float star = smoothstep(0.16, 0.0, d) * step(0.86, h);   // sparse, round
    c += pal(hue + 0.3 + h) * star * 1.4;
  }
  // dim nebula wash
  float neb = 0.04 + 0.06 * hash3(floor(dir * 4.0));
  c += pal(hue + 0.6 + dir.y * 0.3) * neb * 0.45;
  return c;
}

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

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

  // camera: near edge-on to the disk for the lensed-arc look
  vec3 ro = vec3(0.0, tilt * 6.0, 6.2 / zoom);
  vec3 ta = vec3(0.0);
  vec3 ww = normalize(ta - ro);
  vec3 uu = normalize(cross(vec3(0.0, 1.0, 0.0), ww));
  vec3 vv = cross(ww, uu);
  vec3 rd = normalize(uv.x * uu + uv.y * vv + 1.5 * ww);

  // ---- integrate the photon path through curved space ----
  vec3 pos = ro;
  vec3 vel = rd;
  vec3 col = vec3(0.0);
  float trans = 1.0;          // remaining transparency (disk is emissive/absorbing)
  bool  captured = false;
  for (int i = 0; i < 160; i++){
    float r = length(pos);
    if (r < Rs){ captured = true; break; }
    float dt = clamp(0.016 * r, 0.014, 0.11);
    // Newtonian-ish photon deflection toward the mass (tuned, not literal GR)
    vec3 acc = -gravity * 0.95 * pos / (r * r * r);
    vec3 prev = pos;
    vel += acc * dt;
    vel = normalize(vel);
    pos += vel * dt;

    // accretion-disk crossing (the y=0 plane), within the annulus
    if (prev.y * pos.y < 0.0){
      float u = prev.y / (prev.y - pos.y);
      vec3 hit = mix(prev, pos, u);
      float rr = length(hit.xz);
      if (rr > Rin && rr < Rout){
        float ang = atan(hit.z, hit.x);
        float swirl = ang * 2.0 - th * 3.0 + 6.0 / rr;   // spiral, loops (integer in th)
        float band = 0.6 + 0.4 * sin(swirl * 3.0);
        float radial = smoothstep(Rout, Rin, rr) * smoothstep(Rin, Rin + 0.3, rr);
        // Doppler-ish brightening on the approaching side
        float dopp = 0.6 + 0.8 * smoothstep(-1.0, 1.0, sin(ang - th));
        vec3 dc = pal(hue + ph + 0.12 * rr + 0.05 * band);
        float em = radial * band * dopp * 3.2;
        col += trans * dc * em;
        trans *= 0.35;        // disk is fairly opaque
      }
    }
    if (trans < 0.02) break;
  }

  if (!captured){
    col += trans * sky(normalize(vel), th, hue);
  }

  col *= glow;
  col += vec3(0.9, 0.95, 1.0) * pow(max(max(col.r, col.g), col.b) * 0.4, 3.0); // hot cores

  col = col / (1.0 + col);
  col = pow(col, vec3(0.80));
  col *= 1.0 - 0.18 * dot(uv, uv);

  gl_FragColor = vec4(col, 1.0);
}