/*{
  "DESCRIPTION": "Quaternion Julia: a smooth bulbous organic 4D Julia fractal (q -> q^2 + c) raymarched with the analytic distance estimate, electric orbit-trap colour and hot rim light. The seed c drifts on a circle so the creature slowly morphs and tumbles. Real-time, seamless 16s loop.",
  "CREDIT": "glslop agent (Claude); quaternion Julia DE after Inigo Quilez",
  "CATEGORIES": ["fractal", "raymarch", "neon", "julia"],
  "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": "morph",     "TYPE": "float", "DEFAULT": 0.5,  "MIN": 0.0, "MAX": 1.2 },
    { "NAME": "zoom",      "TYPE": "float", "DEFAULT": 1.0,  "MIN": 0.5, "MAX": 2.0 },
    { "NAME": "iterCount", "TYPE": "long",  "DEFAULT": 10,   "MIN": 5,   "MAX": 14 }
  ]
}*/

#define PI  3.14159265359
#define TAU 6.28318530718

const float PERIOD = 16.0;
const int   MAXI   = 14;
const int   STEPS  = 110;

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(0.55, 0.40, 0.18)));
}

// Quaternion Julia DE.  c = seed quaternion ; trap = orbit trap for colour.
float map(vec3 p, vec4 c, mat2 Rt, out vec4 trap){
  p.xz = Rt * p.xz;
  vec4 z = vec4(p, 0.0);
  float md2 = 1.0;            // running |dz|^2
  float mz2 = dot(z, z);
  trap = vec4(abs(z.xyz), mz2);
  for (int i = 0; i < MAXI; i++){
    if (i >= iterCount) break;
    md2 *= 4.0 * mz2;                                   // |dz| <- |2 z dz|
    z = vec4(z.x * z.x - dot(z.yzw, z.yzw), 2.0 * z.x * z.yzw) + c;  // z <- z^2 + c
    trap = min(trap, vec4(abs(z.xyz), dot(z, z)));
    mz2 = dot(z, z);
    if (mz2 > 6.0) break;
  }
  return 0.25 * sqrt(mz2 / md2) * log(mz2);             // analytic distance estimate
}

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

  float ph = fract(TIME * speed / PERIOD);
  float th = TAU * ph;
  // seed drifts on a small circle (seamless) -> the creature morphs
  vec4 c = vec4(-0.45, 0.55, 0.36, 0.18)
         + morph * 0.12 * vec4(cos(th), sin(th), cos(2.0 * th), sin(2.0 * th));
  mat2 Rt = rot(th);

  float camDist = 2.7 * zoom;
  float az = 0.2 * sin(th);
  float el = 0.25 + 0.12 * sin(th);
  vec3 ro = camDist * vec3(sin(az) * cos(el), sin(el), cos(az) * cos(el));
  vec3 ta = vec3(0.0);
  vec3 fwd = normalize(ta - ro);
  vec3 rgt = normalize(cross(vec3(0.0, 1.0, 0.0), fwd));
  vec3 upv = cross(fwd, rgt);
  vec3 rd  = normalize(uv.x * rgt + uv.y * upv + 1.7 * fwd);

  // ---- surface raymarch ----
  float t = 0.02;
  float tcap = camDist + 3.0;
  vec4 trapHit = vec4(0.0);
  float te = 0.0;
  bool hit = false;
  for (int i = 0; i < STEPS; i++){
    vec3 p = ro + rd * t;
    vec4 tr;
    float d = map(p, c, Rt, tr);
    if (d < 0.0004 * t){ hit = true; trapHit = tr; te = t; break; }
    t += d * 0.85;
    if (t > tcap) break;
  }

  vec3 col = vec3(0.0);

  if (hit){
    vec3 p = ro + rd * te;
    vec2 k = vec2(1.0, -1.0);
    float e = 0.0008;
    vec4 tr;
    vec3 n = normalize(
      k.xyy * map(p + k.xyy * e, c, Rt, tr) +
      k.yyx * map(p + k.yyx * e, c, Rt, tr) +
      k.yxy * map(p + k.yxy * e, c, Rt, tr) +
      k.xxx * map(p + k.xxx * e, c, Rt, tr));

    // electric colour from the orbit trap + a little spatial variation
    vec3 base = pal(hue + 0.30 + 0.55 * trapHit.w + 0.5 * trapHit.x
                    + 0.3 * trapHit.z + 0.18 * length(p));
    vec3 lig  = normalize(vec3(0.5, 0.8, 0.4));
    float dif = max(dot(n, lig), 0.0);
    float fres = pow(1.0 - max(dot(n, -rd), 0.0), 2.2);
    float ao  = clamp(0.5 + 0.5 * n.y, 0.3, 1.0);

    col += base * 0.10 * ao;
    col += base * dif * 0.34 * ao;
    col += base * fres * 3.0 * glow;                 // electric rim
    col += vec3(1.0) * pow(fres, 5.0) * 0.8 * glow;
    col *= exp(-0.18 * max(te - camDist + 1.3, 0.0));
  }

  col = col / (1.0 + col);
  col = pow(col, vec3(0.78));
  col *= 1.0 - 0.22 * dot(uv, uv);

  gl_FragColor = vec4(col, 1.0);
}