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/*{
  "DESCRIPTION": "Undertow — op-art contour lines flowing over an unseen undulating surface, swelling and pinching in warm light.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "freq",  "TYPE": "float", "DEFAULT": 7.0, "MIN": 3.0, "MAX": 16.0 },
    { "NAME": "sharp", "TYPE": "float", "DEFAULT": 2.2, "MIN": 0.6, "MAX": 5.0 },
    { "NAME": "warp",  "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.3, "MAX": 2.0 },
    { "NAME": "warm",  "TYPE": "float", "DEFAULT": 0.5, "MIN": 0.0, "MAX": 1.0 }
  ]
}*/

#define TAU 6.28318530718
float hash(vec2 p){ p = fract(p * vec2(123.34, 456.21)); p += dot(p, p + 45.32); return fract(p.x * p.y); }
float noise(vec2 p){
  vec2 i = floor(p), f = fract(p); f = f * f * (3.0 - 2.0 * f);
  float a = hash(i), b = hash(i + vec2(1.0, 0.0)), c = hash(i + vec2(0.0, 1.0)), d = hash(i + vec2(1.0, 1.0));
  return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);
}

// smooth undulating height field (the unseen surface the contours wrap)
float H(vec2 p){
  float t = TIME;
  float h = 0.6 * sin(p.x * 1.4 + t * 0.25);
  h += 0.5 * sin(p.y * 1.2 - t * 0.2);
  h += 0.4 * sin((p.x + p.y) * 0.9 + t * 0.15);
  h += 0.5 * noise(p * 0.7 + t * 0.05);
  return h * warp;
}

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE) / RENDERSIZE.y;
  vec2 p = uv * 2.4;

  float h = H(p);

  // contour bands; spacing varies with surface slope -> 3D op-art illusion
  float band = pow(0.5 + 0.5 * cos(h * freq * TAU), sharp);

  // gradient (slope) -> shade the implied surface like raked light
  float e = 0.012;
  float gx = H(p + vec2(e, 0.0)) - H(p - vec2(e, 0.0));
  float gy = H(p + vec2(0.0, e)) - H(p - vec2(0.0, e));
  vec3 n = normalize(vec3(-gx, -gy, e * 1.6));
  float shade = 0.4 + 0.6 * max(0.0, dot(n, normalize(vec3(0.5, 0.6, 0.6))));

  vec3 deep = vec3(0.05, 0.02, 0.015);
  vec3 amber = mix(vec3(0.95, 0.45, 0.12), vec3(1.0, 0.7, 0.3), warm);
  vec3 hot = vec3(1.0, 0.9, 0.6);

  vec3 col = mix(deep, amber, band) * shade;
  col += hot * pow(band, 3.0) * shade * 0.6;     // bright crests of the ridges

  col *= 1.0 - 0.3 * dot(uv, uv);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}

/*{
  "DESCRIPTION": "Undertow — op-art contour lines flowing over an unseen undulating surface, swelling and pinching in warm light.",
  "CREDIT": "claude-opus-4-8",
  "INPUTS": [
    { "NAME": "freq",  "TYPE": "float", "DEFAULT": 7.0, "MIN": 3.0, "MAX": 16.0 },
    { "NAME": "sharp", "TYPE": "float", "DEFAULT": 2.2, "MIN": 0.6, "MAX": 5.0 },
    { "NAME": "warp",  "TYPE": "float", "DEFAULT": 1.0, "MIN": 0.3, "MAX": 2.0 },
    { "NAME": "warm",  "TYPE": "float", "DEFAULT": 0.5, "MIN": 0.0, "MAX": 1.0 }
  ]
}*/

#define TAU 6.28318530718
float hash(vec2 p){ p = fract(p * vec2(123.34, 456.21)); p += dot(p, p + 45.32); return fract(p.x * p.y); }
float noise(vec2 p){
  vec2 i = floor(p), f = fract(p); f = f * f * (3.0 - 2.0 * f);
  float a = hash(i), b = hash(i + vec2(1.0, 0.0)), c = hash(i + vec2(0.0, 1.0)), d = hash(i + vec2(1.0, 1.0));
  return mix(mix(a, b, f.x), mix(c, d, f.x), f.y);
}

// smooth undulating height field (the unseen surface the contours wrap)
float H(vec2 p){
  float t = TIME;
  float h = 0.6 * sin(p.x * 1.4 + t * 0.25);
  h += 0.5 * sin(p.y * 1.2 - t * 0.2);
  h += 0.4 * sin((p.x + p.y) * 0.9 + t * 0.15);
  h += 0.5 * noise(p * 0.7 + t * 0.05);
  return h * warp;
}

void main(){
  vec2 uv = (gl_FragCoord.xy - 0.5 * RENDERSIZE) / RENDERSIZE.y;
  vec2 p = uv * 2.4;

float h = H(p);

// contour bands; spacing varies with surface slope -> 3D op-art illusion
  float band = pow(0.5 + 0.5 * cos(h * freq * TAU), sharp);

// gradient (slope) -> shade the implied surface like raked light
  float e = 0.012;
  float gx = H(p + vec2(e, 0.0)) - H(p - vec2(e, 0.0));
  float gy = H(p + vec2(0.0, e)) - H(p - vec2(0.0, e));
  vec3 n = normalize(vec3(-gx, -gy, e * 1.6));
  float shade = 0.4 + 0.6 * max(0.0, dot(n, normalize(vec3(0.5, 0.6, 0.6))));

vec3 deep = vec3(0.05, 0.02, 0.015);
  vec3 amber = mix(vec3(0.95, 0.45, 0.12), vec3(1.0, 0.7, 0.3), warm);
  vec3 hot = vec3(1.0, 0.9, 0.6);

vec3 col = mix(deep, amber, band) * shade;
  col += hot * pow(band, 3.0) * shade * 0.6;     // bright crests of the ridges

col *= 1.0 - 0.3 * dot(uv, uv);
  col = pow(max(col, 0.0), vec3(0.9));
  gl_FragColor = vec4(col, 1.0);
}

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