Lección 7 de 10 · El Sistema Solar en GLSL

Saturno

Anillos de hielo de diez metros de grueso. Y la sombra del planeta encima.

El código

precision highp float;
// The exact sphere: how far along the ray it is hit, or -1 if the ray misses
float sphere(vec3 ro, vec3 rd, float r){
  float b = dot(ro, rd), c = dot(ro, ro) - r*r, h = b*b - c;
  return h < 0.0 ? -1.0 : -b - sqrt(h);
}
// WoGL close: knee at 0.70, sRGB, chroma 1.3
vec3 wogl(vec3 c){
  float l = dot(c, vec3(0.2126, 0.7152, 0.0722)), k = max(l - 0.7, 0.0);
  c *= l > 0.0 ? (l - k + k/(1.0 + k))/l : 0.0;
  c = pow(clamp(c, 0.0, 1.0), vec3(0.4545));
  l = dot(c, vec3(0.2126, 0.7152, 0.0722));
  return clamp(mix(vec3(l), c, 1.3), 0.0, 1.0);
}
// Noise in 3D: the usual fb with one more dimension. On a globe it leaves no seam.
float h3(vec3 p){ return h2(p.xy + p.z*vec2(37.0, 17.0)); }
float n3(vec3 p){
  vec3 i = floor(p), f = fract(p);
  f = f*f*(3.0 - 2.0*f);
  return mix(mix(mix(h3(i), h3(i + vec3(1,0,0)), f.x), mix(h3(i + vec3(0,1,0)), h3(i + vec3(1,1,0)), f.x), f.y),
             mix(mix(h3(i + vec3(0,0,1)), h3(i + vec3(1,0,1)), f.x), mix(h3(i + vec3(0,1,1)), h3(i + vec3(1,1,1)), f.x), f.y), f.z);
}
float f3(vec3 p){
  float v = 0.0, a = 0.5;
  for (int i = 0; i < 5; i++){ v += a*n3(p); p = p*2.03 + vec3(1.7, 9.2, 4.1); a *= 0.5; }
  return v;
}
// The background: black, with a star now and then
vec3 sky(vec2 f){
  float s = h2(floor(f));
  return vec3(0.0002, 0.0003, 0.0006) + vec3(0.7, 0.7, 0.8)*step(0.9982, s)*h2(floor(f) + 3.1);
}
float tilt = 0.47;      // 26.7° = 0.47 radians
float cassini = 1.0;    // the width of the Cassini division
float shadow = 1.0;
// How full the rings are at r Saturn radii: C, B, Cassini, A
float rings(float r){
  float gap = 0.04*cassini;
  float C = smoothstep(1.23, 1.25, r)*smoothstep(1.53, 1.51, r)*0.22;
  float B = smoothstep(1.51, 1.55, r)*smoothstep(1.99 - gap, 1.97 - gap, r)*0.95;
  float A = smoothstep(1.99 + gap, 2.01 + gap, r)*smoothstep(2.27, 2.25, r)*0.62;
  return clamp((C + B + A)*(0.78 + 0.22*sin(r*190.0)*sin(r*67.0)), 0.0, 1.0);
}
void main(){
  vec2 p = (gl_FragCoord.xy*2.0 - R)/min(R.x, R.y);
  vec3 ro = vec3(0.0, 0.0, 6.0), rd = normalize(vec3(p, -2.3));   // the camera looks at the center
  vec3 sun = normalize(vec3(M.x*4.0 - 3.8, M.y*3.0 - 1.0, 1.0));   // you move the sun: it is the pointer
  vec3 axis = vec3(0.0, cos(tilt), sin(tilt));      // the planet's axis, tipped toward you
  vec3 col = sky(gl_FragCoord.xy);
  float tp = sphere(ro, rd, 1.0);
  if (tp > 0.0){
    vec3 P = ro + rd*tp, n = normalize(P);
    float lat = asin(dot(n, axis));
    vec3 albedo = mix(vec3(0.80, 0.66, 0.42), vec3(0.62, 0.48, 0.30), 0.5 + 0.5*sin(lat*14.0 + f3(n*vec3(2.0, 8.0, 2.0))*1.2))*0.6;
    float light = max(dot(n, sun), 0.0);
    // The rings' shadow: from the planet, a ray toward the sun. Does it cross the rings?
    float ts = -dot(P, axis)/dot(sun, axis);
    if (ts > 0.0) light *= 1.0 - rings(length(P + sun*ts))*shadow*0.9;
    col = albedo*light*3.0;
  }
  // The rings: the same ray against a plane through the center
  float tr = -dot(ro, axis)/dot(rd, axis);
  if (tr > 0.0 && (tp < 0.0 || tr < tp)){
    vec3 X = ro + rd*tr;
    float density = rings(length(X));
    float lit = 1.0;
    if (sphere(X, sun, 1.0) > 0.0) lit = 1.0 - shadow;          // the planet's shadow on the rings
    vec3 ice = vec3(0.86, 0.76, 0.60)*(0.35 + 0.65*abs(dot(axis, sun)))*lit*2.6;
    col = mix(col, ice*0.6, density);
  }
  gl_FragColor = vec4(wogl(col), 1.0);
}