/*  sky shader, works better with a surface color, Cs of
(0.05, 0.02, 0.5), a skyish blue. Ink_opac is changing wrt t,
from up to down. Looks like it is 6 o'clock sky, just after
sunset.  
By Cemil Turun, June 2, 1995 */

#include "rmannotes.sl"

surface sky(float turbfreq = 5)
{
  color ink, paper;
  point PP;
  float pixelsize, twice, ink_opac, ss, tt;
  float scale, turb, y;

  paper = Cs;
  PP = transform("shader", P) * turbfreq;

  pixelsize = sqrt(area(PP));
  twice = pixelsize * 2;

  turb = 0;
  rotate2d(s, t, noise(s +.5) + t, 0.5, 0.5, ss, tt);
  for (scale = 1; scale > twice; scale /= 2)
    turb += abs(noise(PP / scale) - 0.2) * scale;
  turb = turb * 3;
  ink = color(.6, .4, .5);
  ink_opac = turb * t - .1;
  paper = mix(paper, ink, ink_opac); 
  ss = s; tt = t;

  Oi = Os;
  Ci = Oi * paper;
  

}