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(x,y,&r,&g,&b)

// Procedural penrose tiling
// 2010 (C) Tomasz Dobrowolski
// http://moonedit.com/tom

// This is based on Robinson substitution,
// prototiles are two iosceles triangles,
// type 0 with angles: 36, 36, 108 deg.
// type 1 with angles: 72, 72, 36 deg.

enum { 
   levels = 9,         // number of substitutions
   sc = 2/(sqrt(5)-1),  // inflation scale
   
   // transformations constants:
   d1 = tan(54*PI/180),
   d2 = tan(18*PI/180),
   a1 = .5/cos(36*PI/180),
   a2 = (1+a1)*.5,
   a3 = tan(36*PI/180)*a2, 
   cos1 = cos(144*PI/180)*sc,
   sin1 = sin(144*PI/180)*sc,
   cos2 = cos(108*PI/180)*sc,
   sin2 = sin(108*PI/180)*sc
};

type = 0; // starting triangle type

for(k=0; k<levels; k++)  // iterate all subsitutions
{
   if (type == 0) 
   {
      // We substitute triangle type 0
      // with three possible triangles.
      // We detect in which of those three
      // triangle our current (x,y) lies
      // by checking line equations separating them:
      
      if (1 - d1*y - x > 0) // left triangle
      {         
         // only translation/mirror here:
         x = (1 - x)*sc;
         y = y*sc;            
      } 
      else if (1 - d2*y - x > 0) // middle triangle
      {
         
         // translate:
         xx = x - a2;     
         yy = y - a3;
         
         // rotate:
         x = xx*cos1 - yy*sin1;
         y = xx*sin1 + yy*cos1;
         
         type = 1; // tile type changes here!         
      } 
      else // right triangle
      {      
         // translate (x only):
         xx = x - (1+a1);
         yy = y;
      
         // rotate:
         x =  xx*cos1 + yy*sin1;
         y = -xx*sin1 + yy*cos1;         
      }      
   } 
   else 
   {
      // We substitute triangle type 1 with 
      // two possible triangles (analogically).

      if (d1*y - x > 0) { // upper triangle
      
         // rotate only
         xx = x; 
         yy = y;
         x = -xx*cos2 + yy*sin2;
         y =  xx*sin2 + yy*cos2;
                  
         type = 0; // tile type changes here!
         
      } else { // lower triangle
      
         // translate (x only):
         xx = x - a1;
         yy = y;
         
         // rotate:
         x =  xx*cos2 + yy*sin2;
         y = -xx*sin2 + yy*cos2;         
      }
   }   
}

// Now we found resulting tile type
// after given number of substitution:

// We can use (x,y) for some additional
// texture effects!

if (type == 0) {
   // return tile type 0 color (yellowish orange):
   r = 218;
   g = 213;
   b = 118;
} else {
   // return tile type 1 color (marine blue):
   r = 74;
   g = 101;
   b = 166;
}

(x,y,t)
static ot = -1;
static co = 1;
static si = 0;
if (t != ot) {
   s = sin(t)*.5;
   co = cos(-pi/4+s);
   si = sin(-pi/4+s);
   ot = t;
}
xp = x;
yp = abs(y);
s = .5;
d = .25;
c = 0;
k=0;
while(1) {      
   if (abs(xp)<d && abs(yp-s)<s) {
      c2 = .5-abs(xp)/d;
      if (c < c2) c = c2;
   }
   if (k>16)
      break;
   xp = abs(xp)*1.2;
   yp -= s*2; //1.8;
   x2 = co*xp + si*yp;
   y2 = co*yp - si*xp;
   xp = x2; 
   yp = y2;
   k++;
   s *= 2/3;   
   d *= 2/3;   
}
c

(x,y,t)
static ot = -1, co, si, si2, co2;
if (t != ot) { ot = t;
   s = -.5+sin(t*.6174)*.5;
   co = cos(-pi/4+s);
   si = sin(-pi/4+s);
   si2 = sin(t);
   co2 = cos(t*.374325);
}
xp = x; yp = abs(y);
ys = 2+si2*.5; xs = co2*.5-.5;
s = .5; d = .25; c = 0;
for(k=0; k<16; k++) {
   c2 = 1-(xp^2 + (yp-s)^2);
   if (c < c2) c = c2;
   xp = abs(xp-xs)*1.2;
   yp = abs(yp-ys)*1.2 - s*2;
   x2 = co*xp + si*yp;
   y2 = co*yp - si*xp;
   xp = x2; 
   yp = y2;
   s *= 2/3;   
   d *= 2/3;   
}
c

(x,y,t)
static ot = -1, co, si, si2, co2;
if (t != ot) { ot = t;
   s = -.5+sin(t*.6174)*.5;
   co = cos(-pi/4+s);
   si = sin(-pi/4+s);
   si2 = sin(t);
   co2 = cos(t*.374325);
}
xp = x; yp = abs(y);
ys = 2+si2*.5; xs = co2*.5-.5;
s = .5; d = .25; c = 0;
scl=1.75;
for(k=0; k<30; k++) {
   xp = abs(xp-xs)*scl;
   yp = abs(yp-ys)*scl;
   x2 = co*xp + si*yp;
   y2 = co*yp - si*xp;
   xp = x2; 
   yp = y2;  
}
distance=sqrt(xp^2+yp^2)/scl^k;
distance^0.25//for better contrast