/* Pete Hriso  Assignment #6 d */

#include "rmannotes.sl"

surface smoken (float Ka = 1, Kd = .5;
		float offset = 0.25;
		float amountofsmoke = 8 ;
		float scale = 1;
		float fadeaway = 2;
		color black = color (0, 0, 0);)
{

  color paper, ink, smoke;
  float paper_opac, ink_opac; 
  point Nf, V;      
  float variable;           
  float limit;    
  float amplitude = 1;   
  float sum = 0;
  point PP;      

  /* init */

  V = normalize(I);
  Nf = faceforward (normalize(N),V);
  
  paper = Cs;
  paper_opac = 0;

 /* transform P to shader */


  PP = scale * transform ("shader", P);
  limit = sqrt (area(PP));
  for(variable = 0; variable < amountofsmoke && amplitude > limit; variable += 1){ 
    sum += amplitude * snoise(PP);
	PP *= 2;
	amplitude *= 0.5;
    }
  /* layer 1 */
  
  
  smoke = spline(s, (color (1, 1, 1)), (color (1, 1, 1)), 
		 (color (.789, .789, 1)), 
		 (color (.960, .960, .960)), (color (1, 1, .89)), 
		 (color (1, 1,.89)));
  ink= smoke;
  ink_opac=clamp(sum+offset, 0, 1) * pow(abs(Nf . V), fadeaway);
  paper = mix(paper, ink, ink_opac);


   /*  illum for all layers */

   paper = paper * Ka * ambient() +
     paper * Kd * diffuse(Nf);
  
  Oi = ink_opac;
  Ci = paper * ink_opac ;
 
}