/***************************************************************************
        Gardner's cloud shader
*****************************************************************************/

surface
gcloud (float 	octaves		= 8.0,
		lacunarity	= 2.2,
		txts		= 1.0;
     	float 	fading		= 4.0,
     		Kd		= 1.0,
     		Ka		= 1.0)
{
  	float 	i, value, frequency, pixel_size;
  	point 	p0 = txts * transform("shader", P);
  	vector 	Nf;

  	/*pixel size is the limit for iteration */
  	pixel_size	= sqrt(area(p0));   
  	value		= 0.0;
  	frequency	= 1.0;
  
      /*fBm*/
      for ( i=0; i<octaves && 1/frequency>pixel_size; i+=1 )
      {
         value += (2.0 * noise(p0*frequency) - 1.0) / frequency;
         frequency *= lacunarity;        
      }
       
      value=clamp(abs(value),0,1);
        
      Nf = faceforward( normalize(N), I );   

      /*opacity*/
      Oi = value * pow(abs(Nf . normalize(I)),fading); 

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