/* test swap parameter color shader */
surface
col_swap(float	Kd = 1,	/* basic brightness */
		 		swap = 0;
		 color color1 =(1,1,1),
		 	   color2 =(0,0,0))
{
color	surfcolor = color2;

/* make a copy of the surface normal one unit in length */
normal	n = normalize(N);
normal	nf = faceforward(n, I);

/* set the apparent surface opacity */
Oi = Os;
if (swap == 0)
{

	if(n == nf)
		surfcolor = color1;
		
}else {
	surfcolor = color1;
	
	if(n == nf)
		surfcolor = color2;
}

/* calculate the diffuse lighting component */
color	diffusecolor = Kd * diffuse(nf);


/* calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * diffusecolor;
}