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


/* 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;

surfcolor = mix(color1, color2, blend);

if(n == nf){
	surfcolor = mix(color2,color1, blend);
	}

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

/* STEP 4 - calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * diffusecolor;
}