surface
smoothcolor3(float	Ka = 1,
					Kd = 0.5,
					min_s = 0.1,
					max_s = 0.8,
					min_t = 0.1,
					max_t = 0.8,
					fuzz = 0.1;
			color	color1 = color(1,0,0),
					color2 = color(1,1,0))
{
normal	nf, n;
color	ambientcolor, diffusecolor, surfcolor;

/* STEP 1 - make a copy of the surface normal one unit in length */
n = normalize(N);
nf = faceforward(n, I);

/* STEP 2 - set the apparent surface opacity */
Oi = Os;

/* STEP 3 - calculate the lighting components */
ambientcolor = Ka * ambient();
diffusecolor = Kd * diffuse(nf);

/* STEP 4 - calculate the color blending factor
			and mix the colors appropriately */
float	blend = smoothstep(min_s, min_s + fuzz, s) *
				(1 - smoothstep(max_s, max_s + fuzz, s)) *
				smoothstep(min_t, min_t + fuzz, t) *
				(1 - smoothstep(max_t, max_t + fuzz, t));
surfcolor = mix(color1, color2, blend);

/* STEP 5 - calculate the apparent surface color */
Ci = Oi * Cs * surfcolor * (ambientcolor + diffusecolor);
}