surface
metal (
	float	Ka		= 1, 
		Ks		= 3, 
		roughness	= .25)
{
	normal Nf = faceforward(normalize(N), I);
	vector V = normalize(-I);
	
	Oi = Os;
	Ci = Os * Cs * ( Ka*ambient() + Ks*specular(Nf,V,roughness) );
}