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