surface ptc_occlusion(
string ptc_file = "default.ptc";
float bias = 0.01, maxdist = 10; )
{
normal Nf = faceforward( normalize(N), I );
Ci = 1 - occlusion( P, Nf,
"filename", ptc_file, "pointbased", 1,
"bias", 0.1, "clamp", 0, "hitsides", "front" );
}