The RenderMan Interface - Paul Bourke

}
Oi = sum;
Ci = Cs * Oi * (Ka + Kd * I.N * I.N / (I.I * N.N) );
16.1.2 Ray tracer
The following is a procedure to implement a Turner Whitted-style ray tracer.
surface
whitted(
float eta = 1.5; /* index of refraction */
float Kr =.8; /* reflective coefficient */
float Kt =.2; /* transmissive coefficient */
float Ks =.2; /* specular coefficient */
float Kss = 2; /* specular exponent */
float eta = 1.5; ) /* index of refraction */
{
normal Nn = faceforward(normalize(N), I);
/* ambient term */
Ci = Kd * ambient();
/* diffuse and specular terms */
illuminance( P, Nn, PI/2 ) {
/* diffuse */
Ci += Kd * Cl * (L . Nn);
}
/* specular */
vector H = normalize(normalize(L)+I);
Ci += Ks * Cl * pow(max(0,0, Nn.H), Kss);
/* reflection */
Ci += Ks * trace( reflect( I, Nn ) );
}
/* transmittance */
vector T = refract( I, Nn, (N.I)