// Measures the angle between the surface
// normal and the camera ray - called the
// viewing vector.

shader
SEM_Edge(
   color frontColor = color(0.1,0.1,0.1),
   color edgeColor = color(1,1,1),
   float strength = 3,

    output color resultRGB = 0)
{
vector i = normalize(I);
vector n = normalize(N);
float d = 1 - fabs(dot(n, -i)); // returns the angle expressed
                             // as a cosine

// front cosine  0.0, at the edge it will be 1.0
// we use fabs() to ensure that d is always in
// the range zero to positive one. 
resultRGB = mix(frontColor, edgeColor, pow(d,strength));

}