The RenderMan Interface - Paul Bourke

Section 16
EXAMPLE SHADERS
16.1 Surface Shaders
Surface shaders inherit the surface variables of the surfaces to which they are attached. A
surface shader should always set the output color Ci and optionally the output opacity Oi
that is emitted in the direction -I. I is the direction of the ray incident to the surface. The
length of this vector is equal to the distance between the origin of the ray and the point on
the surface. Thus the actual origin of the ray is available as P-I.
16.1.1 Turbulence
The following surface shader implements a simple turbulence procedural texture. The
shader computes the texture by adding various octaves of noise, weighting each octave
by 1/f, where f is the cutoff frequency of that octave. This texture is then used to modulate
the opacity of the surface. The texture is generated in the named coordinate system
”marble”, which must have been established with by the use of an RiCoordinateSystem
(”marble”) call before the instantiation of the turbulence shader. Notice that after the opacity
has been computed, it is multiplied into the color, so that the colors and opacities output
by the shader are premultiplied for use by pixel compositors.
surface
turbulence ( float Kd=.8, Ka=.2 )
{
float a, scale, sum ;
float IdotN;
point M; /* convert to texture coordinate system */
M = transform( ”marble”, P );
scale = 1;
sum = 0;
a = sqrt(area(M));
while( a < scale ) {
sum += scale * float noise(M/scale);
scale *= 0.5;
}
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