The RenderMan Interface - Paul Bourke

Surface Element
to Illuminate
L
N
Illuminate Cone
Area Light Primitive
P
(u,v)
dPdu
dPdv
Figure 12.2: Light Source Shader State
12.3 Volume Shaders
A volume shader is not associated with a surface, but rather attenuates a ray color as it
travels through space. As such, it does not have access to any geometric surface parameters,
but only to the light ray I and its associated values. The shader computes the new
ray color at the ray origin P −I. The length of I is the distance traveled through the volume
from the origin of the ray to the point P.
12.4 Displacement Shaders
The displacement shader environment is very similar to a surface shader, except that it only
has access to the geometric surface parameters. It computes a new P and/or a new N. In
rendering implementations that do not support the Displacement capability, modifications
to P will not actually move the surface (change the hidden surface elimination calculation);
however, modifications to N will still occur correctly.
12.5 Imager Shaders
An imager shader manipulates a final pixel color after all of the geometric and shading
processing has concluded. In the context of an imager shader, P is the position of the pixel
center in raster space, with the z component being 0.
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