RenderMan Volume Shader

From K-3D

Global Variables

The following table lists the set of global variables that can be used by volume shaders:

Name	Type	Storage	Description
Ci	color	varying	Attenuated ray color
Oi	color	varying	Attenuated ray opacity
P	point	varying	Light ray endpoint
I	vector	varying	Ray direction (pointing toward P)
E	point	uniform	Position of the eye
Ci	color	varying	Ray color
Oi	color	varying	Ray opacity
ncomps	float	uniform	Number of color components
time	float	uniform	Current shutter time
dtime	float	uniform	The amount of time covered by this shading sample.

Geometry of Shading

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