The RenderMan Interface - Paul Bourke

statements are assumed to be non-directional, or ambient. The total amount of ambient
light incident on a surface is normally returned to a surface shader through ambient. A solar
statement indicates that the light source is a directional light source, while an illuminate
statement indicates that the light source is a local light source. Local light sources have a
position. The position can be a property of the shader or can be inherited from a surface. If
the light source is attached to a geometric primitive the light source is an area light source.
Light sources set Cl inside a solar or illuminate block unless they are defining an ambient
light. Inside these blocks the direction L points towards the surface. This variable is
available so that light source output intensities can be directional. If the light source has a
position, the length of L is the distance from the light source to the surface being shaded.
For example, consider the following light source:
light
phong(
float intensity = 1.0;
color color = 1;
float size = 2.0;
point from = point ”shader” (0,0,0);
point to = point ”shader” (0,0,1); )
{
uniform vector R = normalize(to-from);
}
solar( R, PI/2 )
Cl = intensity * color * pow( R.L/length(L), size );
The Phong shading model can be interpreted to be a procedural directional light source.
The light source has a direction R and a size parameter that controls its fall-off. The solar
statement specifies that the light source casts light in the forward facing hemisphere.
An environment background light source would be specified as follows:
light
reflection( string texturename = ””; float intensity = 1.0 )
{
solar()
Cl = intensity * color environment( texturename, -L );
}
The solar statement implies the light is cast from infinity in all directions. The color of the
light is given by the environment map.
16.3 Volume Shader
Volume shaders change Ci and Oi due to volumetric scattering, self-luminosity, and attenuation.
A volume shader is called once per ray so it should explicitly integrate along the
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