What is Primvar¶
A primitive variable (primVar) is a mechanism in RenderMan that allows you to
attach arbitrary data to objects (primitives) in your scene. These values
passe from your object to its shader at render time. Their power lies in their
ability to overwrite shader parameters.
For example, an object can modify the shader attached to it. For a single
object, we can just set the shader parameter ourselves. But if we have 10,000
objects -using the same base shader but with small differences in diffuse
color- we don’t want to make 10,000 instances of our shader and set the
diffuse color in each of them. Instead, we harness the power of primVars and
attach a color primVar to each object. If we set up our base shader to read
this primVar value, we can use just one shader on all 10,000 objects.
