The RenderMan Interface - Paul Bourke

Section 10
RELATIONSHIP TO THE RENDERMAN INTERFACE
The Shading Language is designed to be used with the RenderMan Interface described
in Part I of this document. This interface is used by modeling systems to describe scenes
to a rendering system. Like most graphics systems, the RenderMan Interface maintains a
current graphics state. This state contains all the information needed to render a geometric
primitive.
The graphics state contains a set of attributes that are attached to the surface of each geometric
primitive. These shading attributes include the current color (set with RiColor and
referred to as Cs) and current opacity (set with RiOpacity and referred to as Os). The geometric
primitive also contains a current surface shader (RiSurface) and several volume
shaders: the current atmosphere shader (RiAtmosphere), current interior shader (RiInterior)
and a current exterior shader (RiExterior). All geometric primitives use the current surface
shader for computing the surface shading at their surfaces. Light directed toward the
viewer is attenuated with the current atmosphere shader. If the surface shader of a primitive
causes rays to be traced (with the trace() function), the ray color will be attenuated
with either the current exterior shader or current interior shader, depending on whether the
traced ray is in the direction of, or opposite to, the surface normal.
The graphics state also contains a current list of light sources that contains the light sources
that illuminate the geometric primitives. Light sources may be added to or removed from
this list using RiIlluminate. Light sources can be attached to geometric primitives to define
area light sources (RiAreaLightSource) or procedurally define their geometric properties
(RiLightSource). The current area light source contains a list of geometric primitives that
define its geometry. Defining a geometric primitive adds that primitive to this list.
The graphics state also maintains the current transformation that is the transformation corresponding
to the modeling hierarchy. The graphics state also contains a current displacement
shader (RiDisplacement) and an imager (RiImager).
The RenderMan Interface predefines standard shaders for light sources, surfaces, and volumes.
These standard shaders are available in all rendering programs that implement the
RenderMan Interface, even if some algorithmic limitation prevents them from supporting
programmable shaders. Standard and implementation-dependent shaders should always
be specified in the Shading Language, even if they are built in. The predefined shaders
provided by the Shading Language are listed in Table 10.1, Standard Shaders. There is also
a null shader that is used as a placeholder. Shading Language definitions for these shaders
are given in Appendix A.
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