The RenderMan Interface - Paul Bourke

These functions access the value of the parameter named paramname of one of the
shaders attached to the geometric primitive that is currently being shaded. If the
appropriate shader exists, and a parameter named paramname exists in that shader,
and the parameter is the same type as variable, then the value of that parameter is
stored in variable and the function returns 1; otherwise, variable is unchanged and
the function returns 0.
Note that if the data corresponding to name is uniform, but the variable variable is
varying, the appropriate uniform-to-varying conversion will take place, however, the
reverse case is considered failure.
The lightsource function is only available inside illuminance blocks and refers to the
light being examined by the current iteration.
float incident( string paramname; output type variable )
float opposite( string paramname; output type variable )
These functions access the value of the volume shader parameter paramname that
is stored in the volume shaders attached to geometric primitive surface. incident
accesses values from the volume shader that describes the volume which contains
the incident ray I. opposite accesses values from the volume shader that describes
the volume on the other side of the surface. If the named variable exists and is of the
correct type, the value is stored in value and the function returns 1; otherwise, value
is unchanged and the function returns 0.
Note that if the data corresponding to name is uniform, but the variable variable is
varying, the appropriate uniform-to-varying conversion will take place, however, the
reverse case is considered failure.
float attribute ( string name; output type variable )
Returns data that is part of the primitive’s attribute state, either from individual RenderMan
Interface calls that set attributes or from the RiAttribute call. The name specifies
the piece of RenderMan Interface attribute state that will be returned in variable.
If the data name is known to the renderer and its type and storage class match that
of variable, the named data will be written into variable and attribute() will return
a value of 1.0. If the data is unknown or the types do not match, variable will be
unchanged and attribute() will return 0.0.
Note that if the data corresponding to name is uniform, but the variable variable is
varying, the appropriate uniform-to-varying conversion will take place, however, the
reverse case is considered failure.
The standard data names supported by attribute are listed in Table 15.4. A particular
implementation may support additional attribute queries.
float option ( string name; output type variable )
Returns data that is part of the renderer’s global option state, either from individual
RenderMan Interface calls that set options or from the RiOption call. The name specifies
the piece of RenderMan Interface option state that will be returned in variable. If
the data name is known to the renderer and its type and storage class match that of
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