The RenderMan Interface - Paul Bourke

• No attribute inheritance should be assumed unless implicit in the definition of the
User Entity (i.e., within a hierarchy).
• No attribute should be exported except to establish either global or local defaults.
The RenderMan Specification provides block structuring to organize the components of a
RIB file. Although the use of blocks is only required for frame and world constructs by
the Specification, the liberal use of attribute and transform blocks is encouraged. A modeler
enables a Render Manager to freely manipulate, rearrange, or delete scene elements
(frames, cameras, lights, User Entities) by carefully bounding these elements in the RIB file
according to scope. A Render Manager might, for example, strip all of the frames out of
a RIB file and distribute them around a network of rendering servers. This, of course, is
only possible if the RIB file has been structured in such a way as to bound those things
pertaining to a given frame within its frame block and those things pertaining to all frames
outside and before all frame blocks.
User Entities
A User Entity couples a collection of geometric primitives and/or User Entities with shading
and geometric attributes. As such it introduces a level of scope that is more local than
that implied by the RenderMan world block. Typically, the term User Entity refers to a
geometric element within a scene whose attributes or position a user may wish to modify
or tweak. Because there is some computational expense associated with attribute block
structuring, there is an intrinsic trade-off between control over individual User Entities and
rendering time/memory requirements. At one extreme, the entire scene is made up of one
User Entity within one attribute block. At the other extreme, each polygon is a User Entity
and the renderer is forced to spend most of its time managing the graphics state. Modeling
programs and their users may wish to carefully weigh this trade-off.
The Scoping Conventions above prescribe the following User Entity Conventions:
• All User Entities must be delimited by an attribute block.
• All User Entities must have an identifier attribute that uniquely characterizes that Entity
to the user. Two special identifier attributes are provided to distinguish between
Entities organized by a geometric relationship (the name identifier) and Entities organized
according to material makeup (the shadinggroup identifier).
• A User Entity must be completely described within its attribute block.
Nonportable options and attributes
The following list of RIB requests are restricted as they either limit the device independence
of the file or they control rendering quality or speed parameters. Render managers
should provide this kind of control to users at render time. The inclusion of these restricted
requests by a modeler should indicate to a Render Manager that they are, in some sense,
mandatory. When including nonportable options or attributes in the RIB file, they should
be located contiguously (according to scope) in a RIB file.
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