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Reference: Gritz, Larry and James K. Hahn, “BMRT: A Global Illumination Implementation of the RenderMan Standard,” Journal of Graphics Tools, Vol. 1, No. 3, pp. 29-47 (1996).

BMRT: A Global Illumination Implementation of the RenderMan Standard
Larry Gritz† and James K. Hahn The George Washington University*

ABSTRACT
The RenderMan Interface specification proposed by Pixar is a standard forcommunication between modeling software and rendering software or devices. This standard has proven very powerful and is extremely popular in production work. Although the standard itself claims not to specify a rendering algorithm, people have speculated RenderMan and global illumination are mutually incompatible. We have implemented a rendering system which fully adheres to the RenderManInterface and uses global illumination algorithms. Specifically, this implementation supports progressive refinement radiosity and distribution ray tracing in a two-pass approach. This rendering system is widely distributed, very popular, and has been used in production (three properties usually not found in global illumination renderers). We discuss how we overcame problems in mating globalillumination algorithms with the RenderMan standard, and make recommendations for future versions of the standard to better accommodate such algorithms. We also present a summary of important lessons we learned by creating and distributing this tool.

declare geometric primitives, which are bound to the current graphics state at the time of their declaration. The exact nature of the API calls and RIBprotocol are well documented elsewhere [14][1], and will not be described further in this paper. RenderMan allows arbitrarily complex descriptions of surface appearances (including geometric displacements), illumination distributions from light sources, and attenuation by volumes in RenderMan Shading Language (SL) [9][2]. This powerful ability allows the user to extend the surface and lightdescriptions in a way which is more flexible than generally allowed by rendering systems. The RenderMan standard does not dictate which rendering algorithms a particular implementation should use. Minimally, an implementation must support all of the geometric primitives (including polygons and polyhedra, bilinear and bicubic patches and patch meshes, NURBS, and quadrics), perform hidden surface removal,provide antialiasing and filtering, and support the hierarchical graphics state. In addition, optional capabilities may be supported in a particular implementation, including CSG, motion blur, depth of field, ray tracing, radiosity, displacements, texture and environment mapping, area light sources, and programmable shading in Shading Language. Implementations are not expected to support optionalcapabilities which are not possible or are impractical given their rendering algorithms. Until recently the only available implementation of the RI standard has been Pixar’s PhotoRealistic RenderMan (PRMan). This implementation has become very popular in the production community, arguably becoming the de facto standard in rendering software for feature film work. Several other proprietaryimplementations of the RI standard no doubt exist, but remain undocumented and are of unknown levels of compliance to the standard (an exception is [13]). PRMan is based on the REYES algorithm [6]. Indeed, it is apparent that this implementation and the standard itself were developed handin-hand. Though it claims to be independent of any particular rendering algorithm, many aspects of the standard seem torely

1. Introduction
The RenderMan® Interface was designed by Pixar to be a standard communication protocol between modeling/animation software and rendering software or devices [14][10]. RenderMan gives a means of describing a photorealistic 3-D scene in terms of what should be rendered, but without dictating which algorithms or rendering methods should be used. The RenderMan Interface has two...