The RenderMan Interface Specification,[1] or RISpec in short, is an open API developed by Pixar Animation Studios to describe three-dimensional scenes and turn them into digital photorealistic images.
As Pixar's technical specification for a standard communications protocol (or interface) between modeling programs and rendering programs capable of producing photorealistic-quality images, RISpec is a similar concept to PostScript but for describing 3D scenes rather than 2D page layouts.
The interface was first published in 1988 (version 3.0) and was designed to be sufficiently future proof to encompass advances in technology for a significant number of years.
What set the RISpec apart from other standards of the time was that it allowed using high-level geometric primitives, like quadrics or bicubic patches, to specify geometric primitives implicitly, rather than relying on a modeling application to generate polygons approximating these shapes explicitly beforehand.
The RenderMan shading language allows material definitions of surfaces to be described not only by adjusting a small set of parameters, but in an arbitrarily complex fashion by using a C-like programming language to write shading procedures commonly known as procedural textures and shaders.