Shading Language Overview

Programmable Graphics Processing Units (GPUs) have become widespread. They now support floating-point computations and general programming models. A shading language is a domain-specific programming language for specifying shading computations. In this section of the course notes, we will review several high-level real-time shading languages for programming modern GPUs. These languages have evolved from academic experiments into necessary tools for real-time graphics. Historically, the idea of a shading language is often credited to Cook [3]. Pixar’s RenderMan shading language was developed shortly thereafter and has become a standard in the offline domain [4, 14, 1, 11]. The RenderMan shading language has strongly influenced the design of modern real-time shading languages, particularly the idea of uniform and varying parameters. However, the RenderMan standard, although originally intended as a hardware API, is no longer used as such, and modern GPU shading languages need to target the hardware architecture of modern GPUs. The SGI Interactive Shading Language [10] compiles a shader specification to a multipass implementation, but does not generate complex shader kernels, only primitive passes. The Stanford Real-Time Shading Language was one of the first attempts to build a shading language specifically designed for the shading units of graphics processors [12, 7]. It is so far unique in that it supported a single shader for programming both vertex and fragment units. NVIDIA’s Cg [6] language was one of the earlier commercially available shading languages to run on commodity GPU hardware. Although developed by NVIDIA, it supports multiple hardware platforms, and is the only commercial shading language to support both DirectX and OpenGL. It is related to a number of other tools, including CgFX, which is used to specify multipass algorithms. Microsoft’s HLSL for DirectX was originally similar to Cg but has since then diverged. The Direct3D Effects system is similar to CgFX, and permits the specification of multipass algorithms. The OpenGL standards group recently approved the OpenGL 2.0 Shading Language [5, 13], frequently called GLSL. This shading language will be integrated into the next generation of the OpenGL API and is intended to become the standard-supported way to program GPUs under the OpenGL API.