The Shader Binding Table Demystified

Abstract

When a ray hits a geometry in the scene, the ray tracing hardware needs some way to determine which shader to call to perform intersection tests or shading calculations. The Shader Binding Table (SBT) provides this information. The SBT is a lookup table that associates each geometry in the scene with a set of shader function handles and parameters for these functions. Each set of function handles and parameters is referred to as a shader record. When a geometry is hit by a ray, a set of parameters specified on both the host and device sides of the application combine to determine which shader record is called. The three GPU ray tracing APIs (DirectX Raytracing, Vulkan KHR Ray Tracing, and OptiX) use the SBT to run user code when tracing rays in the scene. These APIs provide a great deal of flexibility as to how the SBT can be set up and indexed during rendering, providing a number of options to applications. However, this generality makes properly setting up the SBT an especially thorny part of these APIs for new users, as incorrect SBT access often leads to crashes or difficult bugs. Translating knowledge from one API to another can also be difficult due to subtle naming differences between them. This chapter looks at the similarities and differences of each API's SBT to develop a holistic understanding of the programming model. A few common configurations of the scene, SBT, and renderer are also discussed to provide guidance through hands-on examples.