In this chapter, we will present pbrt’s abstraction for geometric primitives such as spheres and triangles. Careful abstraction of geometric shapes in a ray tracer is a key component of a clean system design, and shapes are the ideal candidate for an object-oriented approach. All geometric primitives implement a common interface, and the rest of the renderer can use this interface without needing any details about the underlying shape. This makes it possible to separate the geometric and shading subsystems of pbrt.
pbrt hides details about primitives behind a two-level abstraction. The Shape class provides access to the raw geometric properties of the primitive, such as its surface area and bounding box, and provides a ray intersection routine. The Primitive class encapsulates additional nongeometric information about the primitive, such as its material properties. The rest of the renderer then deals only with the abstract Primitive interface. This chapter will focus on the geometry-only Shape class; the Primitive interface is a key topic of Chapter 4.