C Processing the Scene Description

In the discussion of pbrt’s main() function in Section 1.3.2 at the start of the book, we wrote that after the command-line arguments are processed, the scene description is parsed and converted into corresponding Shapes, Lights, Materials, and so forth. Thereafter, as we have discussed the implementations of those classes, we have not worried about when they are created or where the parameter values for their constructors come from. This appendix fills in that gap and explains the path from human-readable scene description files to corresponding C++ objects in memory.

The scene description is processed in three stages, each of which is described in successive sections of this appendix:

• The text file format that describes the scene is parsed. Each statement in the file causes a corresponding method to be called in a ParserTarget class implementation.
• An instance of the BasicSceneBuilder class, which implements the ParserTarget interface, tracks graphics state such as the current material and transformation matrix as the file is parsed. For each entity in the scene (the camera, each light and shape, etc.), it produces a single object that represents the entity and its parameters.
• A BasicScene instance collects the objects produced by the BasicSceneBuilder and creates the corresponding object types that are used for rendering.

Once the BasicScene is complete, it is passed to either the RenderCPU() or RenderWavefront() function, as appropriate. Those functions then create the final representation of the scene that they will use for rendering. For most types of scene objects (e.g., the Sampler), both call a BasicScene method that returns the object that corresponds to what was specified in the scene description. Those two functions diverge in how they represent the intersectable scene geometry. In RenderCPU() as well as when the wavefront renderer is running on the CPU, the primitives and accelerators defined in Chapter 7 are used to represent it. With GPU rendering, shapes are converted to the representation expected by the GPU’s ray-tracing API.