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Difficulty

1

Reading material

primitives/bounding-box-accelerator

1. Implementation

  • Create the files primitives/bbacc.cpp and primitives/bbacc.h.

  • Define a new PrimitiveImplementation subclass.

    • Its constructor should accept a Primitive, which it stores in a field.

  • Implement find_all_hits as described in the reading material.

  • Define a factory method Primitive raytracer::primitives::bounding_box_accelerator(Primitive).
Tip

You don’t want to repeatedly recompute the child’s bounding box. Have the BoundingBoxAccelerator's constructor compute it once and cache it in a field.

2. Evaluation

Visually, a bounding box accelerator should have no effect. Its sole purpose is to speed up rendering. In other words, in order to find out whether your implementation of BoundingBoxAccelerator is correct, you need to set up a scene that greatly benefits from bounding box accelerators.

Say you have a scene with many (thousands) of spheres. Under normal circumstances, the ray intersection algorithm would iterate over all these sphere, one by one, and check whether a given ray hits any one of them. The more spheres there are, the slower this will be.

What would happen if we were to put all these spheres in one big bounding box accelerator? Well, if those spheres are actually visible on the rendering, not much: the rays would hit the bounding box and again we would have to check all spheres one by one. If, however, we can make it so that rays don’t hit the bounding box, thereby avoiding the need to process each sphere in turn, we can speed up rendering a lot.

So, create thousands of spheres, but place them above (or below, whichever you fancy the most) the camera.

  • Without bounding box accelerator, the ray tracer will check each sphere in turn and realize none of them gets hit by the ray.

  • If all spheres are enclosed within a bounding box accelerator, the ray tracer will only check the bounding box, find out that the ray misses it, and won’t bother looking at the spheres.

For example, on our machine, rendering 10,000 spheres took approximately 30 seconds without bounding box accelerator and half a second with one.

Create two scripts that only differ in whether a bounding box accelerator is present or not. Render both and compare rendering times.