Add CSG to your ray tracer: Assume that your scene is built of solid "basic" objects, such as the solid interiors of spheres and ellipsoids (or even cylinders, and cubes). You can define new derived volumetric objects as the volumetric unions, differences, and intersections of these basic objects. This representation is called constructive solid geometry (CSG). You can continue recursively and define CSG objects applying set operations to other CSG objects. In full generality, a CSG object is defined by an expression tree with set operations in the interior nodes and basic objects at the leaves. One can easily ray trace a CSG object, as long one has code that computes ray intersections with the basic objects. When computing the intersection of a ray with a basic object, one stores not just the point of nearest intersection but instead computes and stores the entire interval of intersection. This interval starts where the ray enters the object and ends where the ray exits the object. To compute the ray intersection with, say, the intersection of two basic objects (each represented with its own interval), one simply compute the interval-intersections of the two input intervals. This same idea can be applied to a general CSG expression tree. (Note that a general CSG object may be non-convex and thus the intersection between a ray and a general CSG object may be composed of several intersection intervals.)