Unfortunately, that's not possible in the current version - originally I had a demo with larger scope in mind, but WebGL turned out to be a lot more limited than I expected.

I wasn't able to implement an acceleration structure in WebGL, so the amount of primitives it can trace in real-time are quite limited, and all scenes had to be hardcoded in GLSL. This makes modifying scenes a real pain, since adding a new primitive amounts to generating and compiling new shader code.

The original C++ version just constructs the scene from very small (~1px long) line segments and then builds a BVH acceleration structure over them, so you have practically unlimited freedom in the type and number of primitives. Unfortunately traversing a BVH inside a shader in WebGL is almost not doable, so I had to improvise.