subdivision subsampling: this is one of the main optimisations which allows it to run in hi-res and realtime. This allows detail to be calculated only where needed.

Hardware acceleration using OpenGL: The image is divided up into small squares, each of which is drawn as a textured quad in OpenGL, with the colour at each vertex determined by the ray-tracer lighting calculations. This is the other main optimisation.

Phong lighting: The lighting model is a simple ambient + diffuse + specular thing.

Geometry: Objects currently included are spheres, planes, rectangles, and cuboids.

Light flares: Somewhat gimmicky flares around the light sources, and around primary reflections of the light off objects in the scene. Done with an additive blended OpenGl quad sprite of course :). The positions of the flares are solved analytically.

Quick approximate Sphere intersect test: This is my own invention, an approximate test for primary ray/sphere intersection that requires only one dot product and then a scalar compare.

Another optimisation is that not all objects need to cast shadows. In this way the ground plane does not need to be involved in shadow checks.

Nice code: Code still uses operator overloading on my Vec3 class in most places and uses object-orientated polymorphism on the different types of primitives. == readable code. Of course this is slower but it shows that high-level algorithmic optimisations are sufficient to acheive hi-res realtime raytracing.