Their "search" algorithm is ray tracing. It must be, to some extend. They claim they only process each pixel on the screen, so they must trace a ray and find out which voxel/point/whatever to draw. The "unlimited" part breaks down as soon as you do any sort of reflection, refraction, or diffuse lighting. You can't just trace one ray for each screen pixel; you need to handle branching and scattering and the number of rays to trace becomes exponential.