The simulation of diffusion in 3d voxelized cell geometry can be simplified as at most three interactions between a random walker and membranes in each step, when the step size is smaller than the voxel size; the interactions include either an elastic collision with or a permeation through the membrane. This simplification is similar to the concept of the corner reflector (three consecutive coordinate reflections send vector x → −x, after which no wall can be encountered within the corner, representing a given voxel), which has been applied to radar sets, glass prisms, and bicycle reflectors.