The basic concepts of beam-tracing were presented by Stephenson in
1996 [1], but the first functional implementation
was published by Funkhouser et al in 1998 [2], [3].
The improved image-source technique by Mechel has similar approach than
beam-tracing, but it still is closer to the original image-source
technique [4]. Further optimization on the technique has been presented e.g. by Laine et al [5].