The early-out feature of the beam tree - stopping when the beam tree becomes solid - seems appealing, because it appears to cap worst-case performance. Unfortunately, there are still scenes where it’s possible to see all the way to the sky or the back wall of the world, so in the worst case, all polygons in the frustum will still have to be tested against the beam tree. Similar problems can arise from tiny cracks due to numeric precision limitations. Beam tree clipping is fairly time-consuming, and in scenes with long view distances, such as views across the top of a level, the total cost of beam processing slowed Quake’s frame rate to a crawl. So, in the end, the beam-tree approach proved to suffer from much the same malady as the painter’s algorithm: The worst case was much worse than the average case, and it didn’t scale well with increasing level complexity.
Once the beam tree was working, John [Carmack] relentlessly worked at speeding up the 3-D engine, always trying to improve the design, rather than tweaking the implementation. At least once a week, and often every day, he would walk into my office and say "Last night I couldn’t get to sleep, so I was thinking..." and I’d know that I was about to get my mind stretched yet again. John tried many ways to improve the beam tree, with some success, but more interesting was the profusion of wildly different approaches that he generated, some of which were merely discussed, others of which were implemented in overnight or weekend-long bursts of coding, in both cases ultimately discarded or further evolved when they turned out not to meet the design criteria well enough. Here are some of those approaches, presented in minimal detail in the hopes that, like Tom Wilson with the Paradise FIFO, your imagination will be sparked.
In other heavy-nerd news today, a paper on Google's massively-parallel MapReduce architecture.