"What, exactly, does VIS do with a BSP, and what affects the VIS calculation time?"

- nofxsux

...BSP, or the binary space partitioner, breaks the level up into bounding polygons and the areas that these polygons define. The VIS process calculates what is visible from evry possible player position in the level. It determines "portals", or viewing positions, and everything that can be seen through each of them. VIS greatly increases the performance (framerate) and memory usage of the level. You've probably noticed the decrease in speed of a level that has not been VIS'ed and also the "graying-out" of more distant areas. Of the three major processes in turning a level text file into a runnable map file (BSP, VIS, and LIGHT), VIS'ing definitely takes the longest time.

VIS'ing is greatly hampered by big, open areas. It has to spend much more time calculating all the permutations of the multiple portals in these large areas and this takes alot of time. You'll notice most Quake levels are set in interior rooms with tight, winding hallways. I tried, in our pack, to create more expansive areas because I love rocket fights in them. I paid the price, though, in that most of my levels took 6-18 hours to VIS on a dual Pentium with 128 meg of RAM!

- Levelord