HL2 Map Design Question: Brush or Mesh?

[zaphod]

yea, I'm sure it was for a variety of reasons, probably to try and make vis information more accurate mostly. Hovever the engine uses portals for more than just calculating the PVS, for instance - sorting 32 transparent textures. If you have two 32 bit transparencies in the same leaf, often times the sorting will be incorrect, the engine sorts them by which leaf is closer to you. I have had a lot of instances where I would have to correct sorting issues by hand by putting a hint brush to make a bsp cut between two transparent objects.