Why?

The simple answer is that BSP''s subdivide space, making it fast to check collision. You only have to check collision with a few shapes rather than the entire level.

A regular PRO file has no such organization. It''s just a big list of triangles. It would be possible to check against every triangle in the list but it wouldn''t be fast enough for a game.

The BSP collision actually checks against the convex brushes that you create in the editor. These are shapes defined by planes similar to the polytope routines I have in PR, and aren''t the actual polygons drawn on the screen.

To get technical about it, the brush/polytope method stores sides of the convex shapes using the plane equation (Ax+By+Cz=d) and uses dot products to find the distance from the plane. This different representation is much faster to find collisions than using a ray/triangle intersection test.