I've been a level designer for a few years, professionally only recently. The last three years were spent studying CSI. I never got into game programming but I understand a large amount of the programming side and theory because of my CSI background. Yet still I am unable to answer this concretely. It seems more and more BSP based level design/engines are becoming more and more useless, seeing Quake style engines rapidly disappearing. Obviously a mesh/model cached into video card memory will render much faster, but when an engine is entirely mesh the method of culling becomes questionable. I believe a heavy usage of mesh and a light mixture of BSP for culling, allowing for tons of detail in indoor areas with high frames, will never go instinct. However GPU's are becoming so fast that wasting that extra time rendering is becoming faster and faster it seems that it might even be faster then wasting CPU time deciding what to and what not to render. The answer I'm looking for is that BSP will never go away, not because I like level design with BSP, but because, and I say this at the end of my post, I am much more of a fan boy of the Quake engine and games such as half-life, and there is a growing rivalry with fans of the Unreal engine at our office. Arguments between my co-workers and I often lead to mindless debate and noob calling.

Hate to break it to you, but...
Yann Gives some great reasons why BSP's are out dated..

BSPs were great for rendering when there was no or little hardware Z-buffer support, because they can give you perfect Z-sorting for free when walking down the tree to render it.
Quake3 already dropped the fully convex leaves concept, but still kept BSPs, same for lots of other engines (unreal), I'm not sure why, probably because using something else would have meant making a whole new bunch of tools.

and about that mesh/culling, you really should use massive culling (frustum + occlusion), if you want highly detailed levels on today's hardware... thinking that a whole mesh for a whole level can run OK today is the symptom: "we've got fast CPUs and GPUs, so we don't need to bother optimizing". but if we do bother, we can display much higher quality, detailed graphics.

BSPs were also probably kept for collision detection, but as time goes on, they're becoming more and more useless and obsolete (it's been quite a while now in fact...)

the big problem with BSPs is that they also are completely static. of course you could rebuild at runtime a whole sub-branch of the tree... but it's damn slow, and that time could be spent doing some _useful_ things, using another more appropriate space partitionning technique, such as octrees, ABTs, KD-trees, whatever...

Quote:However GPU's are becoming so fast that wasting that extra time rendering is becoming faster and faster it seems that it might even be faster then wasting CPU time deciding what to and what not to render.

that's true for some things, but definitely not for a whole 500 000 triangles level :)
it'll be true if you partition too much and if you don't have enough triangles in each batch (and still, it's not the time needed to do a frustum test that will become annoying, it's the CPU overhead to render the batch, rendering 1000 10 triangles batches will be a _lot_ slower than rendering 10 1000 triangles batches, even without any culling tests whatsoever)

It probably has alot to do with their lighting engine. Per pixel lighting and alpha blended triangles have trouble with each other (from what I saw of the doom 3 lighting shaders). As for BSP, people forget BSP has alot more use than just what needs to be rendered and what doesn't. Things like sounds being in correct areas, lights only affecting rooms they are in, etc also is part of the BSP process. All the culling you do isn't just for what polygons need rendering, I think BSPs become more and more useful as time goes on, and with the Leafy-bsp system (ALA Quake 3) they're getting better too. BSPs may be static, but you can always put dynamic meshes within leaf areas, there's nothing against doing that.

btw, after reading ffx's post, I need to add a precision to my above post: when talking about BSPs, I referenced what the term "BSP" usually describes: quake-like leafy BSP trees, and not all binary space partitionning techniques of course.

GodBeastX> since Q3 (and maybe Q2), Id doesn't use convex leaves in their BSPs, and cannot use the sort order to sort transparent polygons. having strictly convex leaves in a doom3 level would produce far too many polygon splits, and far too many GCs/render calls.