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BSP and CSG Engines.

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BSP involves creating your world out of planes. In practice, map editors let you work with a system closer to CSG, and then you "compile" the map into BSP.

CSG involves creating your world out of solids and fluids ("gasses", like air, are fluids) of varying primitive shapes (spheres, cubes, cylinders, etc). In practice we allow people to deform these primitives, and then we triangularize them at some point.

I know how Valve did their system for Half-Life, but I'm not familiar with the internal structure of the Unreal engine, so I can't really compare them.

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Are you talking about the original Half-Life? If so, its editor also used CSG-based .map files, like all other quake derivatives.
I don't know about Half-Life 2, but if it doesn't use CSG, it probably just generated the BSP-tree from a polygon/triangle-soup.

CSG-based editor pros:

- It's pretty easy to model
- You're pretty sure that the input data for the BSP generator is well-behaved
- There's a large selection of editors you can use for your game

CSG-based editor cons:

- It takes some extra work to read, parse, and calculate the CSG (it's important to get that piece of code very stable)
- Your a bit limited when it comes to modelling high-detail levels

Polygon-based editor pros:

- You can use 3dsmax or a similar modelling program to make your levels.
- You can create any kind of geometry with pure polygon modelling

Polygon-based editor cons:

- You need to be very careful not to create geometry that will upset your BSP generator. For instance it can be quite a job to keep all polygons and stuff perfectly aligned.
- BSP-trees are not suited for high-detail geometry, and if you're using 3dsmax (for example) you'll probably quickly end up with a lot of polygons.

Well, that was what I could come up with right now :)

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