Brushes that meet, splitting polygons?

Quote:The Quake engine (that Half-Life uses a modified version of) splits polygons where two brushes meet, provided neither of them are entities. That is, if two brushes were to meet, it would effectively cut one of them into sections on the surface the brushes are touching, just so the textures can be applied effectively.

In the diagram, the far left image is what you would see in the game. If both brushes are worldspawn (ie, not entities) they are split by QBSP into the polygons shown by the centre image. However, should one of them be an entity (the smaller brush in this example) like a func_wall QBSP will not make the entity split the faces of other brushes, or be split itself, like in the third image.

Although the polygons for the smaller brush aren't shown for the third image, that doesn't mean they aren't drawn. Instead, they are drawn as entity polygons, separate from world polygons. Entity polygons include those on monsters and doors, and buttons etc. World polygons are solely non-entity brushes, the sky, and water that isn't created via func_water (rather a brush with only !blah textures such as !C2A5).

Use this rule as best you can - for example if you are adding small details (like several tiny little lights on a huge surface) make them func_wall or func_illusinary so they don't split the huge face they are attached to. This keeps r_speeds down.