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Portal/sector visibility is based on 2D clipping. The 2D projected outline (in screen coordinates) of the last portal is used as a clipper for all geometry that can be seen through that portal.
This approach obviously has some disadvantages. It means we have to transform all geometry to camera space and then to screen space. Given the current advances in hardware (i.e. hardware accelerated transforms) this is no longer a good thing to do. In the future it might be possible to change this system to 3D clipping. For now this is a fundamental change and we'll try to get around this limitation in other ways.
Detail objects like 3D sprites are only clipped in total. i.e. the individual polygons are not clipped but either the object is visible or not. The Z-buffer takes care of the rest. To do this clipping we take the bounding box of the object and test if that bounding box is visible through the portal.
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