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A sector is the basic building block in Crystal Space. A world will be made out of several sectors connected with portals (see section Portal Engine). Normally adjacent sectors can be connected with a straight portal but portals can also warp space so that you can connect any point in space to any other point in space. This effect can be used to create mirrors for example.
This effect of portals makes them very powerful. It also means that you can have overlapping sectors and in general other weird portal constructs.
Portals starting in arbitrary space (i.e. in the middle of a sector) are called floating portals. Some special care needs to be taken for those because we can't rely on the Z-buffer to be useful here. So this is a case where clipping to the 2D boundary of the portal must happen.
Important for this discussion is the fact that we have two important areas where culling happens. We have in-sector visibility culling which is all visibility testing that ignores portal boundaries and we have portal/sector visibility which controls what you can see through a portal. Discussing both will be the main focus of this document later.
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