Black Hole Portals
Question submitted by (15 November 1999)

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  I've just finished my very first portal renderer (more or less). There is, however, one bug that I don't really know how to get rid of. I clip my portals' vertices against the view frustum, so if you walk up really close to a portal, the portal covers the entire screen, but none of its vertices are inside the frustum! Hence, the engine thinks the portal can be clipped, and a black hole appears where the next sector used to be. Can you tell me how to catch this degenerate case?  

  If I am understanding you correctly, it sounds like you're not clipping your portals to the screen before you clip the polygons "behind" that portal to the portal itself.

However, if you're having problems where polygons that fill the screen end up not being clipped properly, you should read up on the 3D clipping document I wrote a while back. It's right here on flipCode.

Response provided by Paul Nettle

This article was originally an entry in flipCode's Fountain of Knowledge, an open Question and Answer column that no longer exists.


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