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wanzi

HZB & HOM question

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I don''t understand why hierarchical maps are used( why not just one map?), say HZB, the lowest level of Z-buffer pyramid only stores the maxmum Z buffer of the whole map. Is it because that some faces can be rejected in lower map comparison, which is cheaper to performe? However it doesn''t garantee the faces rejected by this level are definately invisible.

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quote:
Original post by wanzi
I don''t understand why hierarchical maps are used( why not just one map?), say HZB, the lowest level of Z-buffer pyramid only stores the maxmum Z buffer of the whole map.
Is it because that some faces can be rejected in lower map comparison, which is cheaper to performe?
However it doesn''t garantee the faces rejected by this level are definately invisible.


You do it for performance, this way the cost of testing scales with O(log(p)) (where p is the numbre of pixels) instead of O(p).

There is no need to make sure the tested object is guaranteed hidden, after all you couldn''t do this even if you did all tests on the highest level (your term, normally the lowest level is considered the full resolution one) as long as you use some kind of bounding volume for testing.

The whole point of consetrvative visibility algorithms is to chepaly avoid doing exact visibility determination to increase performance. There''s no point in doing exact visibility determination twice.

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