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Occlusion Systems
Posted on Nov 20, 2007, 1:00 pm
In a Tech presentation, Al Hastings (CTO) shared the details of both the static and dynamic occlusion systems that we used for both Resistance: Fall of Man and Ratchet and Clank Future: Tools of Destruction.
Our static occlusion system is basically a straightforward grid-based PVS which has the advantage of being:
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Very fast to query at run-time - almost free
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Relatively low memory footprint
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Doesn't impose any restrictions on level construction techniques
But this method also comes with some disadvantages:
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Labor intensive and time intensive process
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Hard to keep it up-to-date during production
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Can't handle moving geometry, deforming geometry, destructible geometry
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Suffers from some over-inclusion (large ties are the biggest problem, compression error is a secondary problem)
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Monolithic database (hard to stream, etc)
Our dynamic occlusion system is another straightforward system. We pre-render the bounding volumes on the GPU to determine visibility. The advantages:
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Relatively lightweight system (rendering the bounding volumes usually takes a fraction of a millisecond)
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Works with any level design
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Typically it manages to cull that vast majority of objects that should be culled.
And the disadvantages:
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Basing occlusion on bounding volume visibility works poorly for some objects (long, thin mobys in particular)
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One frame lag issues
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Visibility spikes can occur after camera cuts and mode transitions
Check out Al's slides to find out more about how they're used and implemented in practice.
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