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CS 551/651:
Visibility Culling Cont
David Luebke
[email protected]
http://www.cs.virginia.edu/~cs551dl
DPL
7/27/2016
Administrivia
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Hand back midterm for real
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Recap
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Visibility culling: don’t render what
can’t be seen
– Off-screen: view-frustum culling
– Z-buffered away: occlusion culling
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Potentially Visible Set (PVS)
– Conservative overestimate of visible
polygons
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Recap
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Cells & Portals
– Works well for architectural models
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Buildings, cities, catacombs…
– These divide naturally into cells
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Rooms, alcoves, corridors…
– Transparent portals connect cells
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Doorways, entrances, windows…
Key observation: cells only see each
other through portals!
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Recap
Create an adjacency graph of cells
 A cell is only visible if it can be seen
through a sequence of portals
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– Need a line of sight
– So cell visibility reduces to testing
portal sequences…
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Recap
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Airey: view-independent PVS
– Ray-casting for portal-portal visibility
– Slow, possibility of inaccuracies
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Recap
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Teller: view-independent PVS, viewdependent runtime refinement
– Line stabbing as linear program
– Accurate, elegant, slow
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Recap
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Luebke: view-dependent PVS
– Depth-first traversal of adjacency graph
Render cell w/ portals
 If portal is visible, render adjacent cell, but
clip to portal boundaries
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– Cull boxes overestimate portal visibility,
simplify portal intersection
– Fast, easy, somewhat conservative
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pfPortals
Q: How badly does the cull box
approximation overestimate PVS?
 Q: How to represent and intersect
cull boxes?
 Note: Can implement mirrors as
portals with an extra transformation!
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– Some clipping & Z-buffering issues
(Like what?)
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pfPortals
Show pictures (web page)
 Show video
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pfPortals
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Intriguing thought: extend pfPortals
to view-independent solutions
– pfPortals based on simple box to
overestimate portal in screenspace
– Idea: use 3-D box to overestimate
portal in world space
Portal-portal visibility now overestimated
by line-box stabbing
 Amenti: O(n) line-box intersection test
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Cells and Portals: Details
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Usually separate model into
occluders and detail objects
– Occluders: walls, floors
– Detail objects: desks, chairs, pencils
– Cell creation process only accounts for
occluders (Why?)
Compute detail object visibility
through portal sequences at run time
 Teller: also precompute into PVS
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Why View-Independent?
Q: If view-dependent techniques like
pfPortals can calculate a reasonable
PVS fast enough, why bother with
view-independent solutions?
 A: Intelligent prefetching
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– Soda Hall walkthrough (Funkhouser)
Whole model doesn’t fit in memory
 Use Teller stab trees to load in only cells
that might be visible
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Creating Cells and Portals
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Q: Given a model, how might you
extract the cells and portals?
– Airey: k-D tree (axis-aligned cells)
– Teller: BSP tree (general convex cells)
– Luebke: modeler (any cells at all)
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Problems and issues
– Running time
– Free cells
– Intra-wall cells
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Cells & Portals: Summary
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Architectural environments: mostly
considered a solved problem
– Got the “right” solution
– Got the good-enough hack
Imagine creating cells and portals for
a forest environment…
 Next up: general occlusion culling
using the hierarchical Z-buffer
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DPL
7/27/2016