Visibility Culling David Luebke Computer Science Department University of Virginia

D2 Motivation Like most other rendering acceleration techniques, the goal is to avoid rendering redundant geometryLike most other rendering acceleration techniques, the goal is to avoid rendering redundant geometry The basic idea: don’t render what can’t be seenThe basic idea: don’t render what can’t be seen –Off-screen: view-frustum culling –Occluded by other objects: occlusion culling

D3 Motivation The obvious question: why bother?The obvious question: why bother? –Off-screen geometry: solved by clipping –Occluded geometry: solved by Z-buffer The (obvious) answer: efficiencyThe (obvious) answer: efficiency –Clipping and Z-buffering take time linear to the number of primitives

D4 The Goal Our goal: quickly eliminate large portions of the scene which will not be visible in the final imageOur goal: quickly eliminate large portions of the scene which will not be visible in the final image –Not the exact visibility solution, but a quick-and-dirty conservative estimate of which primitives might be visible Z-buffer& clip this for the exact solution Z-buffer& clip this for the exact solution –This conservative estimate is called the potentially visible set –This conservative estimate is called the potentially visible set or PVS

D5 Visibility Culling The remainder of this talk will cover:The remainder of this talk will cover: –View-frustum culling (briefly) –Occlusion culling in architectural environments –General occlusion culling

D6 View-Frustum Culling An old idea (Clark 76):An old idea (Clark 76): –Organize primitives into clumps –Before rendering the primitives in a clump, test a bounding volume against the view frustum If the clump is entirely outside the view frustum, don’t render any of the primitives If the clump is entirely outside the view frustum, don’t render any of the primitives If the clump intersects the view frustum, add to PVS and render normally If the clump intersects the view frustum, add to PVS and render normally

D7 Efficient View-Frustum Culling How big should the clumps be? How big should the clumps be? –Choose minimum size so: cost testing bounding volume << cost clipping primitives –Organize clumps into a hierarchy of bounding volumes for more efficient testing If a clump is entirely outside or entirely inside view frustum, no need to test its children If a clump is entirely outside or entirely inside view frustum, no need to test its children

D8 Efficient View-Frustum Culling What shape should bounding volumes be? What shape should bounding volumes be? –Spheres and axis-aligned bounding boxes: simple to calculate, cheap to test – Oriented bounding boxes converge asymptotically faster in theory –Lots of other volumes have been proposed, but most people still use spheres or AABBs.

D9 Cells & Portals Goal: walk through architectural models (buildings, cities, catacombs)Goal: walk through architectural models (buildings, cities, catacombs) These divide naturally into cellsThese divide naturally into cells –Rooms, alcoves, corridors… Transparent portals connect cellsTransparent portals connect cells –Doorways, entrances, windows… Notice: cells only see other cells through portalsNotice: cells only see other cells through portals

D10 Cells & Portals An example:An example:

D11 Cells & Portals Idea:Idea: –Cells form the basic unit of PVS –Create an adjacency graph –Create an adjacency graph of cells –Starting with cell containing eyepoint, traverse graph, rendering visible cells –A cell is only visible if it can be seen through a sequence of portals So cell visibility reduces to testing portal sequences for a line of sight… So cell visibility reduces to testing portal sequences for a line of sight…

D12 Cells & Portals A D H F CB E G H BCDFG EA

D13 Cells & Portals A D H F CB E G H BCDFG EA

D14 Cells & Portals A D H F CB E G H BCDFG EA

D15 Cells & Portals A D H F CB E G H BCDFG EA

D16 Cells & Portals A D H F CB E G H BCDFG EA

D17 Cells & Portals A D H F CB E G H BCDFG EA ? ?

D18 Cells & Portals A D H F CB E G H BCDFG EA X X

D19 Cells & Portals View-independent solution: find all cells a particular cell could possibly see: View-independent solution: find all cells a particular cell could possibly see: C can only see A, D, E, and H A D H F CB E G A D H E

D20 Cells & Portals View-independent solution: find all cells a particular cell could possibly see: View-independent solution: find all cells a particular cell could possibly see: H will never see F A D H F CB E G A D CB E G

D21 Cells and Portals Questions:Questions: – How can we detect whether a given cell is visible from a given viewpoint? – How can we detect view-independent visibility between cells? The key insight :The key insight : –These problems reduce to eye-portal and portal-portal visibility

D22 Cells and Portals Airey (1990): view-independent onlyAirey (1990): view-independent only –Portal-portal visibility determined by ray-casting Non-conservative portal-portal test resulted in occasional errors in PVS Non-conservative portal-portal test resulted in occasional errors in PVS –Slow preprocess –Order-of-magnitude speedups

D23 Cells and Portals Teller (1993): view-independent + view-dependentTeller (1993): view-independent + view-dependent –Portal-portal visibility calculated by line stabbing using linear program Cell-cell visibility stored in stab trees Cell-cell visibility stored in stab trees View-dependent eye-portal visibility stage further refines PVS at run time View-dependent eye-portal visibility stage further refines PVS at run time –Slow preprocess –Elegant, exact scheme

D24 Cells and Portals Luebke (1995): view-dependent onlyLuebke (1995): view-dependent only –Eye-portal visibility determined by intersecting portal cull boxes –No preprocess (integrate w/ modeling) –Quick, simple hack –Public-domain library: pfPortals

D25 pfPortals Algorithm Depth-first adjacency graph traversalDepth-first adjacency graph traversal –Render cell containing viewer –Treat portals as special polygons If portal is visible, render adjacent cell If portal is visible, render adjacent cell But clip to boundaries of portal!But clip to boundaries of portal! Recursively check portals in that cell against new clip boundaries (and render) Recursively check portals in that cell against new clip boundaries (and render) –Each visible portal sequence amounts to a series of nested portal boundaries Kept implicitly on recursion stack Kept implicitly on recursion stack

D26 pfPortals Algorithm Recursively rendering cells while clipping to portal boundaries not newRecursively rendering cells while clipping to portal boundaries not new –Visible-surface algorithm (Jones 1971): general polygon-polygon clipping Elegant, expensive, complicated Elegant, expensive, complicated –Conservative overestimate (pfPortals): use portal’s cull box Cull box = x-y screenspace bounding box Cull box = x-y screenspace bounding box Cheap to compute, very cheap (constant time) to intersect Cheap to compute, very cheap (constant time) to intersect

D27 pfPortals Algorithm How badly does the cull box approximation overestimate PVS? How badly does the cull box approximation overestimate PVS? –Not much for most architectural scenes Note: Can implement mirrors as portals with an extra transformation!Note: Can implement mirrors as portals with an extra transformation! –Some clipping & Z-buffering issues

D28 Cells and Portals: Details Usually separate model into occluders and detail objectsUsually separate model into occluders and detail objects –Occluders: walls, floors –Detail objects: desks, chairs, pencils –Cell creation process only accounts for occluders ( Why? ) pfPortals: find detail object visibility through portal sequences at run timepfPortals: find detail object visibility through portal sequences at run time Teller: also precompute into PVSTeller: also precompute into PVS

D29 Why View-Independent? If view-dependent techniques can often calculate a reasonable PVS fast enough, why bother with view-independent solutions? If view-dependent techniques can often calculate a reasonable PVS fast enough, why bother with view-independent solutions? One good answer: smart prefetchingOne good answer: smart prefetching –Soda Hall walkthrough (Funkhouser) Whole model doesn’t fit in memory Whole model doesn’t fit in memory Use Teller stab trees to load in only cells that might be visible Use Teller stab trees to load in only cells that might be visible

D30 Creating Cells and Portals Given a model, how might you extract the cells and portals? Given a model, how might you extract the cells and portals? –Airey: k-D tree (axis-aligned boxes) –Teller: BSP tree (general convex cells) –Luebke: modeler (any cells at all) Problems and issuesProblems and issues –Running time –Free cells –Intra-wall cells

D31 Cells and Portals: Discussion Good solution for most architectural or urban modelsGood solution for most architectural or urban models –Use the simplest algorithm that suffices for your needs: pfPortals-style algorithm: view-dependent solution, reasonably tight PVS, no preprocess necessary (except partition) pfPortals-style algorithm: view-dependent solution, reasonably tight PVS, no preprocess necessary (except partition) Teller-style algorithm: tighter PVS, somewhat more complex, can provide view-independent solution for prefetching Teller-style algorithm: tighter PVS, somewhat more complex, can provide view-independent solution for prefetching

D32 Cells and Portals: Discussion Public-domain code I’m aware of:Public-domain code I’m aware of: –pfPortals: A very simple set of Performer callbacks that implements cull-box portal culling A very simple set of Performer callbacks that implements cull-box portal culling –pfWalkthru: Includes code to extract cells and portals Includes code to extract cells and portals –Game engine sites Lots of “level builders” and “level compilers” Lots of “level builders” and “level compilers” Treat these with a grain of salt Treat these with a grain of salt