Visibility Culling II: Beyond Cells & Portals David Luebke Computer Science Department University of Virginia

D2 Review: 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…

D3 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA

D4 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA

D5 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA

D6 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA

D7 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA

D8 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA ? ?

D9 Cells & Portals: View-Dependent Solution A D H F CB E G H BCDFG EA X X

D10 Cells & Portals: View-Independent Solution 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

D11 Cells & Portals: View-Independent Solution 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

D12 Review: 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

D13 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.cs.virginia.edu

D14 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

D15 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

D16 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 –Must limit recursion

D17 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

D18 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

D19 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

D20 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

D21 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

D22 General Occlusion Culling When cells and portals don’t work…When cells and portals don’t work… –Trees in a forest –A crowded train station Need general occlusion culling algorithms:Need general occlusion culling algorithms: –Aggregate occlusion –Dynamic scenes –Non-polygonal scenes

D23 General Occlusion Culling I’ll discuss three algorithms:I’ll discuss three algorithms: –Loose front-to-back sorting –Hierarchical Z-Buffer Ned Greene, SIGGRAPH 93 Ned Greene, SIGGRAPH 93 –Hierarchical Occlusion Maps Hansong Zhang, SIGGRAPH 97 Hansong Zhang, SIGGRAPH 97 I’ll also describe current hardware supportI’ll also describe current hardware support

D24 Loose Front-To-Back Sorting Can sort your geometry in roughly front-to- back order, e.g. by:Can sort your geometry in roughly front-to- back order, e.g. by: –Using an octree/BSP tree –Sorting centroids or near points of bounding volumes Why would this help? Why would this help? –A: Early rejection helps whole fragment pipeline Why might this be hard? Why might this be hard? –A: could conflict with sorting by render state

D25 Image-Space Occlusion Culling Most general occlusion culling algorithms use an image-space approachMost general occlusion culling algorithms use an image-space approach Idea: solve visibility in 2D, on the image planeIdea: solve visibility in 2D, on the image plane

D26 Hierarchical Z-Buffer Replace Z-buffer with a Z-pyramidReplace Z-buffer with a Z-pyramid –Lowest level: full-resolution Z-buffer –Higher levels: each pixel represents the maximum depth of the four pixels “underneath” it Basic idea: hierarchical rasterization of the polygon, with early termination where polygon is occludedBasic idea: hierarchical rasterization of the polygon, with early termination where polygon is occluded

D27 Hierarchical Z-Buffer Idea: test polygon against highest level firstIdea: test polygon against highest level first –If polygon is further than distance recorded in pixel, stop—it’s occluded –If polygon is closer, recursively check against next lower level –If polygon is visible at lowest level, set new distance value and propagate up

D28 Hierarchical Z-Buffer Z-pyramid exploits image-space coherence :Z-pyramid exploits image-space coherence : –Polygon occluded in a pixel is probably occluded in nearby pixels HZB also exploits object-space coherenceHZB also exploits object-space coherence –Polygons near an occluded polygon are probably occluded

D29 Hierarchical Z-Buffer Exploiting object-space coherence:Exploiting object-space coherence: –Subdivide scene with an octree –All geometry in an octree node is contained by a cube –Before rendering the contents of a node, “render” the faces of its cube (i.e., query the Z-pyramid) –If cube faces are occluded, ignore the entire node

D30 Hierarchical Z-Buffer HZB can exploit temporal coherenceHZB can exploit temporal coherence –Most polygons affecting the Z-buffer last frame will affect Z-buffer this frame –HZB also operates at max efficiency when Z-pyramid already built So start each frame by rendering octree nodes visible last frameSo start each frame by rendering octree nodes visible last frame

D31 Hierarchical Z-Buffer: Discussion HZB needs hardware support to be really competitiveHZB needs hardware support to be really competitive Hardware vendors haven’t entirely bought in:Hardware vendors haven’t entirely bought in: –Z-pyramid (and hierarchies in general) unfriendly to hardware –Unpredictable Z-query times generate bubbles in rendering pipe But there is a promising trend…But there is a promising trend…

D32 Hierarchical Z-Buffer Recent hardware supports Z-query operationRecent hardware supports Z-query operation –Allows systems to exploit: Object-space coherence (bounding boxes) Object-space coherence (bounding boxes) Temporal coherence (last-rendered list) Temporal coherence (last-rendered list) –Examples in OpenGL: HP_OCCLUSION_QUERY HP_OCCLUSION_QUERY NV_OCCLUSION_QUERY NV_OCCLUSION_QUERY Go to NVIDIA occlusion presentation… Go to NVIDIA occlusion presentation… –An aside: applies to cell-portal culling!