Afrigraph 2004 Tutorial A: Part I Rasterization Based Approaches Andreas Dietrich Computer Graphics Group, Saarland University Saarbrücken, Germany

Afrigraph 2004 Tutorial A:Afrigraph 2004 Tutorial A:

Part I Part I Rasterization Rasterization BBased Approachesased Approaches

Andreas DietrichAndreas Dietrich

Computer Graphics Group, Saarland UniversityComputer Graphics Group, Saarland University

Saarbrücken, GermanySaarbrücken, Germany

Afrigraph 2004Afrigraph 2004 State of the Art in Massive Model VisualizationState of the Art in Massive Model Visualization 22

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OverviewOverview

• Part I – Rasterization Based ApproachesPart I – Rasterization Based Approaches– Visibility CullingVisibility Culling

- Hierarchical Z-BufferHierarchical Z-Buffer- Hierarchical Occlusion MapsHierarchical Occlusion Maps- Prioritized-Layered ProjectionPrioritized-Layered Projection

– Simplification TechniquesSimplification Techniques- LODs / HLODs,LODs / HLODs,- Textured Depth MeshesTextured Depth Meshes

– Existing ArchitecturesExisting Architectures- MMRMMR- GigawalkGigawalk- iWalkiWalk


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OverviewOverview

• Part I – Rasterization Based ApproachesPart I – Rasterization Based ApproachesVisibility CullingVisibility Culling





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Visibility CullingVisibility Culling

• Large scenes often densely occludedLarge scenes often densely occluded– Only a fraction of the total dataset visibleOnly a fraction of the total dataset visible

Visibility cullingVisibility culling– Try to find the Try to find the visible setvisible set

i.e. objects that contribute to the imagei.e. objects that contribute to the image– Goal:Goal:

- Rejecting large parts of the scene Rejecting large parts of the scene beforebefore actual HSR actual HSR- Reduce rendering cost to complexity of visible portionReduce rendering cost to complexity of visible portion- Ideally Ideally output sensitive output sensitive ::

Running time proportional to visible set sizeRunning time proportional to visible set size


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Visibility CullingVisibility Culling

• Culling techniquesCulling techniques– View-frustumView-frustum cullingculling

- Reject geometryReject geometryoutside the viewingoutside the viewingvolumevolume

– Back-faceBack-face cullingculling- Reject geometryReject geometry

facing away fromfacing away fromthe observerthe observer

– OcclusionOcclusion cullingculling- Reject objectsReject objects

occluded by othersoccluded by others


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Visibility CullingVisibility CullingOcclusion CullingOcclusion Culling

• Occlusion cullingOcclusion culling– Not as trivial as view-frustum or back-face Not as trivial as view-frustum or back-face

cullingculling– Often requires preprocessingOften requires preprocessing– Usually involving some scene hierarchyUsually involving some scene hierarchy

- Occlusion tests performed top-downOcclusion tests performed top-down

– Difference to Difference to Hidden surface removalHidden surface removal (HSR) (HSR)- Does not identify exact potion of visible polygonsDoes not identify exact potion of visible polygons- Tries to identify objects Tries to identify objects notnot visible visible- Often exact HSR follows after culling stepOften exact HSR follows after culling step

– However, distinction not that clearHowever, distinction not that clear- Some HSR algorithms feature built-in occlusion cullingSome HSR algorithms feature built-in occlusion culling

e.g. Ray casting (see Part II)e.g. Ray casting (see Part II)


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• Main classification [Cohen-Or 03]Main classification [Cohen-Or 03]– From-pointFrom-point methods methods

- Computation with respect to current viewpointComputation with respect to current viewpoint– Image precisionImage precision variants: Operate on fragments variants: Operate on fragments– Object precisionObject precision variants: Operate on raw objects variants: Operate on raw objects

– From-regionFrom-region methods methods- Bulk computations valid for a specific regionBulk computations valid for a specific region

– Cell-and-portalCell-and-portal variants: Exploit scene characteristics variants: Exploit scene characteristics– Generic sceneGeneric scene variants: Work with arbitrary scenes variants: Work with arbitrary scenes


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• Additional classification criteria [Cohen-Or Additional classification criteria [Cohen-Or 03]03]– Conservative vs. approximate techniquesConservative vs. approximate techniques– Tightness of approximationTightness of approximation– All objects vs. subset of occludersAll objects vs. subset of occluders– Convex vs. generic occludersConvex vs. generic occluders– Individual vs. fused occludersIndividual vs. fused occluders– 2D vs. 3D2D vs. 3D– Special hardware requirementsSpecial hardware requirements– Need of precomputationNeed of precomputation– Dynamic scenesDynamic scenes


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Visibility CullingVisibility CullingHierarchical Z-BufferHierarchical Z-Buffer

• Hierarchical Z-Buffer (HZB) [Greene 93]Hierarchical Z-Buffer (HZB) [Greene 93]– ExploitsExploits object-space object-space coherence: Octree subdivision coherence: Octree subdivision– ExploitsExploits Image-space Image-space coherence: Z-pyramid coherence: Z-pyramid

• Octree used forOctree used for– View-frustum cullingView-frustum culling– Hierarchic top-down rendering / occlusionHierarchic top-down rendering / occlusion– Front-back renderingFront-back rendering

• Z-PyramidZ-Pyramid– Use original Z-buffer as finest levelUse original Z-buffer as finest level– Combine 2x2 samples by choosing farthest Z valueCombine 2x2 samples by choosing farthest Z value


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Visibility CullingVisibility CullingHierarchical Z-BufferHierarchical Z-Buffer

• Z-pyramid visibility queryZ-pyramid visibility query– Polygon visibility test:Polygon visibility test:

1.1. Find finest-level pyramid sample coveringFind finest-level pyramid sample coveringscreen-space bounding box of polygonscreen-space bounding box of polygon

2.2. If nearest polygon Z value farther away than sample Z If nearest polygon Z value farther away than sample Z valuevalue

Polygon hiddenPolygon hidden

Otherwise subdivide polygon and recurseOtherwise subdivide polygon and recurse

– Allows for fast octree node occlusion query:Allows for fast octree node occlusion query:1.1. Test projected octree node facesTest projected octree node faces

2.2. If node is hiddenIf node is hidden All polygons inside hiddenAll polygons inside hidden

Otherwise subdivide node and recurseOtherwise subdivide node and recurse


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Visibility CullingVisibility CullingHierarchical Occlusion Hierarchical Occlusion

MapsMaps• Hierarchical Occlusion Map (HOM) [Zhang Hierarchical Occlusion Map (HOM) [Zhang

97]97]– Pixels record opacity of screen space regionsPixels record opacity of screen space regions– Construction:Construction:

1.1.Select occluders:Select occluders:E.g. visible objects from previous frameE.g. visible objects from previous frame

2.2.Render occluders:Render occluders:Pure white pixels on black backgroundPure white pixels on black background

3.3.Form next coarser level:Form next coarser level:Average 2x2 pixel regionsAverage 2x2 pixel regions


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Visibility CullingVisibility CullingHierarchical Occlusion Hierarchical Occlusion

MapsMaps• Object occlusion testObject occlusion test

1.1. Find hierarchy level with pixels approximately the sameFind hierarchy level with pixels approximately the samesize as screen-space object bounding boxsize as screen-space object bounding box

2.2. Examine each pixel in map overlapping bounding Examine each pixel in map overlapping bounding rectangle:rectangle:If all pixel completely opaqueIf all pixel completely opaque

Objects projection inside occludersObjects projection inside occluders Z-test:Z-test:

- Single Z-plane behind all occludersSingle Z-plane behind all occluders- Depth estimation bufferDepth estimation buffer

(Z-planes for separate screen regions)(Z-planes for separate screen regions)

Otherwise check next level for not completely opaque Otherwise check next level for not completely opaque pixelspixels

Use Use transparency thresholdtransparency threshold to terminate recursion to terminate recursion Render object using Z-bufferRender object using Z-buffer


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Visibility CullingVisibility CullingPLPPLP

• Prioritized-Layered Projection [Klosowski 00]Prioritized-Layered Projection [Klosowski 00]– Approximate occlusion cullingApproximate occlusion culling

- Estimates visible primitivesEstimates visible primitives- Renders the primitives most likely visible up to a given Renders the primitives most likely visible up to a given

budgetbudget

– Scene partitioned into cellsScene partitioned into cells– Basic Idea:Basic Idea:

Cells containing much geometry are likely to occlude Cells containing much geometry are likely to occlude other cellsother cells

Render cells front-to-back in layersRender cells front-to-back in layersUse probabilistic values to prioritize cell rendering Use probabilistic values to prioritize cell rendering

order order (depending on viewpoint settings)(depending on viewpoint settings)


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• Priority-Based Cell TraversalPriority-Based Cell Traversal– Maintain priority queue (Maintain priority queue (front front ))

- Contains cells to be rendered nextContains cells to be rendered next- Front „advances“ from the viewpoint into the view Front „advances“ from the viewpoint into the view

frustum:frustum:1.1. Remove cell (depending on priority) from the frontRemove cell (depending on priority) from the front

2.2. Render it (using z-Buffer)Render it (using z-Buffer)

3.3. Add adjacent cells to the front with Add adjacent cells to the front with updatedupdated priority priority

4.4. Continue until triangle budget reachedContinue until triangle budget reached


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• Priority-Based Cell TraversalPriority-Based Cell Traversal– As priority use As priority use soliditysolidity

- Heuristic to determine how difficult it is to see a Heuristic to determine how difficult it is to see a particular cellparticular cell

- No inherent property of a cellNo inherent property of a cell- Accumulated during rendering, depending on viewpointAccumulated during rendering, depending on viewpoint

– Solidity is transferSolidity is transferrred to neighboring cellsed to neighboring cells


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OverviewOverview



Simplification TechniquesSimplification Techniques- LODs / HLODs,LODs / HLODs,- Textured Depth MeshesTextured Depth Meshes



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SimplificationSimplificationPolygonal SimplificationPolygonal Simplification

• Idea:Idea:– Simplify small or distant model parts Simplify small or distant model parts withoutwithout

significant loss in the scene‘s visual appearancesignificant loss in the scene‘s visual appearance– Switch at runtime between different complexity Switch at runtime between different complexity

levelslevels

Reduce I/O bandwidthReduce I/O bandwidthImprove runtime performanceImprove runtime performance


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• ClassificationClassification– Static simplificationStatic simplification

- Offline computation of discrete versions of each object:Offline computation of discrete versions of each object:Levels-of-DetailLevels-of-Detail (LODs) (LODs)

– Dynamic simplificationDynamic simplification- Data structures encoding a continuous detail spectrumData structures encoding a continuous detail spectrum

– View-dependent simplificationView-dependent simplification- Single objects can span multiple simplification levelsSingle objects can span multiple simplification levels


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• TechniquesTechniques– SamplingSampling

- Sample initial model e.g. with points on surfaceSample initial model e.g. with points on surface

– Adaptive subdivisionAdaptive subdivision- Find base mesh approximating initial model and subdivideFind base mesh approximating initial model and subdivide

– DecimationDecimation- Remove vertices and retriangulate resulting holesRemove vertices and retriangulate resulting holes

– Vertex-mergingVertex-merging- Collapse two or more vertices,Collapse two or more vertices,

remove degenerated trianglesremove degenerated triangles- Special case: Special case: Edge collapseEdge collapse

(merge 2 vertices per step)(merge 2 vertices per step)- Use error metric to decide which vertices to collapse,Use error metric to decide which vertices to collapse,

e.g. Quadratic Error Metrics (QEM) [Garland 97]e.g. Quadratic Error Metrics (QEM) [Garland 97]


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• Hierarchical Level of Detail (HLOD) [Erikson Hierarchical Level of Detail (HLOD) [Erikson 01]01]– Use LODs for each scene graph nodeUse LODs for each scene graph node– HLODs:HLODs:

Simplification of entireSimplification of entirescene graph branchesscene graph branches

Higher fidelity thanHigher fidelity thanseparate approximationsseparate approximations

Simplify scene graphSimplify scene graphtraversaltraversal


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SimplificationSimplificationImage-Based MethodsImage-Based Methods

• Polygonal simplification sometimes difficultPolygonal simplification sometimes difficult– E.g. because of unsuitable scene structureE.g. because of unsuitable scene structure

Image-Based RenderingImage-Based Rendering (IBR) (IBR)- Image-based entities (Image-based entities (impostorsimpostors) as alternative ) as alternative

representation for scene partsrepresentation for scene parts- Simple variant: Simple variant: BillboardsBillboards


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SimplificationSimplificationImage-Based MethodsImage-Based Methods

• Textured Depth Meshes (TDM) [Sillion 97]Textured Depth Meshes (TDM) [Sillion 97]– Simple polygon mesh of rough scene structureSimple polygon mesh of rough scene structure– Textured with detailed model imagesTextured with detailed model imagesBetter parallax movementBetter parallax movementLonger valid when viewpoint changesLonger valid when viewpoint changes


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OverviewOverview




Existing ArchitecturesExisting Architectures- MMRMMR- GigawalkGigawalk- iWalkiWalk


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Existing Existing ArchitecturesArchitectures

MMRMMR• MMR [Aliaga 99]MMR [Aliaga 99]

– Massive Model RenderingMassive Model Rendering framework framework– One of first systems capable of interactive display One of first systems capable of interactive display

of aof a12.5 million power plant scene (out-of-core 12.5 million power plant scene (out-of-core rendering)rendering)

– Modular System:Modular System:Incorporates a variety of techniquesIncorporates a variety of techniques- Visibility CullingVisibility Culling- Mesh simplificationMesh simplification- Static LODsStatic LODs- Hierarchical occlusion mapsHierarchical occlusion maps- Textured depth meshesTextured depth meshes


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MMRMMR• Basic Idea:Basic Idea:

– Partition scene into Partition scene into viewpoint cells viewpoint cells (not (not automatic)automatic)

– Associate Associate cull boxcull box with each viewpoint-cell with each viewpoint-cell– For every observer position inside a viewpoint For every observer position inside a viewpoint

cellcell- Clip geometry against cull boxClip geometry against cull box- Replace clipped geometry with TDMsReplace clipped geometry with TDMs


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MMRMMR• Textured depth meshesTextured depth meshes

– Pre-generated images of geometry outside cull Pre-generated images of geometry outside cull box viewed from cell centerbox viewed from cell center

– Images projected onto simplified depth meshesImages projected onto simplified depth meshes


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MMRMMR• Run-time pipelineRun-time pipeline


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MMRMMR• Multiprocessor pipelined implementationMultiprocessor pipelined implementation

– Interframe phaseInterframe phase– Cull phaseCull phase– Render phaseRender phase– Prefetch phasePrefetch phase

Only a single rendering pipeline usedOnly a single rendering pipeline used


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MMRMMR• ResultsResults

– SGI Onyx with Infinite Reality graphicsSGI Onyx with Infinite Reality graphics– Frame rates: 5-15 fpsFrame rates: 5-15 fps– Only 0.9% of original polygons need to be Only 0.9% of original polygons need to be

renderedrendered– However: Popping and distortion when switching However: Popping and distortion when switching

cellscells– Preprocessing time:Preprocessing time:

- 17 hours for cells on selected sample camera paths17 hours for cells on selected sample camera paths- 525 hours for complete model (estimated)525 hours for complete model (estimated)


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GigawalkGigawalk• Gigawalk [Baxter 02]Gigawalk [Baxter 02]

– Fully automatic scene organization (BVH)Fully automatic scene organization (BVH)– Hierarchical Z-BufferHierarchical Z-Buffer– Static LODs / HLODsStatic LODs / HLODs– Uses two rendering pipelinesUses two rendering pipelines

- Parallel rendering of occluders and visible geometryParallel rendering of occluders and visible geometry

– Can exploit temporal cohCan exploit temporal coheerencerence- Use of visible geometry from a previous frame as Use of visible geometry from a previous frame as

occludersoccluders


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GigawalkGigawalk• System architectureSystem architecture

– 3 Processes running in parallel3 Processes running in parallel- Occluder Rendering (OC)Occluder Rendering (OC)- Scene Traversal, Culling, LOD selection (STC)Scene Traversal, Culling, LOD selection (STC)- Rendering Visible Scene Geometry (RVG)Rendering Visible Scene Geometry (RVG)


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GigawalkGigawalk• Timing relationshipTiming relationship

– Frame i uses occluders from frame i-2Frame i uses occluders from frame i-2


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GigawalkGigawalk• ResultsResults

– SGI Onyx with Infinite Reality graphics,SGI Onyx with Infinite Reality graphics,dual graphics rasterization pipelinesdual graphics rasterization pipelines

– Frame rates: 11-50 fpsFrame rates: 11-50 fps(tanker model: 82 million triangles)(tanker model: 82 million triangles)

– Preprocessing time: 35 hours (Pentium IV, 2GHz)Preprocessing time: 35 hours (Pentium IV, 2GHz)– However: No out-of-core renderingHowever: No out-of-core rendering


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iWalkiWalk• iWalk [Correa 03]iWalk [Correa 03]

– Interactive frame rates on a single commodity Interactive frame rates on a single commodity PCPC

– Prioritized-Layered ProjectionPrioritized-Layered Projection- Approximate visibilityApproximate visibility- Budget-based renderingBudget-based rendering- Optionally conservative variant (cPLP)Optionally conservative variant (cPLP)

– Efficient prefetchingEfficient prefetching– Out-of-core renderingOut-of-core rendering


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iWalkiWalk• Multi-threaded rendering pipeline Multi-threaded rendering pipeline

(simplified)(simplified)– PLP used for visibility culling and prefetchingPLP used for visibility culling and prefetching


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iWalkiWalk• ResultsResults

– Commodity PC (Pentium IV, 2.8 GHz), 512 MByte Commodity PC (Pentium IV, 2.8 GHz), 512 MByte RAMRAM

– Rendering budget: 280.000 triangles per frameRendering budget: 280.000 triangles per frame– Average frame rate: 9.3 fps (power plant)Average frame rate: 9.3 fps (power plant)– Median accuracy: 99.2% pixels correctMedian accuracy: 99.2% pixels correct– Preprocessing time: 3 minutesPreprocessing time: 3 minutes

End of Part IEnd of Part I

Questions ?Questions ?

Documents

Afrigraph 2004 Tutorial A: Part I Rasterization Based Approaches Andreas Dietrich Computer Graphics Group, Saarland University Saarbrücken, Germany