“Level of Detail Management for 3D Games” Martin Reddy Pixar Animation Studios Terrain Level Of Detail

““Level of Detail Management Level of Detail Management for 3D Games”for 3D Games”

Martin ReddyPixar Animation Studios

Terrain Level Of DetailTerrain Level Of Detail

ContentsContents

BackgroundBackground– History, applications, data sizesHistory, applications, data sizes

Important ConceptsImportant Concepts– regular grids v TINs, quadtrees v bintrees, regular grids v TINs, quadtrees v bintrees,

out-of-core paging, web streamingout-of-core paging, web streaming ImplementationsImplementations

– Lindstrom 96, Duchaineau 97, RLindstrom 96, Duchaineau 97, Rööttger 98, ttger 98, Hoppe 98, DeFloriani 00, Lindstrom 01 Hoppe 98, DeFloriani 00, Lindstrom 01

Available softwareAvailable software

Gameplay Goals for TerrainGameplay Goals for Terrain

Large enough to travel around for hoursLarge enough to travel around for hours Detailed when seen at human scaleDetailed when seen at human scale Dynamic modification of terrain dataDynamic modification of terrain data Runs at high, stable frame ratesRuns at high, stable frame rates Need fast rotation of viewpointNeed fast rotation of viewpoint

From Jonathan Blow’s SIGGRAPH 2000 Course:

BackgroundBackground

One of the first real uses of LODOne of the first real uses of LOD Important for applications such asImportant for applications such as

– Flight simulatorsFlight simulators– Terrain-based computer gamesTerrain-based computer games– Geographic Information Systems (GIS)Geographic Information Systems (GIS)– Virtual tourism, real-estate, mission planningVirtual tourism, real-estate, mission planning

Sustained R&D since the 1970sSustained R&D since the 1970s Other terms includeOther terms include

– generalizationgeneralization (GIS) (GIS)

Terrain LOD ExampleTerrain LOD Example

Screenshot of Screenshot of the Grand the Grand Canyon with Canyon with debug view debug view using the using the Digital Dawn Digital Dawn Toolkit, now Toolkit, now incorporated incorporated into Crystal into Crystal SpaceSpace

Terrain LOD vs Generic LODTerrain LOD vs Generic LOD

Terrain is easier...Terrain is easier...– Geometry is more constrainedGeometry is more constrained– Normally uniform grids of height valuesNormally uniform grids of height values– More specialized and simpler algorithmsMore specialized and simpler algorithms

Terrain is more difficult...Terrain is more difficult...– Continuous and very large modelsContinuous and very large models– Simultaneously very close and far awaySimultaneously very close and far away– Necessitates view-dependent LODNecessitates view-dependent LOD– Often requires paging from disk (out-of-core) Often requires paging from disk (out-of-core)

Large Terrain DatabasesLarge Terrain Databases

USGS GTOPO30USGS GTOPO30– 30 arc-second (~1 km) resolution elevation30 arc-second (~1 km) resolution elevation– 43,200 x 21,600 = 1.8 billion triangles43,200 x 21,600 = 1.8 billion triangles

NASA EOS satellite ASTERNASA EOS satellite ASTER– 30-m resolution elevation data30-m resolution elevation data– from 15-m near infrared stereo imageryfrom 15-m near infrared stereo imagery

USGS National Elevation Dataset (NED)USGS National Elevation Dataset (NED)– 50,000 quads at around 50 GB50,000 quads at around 50 GB

Regular GridsRegular Grids

Uniform array of height valuesUniform array of height values Simple to store and manipulateSimple to store and manipulate Encode in raster formats (DEM, GeoTIFF)Encode in raster formats (DEM, GeoTIFF) Easy to interpolate to find elevationsEasy to interpolate to find elevations Less disk/memory (only store z value)Less disk/memory (only store z value) Easy view culling and collision detectionEasy view culling and collision detection Used by most implementers Used by most implementers

TINsTINs

Triangulated Irregular NetworksTriangulated Irregular Networks Fewer polygons needed to attain required Fewer polygons needed to attain required

accuracyaccuracy Higher sampling in bumpy regions and Higher sampling in bumpy regions and

coarser in flat onescoarser in flat ones Can model maxima, minima, ridges, Can model maxima, minima, ridges,

valleys, overhangs, cavesvalleys, overhangs, caves Used by Hoppe 98 & DeFloriani 00Used by Hoppe 98 & DeFloriani 00

LOD Hierarchy StructuresLOD Hierarchy Structures

QuadTree Hierarchy

BinTree Hierarchy

QuadtreesQuadtrees

Each quad is actually two trianglesEach quad is actually two triangles Produces cracks and T-junctionsProduces cracks and T-junctions Easy to implementEasy to implement Good for out-of-core operationGood for out-of-core operation

BintreesBintrees

TerminologyTerminology– binary triangle tree (bintree, bintritree, BTT)binary triangle tree (bintree, bintritree, BTT)– right triangular irregular networks (RTIN)right triangular irregular networks (RTIN)– longest edge bisectionlongest edge bisection

Easier to avoid cracks and T-junctionsEasier to avoid cracks and T-junctions Neighbor is never more than 1 level awayNeighbor is never more than 1 level away Used by Lindstrom 96 & Duchaineau 97Used by Lindstrom 96 & Duchaineau 97

Cracks and T-JunctionsCracks and T-Junctions

Avoid cracks:Avoid cracks:– Force cracks into T-junctions / remove floating vertexForce cracks into T-junctions / remove floating vertex– Fill cracks with extra trianglesFill cracks with extra triangles

Avoid T-junctions:Avoid T-junctions:– Continue to simplify ...Continue to simplify ...

Avoiding T-junctionsAvoiding T-junctions

Avoiding T-junctionsAvoiding T-junctions

Split(BinTri *tri) { if tri->BottomNeighbor is valid { if tri->BottomNeighbor->BottomNeighbor != tri { Split(tri->BottomNeighbor) } Split2(tri) Split2(tri->BottomNeighbor) tri->LeftChild->RightNeighbor = tri->BottomNeighbor->RightChild tri->RightChild->LeftNeighbor = tri->BottomNeighbor->LeftChild tri->BottomNeighbor->LeftChild->RightNeighbor = tri->RightChild tri->BottomNeighbor->RightChild->LeftNeighbor = tri->LeftChild } else { Split2(tri) tri->LeftChild->RightNeighbor = 0; tri->RightChild->LeftNeighbor = 0; }}

Split2(tri) { tri->LeftChild = AllocateBinTri(); tri->RightChild = AllocateBinTri(); tri->LeftChild->LeftNeighbor = tri->RightChild tri->RightChild->RightNeighbor = tri->LeftChild tri->LeftChild->BottomNeighbor = tri->LeftNeighbor if tri->LeftNeighbor is valid { if tri->LeftNeighbor->BottomNeighbor == tri { tri->LeftNeighbor->BottomNeighbor = tri->LeftChild } else { tri->LeftNeighbor->RightNeighbor = tri->LeftChild } } tri->RightChild->BottomNeighbor = tri->RightNeighbor if tri->RightNeighbor is valid { if tri->RightNeighbor->BottomNeighbor == tri { tri->RightNeighbor->BottomNeighbor = tri->RightChild } else { tri->RightNeighbor->LeftNeighbor = tri->RightChild } } tri->LeftChild->LeftChild = 0 tri->LeftChild->RightChild = 0 tri->RightChild->LeftChild = 0 tri->RightChild->RightChild = 0}

Pseudo code to do triangle splitting and avoid T-junctions

Code by S. McNally on GameDev.net, optimized by A. Ögren

Out-of-core operationOut-of-core operation

Virtual memory solutionsVirtual memory solutions– mmap() used by Lindstrom 01mmap() used by Lindstrom 01– VirtualAlloc() / VirtualFree() used by Hoppe 98VirtualAlloc() / VirtualFree() used by Hoppe 98

Explicit paging from diskExplicit paging from disk– NPSNET (NPS): Falby 93NPSNET (NPS): Falby 93– VGIS (GVU): Davis 99VGIS (GVU): Davis 99– OpenGL Performer Active Surface Def (ASD)OpenGL Performer Active Surface Def (ASD)– SGI InfiniteReality (IR) ClipmappingSGI InfiniteReality (IR) Clipmapping

Streaming over the WebStreaming over the Web

TerraVision (SRI) – Leclerc 94, Reddy 99TerraVision (SRI) – Leclerc 94, Reddy 99

TerraVision MovieTerraVision Movie

Texture issuesTexture issues

Need to handle paging of imagery as well Need to handle paging of imagery as well as geometry (satellite imagery resolution as geometry (satellite imagery resolution is generally > than elevation resolution)is generally > than elevation resolution)

Hardware support for paging (clipmaps)Hardware support for paging (clipmaps) Detail textures for close-to-ground detailDetail textures for close-to-ground detail Texture compression useful?Texture compression useful?

Lindstrom et al. 1996Lindstrom et al. 1996

One of first real-time view-dependent algorithms, One of first real-time view-dependent algorithms, referred to as continuous LOD (CLOD)referred to as continuous LOD (CLOD)

Regular grid, bintree, quadtree blocksRegular grid, bintree, quadtree blocks– Mesh broken into rectangular blocks with a top-down Mesh broken into rectangular blocks with a top-down

coarse-grained simplificationcoarse-grained simplification– Then per-vertex simplification performed within each Then per-vertex simplification performed within each

blockblock

Frame-to-frame coherence:Frame-to-frame coherence:– Maintain an active cut of blocksMaintain an active cut of blocks– Only visit vertices if could change in frameOnly visit vertices if could change in frame


Vertex removal schemeVertex removal scheme Merge based upon a Merge based upon a

measure of screen-measure of screen-space error between space error between the two surfaces, the two surfaces,

Used a nonlinear Used a nonlinear mapping of mapping of to to represent 0..65535 in represent 0..65535 in only 8 bitsonly 8 bits


Hunter-Liggett US Army base

2-m res

8 x 8km

32 M polys


Duchaineau et al. 1997Duchaineau et al. 1997

Real-time Optimally Adapting Meshes Real-time Optimally Adapting Meshes (ROAM)(ROAM)

Regular grid, bintree, 2 priority queues:Regular grid, bintree, 2 priority queues:– 1 priority-ordered list of triangle splits1 priority-ordered list of triangle splits– 1 priority-ordered list of triangle merges1 priority-ordered list of triangle merges

Frame coherenceFrame coherence– pick up from previous frame’s queue statepick up from previous frame’s queue state

Very popular with source code and Very popular with source code and implementation notes availableimplementation notes available


““Split-Only” ROAM often used in gamesSplit-Only” ROAM often used in games– Used by Seumas McNally in TreadMarksUsed by Seumas McNally in TreadMarks

Removes the frame coherence portionRemoves the frame coherence portion– Don’t need 2 priority-sorted queuesDon’t need 2 priority-sorted queues

Implementation available as SMTerrain in Implementation available as SMTerrain in the vterrain.org VTP softwarethe vterrain.org VTP software

Good article on Split-Only ROAM at Good article on Split-Only ROAM at Gamasutra by Bryan TurnerGamasutra by Bryan Turner


ROAM Split and Merge “Diamonds”


Principal metric was Principal metric was screen-based screen-based geometric error with geometric error with guaranteed bound on guaranteed bound on the errorthe error

Hierarchy of volumes Hierarchy of volumes called called wedgieswedgies

Covers the Covers the (x,y)(x,y) extent extent of a triangle and of a triangle and extends over the extends over the height range height range z-ez-eTT through through z+ez+eTT


RRöttger et al. 1998öttger et al. 1998

Extended Lindstrom’s CLOD workExtended Lindstrom’s CLOD work Regular grid, quadtree, top-downRegular grid, quadtree, top-down World space metric considered:World space metric considered:

– viewer distance & terrain roughnessviewer distance & terrain roughness Integrated vertex geomorphingIntegrated vertex geomorphing Deal with tears by skipping center vertex Deal with tears by skipping center vertex

of higher resolution adjacent edgeof higher resolution adjacent edge

RRöttger et al. 1998öttger et al. 1998

Hawai’i

Hoppe 1998Hoppe 1998

View-Dependent Progressive Meshes View-Dependent Progressive Meshes (VDPM) from Hoppe 97 applied to terrain(VDPM) from Hoppe 97 applied to terrain

TIN-based, out-of-core (VirtualAlloc/Free)TIN-based, out-of-core (VirtualAlloc/Free) Integrated vertex geomorphingIntegrated vertex geomorphing Tears between blocks avoided by not Tears between blocks avoided by not

simplifying at block boundariessimplifying at block boundaries Notes that larger errors can occur Notes that larger errors can occur

between grid points and precomputes between grid points and precomputes maximum height deviationsmaximum height deviations

Hoppe 1998Hoppe 1998

Grand Canyon, Arizona

4,097 x 2,049

8 x 4 blocks of 513 x 513

Hoppe VideoHoppe Video

QuickTime™ and a YUV420 codec decompressor are needed to see this picture.

DeFloriani et al. 2000DeFloriani et al. 2000

VARIANT. Uses Multi-Triangulation (MT)VARIANT. Uses Multi-Triangulation (MT) General TIN approach applied to terrainGeneral TIN approach applied to terrain Plug in different simp. & error routinesPlug in different simp. & error routines Supports analyses: visibility, elevation Supports analyses: visibility, elevation

along a path, contour extraction, viewshedalong a path, contour extraction, viewshed Frame coherence (use previous state)Frame coherence (use previous state) Freely available C++ library for MTFreely available C++ library for MT

DeFloriani et al. 2000DeFloriani et al. 2000

Lindstrom & Pascucci 2001Lindstrom & Pascucci 2001

Visualization of Large Terrains Made EasyVisualization of Large Terrains Made Easy Regular gridded, top-down, bintreeRegular gridded, top-down, bintree Out-of-core with mmap() and spatial org.Out-of-core with mmap() and spatial org. Fast hierarchical culling, triangle stripping, Fast hierarchical culling, triangle stripping,

and optional multithreading of refinement and optional multithreading of refinement and rendering tasksand rendering tasks

Uses a nesting of error metric terms Uses a nesting of error metric terms (bounding spheres) (bounding spheres)

Lindstrom & Pascucci 2001Lindstrom & Pascucci 2001

Puget Sound, Washington

16,385 x 16,385

512 MB

Lindstrom & Pascucci VideoLindstrom & Pascucci Video

Available SoftwareAvailable Software

Virtual Terrain Project (VTP)Virtual Terrain Project (VTP)– http://www.vterrain.org/http://www.vterrain.org/

TerraVisionTerraVision– http://terravision.sourceforge.net/http://terravision.sourceforge.net/

SOARSOAR– http://www.cc.gatech.edu/~lindstro/software/soar/http://www.cc.gatech.edu/~lindstro/software/soar/

ROAMROAM– http://www.cognigraph.com/ROAM_homepage/http://www.cognigraph.com/ROAM_homepage/

Source code links (ROAM, VTP, MT, etc.)Source code links (ROAM, VTP, MT, etc.)– http://www.LODBook.com/source/http://www.LODBook.com/source/

Terrain Data ResourcesTerrain Data Resources

Large Terrain DatabasesLarge Terrain Databases– http://www.cc.gatech.edu/projects/large_models/http://www.cc.gatech.edu/projects/large_models/

Terrain Models on LODBook.comTerrain Models on LODBook.com– http://www.LODBook.com/terrain/http://www.LODBook.com/terrain/

GeoVRML Data Resources PageGeoVRML Data Resources Page– http://www.geovrml.org/data/http://www.geovrml.org/data/

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“Level of Detail Management for 3D Games” Martin Reddy Pixar Animation Studios Terrain Level Of Detail