Terrain Level Of Detail

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Terrain Level Of Detail
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Contents

• Background
• History, applications, data sizes
• Important Concepts
• regular grids v TINs, quadtrees v bintrees,
  out-of-core paging, web streaming
• Implementations
• Lindstrom 96, Duchaineau 97, Röttger 98, Hoppe
  98, DeFloriani 00, Lindstrom 01
• Available software

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Gameplay Goals for Terrain
From Jonathan Blows SIGGRAPH 2000 Course

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

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Background

• One of the first real uses of LOD
• Important for applications such as
• Flight simulators
• Terrain-based computer games
• Geographic Information Systems (GIS)
• Virtual tourism, real-estate, mission planning
• Sustained RD since the 1970s
• Other terms include
• generalization (GIS)

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Terrain LOD Example

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

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Terrain LOD vs Generic LOD

• Terrain is easier...
• Geometry is more constrained
• Normally uniform grids of height values
• More specialized and simpler algorithms
• Terrain is more difficult...
• Continuous and very large models
• Simultaneously very close and far away
• Necessitates view-dependent LOD
• Often requires paging from disk (out-of-core)

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Large Terrain Databases

• USGS GTOPO30
• 30 arc-second (1 km) resolution elevation
• 43,200 x 21,600 1.8 billion triangles
• NASA EOS satellite ASTER
• 30-m resolution elevation data
• from 15-m near infrared stereo imagery
• USGS National Elevation Dataset (NED)
• 50,000 quads at around 50 GB

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Regular Grids

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

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TINs

• Triangulated Irregular Networks
• Fewer polygons needed to attain required accuracy
• Higher sampling in bumpy regions and coarser in
  flat ones
• Can model maxima, minima, ridges, valleys,
  overhangs, caves
• Used by Hoppe 98 DeFloriani 00

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LOD Hierarchy Structures
QuadTree Hierarchy
BinTree Hierarchy
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Quadtrees

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

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Bintrees

• Terminology
• binary triangle tree (bintree, bintritree, BTT)
• right triangular irregular networks (RTIN)
• longest edge bisection
• Easier to avoid cracks and T-junctions
• Neighbor is never more than 1 level away
• Used by Lindstrom 96 Duchaineau 97

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Cracks and T-Junctions

• Avoid cracks
• Force cracks into T-junctions / remove floating
  vertex
• Fill cracks with extra triangles
• Avoid T-junctions
• Continue to simplify ...

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Avoiding T-junctions
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Avoiding T-junctions
Pseudo code to do triangle splitting and avoid
T-junctions

• Split(BinTri tri)
• if tri-gtBottomNeighbor is valid
• if tri-gtBottomNeighbor-gtBottomNeighbor ! tri
  
• Split(tri-gtBottomNeighbor)
• Split2(tri)
• Split2(tri-gtBottomNeighbor)
• tri-gtLeftChild-gtRightNeighbor
• tri-gtBottomNeighbor-gtRightChild
• tri-gtRightChild-gtLeftNeighbor
• tri-gtBottomNeighbor-gtLeftChild
• tri-gtBottomNeighbor-gtLeftChild-gtRightNeighbor
  
• tri-gtRightChild
• tri-gtBottomNeighbor-gtRightChild-gtLeftNeighbor
  
• tri-gtLeftChild
• else
• Split2(tri)
• tri-gtLeftChild-gtRightNeighbor 0
• tri-gtRightChild-gtLeftNeighbor 0

Split2(tri) tri-gtLeftChild
AllocateBinTri() tri-gtRightChild
AllocateBinTri() tri-gtLeftChild-gtLeftNeighbor
tri-gtRightChild tri-gtRightChild-gtRightNeighbor
tri-gtLeftChild tri-gtLeftChild-gtBottomNeighbor
tri-gtLeftNeighbor if tri-gtLeftNeighbor is
valid if tri-gtLeftNeighbor-gtBottomNeighbor
tri tri-gtLeftNeighbor-gtBottomNeighbor
tri-gtLeftChild else
tri-gtLeftNeighbor-gtRightNeighbor
tri-gtLeftChild tri-gtRightChild-gtBottom
Neighbor tri-gtRightNeighbor if
tri-gtRightNeighbor is valid if
tri-gtRightNeighbor-gtBottomNeighbor tri
tri-gtRightNeighbor-gtBottomNeighbor
tri-gtRightChild else
tri-gtRightNeighbor-gtLeftNeighbor
tri-gtRightChild tri-gtLeftChild-gtLeftCh
ild 0 tri-gtLeftChild-gtRightChild 0
tri-gtRightChild-gtLeftChild 0
tri-gtRightChild-gtRightChild 0
Code by S. McNally on GameDev.net, optimized by
A. Ögren
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Out-of-core operation

• Virtual memory solutions
• mmap() used by Lindstrom 01
• VirtualAlloc() / VirtualFree() used by Hoppe 98
• Explicit paging from disk
• NPSNET (NPS) Falby 93
• VGIS (GVU) Davis 99
• OpenGL Performer Active Surface Def (ASD)
• SGI InfiniteReality (IR) Clipmapping

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Streaming over the Web

• TerraVision (SRI) Leclerc 94, Reddy 99

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TerraVision Movie
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Texture issues

• Need to handle paging of imagery as well as
  geometry (satellite imagery resolution is
  generally gt than elevation resolution)
• Hardware support for paging (clipmaps)
• Detail textures for close-to-ground detail
• Texture compression useful?

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Lindstrom et al. 1996

• One of first real-time view-dependent algorithms,
  referred to as continuous LOD (CLOD)
• Regular grid, bintree, quadtree blocks
• Mesh broken into rectangular blocks with a
  top-down coarse-grained simplification
• Then per-vertex simplification performed within
  each block
• Frame-to-frame coherence
• Maintain an active cut of blocks
• Only visit vertices if could change in frame

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Lindstrom et al. 1996

• Vertex removal scheme
• Merge based upon a measure of screen-space error
  between the two surfaces, d
• Used a nonlinear mapping of d to represent
  0..65535 in only 8 bits

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Lindstrom et al. 1996
Hunter-Liggett US Army base 2-m res 8 x 8km 32
M polys
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Lindstrom et al. 1996
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Duchaineau et al. 1997

• Real-time Optimally Adapting Meshes (ROAM)
• Regular grid, bintree, 2 priority queues
• 1 priority-ordered list of triangle splits
• 1 priority-ordered list of triangle merges
• Frame coherence
• pick up from previous frames queue state
• Very popular with source code and implementation
  notes available

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Duchaineau et al. 1997

• Split-Only ROAM often used in games
• Used by Seumas McNally in TreadMarks
• Removes the frame coherence portion
• Dont need 2 priority-sorted queues
• Implemenation available as SMTerrain in the
  vterrain.org VTP software
• Good article on Split-Only ROAM at Gamasutra by
  Bryan Turner

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Duchaineau et al. 1997
ROAM Split and Merge Diamonds
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Duchaineau et al. 1997

• Principal metric was screen-based geometric error
  with guaranteed bound on the error
• Hierarchy of volumes called wedgies
• Covers the (x,y) extent of a triangle and extends
  over the height range z-eT through zeT

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Duchaineau et al. 1997
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Röttger et al. 1998

• Extended Lindstroms CLOD work
• Regular grid, quadtree, top-down
• World space metric considered
• viewer distance terrain roughness
• Integrated vertex geomorphing
• Deal with tears by skipping center vertex of
  higher resolution adjacent edge

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Röttger et al. 1998
Hawaii
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Hoppe 1998

• View-Dependent Progressive Meshes (VDPM) from
  Hoppe 97 applied to terrain
• TIN-based, out-of-core (VirtualAlloc/Free)
• Integrated vertex geomorphing
• Tears between blocks avoided by not simplifying
  at block boundaries
• Notes that larger errors can occur between grid
  points and precomputes maximum height deviations

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Hoppe 1998
Grand Canyon, Arizona 4,097 x 2,049 8 x 4
blocks of 513 x 513
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Hoppe Video
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DeFloriani et al. 2000

• VARIANT. Uses Multi-Triangulation (MT)
• General TIN approach applied to terrain
• Plug in different simp. error routines
• Supports analyses visibility, elevation along a
  path, contour extraction, viewshed
• Frame coherence (use previous state)
• Freely available C library for MT

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DeFloriani et al. 2000
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Lindstrom Pascucci 2001

• Visualization of Large Terrains Made Easy
• Regular gridded, top-down, bintree
• Out-of-core with mmap() and spatial org.
• Fast hierarchical culling, triangle stripping,
  and optional multithreading of refinement and
  rendering tasks
• Uses a nesting of error metric terms (bounding
  spheres)

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Lindstrom Pascucci 2001
Puget Sound, Washington 16,385 x 16,385 512 MB
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Lindstrom Pascucci Video
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Available Software

• Virtual Terrain Project (VTP)
• http//www.vterrain.org/
• TerraVision
• http//terravision.sourceforge.net/
• SOAR
• http//www.cc.gatech.edu/lindstro/software/soar/
• ROAM
• http//www.cognigraph.com/ROAM_homepage/
• Source code links (ROAM, VTP, MT, etc.)
• http//www.LODBook.com/source/

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Terrain Data Resources

• Large Terrain Databases
• http//www.cc.gatech.edu/projects/large_models/
• Terrain Models on LODBook.com
• http//www.LODBook.com/terrain/
• GeoVRML Data Resources Page
• http//www.geovrml.org/data/

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End of Presentation
End of Presentation