Terrain Level Of Detail

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Terrain Level Of Detail

• Hyunseok Oh
• 2007.3.30

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Contents

• Terrain LOD
• Background
• History
• Applications
• Data sizes
• Important Concepts
• Regular grids vs TINs
• Quadtrees vs Bintrees
• Crack,T-Junctions
• Out-of-core paging
• Related Works
• ROAM
• Research Plans

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Terrain LOD

• LOD
• Terrain LOD
• General LOD vs Terrain LOD
• Easier
• Constrained geometry (generally).
• More specialized and simpler algorithms.
• Harder
• Continuous and large models.
• Simultaneously very close and far away.
• Necessitates view-dependent LOD.
• Out-of-core.

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Terrain LOD Example
Screenshot of the Grand Canyon with debug view
using the Digital Dawn Toolkit.
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Background

• History
• Sustained RD since the 1970s.
• The first system to use LOD Flight simulators.
• Applications
• Visual tourism travel planning.
• Education geographical general reference.
• Planning land panning/land usage, urban
  planning.
• Terrain-based computer games.
• Geographic Information Systems (GIS).
• Military mission planning applications.

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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.

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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.

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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).
• longest edge bisection.
• Easier to avoid cracks and T-junctions.
• Neighbor is never more than 1 level away.

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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.
• Forced spiltting.

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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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Related Works

• Papers.
• Right Triangular Irregular Networks , 1997.
• ROAMing Terrain Real-time Optimally Adapting
  Meshes, 1997.
• Real-Time Generation of Continuous Levels of
  Detail for Height Fields, 1998.
• Terrain Rendering at High Levels of Detail (pdf),
  2000 .
• A Fast Algorithm For Large Scale Terrain
  Walkthrough, 2001 (pdf) .
• Visualization of Large Terrains Made Easy, 2001 .
• Diamond Terrain Algorithm CLOD for Height
  Fields, 2001 .
• Real-time Terrain Rendering using Smooth Hardware
  Optimized Level of Detail, 2003 pdf.
• Planet-Sized Batched Dynamic Adaptive Meshes
  (P-BDAM), IEEE Visualization 2003.
• Geometry clipmaps Terrain rendering using nested
  regular grids. Siggraph 2004.
• Rendering Procedural Terrain by Geometry Image
  Warping. Eurographics 2004.
• Adaptive Streaming and Rendering of Large
  Terrains using Strip Masks, 2005.
• Terrain Rendering Engine (TRE) Cell Broadband
  Engine Optimized Real-time Ray-caster, 2005.
• GPU-Friendly High-Quality Terrain Rendering
  Journal of WSCG 2006.
• Terrain Rendering using Spherical Clipmaps,
  Eurographics 2006.
• Seamless Patches for GPU-Based Terrain Rendering,
  WSCG 2007.

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ROAM

• Real-Time Optimally Adapting Meshes.
• Mark Duchaineau, 1997 (LLNL).
• Binary Triangle Tree Structure.
• No need to worry about cracks, etc.
• Can specify the desired number of triangles.

Example of ROAM terrain.
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ROAM

• Main Concepts
• Split and Merge.
• Two priority queues.
• One for splits and one for merge.
• Allows for frame-to-frame coherence.
• Error Metrics for Splits and Merges.
• Incremental triangle stripping introduced.

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ROAM

• Splitting and Merging diamonds.

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ROAM

• Avoiding T-junctions.

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ROAM Priority Queues

• One priority queue for splits, one for merges,
  and use a greedy algorithm to triangulate.
• Priority error metric( using wedgies ).

Illustrating nested ROAM wedgies for the 1D case.
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Research Plans

• 4.20 - Survey papers select a paper.
• 5.31 Implementation.

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References

• http//www.vterrain.org
• David Luebke et al. Level of Detail for 3D
  Graphics. p185-228, Morgan Kaufmann, 2003.