Terrain Rendering and LOD in Games

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Terrain Rendering and LOD in Games

• Abdennour El Rhalibi

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Outline

• overview of terrain algorithms
• optimization criteria
• Lindstrom algorithm
• progressive meshes
• ROAM
• performance

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Terrain Model
triangle mesh
texture image
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The Data

• Triangulated Irregular Networks (TIN)

• Regular Grid Height Field

• Tradeoffs?

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Simplify!

• We need to simplify our meshes!
• Go from
• Full Mesh -gt Simplified Mesh

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Simplify!

• Using view dependent metrics

actual view
overhead view
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Overview of Terrain Algorithms

• grid vs. TIN

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Overview of Terrain Algorithms

• Discrete LOD
• Continuous LOD
• grid-based
• continuous LOD rendering by Lindstrom
• Real-time Optimally Adapting Meshes (ROAM)
• TIN-based
• progressive meshes

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Terrain LOD vs Traditional 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

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A Discrete LOD Approach

• View-Independent, camera location-dependent
• Still involves subdividing terrain
• Render closer subdivisions at higher resolution
• Popping
• Will get cracks and T-junctions

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Discrete LOD

• The End Result of what were achieving
• Low Detail Medium Detail Full Detail

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Terrain LOD Basics

• Cracks, T-junctions
• How do we solve this?

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Terrain LOD Basics 2

• What can you do?
• Ensure common vertices on edge of subdivision
• Draw a triangle to fill that spot
• Force a crack into a T-junction
• There must be an easier way
• Subdivide the terrain such that this is easier or
  done for free

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Terrain LOD Basics 3

• Quadtrees and BinTrees

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Quadtrees

• Easy to implement

• Each quad is actually two triangles
• Still have cracks and T-junctions

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BinTrees (Binary Triangle Trees)

• Cracks and T-junctions are solved!
• Any two triangles differ by no more than one
  resolution level

• A little harder to implement
• Forced Splitting

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Several Algorithms

• Lindstroms Continuous LOD
• Roettgers CLOAD Top-down version of Lindstrom
  CLOAD
• ROAM (Duchaineau)
• 3D Bounding Isosurfaces (Blow)
• Caching geometry (Vis2002)
• Visualization of Large Terrains Made Easy
• SOAR

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Optimization Criteria

• time required to achieve a given triangle count
• user specified view-dependent metrics
• simplicity
• strict frame rates
• dynamic terrain
• continuous changes
• etc.

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Continuous LOD for Height Fields

• Peter Lindstrom et al., 1996
• Used a binary vertex tree
• Frame-to-frame coherence

• Introduced user-controllable screen space error
  threshold

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Continuous LOD by Lindstrom

• Quad tree
• bottom-up
• block-basedvs.vertex-based

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Progressive Meshes

• PM representation
• only single operationmerge (edge collapse)
• split (vertex split)
• view-dependent PM

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ROAM

• Real-Time Optimally Adapting Meshes
• Mark Duchaineau, 1997
• Binary Triangle Tree Structure
• No need to worry about cracks
• Can specify the desired number of triangles
• Probably the most popular algorithm today

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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
• Geomorphing introduced, but rarely needed
• Incremental triangle stripping introduced

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ROAM (1)

• triangle bintree
• top-downalgorithm
• 2 priority queues
• merge
• split

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Roam Structure

• Triangle Subdivision

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Binary Triangle Tree

• Whats represented in each node of the tree

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ROAM (2)

• (mergeable) diamond
• forced split gt
• no effort to avoiddiscontinuities

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ROAM Splitting and Merging diamonds
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Splitting Nodes

• Imagine the Diamond

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Managing Splitting

• The Recursive Split

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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
• Nested world space bounds
• Geometric screen distortion
• Line of site

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What Cracks?

• The T-Junction.

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ROAM Wedgies!

• Wedgie basically a bounding volume
• Covers the (x,y) extent of a triangle and extends
  over the height range z-eT through zeT

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ROAM Splitting Algorithm

• Let T the base triangulation
• For all t in T, insert t into Q
• While T is too small or inaccurate
• Identify highest priority t in Q
• Force-split t
• Update split queue as follows
• Remove t and other split triangles from Q
• Add any new triangles to Q

Adapted from Duchaineaus original ROAMing
Terrain paper (96)
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ROAM Merging AND Splitting

• Splitting is straightforward, but so is merging
• Make two priority queues, Qs and Qm
• Add another check
• if T is too large or too accurate
• identify lowest priority elements T and TB in
  Qm
• Merge (T, TB)
• Update queues
• Remove all merged children from Qs
• Add merge parents T, TB to Qs
• Remove T, TB from Qm
• Add all newly-mergeable diamonds to Qm

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Comparison
Lindstrom ROAM PM
elementary operation complex,1 vertex reduction basic,1 vertex reduction complex,2 vertices reduction
triangles rigth-isosceles rigth-isosceles introduces slivers
complexity O(mesh size) O(triangle changes) O(mesh size)
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LOD for Games

• Bryan Turner, gamasutra.com
• Split-Only ROAM
• No frame to frame coherence, but still performs
  very well
• Seamus McNally, SMTerrain uses this same approach
• GDC 2003 Course Materials Many notes
• http//lodbook.com/course/2003/