1. Terrain Level of Detail
John Tran
Computer Science Department
University of Virginia

2. Why Terrain LOD?
“It seems you can’t shake a stick in the world of terrain visualization without hitting a reference to LOD Terrain Algorithms” –Bryan Turner (gamasutra.com)

3. The Data Regular Grid Height Field Tradeoffs?
Triangulated Irregular Networks (TIN)
Tradeoffs?

4. 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
Out-of-core
From Martin Reddy’s 2002 SIGGRAPH course

5. 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

6. Terrain LOD Basics
Cracks, T-junctions
How do we solve this?

7. Terrain LOD Basics 2
Can’t use edge/vertex collapse techniques from last lecture – Why not?
What can you do?
Ensure common vertices on edge of subdivision
Looks awkward
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

8. Terrain LOD Basics 3
Quadtrees and BinTrees

9. Quadtrees Each quad is actually two triangles
Still have cracks and T-junctions
Easy to implement

10. 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

11. Several Algorithms Lindstrom’s Continuous LOD ROAM (Duchaineau)
3D Bounding Isosurfaces (Blow)
Caching geometry (Vis2002)
Visualization of Large Terrains Made Easy
SOAR

12. 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

13. ROAM Mark Duchaineau, 1997 (LLNL) Binary Triangle Tree Structure
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
Probably the most popular algorithm today

14. 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

15. ROAM – Splitting and Merging diamonds

16. 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
How do we calculate these error metrics?

17. ROAM – Wedgies! Wedgie – basically a bounding volume
Covers the (x,y) extent of a triangle and extends over the height range z-eT through z+eT

18. 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 Duchaineau’s original ROAMing Terrain paper (96)

19. 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

20. ROAM – Notes
Also need to check if the previous frame is finished rendering and update priorities for all elements if not
For more details on algorithm, read the ROAMing Terrain paper

21. ROAM – Demo Bryan Turner, gamasutra.com “Split-Only ROAM”
No frame to frame coherence, but still performs very well
Seamus McNally, SMTerrain uses this same approach

22. Bounding ROAM with 3D Isosurfaces
Jonathon Blow (2000)
ROAM doesn’t work well for densely sampled data – large number of unnecessary wedgie calculations
Screen-space error metrics compress 3D geometric data into a 1D scalar value
Instead, use all 3 dimensions, and have bounding volumes (spheres) determine visibility
65% less triangles than ROAM

23. Caching Geometry Josh Levenberg, Vis2002
“Fast View-Dependent Level-of-Detail Rendering Using Cached Geometry”
Not yet published
Uses ROAM, but also caches geometry of “aggregate triangles” on the video card (VAR)
Claim 4x faster with caching

24. Visualization of Large Terrains Made Easy
P. Lindstrom and V. Pascucci (Vis2001)
Few dozen lines of C-code
Uses regular grid bintree
Now implemented as SOAR (Stateless, One-pass adaptive Refinement)

25. Visualization of Large Terrains Made Easy (2)
Focus on “the manner in which the data is laid out to achieve good memory coherency”
Using mmap system call
Let the OS take care of paging

26. Visualization of Large Terrains Made Easy (3)
“Longest edge bisection”
Monotonic!
Implicit parent-child relationships – no need for priority queues
Represent this mesh using a DAG of the vertices
They used a nested sphere hierarchy for object space and screen space testing (similar to Blow)

27. Visualization of Large Terrains Made Easy (4)
Separate threads for rendering and geometry updates
Mesh refinement, view frustum culling, and FULL triangle stripping
All done in one pass over the mesh
No Frame-to-frame coherence needed!

28. Visualization of Large Terrains Made Easy
τ = 2 pixels
79,382 triangles
τ = 4 pixels
25,100 triangles
τ = screen space error threshold

29. Implementing a terrain in your Scene Graph
Anyone have tips?
Most games use a modified ROAM algorithm
Although a static approach may be easy, it will be inaccurate and it will show
Keep the terrain fairly small if possible
i.e. Don’t have a 10k x 10k grid if you only want to show a single mountain

30. Implementing a terrain in your Scene Graph
Where to go for more info?
LODbook.com
Virtual Terrain Project (
Duchaineau’s ROAM homepage (
SOAR (
There are other basic algorithms
ie TIN-based algorithms

31. More Problems with Terrain LOD
QuadTIN (VIS2002)
Large Textures
Paging/Streaming and Out-of-core Techniques

32. Summary
Any questions?