Terrain Level of Detail

1. Terrain Level of Detail John Tran Computer Science Department University of Virginia <jdt6a@cs.virginia.edu>

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

10. A little harder to implement Forced Splitting Cracks and T-junctions are solved! Any two triangles differ by no more than one resolution level BinTrees (Binary Triangle Trees)

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 • 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 (www.vterrain.org) • Duchaineau’s ROAM homepage (www.cognigraph.com/ROAM_homepage/) • SOAR (www.cc.gatech.edu/~lindstro/software/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?