Terrain LOD: Runtime TIN Approaches

• one approach to LOD is called VIPM, for View Independent Progressive Mesh, or VDPM, View Dependent Progressive Mesh
  • VIPM is where your collapse order is fixed - the only thing the observer's position does is choose how many tris to expand/collapse to.
  • VDPM varies the order of collapse depending on where the view is, so bits further away from the camera get collapsed before bits near the camera.
    • VDPM theoretically has fewer screenspace collapse artifacts
    • generally has higher realtime computation costs
  • these approaches are widely used for static isolated objects, but are not generally well suited for terrain heightfields
• Hugues Hoppe at Microsoft
  • most of his work is on general mesh simplification, but he does tackle terrain:
  • Smooth view-dependent level-of-detail control and its application to terrain rendering, IEEE Visualization `98
  • using several techniques to minimize memory requirements, this VDPM approach still needs 48 bytes/height field element
  • there is a executable demo (Windows, OpenGL): Rendering of Terrains Using Geometry Clipmaps - Puget Sound Flythrough Demo
    • it actually works quite well and smoothly on a 16k*16k grid
    • even with paging, RAM consumption is very high
• Temporal Continuity of Levels of Detail in Delaunay Triangulated Terrain
  • Cohen-Or, D. and Levanoni, Y. from IEEE Visualization '96 Proceedings
  • "all the advantages of a TIN, yet no artifacts because of runtime morphing between tessellations"
  • claims that a 150,000 poly database runs well on an old platform (P120 with Renderware under Win95)
  • at the highest level of detail, exposes the regular mesh! - a desirable trait, but not likely to be memory-efficient
• SGI / Lockheed-Martin's CAT / ASD
  • CAT is "Continuously Adaptive Terrain"
  • ASD is "Active Surface Definition"
  • "fine grained adaptive LOD"
  • part of Performer
  • Quadtree is based on iteratively divides each triangle into 4, morphs at runtime.  Doesn't seem to take screen error into consideration.
  • "terrain cutouts to fit cultural features"
  • old MultiGen II (SGI only) had a "TerrainPro" option with "CAT":
    • Continuous Adaptive Terrain - CAT
    • "the most advanced culling and Level of Detail (LOD) morphing available"
    • "compatibility with SGI Active Surface Definition"
    • Because it is very CPU intensive, it requires a multi-CPU machine.  CAT also requires a custom runtime. SGI Performer is the only system to support it.  It proves very difficult to add cultural features such as roads and rivers to the terrain skin. No way exists to morph cultural features out of the high resolutions. Thus any culture introduced into the scene would persist across all levels of detail.  The runtime component part of this technology is under patent contention between SGI and Lockheed Martin.
• reportedly runtime TIN LOD is used in some commercial game titles like Flight Unlimited II