Terrain LOD: Runtime TIN Approaches
- one approach to LOD is called VIPM, for View
Independent Progressive Mesh, or VDPM, View Dependent Progressive
- VIPM is where your collapse order is fixed - the only thing
the observer's position does is choose how many tris to expand/collapse
- VDPM varies the order of collapse depending on where the view
is, so bits further away from the camera get collapsed before bits near
- 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
- Temporal Continuity of Levels of Detail in Delaunay Triangulated
- 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 /
- CAT is "Continuously Adaptive Terrain"
- ASD is "Active Surface Definition"
- "fine grained adaptive LOD"
- part of
- 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
- "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