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Submitted by Jon Harrop, posted on May 30, 2002




Image Description, by Jon Harrop



These images are snapshots of a real-time level of detail planet renderer which I wrote last year.

The complete source code is available on my web site:

http://jdh30.ch.cam.ac.uk/~jdh30/programming/opengl/lod/index.html

Please feel free to use/abuse it for anything you want.

I did the project for fun and the technique used was invented on-the-fly. Basically, the program starts with an icosahedral mesh represented as a tree of triangular faces, edges and vertices. Split and merge operations are defined which allow the creation of a quad-tree (splitting an approximately equilateral triangle into four more). Split and merge operations are only allowed to result in -1/0/+1 change in LOD across any edge. Lists of faces which can currently be split or merged are maintained.

When an edge is split the central vertex is pushed outwards so that infinite subdivision would create a sphere. Terrain is introduced as perturbations to this edge splitting, either to the distance pushed out or to the surface normal (which affects future splittings). These random perturbations are generated using pseudo-random number generators seeded by the result of the parent in the tree. Therefore, the planet is defined to infinite resolution as an implicit surface and (at any time) only the information required to render the planet needs to be stored.

The program uses AGP memory and includes an interesting little library which maintains "render buffers" of triangles in the mesh. Each render buffer of triangles has an associated texture map part of the vertex array. The render buffers are plotted in turn, memcpy'ing the new texture map coordinates into AGP memory each time. This can sustain huge triangle throughput for meshes but isn't currently pushed by the LOD engine which requires the CPU to perform complicated per-triangle calculations. Switching to a LOD engine which handles bunches of ~100 triangles at a time would give (I estimate) a 50x increase in performance, smashing through 1Mtris/sec.

Cheers,
Jon.


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