Add example 42 : BunnyLOD

[cloudwu]

I reimplement https://github.com/dougbinks/BunnyLOD in bgfx today.

There are still some issues:

1. I use the shaders vs_picking_shaded and fs_picking_shaded of 30-picking.
2. Init is too slow ( 8 seconds in my computer) , because the algorithm is O(n^2) , and bunny.bin have more than 100k vertices. A simpler mesh would be much faster.
3. This LOD algotithm need removing the vertices with the same position. The exist bunny.bin have many vertices with the same posiotions (different normals). I wrote functions findDuplicateVertices and mergeVertices to merge these vertices. It's O(n^2) , too. But I think it can be done offline.

[bkaradzic]

One thing you could do is cache result like 40-svt does. It's slow only first time, and on second load it just uses cached results.

There is issue with vertex count when lowering LOD level:

[bkaradzic]

This LOD algotithm need removing the vertices with the same position. The exist bunny.bin have many vertices with the same posiotions (different normals). I wrote functions findDuplicateVertices and mergeVertices to merge these vertices. It's O(n^2) , too. But I think it can be done offline.

There is utility function bgfx::weldVertices that should output only unique vertices.

https://bkaradzic.github.io/bgfx/bgfx.html#_CPPv4N4bgfx12weldVerticesEP8uint16_tRK12VertexLayoutPKv8uint16_tf

[cloudwu]

I'll improve this example tomorrow.

There is issue with vertex count when lowering LOD level:

If there are holes in the mesh, the hole would enlarge when the mesh collapse to lower version. The mesh bunny.bin is not closed at the bottom. I think it's a defect of this LOD algorithm.

[bkaradzic]

is not closed at the bottom. I think it's a defect of this LOD algorithm.

Makes sense. Switched to bunny_patched and it works now.

[cloudwu]

I want to use bgfx::weldVertices today, but I found it use uint16_t for index. Is there any reason for this limit ?

bunny.bin has about 100K (>64K) vertices.

[bkaradzic]

None. I'll update API to be 32-bit.

[bkaradzic]

Updated: 977188e

[cloudwu]

Done #2156