Voxel "chunk" storage
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Posted 03 May 2012 - 02:10 PM
My issue is the following: in games like MineCraft where the voxels are cubes, N by N by N voxels actually contains (and completely defines) exactly N by N by N units of space. However, in systems where a mesh is extracted, it actually takes 8 voxels (centered on corners of a cube) to define a "patch" of the mesh. This means that to completely generate the region of N by N by N voxels, (N+1) by (N+1) by (N+1) voxels are actually required. This means that if each chunk is stored/generated in isolation, there is a duplicated row (or rather, "slice") of voxels stored. Furthermore, it is nice to use a power of 2 for N since it works well with LOD data structures (e.g. chunked-LOD) or octrees. However, this means that the actual stored voxel volume won't be a power of 2, and won't have nice cache alignment properties. It also means that updating voxels isn't as simple - you might need to update the data in multiple places - potentially in up to 8 chunks if it's a corner!
Actually though, the problem doesn't stop there - to generate smooth normals for lighting, you need to examine the surrounding voxel density values, so you actually need an extra row of voxels on each edge - this means you actually need (N + 3)^3 voxels!
A few possible approaches I've thought about:
1) Just deal with the extra storage. I really don't like this solution though, it seems messy and probably slow.
2) Require that 3x3x3 chunks be present in memory before generating the center chunk. I don't really like this though because it seems very wasteful and adds weird dependencies between chunks that shouldn't be there.
3) When generating the mesh for a chunk, leave off a row - that is, only generate a mesh filling (N-1)^3 units of space. When there are two adjacent chunks present, generate the strip connecting them separately.
I think solution 3 is the best, and it also allows for LOD stitching between chunks to be handled very nicely, since if one of the chunk's LOD level changes, only the strips connecting chunks need to be regenerated, and algorithms like http://www.terathon.com/voxels/ could be easily implemented. However, it still does not solve the problem of needing an extra row of voxels for normal generation! I can think of 2 possible solutions for this: either regenerate the normals on the edges of meshes when the adjacent chunk is loaded, or make the connecting "strips" 3 units wide instead of 1 unit wide, and just leave off an extra strip from the "main" chunk meshes. Neither of these seem that good though: the first solution would require re-uploading the entire mesh, or portions of the mesh, to the GPU, and would require keeping track of which vertices need to have their normals recomputed; in the second solution, an "isolated" chunk would be smaller in size, and the size of the strips connecting chunks would be larger and take longer to generate, but this seems like the better of the two.
So, how is this usually handled? Is solution 3 the way to go? And how are normals dealt with?
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Posted 04 May 2012 - 01:30 AM
So basically we focus on just providing fast volume data structures which are independant of the algorithm which is executed on them. Surface extraction is just one task you need to perform, and raycasting (for example) can have a different set of characteristics.
That doesn't exactly answer your question... but maybe it's something to think about.
Edited by PolyVox, 04 May 2012 - 01:30 AM.
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Posted 05 May 2012 - 01:10 AM
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Posted 08 May 2012 - 12:56 PM
Eric Lengyel - you're right, 16x16x16 doesn't sound too bad; I was thinking about chunks as being larger, but perhaps that isn't optimal (though obviously that's up to profiling to decide). My idea was actually to have a mipmap/chunked-LOD system, where a single mipmap pyramid would correspond to a single chunk (my reasoning for larger chunks). But maybe smaller chunks would work with this as well (the lower detail levels would just span multiple chunks). Are you using a LOD system?