Fast GPU voxelization update:

• new algorithm for .stl mesh GPU voxelization: ~500x faster now, from minutes to milliseconds
• added unvoxelize kernel, to quickly remove all boundaries in the mesh bounding box.
• removed old hull voxelization algorithm

Old: naive GPU voxelization

• For each voxel in the 3D grid, cast a ray from the voxel center in an arbitrary direction, and check with all mesh triangles for intersection.
• Count the number of intersections.
• Odd number of intersections means the voxel is inside.
• Runtime: N³×Triangles

New: fast GPU voxelization

• Only for the 2D bottom layer of grid points, shoot vertical rays upward and check with all mesh triangles for intersection.
• The vertical rays pass through all voxels in the columns above, so these don't have to be checked for ray-mesh intersection at all.
• Store all intersection distances in a short array in registers.
• Sort this array with insertion sort.
• Iterate through the vertical column of voxels.
  • The first voxel is inside/outside depending on odd/even total intersection count.
  • Each time one of the stored distances in the sorted array is passed, switch inside/outside state.
• Optimizations
  • Only check inside the bounding box of the mesh.
  • Don't always start from the bottom (z-direction), but from the direction where the mesh bounding box has the smallest cross-section area, so the smallest number of ray-mesh intersections have to be tested.
  • To avoid errors on the odd/even total number of intersections, shoot a second ray in the opposite direction and only count the intersection number. Both have to be odd for the bottom voxel to start in inside state.
• Runtime: N²×Triangles, if N=500, this is 500x faster than naive voxelization

Known issues:

• voxelization might not always produce binary identical results in multi-GPU (floating-point round-off on ray-triangle instersection distances may differ for different ray origin) --> fixed in v2.16!