Resolve recalculation of mesh triangulation #121240

New Issue

Hans Goudey · 2024-04-30T03:59:21+02:00

Hans Goudey commented

2024-04-30 03:59:21 +02:00

Currently the PBVH stores a separate array of mesh triangles, PBVH::corner_tris rather than reusing the cached triangles that are stored on the mesh already.

It does this because the triangulation depends on mesh positions, and every sculpt brush stroke changes positions.

However, duplicating the entire array of triangles adds an extra 24 bytes of memory usage per quad, assuming the based mesh's triangles are also calculated. Lowering memory usage overall is a good goal and can also improve performance, so we should resolve this somehow. Reusing the cached triangulation could also make entering sculpt mode faster.

Sculpt mode drawing is performance sensitive-- we don't want to recalculate the triangulation while sculpting. Shuffling triangle corner indices while sculpting would probably require updating the PBVH nodes as well.

Some options:

Try to delete the base mesh triangles SharedCache when entering sculpt mode. This would prevent storing the triangles twice (unless some other active code also depends on the mesh's triangles too). However, it doesn't bring any benefit from caching.
Extend the SharedCache system to take ownership from it when entering sculpt mode.
Remove the corner_tris storage in PBVH and only use the cached triangles. There are a few sub-options here:
1. Tag the cache dirty, but extend SharedCache so that we can still use the invalid state for sculpt mode drawing and for PBVH raycasting, etc.
2. Tag the cache dirty, but recalculate triangulation of affected faces and adjust internal PBVH node indices as necessary.
3. Don't tag the cache dirty. Tag it dirty when leaving sculpt mode to avoid longer term invalid triangulation.
4. Tag the cache dirty. Use potentially invalid triangulation during a stroke, update triangulation and PBVH node indices after strokes.

I think option 3 is far better and progresses towards the goal of #118145. But I'm not sure about the specifics.

Currently the PBVH stores a separate array of mesh triangles, `PBVH::corner_tris` rather than reusing the cached triangles that are stored on the mesh already. It does this because the triangulation depends on mesh positions, and every sculpt brush stroke changes positions. However, duplicating the entire array of triangles adds an extra 24 bytes of memory usage per quad, assuming the based mesh's triangles are also calculated. Lowering memory usage overall is a good goal and can also improve performance, so we should resolve this somehow. Reusing the cached triangulation could also make entering sculpt mode faster. Sculpt mode drawing is performance sensitive-- we don't want to recalculate the triangulation while sculpting. Shuffling triangle corner indices while sculpting would probably require updating the PBVH nodes as well. Some options: 1. Try to delete the base mesh triangles `SharedCache` when entering sculpt mode. This would prevent storing the triangles twice (unless some other active code also depends on the mesh's triangles too). However, it doesn't bring any benefit from caching. 2. Extend the `SharedCache` system to take ownership from it when entering sculpt mode. 3. Remove the `corner_tris` storage in `PBVH` and only use the cached triangles. There are a few sub-options here: 1. Tag the cache dirty, but extend `SharedCache` so that we can still use the invalid state for sculpt mode drawing and for PBVH raycasting, etc. 2. Tag the cache dirty, but recalculate triangulation of affected faces and adjust internal PBVH node indices as necessary. 3. Don't tag the cache dirty. Tag it dirty when leaving sculpt mode to avoid longer term invalid triangulation. 4. Tag the cache dirty. Use potentially invalid triangulation during a stroke, update triangulation and PBVH node indices after strokes. I think option 3 is far better and progresses towards the goal of #118145. But I'm not sure about the specifics.

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Hans Goudey added the

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label 2024-04-30 03:59:22 +02:00

Hans Goudey added this to the Sculpt, Paint & Texture project 2024-04-30 03:59:22 +02:00

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Resolve recalculation of mesh triangulation #121240