Prior to the brush refactor in 4.3, the cavity automasking feature
had an implicit dependency on stable position values. In many cases,
such as during brush evaulation, this was upheld by the usage of the
proxy system, which meant that even if a vertex's neighbor was changed
during a stroke in paralell, these changes would not be accessed until
after a stroke finished.
With the new method of brush evaulation removing the proxy system, this
introduced subtle artifacts on BVH node boundaries, as vertices were
guaranteed to change and cause differences as there was no temporary
storage to use the "old" position data. This effect was most noticable
in 4.3 and beyond in multires sculpting, though similar artifacts can be
seen when using the mesh filter in all prior versions.
To fix these issues, instead of calculating the internal cavity factor
during a stroke or during deformations, we calculate it ahead of time
based on the affected nodes of a stroke. This has the benefit of now
behaving consistently for a given mesh and given brush or filter
application.
From a performance perspective, this change should have no noticeable
impact, as each BVH node, and by extension each vertex, only has its
corresponding cavity factor calculated once, when the stroke would
affect the node.
Related: #102745
Pull Request: https://projects.blender.org/blender/blender/pulls/133224
6b0fa709fa