When sculpting high-res meshes, the total number of nodes in the BVH
tree can get quite high and — especially when the brush radius is low —
comparable to the number of vertices influenced by the brush. Under
these conditions, algorithms that run on each node of the BVH,
especially if single-threaded, can become a bottleneck. This issue is
particularly prominent in multires, where the leaf limit is 2500.
The main culprit is `BKE_pbvh_redraw_BB`, which is used when flushing an
update to calculate the redraw bounds. Because partial drawing is
currently broken, and because a future reimplementation of partial
drawing is unlikely to adopt the existing approach, this PR removes the
partial drawing logic from `flush_update_step`.
This patch provides a speedup ranging from 1.25x to 1.45x, depending on
the brush size, when sculpting a small portion of a multires mesh with
roughly 8.1 million faces and 12.6 million vertices using the draw
brush.
Pull Request: https://projects.blender.org/blender/blender/pulls/136471
d001e143a9