The multithreaded algorithm works by atomically assigning each face's
group ID to the surrounding edges. If the ID for the edge is different
than one set previously, the edge becomes a boundary.
Using the edge to face topology map was also tested, but it wasn't
faster, and given the large memory usage of the map, the increased
complexity of this algorithm was considered worthwhile.
Speed improvement for attached example file is listed in table:
| Cube resolution | Main | PR |
| -- | -- | -- |
| 20x20x20 | `71920 ns` | `97400 ns` |
| 100x100x100 | `1.27 ms` | `1.17 ms` |
| 500x500x500 | `79.37 ms` | `23.16 ms` |
| 1000x1000x1000 | `520.31 ms` | `142.21 ms` |
Pull Request: https://projects.blender.org/blender/blender/pulls/115138
5429e006c8