refine distanceSurface proximity filter
The current region-based and proximity-based filtering work adequately for many topologies. However, with a central gap (eg, for double-D pipe), neither works particularly well. Selecting the largest region can fail spectacularly (choosing the gap and the ragged outside, since they have the largest number of faces). Using face-based proximity filtering will properly handle the ragged edges, but fails to detect the inside gap at all.
As suggested by Sven-Eric, should combine region-based and proximity-based. Use regionSplit for the topological connectivity, combined with a proximity metric for the regions.
Since the region filter is expected to occur, can increase the weighting of regions with bad faces by increasing the initial cell bounding boxes. This will increase the number of ragged faces even more, making an even clearer distinction.