We use two different approaches in Slicer for fitting closed surface on an arbitrary closed curve: “ProjectWarp” and “DiskWarp”. They are described here. Baffle planner uses DiskWarp, which is more tolerant to out-of-plane points, but only works for quasi-circular contours. Markups closed curves use “ProjectWarp”, which works for arbitrarily concave shapes, but more sensitive to contour points being out of plane.
If you find the surface computed by baffle planner appropriate then you can go to Models module and get the surface are in the Information section:
