Hi Bruce - that surface has some bends in it which are of tighter radius than that distance.

That will create some unpredictable results - it basically causes the offset surface to kind of "bunch up" in that area. It seems that MoI's offset mechanism is a bit more sensitive in trying to deal with that kind of messy bunching than Rhino for that particular case.

If you look more closely at the result that Rhino gave, you can see that there are still bunching problems in the result, in these marked areas:



An offset of a distance greater than the radius of curvature is something that you generally will need to avoid doing in both MoI and Rhino - even though Rhino gives a result at distance 2.0, and MoI gives one at 2.6 all of those are kind of messed up surfaces in those areas there, you need to stick to an offset distance that is less than the surface's curvature in those areas to get a good quality surface result.

It looks like the radius 2 problem in MoI is that it is getting confused about how to apply the trim curves from the original on to the new messed up offset. I may be able to tune this up in the future so that it creates a result, but it won't actually do a whole lot of good because the result is just not a good surface due to that bunching.

- Michael