Hi nos, the size of the target surface is... well, it's size.

Every NURBS surface has a 4-sided layout to it the same as your plane there, with a closed surface it's just like you have a sheet of paper that is rolled up so that 2 of its edges are touching one another.

If you can imagine that sheet of paper being rolled up into the shape of your closed target surface there, the size is the size of that sheet.

So also the "seam edge" of the surface (where 2 of the edges of the rolled up surface touch each other) is significant - that will map to 2 of the edges of the flat plane. You may want to use the SrfSeam command in Rhino to move the seam to some other spot if you want your placed surface to start in some other spot than the current seam.


re: keep the size the same - that's generally not possible to do with these kinds of morphing transformations. A surface is not restricted to only have one fixed size in a direction, it can easily have things pinched together in some areas and be wider in other areas, and with your bendy surface it looks like you have that kind of a thing since it isn't just one single rigid shape in one direction of it.

If you wanted to preserve the size, that would require some kind of evenly shaped target and not one that was stretching and deformed in different areas of it.

- Michael

Hi nos, also in some cases you might want to use the Flow command in Rhino instead of FlowAlongSrf.

The Flow command uses curve objects to define the transformation, from a base curve to a target curve. That may be a bit easier to visualize the result, I believe the distance as traveled along the base curve will map to distance traveled along the target curve in that case.

FlowAlongSrf on the other hand does not map distance to distance in a direct 3D distance sense - instead it will map between the UV space of the base surface to the UV space of the target surface. So this means that things like compression of the UV parameter space in the target surface (things like surface control points unevenly distributed) will result in uneven distribution of the morphing as well.

With the curve-based Flow command being distance based it may be easier to get a distance preserving result.

- Michael