unrollSrfUV - BUG

i wanted to post a nice workflow for this helix question but i came across a bug in _unrollSrfUV

part of the surface in question (green) behave like i expect - gettting a parallelgram-shap

but as soon as the surface has more then a turn - the result of unrollSrfUV (red) is completetly unexpected - as fare as i hope i did understand _unrollSrfUV - @pascal or @theoutside - it is a bug. (?)
(if it s not a bug - please explain the triangular result)

_unrollSrf (no UV ) behaves like expected.

unrollSrfUV-Bug.3dm (157.8 KB)

kind regards -tom

Interesting and it looks like a bug.

An observation:
The red surface is degree 3 x degree 3. Use ChangeDegree to transform the surface to degree 1 x degree 3 without changing the shape. UnrollSrfUV correctly unrolls the modified surface.

In general degree 1 x degree n surfaces (or degree n x degree 1 surfaces) work more reliably with UnrollSrf and UnrollSrfUV then when the ruled direction is degree 2 or higher. When one direction is degree 1 it is obvious which direction is the ruled direction.

UnrollSrfUV is actually an older version of UnrollSrf. UnrollSrfUV is more reliable at preserving the UV mapping onto the unrolled surface, but is not always as accurate for the unrolled shape as UnrollSrf.