Can not flow along surface

This model is a part of backrest I import from step file. Currently, they contain 5 surfaces, I join into 1 surface. Then I use unfold surface UV. I intend re-model this surface then using Flow along surface in order to wrap it back.
But I only can select a part of surface. detail in photo below:

try roll-rhino6.3dm (9.7 MB)


Your object is a polysurface, not a single surface. FlowAlongSrf only works with a single target surface and a single base surface. It does not work with polysurfaces.

Join when used on surfaces creates a polysurface. It does not merge the surfaces into a single surface. A polysurface is a collection of surfaces “glued” together. Each individual surface retains its own UV parameter system.

Thank @davidcockey for your explanation. Do you have suggest to make it single surface?


I actually had to tackle this very problem this week:

Hello - in this case, you can jump through a couple of hoops and get something that works - it might be easy enough to reconstruct that shape as a single surface as a target for FlowAlongSrf and then Pull the results to the existing polysurface.

Or you can merge the surfaces - I’d do this on a copy - of your polysurface.

  • Explode
  • ShrinkTrimmedSrfToEdge (the surfaces extend slightly past the trimmed edges.
  • UntrimAll
  • ExtendSrf each edge at the top end bottom so they line up exactly


  • MatchSrf each pair of surfaces with Refine -

It makes the surfaces messy- that’s why we’re working on a copy. There are locations on the extended surfaces that fall verry slightly away from the neighboring surfaces.

  • MergeSrf to make a single quite messy surface.

  • UnrollSrfUV to make the base surface.

  • FlowAlongSrf with these two surfaces and then delete or hide that merged thing.




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