FitCurveToSurface fails more often than it works, what am I doing wrong?

I have been playing with the FitCurveToSurface command that is new in Rhino 8. More often than not the command fails and I am trying to understand what is going wrong.

  1. I have had no luck at all trying to get it to work on a doubly curved surface. Is that a limitation with the command or am I doing something wrong?

  2. The Rhino manual says that, “The curve must traverse the entire target surface in the U or V direction.” Which sounds pretty straightforward but I frequently get the error message , “FitCurveToSurface failed. The input curve does not traverse this entire surface in U or V direction.” Even when as far as I can tell the curve is equal to or longer than the surface I am trying to apply it to along the x or y axis (which appears to match up with the U V direction).

I am still relatively new to the software so it is entirely possibly I am missing something obvious.

I don’t have any particular design I am trying to create, I am just trying to understand how the tool works so I can add it to my arsenal. My thought when I first heard about this command was it might be a way to put a curve on a surface with less distortion than other methods but I am not really seeing a way to use it that makes it better than something like Flow Along Curve or Pull. Would appreciate any information to help me better understand its use and what benefit it has over similar commands. Thanks.

Please post a simple example and the detailed steps you’re following to make it fail.
With that, we should be able to repeat the problem and figure out what to do.


Hello - the command works on doubly curved surfaces but must traverse the surface completely like so:

Red= output.


Thank you. On the dome I get the “The input curve does not traverse this entire surface in U or V direction.” I have noticed that when I select the surface in the command it appears to only show one quadrant selected but when I select it outside of the command it looks normal. The dome was made with rail revolve if that matters.

The half pipe sort of shape does project something but it’s not the shape I started with (you can see the result at the top). I tried from above and below. One was a control point curve and the rest are a string of hyperbolas that I joined. Was basically just throwing stuff out to see what might work.

I think I just used the default settings on the FitCurveToSurface command (unless I inadvertently changed something). Maybe I need to adjust something?

fitcrvsrf-test2.3dm (207.5 KB)

Thank you, but I have the problem even when the curve is significantly longer than the surface.

Hello- yep, that is correct - it traverses - due to the polar structure - not strictly in U or V but across both. The U direction is circular and the V is radial in this case - not an ideal one for this command.


Ah, thank you. That is helpful.