V6 - Surface from Planar Curves results in Trimmed Surface - BUG?

I have just landed on something that is giving me cold sweats. Turns out I now know what is going on, but it took a little while to figure it out.

In Rhino 5, I sometimes create a surface by drawing a rectangle, and then use the “Surface from Planar Curve”. Probably not the most efficient, but it has worked fine until now. The surface I get is an untrimmed surface - the control points are on the edges.

If I do the same thing in Rhino 6, I end up with a TRIMMED surface whose control points are outside of the original rectangle. I could understand this would make sense in the case of a very weird set of edge curves, but for simple planar rectangular surface I find this very confusing. The danger is that in our workflow, unless we catch this early, it can lead to all sorts of complications later. The solution is obviously to create surfaces using the create surface command, and not the planar surface command - at least for simple rectangular shapes. Still, I am not sure that it makes sense to create a larger surface and trim it if one could create a surface that matches the original quadrilateral exactly…

Curious to get some thoughts…



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Hi Tom - I see that - this was the case in V5 for a while then was fixed - I don’t know if this V6 behavior is by design or not, I’ll check on it, thanks. I do know that for example FlowAlongSrf has been tuned up to use the ‘shrunk’ version of the input surfaces, so that removes one of the problems that was caused by this - anyway, I’ll see what I can find out.

(Using the Plane command is probably quicker anyway, but hopefully we can get this fixed.)


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Hi Pascal,
Thanks for taking a look. The plane command is surely the better approach. I think the reason I ended up using the rectangle->planarSrf approach is the default behavior to create a plane is 4 points (in the toolbar), which is more data than I care to enter most of the time… Just being lazy!
Now that I know the behavior I can be cautious, and perhaps I’ll just get used to the plane command!

Hi Tom- no… that is SrfPt, if I follow you , from corner points, and not constrained to being planar, but the Plane command has the same UI as Rectangle - (surface menu > Plane or this image icon.


I had this same issue in Rhino 6. I’m so glad I caught the bug early on, as this would have been a very expensive fix had we not picked it up! My workaround was joining the rectangle curves, then using the “boundary surface” command.

If I make an axis-aligned rectangle here and call PlanarSrf, I get an untrimmed surface as a result. (6.4.18100.1591, 10-04-18)