In Rhino 8 by default isocurves are not shown for planar surfaces. This is a per-display-mode setting, there is no global setting that I know of. This is bad, but only tangential to the subject of this post.
So - draw a cube. Put the viewport in shaded mode. Make sure “show isocurves” is checked in the object properties. They will not be shown anyway because of course all the cube faces are planar.
Now, run ExtractWireframe on the cube. It will duplicate all the edges AND all the interior isocurves that are not shown.
I think thats because a planar srf is still a NURBS, and the curves are still needed to help define the srf nurb as mathematical constructs, so I think even if they dont display they still retain their U/V parametrization from a technical point of view, even for planarsrf. @nathanletwory maybe can let us know more regarding this
So I think the command is actually being “correct”, It’s extracting the isocurves, even though clearly in this case It was not intended.
Not sure how you could fix that, but I wanted to share some ideas on why you might face this issue
That’s not accurate as far as I know. On-screen isocurves in Rhino are purely a visual aid. You can make them as dense as you want (by increasing the number in Properties) or not show them at all.
Yes, it’s not expected or intended - it’s extracting stuff that is invisible, the user doesn’t necessarily want them and has no idea why they show up.
So if you have a standard 4-view layout and the perspective viewport is in Shaded and all other in Wireframe, when the user accidentally clicks inside the Top viewport and then runs ExtractWireframe, the result should be something else then when the user had not accidentally activated the top view?
Make2D works that way. If you have a clipped object and run the command from a wireframe viewport, the resulting curves will show an object with a hole. If the viewport is shaded, the hole is covered. This behavior continues to bite me…
Also, lets imagine that the ExtractWireframe command for some reason at some points becomes history-enabled, would you expect the output to change when the display mode changes?
-wim