How to close an open curve to make a solid shape in Rhino?

ByCADbrief Staff

Short Answer

Yes— in Rhino 3D, you can close an open curve and turn it into a solid shape by first making the curve closed, then using PlanarSrf and ExtrudeCrv with Solid=Yes. This is the fastest and most common professional workflow for flat profiles. Limitation: it only works cleanly when the curve is planar.

What You Need to Know Before

Warning: If the curve endpoints are not actually touching within Rhino’s file tolerance, the curve may look closed but still fail to make a surface or solid. Another common failure is trying this on a non-planar curve, which prevents a proper planar surface from being created.

How to Close an Open Curve to Make a Solid Shape in Rhino

  • Command: CloseCrv

  • Shortcut: None by default

  • Quick Steps:

    1. Select the open curve, then run CloseCrv from the command line or Curve tools.
    2. If needed, check the curve with Properties or run What to confirm it is now closed.
    3. Create a surface with PlanarSrf, then extrude it with ExtrudeCrv and set Solid=Yes in the command options.

Variables & Settings

  • Key Setting: Solid=Yes in ExtrudeCrv

  • Expert Setting: This option adds caps automatically during extrusion, creating a closed polysurface instead of an open extrusion. If this is off, you will get walls without a sealed top and bottom.

Why it Fails

  • Cause 1 (Geometry): The curve is non-planar, self-intersecting, or has endpoints too far apart for CloseCrv to produce a valid closed boundary.

  • Cause 2 (layers/Locks): The curve or related geometry is on a locked layer, preventing editing or causing you to select the wrong object.

  • Cause 3 (Command/Logic): CloseCrv only closes the curve; it does not make a solid by itself. You still need PlanarSrf and then ExtrudeCrv with capping enabled.

Quick Fix & Best Practice

  • Quick Fix: Run CrvEnd to inspect the endpoints, then use Join or move the endpoints together before running CloseCrv again.

  • Manager’s Verdict: Use this workflow for 2D planar profiles that need fast solid creation for modeling, fabrication, or concept design. Avoid it for messy imported geometry until you first clean overlaps, gaps, and planarity issues.

FAQ

Can I make a solid directly from an open curve in Rhino?
No, the curve must first become a valid closed boundary.

What if PlanarSrf does not work after closing the curve?
The curve is usually non-planar or self-intersecting.

Is Join better than CloseCrv?
Yes, if the shape is made of multiple curve segments that should form one closed loop.

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