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Problems capping objects

Explanation of common capping failures and suggestions for fixing them

An often asked question on the Rhino newsgroup is “why didn't my solid extrusion get closed?” or “why can't I cap this object?” In the case of the Extrude command, Rhino currently doesn't give any error messages or explanations of why the command didn't work as expected, it just fails silently leaving the inexperienced user to wonder what they did wrong… In the case of the Cap command, you get a message “unable to cap that object”, which isn't very helpful to diagnose the problem. Hopefully the explanations and procedures on this page will help…

Extrusions that won't cap...

Check for bad objects first

All of the following is based on having valid objects. So, before you start, use SelBadObjects and make sure your objects are good (they shouldn't select). If there are bad objects, you need to fix them first. For more info on dealing with bad objects, see here

OK, my objects are good, so what's wrong then?

Rhino needs to have a closed, planar curve to create a cap surface. If an extrusion of a theoretically closed planar curve doesn't “cap”, even though you chose Extrude from the solid menu or clicked on cap=yes in the command line, there are one of two things wrong…


1) the curve being used isn't closed, or
2) it isn't really planar.

Not Closed

If there is no closed loop, Rhino cannot create a complete trim curve to create the cap surfaces. The object will extrude, but since the Rhino can't find a complete closed curve to trim the cap surface, it won't be created, and you will be left with an open extrusion. In Rhino 4, there is no error message given in this case…

How to find out if your curve is open? Several ways:

  • Use the command SelOpenCrv and see if it selects.
  • Use the command SelClosedCrv and see if it doesn't select.
  • Select the curve and look in Properties>Details (or use the command What) - it should tell you if it's closed.

There are many methods to use to close an open curve, depending on what the problem is…

First, you need to find the open spot:

The commands CrvStart or CrvEnd will place a point at the start or end of the curve, which is where the opening will be found.

You may then turn on points and try moving one endpoint on top of the other with point snap or otherwise edit the object to close it.

Not Plana

rhino/capproblems.txt · Last modified: 2015/05/21 (external edit)