Is it possible to Split a SubD with a plane?

Is it possible to split a 3d subD with a plane? Or does it require a different approach?

So imagine I want to have a sort of clipping plane that splits a subD into 2 parts and patches the cross section as a crease.

EDIT: I have managed this by splitting the subD with a planar surface but it turns the subD into a polysurface, This is fine for what I am doing, but is it possible to split the subD with a plane and still have a subD?

no, you’ll need to use a edgeloop

if you set an edgeloop to be planar then split by edge loop, you can achieve this effect.

Thanks for the tip… I can’t see how to set an edge loop to be planar; I only see options for loop or ring. Is it possible to make the edge loop lie on a plane without changing the shape of the SubD?

To make an edgeloop planar use the Align command and then the Fit Plane option when it is selected. This will change the shape but you can also use the Pull command to pull the edgeloop to a surface that defines where you want the edges. That may help account for the change.

If you use Split on a SubD, first run the command ToNURBS on the SubD to pack the faces into less surfaces. Then use Split on the much lighter polysrf that results.

From your SubD + planar surface do:
- ExtractControlPolygon
- MeshSplit with CreateNgons=yes
- join the pieces of the mesh
- convert back to SubD
- select your edge loop with ctrl+shift+ double click
- Crease it

The resulting SubD will have a perfectly planar edge loop where your plane is, and likely/possibly many Ngons. Shape will obviously be different.

Yes, I’ll say it once more:
Rhino mesh tools are many and useful.
SubD tools are way less.
Creases and Ngons are the same on Meshes and SubD.

A toggable function “treat all SubDs as meshes (the control polygon)” would be very useful.
For cases like this one ^

1 Like


I think using a planar polysurface to split the subD does the ToNURBS command on the subD anyway?

The reason I am trying to do this is because I am splitting a shape into separate parts to manufacture so I cannot have any change to the original shape. I need to export STL meshes for manufacture so it does not matter if the workflow is

SubD to NURBS (then split) to STL or

SubD (then split) to STL

as long as the shape does not change.

I just wondered if it was possible to split the subD and have subD solids with creases along the split lines but no change to the overall shape.

The resulting polysrf after the split will have many many more surfaces. Trust me, run ToNURBS on the SubD first and then Split it. Disable isocurve display in shaded mode to see how fewer surfaces you’ll get.

I trust you :slight_smile:

I couldn’t manage to align the edge loop to a plane… even when I created a new edge loop I couldn’t get it planar.

However SubD >> ToNURBS >> Split is fine!

That certainly gets me a planar split but I need the shape to stay the same :frowning:

Actually… Using ToNURBS doesn’t reduce the surfaces by much in this case…

I hope that this tedious workflow is automated in Rhino 8.

Yes, the packing will happen in areas where faces can be combined, more faces will equal more packing. Lots of faces comprising an even grid will make it more apparent.