Creating surfaces primitves from a mesh

Rhino Rhino for Windows
1

I’m starting with scanned meshes and trying to reverse model a part. I’m having good luck with the following workflow for planes:

  • ExtractMeshFaces to select meshes that represent the plane
  • using PlaneThroughPt to create a small primitive surface
  • scaling/translating the primitive to cover the original mesh
    The goal is to use CreateSolid in the end once my volume is defined and then finishing with fillets/chamfers.
    The problem is that not all of the primitives are planes. I’d like to find a hypothetical “CylinderThroughPt” to create another surface. Does such a beast exist or are there other workarounds that use the existing mesh? There are alternative ways to do this but I’d like to stick with existing meshes as the starting point since future primitives may not be so easily generated. For example:
  • create a CPlane on the end of the cylinder by selecting vertices
  • draw a circle on the new plane
  • extrude the circle.
    Below is a screenshot of the work in progress. Pale yellow are the extracted meshes, blue are the planes through the meshes, and red are scaled versions put in place. The problem area is the cylindrical piece towards the back.

Thanks for any help

2

Hi Steve - one way to get a cylinder is to make a section (Section command) through the cylindrical area, then turn on control points for the resulting polyline - use all or some of the points for Circle >FitPoints and extrude the resulting curve.

-Pascal

3

Pascal,

That worked very well!
It just requires a bit of caution to:

  • ensure that the section is normal to the longitudinal axis of the cylinder
  • ensure that the section is created through an appropriate area (i.e. good scan data)
  • that the appropriate control points are selected (I wonder if a curve rebuild would help here)
  • and that the CPlane is parallel to the original section plane.
    Definitely workable but not quite as simple as selecting a whole bunch of meshes to create an “average” cylinder.
    Below is a screenshot of the “section”. The “blobby” curve in the middle is actually a hole in the cylinder and shows what poor scan data looks like in occluded areas.
    Thanks

Steve

blob.jpg686×547 107 KB

4

Right - that may be the tricky bit.

-Pascal