Modifying surface geometry converging into a single point (singularity)?

Hi, I’m having two issues with the geometry, first modeled as a solid symmetrical object,
then cut to half and trimmed away an opening. I figured I’ll work on half and run symmetry later.

  1. I don’t like the converging into a singular point at the top and bottom.
    Any idea how to modify it, how to rebuild the original geometry to eliminate the singular point (having smooth topology) and still remain as a single surface?

  2. I have no idea why are the control points of the part that was trimmed away still there? Even when rebuilt ( trim checked) and also shrink trimmed it. I’d like a clean edge there to be controlled exactly by the relevant control points (not imaginary geometry)._help1_wip_geometry.3dm (424.8 KB)


    What am I missing? Is there some history that needs to be disabled?
    Any idea?

Absolutely nothing to do with history.

All trimmed surfaces in Rhino is an untrimmed surface and trim curves. The control points are from the untrimmed surface. The untrimmed surface has isocurves as edges, and can only shrink until the isocurve edge encounters a trimmed edge.

The only situation where a trimmed surface can shrink and become an untrimmed surface is if the trimmed edges are exactly isocurves.

All untrimmed surfaces in Rhino have 4 edges. One, two or three of the edges can have zero length which results in a singular point where isocurves converge at each zero length edge… Also possible is to have adjacent edges which are tangent to each other at a corner, which results in the U and V isocurves tangent at the corner. Both situations, zero length edges and tangent corners, can have undesirable effects but are commonly used.

It is not possible to model your shape as a single untrimmed surface without zero length edges or tangent corners. This shape requires one or more of:

  • trimmed surface
  • multiple surfaces (ie a polysurface)
  • zero length edges with converging isocurves.
  • tangent corners with tangent isocurves.

An alternative for modeling this shape is SubD modeling. Rhino 6 has limited capabilities and Rhino 7 WIP has much more capability. SubD modeling is in many respects equivalent to using multiple single span surfaces.