Surface control point question (surface pinching/distortion)

Hi ,

Question about surface control points . The way I modeled attached shape ( a small part of bigger model split by isocurves , shrinked) was :

  1. I drew outline of one half and center line with equal amount of points .
  2. Lofted the to curves “straight” to get two control point surface and arranged the curve control points .
  3. Changed the degree of the surface to U=5 V=3 .
  4. Mirrored the shape and raised the center 6 control points (three U from both sides) to get continuity and tweaked them to follow a curve .
  5. Added few knots to define the approximate shape I wanted .

As a result I have few unnecessary control point stacked very close to other points in center line that cause small warp in zebra and curvature analyses and they can’t be deleted .
I can join or match or merge the two halves , joined or merged it is watertight with no bad objects .
I can also stack some pair of points ( or all four pairs ) on top of lower pair of three highlighted pairs of points in the attached picture to reduce the warping .

The problem is probably the way I made the surface - closely stacked points in the start of the center line where the loft goes from vertical to diagonal and finally horizontal . I found a tutorial suggesting this , very easy and small amount of control points for easy shaping . In the actual model I added few knots where I needed more refinement after basic shaping.

Is there a way to delete individual surface control points afterwards or should I construct the shape other way (or try damage control with stacking control point pairs in top of other pairs) ?

Attached is the surface test 3dmRhino6_eval_test.3dm (203.9 KB) with zebra and surface analysis picture where the pairs of closely stacked control points are highlighted ( points on the center line) .

Hello - I would construct it slightly differently I think - with this type of structure:


Thanks Pascal , looks good .

I’ve tried to edit the surface by removing knot and points that tighten the center for mirroring and just extrude the center curve section (removing unnecessary points) and trimming it to fill the top .

Problem is when matching side curvature to the trimmed top , it should work other way - top to side . The center/top section in whole model is a bit complex going from flat angled section (to another flat angled section) and ending to the attached where it starts to curve (the part I posted).

I would be interested to see how you built your version , I’d appreciate to see the 3dm file if you can attach it .

  • P

Hello - My inclination is to simplify - you mentioned above that you moved points around

So, I’d set things up to make this as easy and predictable as possible.

Currently your edge curves are a little messy, curvature-wise

I’d try to avoid that - for example, I’d assume the centerline is meant to be a line or very shallow arc-like curve and not have that spike in curvature at the end.

See if the attached file has anything good for what you need. The general idea is to keep surfaces rectangular/ four-sided with relatively even point spacing, for better curvature control.

Rhino6_eval_test_PG.3dm (618.4 KB)