Closed polysurface has overhanging surfaces when shaded or rendered

Happy Twenty 19 to all.

Version 6 SR12
(6.12.18351.21491, 17/12/2018)

The attached closed polysurface displays overhanging top and bottom surfaces on the inside of the curve in Shaded, Rendered, Ghosted, etc views. It looks OK in Wireframe, and ShowEdges exposes no stray edges.

I started with a polyline. From that, I created the outer vertical surface. I then created an offset curve from the initial curve, to use as the basis for the inner vertical surface. The top and bottom surfaces were created from the edges of the vertical surfaces, as were the end surfaces. At some point, I deleted the original polyline and the offset curve.

At that stage, I noticed the overhangs on the top and bottom surfaces, so I deleted the inner vertical surface and the top and bottom surfaces. I then recreated the inner vertical surface by sweeping the inner vertical edge of the far end surface along the lower edge of the outer vertical surface. The top and bottom surfaces were again created from the edges of the vertical surfaces. The overhang problem was the same.

Given that it is a closed polysurface, I cannot see why there is a rendered overhang.

HouseRemodelFStrayEdges.3dm (5.5 MB)

Hi, I wonder if it is a render mesh setting? You could try and investigate render mesh settings—Mark

Hi Mark. That does not seem to be the culprit. It is set to Custom with high density. It degrades if I change to the other settings.

Explode your polysurf and turn on points. You’ll see that the top and bottom surfaces are quite the mess.

I’m still pretty much a novice in this area, but I’d _DupBorder on the bottom and top surfaces. For each surface turn on points (_POn) for then use _PlaneThoughPt. Trim the new plane with the curve. You should end up with much better surfaces.

The top done with the process:

Hi Garry = Nathan has it right- the top and bottom faces of this thing are a mess. The quick and easy way out is: ExtractSrf and delete the top and bottom faces, then Cap the object that is left.


Thank you Nathan. I have followed your instructions and all is now as expected. A simple rectangular surface would have achieved the same outcome. Lesson learned regarding creating surfaces from the edge of complex surfaces.

Hi Pascal. I tried your suggestion too. I like the Cap feature.

I see that these flat surfaces were created with the Loft command with the “Do not simplify” option. If you want to create a lofted surface with more natural distribution of control points and the edges are not flat (i.e. unable to use the Cap command), then use the “Refit within” option and set the tolerance to be equal to your Absolute tolerance. There are a few alternatives to this:

  1. Sweep 2 rails (also try its “Add slash” button for further control).

  2. Blend surface with “Position” for both sides 1 and 2. It also has a similar way to control the direction of spans with the “Add shapes” button. Blend surface create a bit more control points overall.

For both commands you can try “Remove multiple knots from surface or curve” to see if it will help to simply the surface without destroying it at certain areas (sometimes it’s better to not use this command).

You can also create such surface with the “Network surface” tool, though it uses interpolation and adds too many control points in both directions, so I don’t recommend it.

This is also why Isocurves are turned On by default in several of the working display modes. Then you have visual feedback that the surfaces are a mess and need to be cleaned up.
Turning off isocurve display hid this important information from view.

Wish: I wonder is there a way to select an entire (or partial) row of control points and have a command to distribute them evenly along the control polygon? Especially if it’s possible to do this with just a few control points (i.e. not the entire row), it should greatly help to improve the fine adjustment of surfaces. It would help even with fixing the existing top and bottom surfaces of 3d model from the original post, optionally followed by the “Match surface” command to make sure they will join with the vertical surfaces.