Extrusion surface pulling away from edge

hi all,

I’m using Rhino 7 (7.7.21160.05001) for Windows (Windows 10.0.19043) with a Nvidia GeForce GTX 750 Ti.

I’m having an issue with an extrusion where the perceived surface of the extrusion in becoming polygonal and ‘pulling away’ from the edge that defines the extrusion.

The extrusion was made from a closed polyline made up of a series of curves and straight polylines joined together. I thought this might just be an issue with how the surface is rendered in Shaded mode, but the issue remains even when I run high-quality renders of the affected areas.

I can approximate what I am looking for by extruding the curve as a SubD, which solves the edge issue, and then creating a planar surface from the curves to provide a ‘top’ and ‘bottom’ to this form. These lines represent curbs in a rendering, so it doesn’t matter if the ‘top’ and ‘sides’ are joined or not. But I am more just curious what is happening here and if I can fix it / change my workflow to avoid it.

Any thoughts on the matter? I feel as though this question may have been asked before, but my googling is not producing matching results…

image and .3dm attached:

extrusion-edge-issues.3dm (189.1 KB)

Hello - there are two things you can do to clean up - both are related to how the display mesh is generated - that is the thing that looks ‘off’ in the display.

  1. Set finer/moreaccurate render meshes in either the object properties or globally for the document in DocumentProperties > Mesh page. See Rhino Mesh Settings [McNeel Wiki]

  2. In this case and many others, it is helpful to split single surfaces at tangents - DivideAlongCreases > SplitAtTangents=Yes.


Thanks Pascal! First one works and the second works and makes a lot of sense–will keep that solution in mind in the future.

Thanks also to all at McNeel, you all are so quick and helpful!