Hi Strube, that's just an artifact of the display. It's nothing to really worry about, the actual surface there is fine.

In order for a surface to be displayed it gets broken down into triangles, and in those particular areas the triangles were a bit sparse, leaving some evidence of their linear edged nature in the display result.

Curves are drawn to a higher precision than surfaces, curves are actually broken down dynamically until the on-screen deviation is within a pixel of the true curve so the curve and edge display is actually highly accurate. It's unfortunately too time consuming to do the same dynamic refinement with surfaces, so surfaces get only a fixed world space triangulation instead of a screen adaptive one.

It's just a normal part of this display system (that includes compromises for the sake of speed) that you will have a display that looks like you show there, there is nothing actually wrong there at all.


> Does it mean no face can be created using nurbs?

Nope, the actual face itself is fine, what you're seeing is purely a display artifact in this case, not any actual problem with the surface.



> And can I get the outline curve of this new face?

Yup, the new face should be totally fine and can be used in any way that you use any face. Just ignore that slight deviation between the edge and the triangulation of the shaded part of the surface.

There are other kinds of display artifacts that can indicate some bigger problems like if there are weird black spots in some small region of the surface (probably indicates an area of self intersection or surface folding back over top of itself in a small area), or if there is a sort of "leakage" with a lot of shaded area sticking outside of the apparent boundary which can indicate some problem with the trimming boundary like a self intersecting loop in some spot of the trim curves. But a small area that has a little bit of a gap between an edge and a slightly straight looking portion of the shaded surface is just a lack of triangle density in the display mesh in that spot and not any actual problem.

Since the edges are drawn adaptively to a high accuracy the edge and curve display are going to show you the true shape of the outline more accurately in these cases rather than the edges of the shaded surface.


It's very normal for every CAD program to have display artifacts similar to this, it's basically part of the nature of trying to make the display run quickly.


- Michael