BUG? Trimming produces weird results

I notice this strange behaviour in Rhino for the first time ever. Despite that all curves are set to share the same plane, when I try to trim them together, only a fraction of the expected curve segment is actually trimmed. I can’t see the logic of this. It does not matter if I trim in Perspective view or the Left/Right ones. For some weird reason, Rhino leaves some small portion of the curves untrimmed and I have to click multiple times to remove each unnecessary segment.

The circle curves are built with the “Circle” command and are true arc G2 circles. The straight lines are G1 lines with just 2 control points. Just the bare minimum for such geometry.

Buggy trimming.3dm (132.3 KB)

I’m also forced to replace the resulting 4 arc segments with new manually built arcs, because the former kept the original quad position of the control points left from the full circles (plus added extra control points). The expectation is for those trimmed true arc segments to be automatically converted to simple true arcs with an even distribution of the control points. This is especially important in situations where the trimmed circles (now arcs) will be used for extrusions.

Hi Bobi - here, in our latest at least, trimming is clean. i.e. I do not see the little extra bits - I do not have an explanation for that one…’

You can clean up the trimmed arcs with SimplifyCrv.


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Hi Pascal! Thanks for the tip about the SimplifyCrv command. So far I have never needed it in my practice, because Rhino itself simplifies the arcs on the drawings I work on.

Trimming in Rhino 8 seems better in this case, except that it failed to trim one of the segments at the bottom of the larger circle. No idea why is this happening, since the input circles and lines are the most basic geometry of all. :slight_smile:

Hi Bobi - your circles are Nurbs cirlcles - if you SimplifyCrv before the trim, the results will also be simple. The Circle command makes simple circles not Nurbs ones… I guess these are imported or derived from Nurbs edges or somerthing?


Hi Pascal! I made the circles by duplicating a circle edge from a cylinder, which on the other hand was built from an extruded circle. I didn’t knew that virtually the same circles will behave in such a different way depending on the way they were built. :slight_smile: This is something new to learn.

Hi Bobi - thanks for the details - it seems an extrusion object from a circle returns Nurbs circles on DupEdge, but a surface does not… I will see if we can tune this up.
RH-82422 DupEdge on extrusions and breps


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It would be great to make everything work in a more consistent way, no matter how the input surface or polysurface was created.
By the way, a duplicated circle edge from a revolved polysurface also returns a NURBS circle. :slight_smile:

Probably because ExtendCuttingLines=Yes

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This actually helps to overcome the bug. Nice find.
It’s interesting that setting this option to “Yes” reveals the bug regarding the different type of output circle.