Why does my trim fail here?

I’m just curious because I don’t see the probem.

trim.3dm (60.3 KB)

Trimming with 3D curves in a perspective viewport shoud, I’ve been told, automatically pull the curves to the surface, and since these curves are either G2 or overlapping, that should work?

EDIT: I’m trying to trim away the top right corner!

EDIT: I can’t even split the surface with a view based projection, so now I’m even more confused.

I don’t know why it fails but if you join those 3 curvy lines together it does work.

You are probably right that this should work. but its not smart enough to figure it out.

The problem is the degree2 curve in the middle is not a 3d curve. It is a 2d curve and that means it is projected instead of pulled which produces a result that does not connect with the other 2 curves (they are pulled to the surface) to make a complete trim boundary.

If you pull the 2d curve to the surface and then select the other 2 disconnected curves trim will work. The 4th curve that is off the surface isn’t involved in the trim.

Oh, that’s interesting!

How can the 2 degree curve be considered 2d? It’s made from a variable chamfer, that I then duplicated the edge from, rebuilt and then actually tweaked manually in perspective using the view mode drag… so in my mind, there’s nothing 2D about it…

Wait… does Rhino consider all 2 degree curves to be 2D automatically just because the three control points can always make a plane?

A 2d curve is one that lies all in one plane. You can rebuild the curve to make it higher degree curve but it will still be 2d curve.

Hello eobet,

I just joined the horizontal lines and then did the trim. But I do not know why it failed otherwise.

Thank you,
trim1.3dm (71.2 KB)

Hello - after joining, there is no longer a 2d/planar curve, and the resulting curve is pulled rather than projected to the target. Though you need to monkey with things a little anyway to get a join here…


Hello Pascal,

Thank you for the explanation.


Thank you!

@jim is there a command to detect if a curve is 2D or 3D?

@pascal it would be great to get an explicit “direction” option for trim that includes view (which we’ve talked about before in another thread) and normal (ie pull).

Hello - SelPlanarCrv