How to explicitly control trim direction?

Yes, I have made trim topics before.

Unfortunately, I still don’t fully understand it…

  1. Is there any indication in Rhino which direction the trim happened in or do you just have to know all the hidden rules about trim direction (like planar curves, etc)?
  2. Am I correct in that Trim always trims to World Z, regardless of the current CPlane Z?
  3. Does being in a ortho view always result in a view based trim?
  4. Does two joined curves always result in a pulled curve before trim (and if so, is it a true normal direction based trim, or something else)?

Any help would be appreciated. Trimming what causes me the most issues with Rhino currently (and I know I should trim with surfaces and not curves, but that’s not always feasible when you’re stitching and unstitching trying to fix problems in solids quickly).

Curves tirmming surfaces:

In oblique views:
Non-planar curves are pulled to the surface
Planar curves are projected in the curve plane
Lines are projected in the CPlane Z

In plan views curves are projected in CPlane Z


Ok, so no way to override trim direction then (unless I use split)?

And Rhino never reveals what direction it used?

Any way to get Rhino to tell me what “plane” (if any) it thinks a curve belongs to?

What about my question 4, how does Rhino interpret joined curves when it comes to trimming? Is it the same (ie, a joined curve may still be “planar” in some circumstances)?

Yeah, there is no difference unless part of the set of curves is a line and that becomes part of a planar curve if joined - I could see that making a difference if that plane were skewed, but I have not tested. If in doubt, pull or project your curve ‘manually’ first and trim with the result.


I’m afraid I already tried that and had to make a thread for that too.

Hence why I made this thread to ask if it’s possible to see ideally ahead of time, or at least after, what kind of trim Rhino performed.

EDIT: And, hehe, searching for that post above led me to this even older thread of mine, where I at least was told one way to check if a curve is planar or not. :slight_smile:

Ok, so if Rhino always respects the rules you laid out (and that I’ve apparently failed to understand in several threads), why does this trim fail in perspective view:

non-planar-trim.3dm (68.7 KB)

Because if I do SelPlanarCrv Rhino tells me that nothing gets selected, however if I then Pull that curve onto the surface manually, the Trim succeeds. So what rule did I not understand here?

Hello - This appears to be a bug with trimming polysurfaces - if you explode that thing, trim works on the single surface.