Trim/split from view?

The Project command has a Direction parameter, can take the current view as a parameter.

But the Trim (and Split) command, surprisingly doesn’t expose this parameter, even though it does implicit projection (through CplaneZ it seems, and other times through normal, which feels a bit uncontrolled).

Is there any way to get Trim (or Split) to expose the Direction parameter?

Alternatively, does anyone have a workaround that doesn’t involve changing the construction plane?

I can’t think of any other way. If you want, this is quite easy to script, or even macro could do it for you. Something that sets the Cplane to view, do the trim and restore the CPlane

Hello - one difficulty with building that into the command is how trim works with planar and non-planar curves and ortho and oblique views - it does different things in different cases-

  • When trimming a surface with a curve in a parallel view like the default Top, Front, and Right view, the cutting curve is projected on the surface in the view direction.
  • When trimming a surface with a planar curve in an angled parallel or a perspective view like the default Perspective view, the cutting curve is projected on the surface in a direction perpendicular to the curve plane.
  • When trimming a surface with a 3-D curve in an angled parallel or a perspective view, the cutting curve is pulled on the surface by closest points.


This inconsistency always bothered me and keeps keeping me on my toes with trim command.

I think trim should always trim perpendicular to current CPlane disregarding view, projection or type of curve.

I must admit I don’t see what you describe.

The third option is what I want, but in a recent model that I was working on, what I most often got was CplaneZ with seemingly no option to change it into anything else (except for the manual “direction” parameter), regardless of perspective or view.

I don’t have time to make an example right now, but more trimming control would be very welcome.

Hm - but that is not what you originally asked about - you seemed to want a projection direction not a pull normal to the target surface…?


Sorry, what I meant was from those three options you describe, which apparently already is in Rhino, the third option is the closest to what I want (but again, I didn’t see it… my result was always in CplaneZ).

If I’m allowed a feature request, then I’d very much like the trim direction get a “current view” option as well. :slight_smile:

I encountered this problem again and wanted to do a correction:

I noticed that I had renamed the Split command to “trim convert”, because that’s what I’m used to from Alias. If you use a line to Split a polysurface, you always get CplaneZ with no options, which renders it pretty much useless to me (I need to Project first, and then Split).

The Trim command does pull as you say, it seems, but neither Trim or Split has proper projection options which would be a very nice addition.

Hello - here is a macro you can try for the moment

'CPlane _View _Trim _Multipause _Multipause _Enter _CPlane _Undo


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Thank you!

I will try this, and hopefully it will work for Split as well.

Wow, if you hadn’t posted this, I would really have been confused by why a trim I tried to perform failed just now. Apparently, the curve I have has a “plane” so instead of option 3 happening, it defaults to option 2:

1.3dm (40.5 KB)

(In the above example, the top curve trims the surface very weirdly, and the bottom curve doesn’t trim the surface at all, even though if the direction would have been normal to the surface both would have succeeded).

Unfortunately, your macro also failed in this instance. The only way is to maximize the Front view and trim from there.

Explicit controls for trim direction (as with Project) would be really useful, and Pull should definitely be an explicit option so there’s no confusion as to what is happening.

Yeah, this does not look right - it should pull the curves and trim - thanks for the example. (Pull first works)