Match problem + "TrimConvert" (?)

I actually have two questions, the first Q is

  1. why can’t I (position-) Match the following planar surface (green) against the slightly curved (gray) vertical surface (nearest in front):

Fig 1.

Fig 2. The edge of the green top surface won’t connect with the vertical edge (the small arrow points to a narrow “overhang”). No option in the dialog does away with the overhand:

  1. My second question is if there’s any command in Rhino which can trim and convert a surface to it’s simplest structure within the new boundaries. In Alias there seems to be a command named TrimConvert doing just this (possibly also increasing the complexity of the structure if need be & allowed by the user).

In Rhino I know of only the two Shrink commands, of which neither typically adjust the trimmed surface to curved edges as the final untrimmed srf, leaving it to the user to extract the surface boundaries to be Lofted or Sweeped etc.

The #1 could serve as an example where the surface could benefit from such a command.

Match Edge.3dm (103.3 KB)

// Rolf

Hi Rolf - your green surface has internal control points - those are not moved my MatchSrf and are left hanging out in space, causing a fold in the surface.

V7/WIP has a test command, testSplitRefit that will do what you want here:



To mee it looks like this (only when ticking the option “Match edges by closest points” it folds for me):

It seem like I have to

  1. Rebuild the green top surface to a degree 1 surface,
  2. Move the nearest corner to the corner of the “cube”
  3. Rbuild the top surface again to degree 3
    4 then match the edge.

All this to avoid the surface to fold over itself. Shouldn’t this be an option in the match command (to (re)distribute the control points to make room for the new edge?

// Rolf

Aha (didn’t see this addition to your reply until now).

Yes, that looks a lot like the “TrimConvert” I was onto in the OP.

Thanks for this hint!

// Rolf

good to see this implemented now. An extremely useful feature, I used this very often in other CAD software!


Hi Tom - at the moment it is partially implemented, as a test command with a rather impossible UI, but it does work pretty well.



@pascal, I tried to run the command TestSplitRefit and yes, it splits, but only a straight split “from corner to corner” not following the curvature of the curved edge.

I tried different options but they seem a bit cryptic… so I don’t really know what to expect - is the command expected to be able to follow the curved edge of the vertical surface, or did you run MatchSrf explicitly on the new edge after the TestSplitRefit command?

// Rolf

Hi Rolf - here is what I get at default settings:


Hm, and this is what I get (a straight cut):


// Rolf

Hi Rolf - if your top surface is as in the originally posted file (degree 3 by 3) and you’re using the top edge of the vertical surface as the cutter… ? you should not get a straight line as far as I know…


Doh! I forgot to restore the surface to degree 3… sigh. No bells ringing although it should have. Sorry for that.

// Rolf

Hi @pascal
The testSplitRefit command in Rhino7 doesn’t work. Do you have a replacement?

Hello - that and related commands are in flux right at the moment - I am away from my desk today, but I’ll see if I can sort out what is going on - they should become real commands.


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