Hi Marek, yup that's intentional - in MoI the Trim command is focused just on working on surfaces, including just the surface skin of a solid.

It can sometimes be convenient to work at the surface level like this on individual portions of your object, and then use the Join command to seal all those surface fragments back together into a solid at the end of tweaking the surfaces.

If you instead want to cut a solid and maintain it as a solid volume, then like Steve mentions use the Boolean Difference command to cut your object instead of the Trim command.

The Booleans are focused on processing solid volumes, and they will perform the operation that you were looking for there.


You might want to check out the MoI video tutorials here:
http://moi3d.com/2.0/docs/tutorials.htm
the Crown of clubs tutorial covers using booleans between a solid and a curve in a few different steps in it.

There are also some examples in the help file here:
http://moi3d.com/2.0/docs/moi_command_reference7.htm#booleandifference


> MoI alike results are in Rhino so the author of both is the
> best person to ask about it :)

Rhino is similar but some of the specifics are different - in Rhino you can't use curves in the regular Boolean commands, instead you would first extrude the curve into a surface and use the surface in the Boolean, or use a separate command called "WireCut".

One of the nice things about MoI's Boolean commands is how you can just use curve objects as input into the Booleans and they become extrusions automatically for you.

- Michael