split surface

 From:  Michael Gibson
3408.9 In reply to 3408.7 
Hi Luis,

If I use boolean>diff or merge to split the sphere and then boolean>union the resulting solids, I get back the whole sphere without a cut line. However, when I do a edit>trim on the sphere and then a edit>join, the trim line stays. Is this the correct/expected behavior?

Yes, this is the way it's set up to work - basically Join does not attempt to modify the geometry, it only tries to glue edges together.

Booleans on the other hand, including Boolean union are more intended to be used when objects are going to be cutting each other in some way so that some pieces are going to be changed, like some parts of the model will usually be discarded with Booleans.

One of the things the Booleans do is merge surfaces together when it can see that the pieces have the same underlying surface.

It's mostly meant to clean up things like this, here are 2 boxes being unioned together:

But it also can clean up other situations where pieces come from the same original surface like in that sphere example.

One thing that is nice about that is it can make it possible to carve off a chunk of an object and work with it, and then union it back with other pieces of the original without an extra edge sticking around - you can see that in effect in the "Crown of clubs" video tutorial here:

It's why there are not extra edges at the base of the separate "club" spoke pieces when they are unioned back to the base, since they came from the same larger original revolve surface.

Is there and untrim-like command in MoI.

It sounds like you already figured this one out? Just in case though, the method for untrim is to either delete faces or separate things out so that the edges you want to untrim are not joined to anything else, then you can select an entire trimming boundary (including the "seam edge" if edges touch the seam of a closed surface) and push Delete to remove them and recover the underlying surface in that area.

A couple of shortcuts related to that - if you select 1 edge you can then do Ctrl+A to select all edges of that same object, and there is also a select loop script you can set up on a keyboard shortcut by putting this in as the command part of a shortcut key:

For some examples of select loop, see here:

Also, is the boolean union really the entire sphere or is there a hidden split line that is not displayed?

It's really the whole sphere, there are never any hidden splits that are not displayed, you'll always see edges at trimming boundaries.

- Michael