Hi Luis,

> is there a difference between a boolean operation vs a
> trim to cut a line or a face? If so, what are the advantages
> of one vs the other? thnxs,

In some cases they can be equivalent, but in general the Booleans are more focused on operations between volumes or with curves the equivalent is operations involving closed curves.

Trim is more oriented towards cutting just the surfaces of an object and not preserving volumes.

If you want to work at more of a surface level where you're going to be cutting surfaces up and then joining them later on to make a solid, that's when you'll more often want to use Trim rather than the Booleans.

If you're working with solids and you want to cut a solid up and have the result be a solid, that's when you want to use Booleans instead.


There is some overlap between them though, because the Booleans are sort of like a batch combination version of trimming, where it does trimming + throw away pieces + joining for you as a kind of batch operation.


Here's a bit of illustration to show the different results between Trim or Booleans when solids are involved.

Here's a sphere and a line to cut it:





If you do a Trim, the sphere surface is cut and results in 2 surface fragments (I separated them slightly so you can more easily see):




But when doing a Boolean Difference (or Merge also for this case will produce the same result) the result is 2 solid pieces, where the extruded pieces from the line become part of the result:




Trimming is a somewhat more "low level" mechanism that you tend to use when working more with surface modeling techniques where you kind of work at generating the skin of an object and then join it together at the end.


It can be good to try to work more with solids and booleans when possible though, it can save time because pieces get combined together without extra steps, like in the example above when doing a boolean I don't have to do any extra steps to generate a cap surface to make each of those halves into a solid.


You can mix and match between these techniques though, by using Edit > Separate you can break a solid into individual surfaces and use surface modeling techniques, and you can use Edit > Join to glue together individual surfaces into a solid and then use solid modeling techniques with it after that.

But often staying more in solids can be more efficient.

- Michael

thanxs Michael,

Great explanation! I noticed something in your example after trying it myself. 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? Is there and untrim-like command in MoI.

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

Its very interesting, Im learning a lot here. thanxs!

--Luis

I found this relevant topic for untrimming surfaces/holes:

http://moi3d.com/forum/index.php?webtag=MOI&msg=2671.1

It's very helpful. thnxs!

Hi Luis,


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:
http://moi3d.com/1.0/docs/tutorials.htm

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.


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:
script:moi.geometryDatabase.selectLoop();

For some examples of select loop, see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3112.22
http://moi3d.com/forum/index.php?webtag=MOI&msg=3112.32



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