Intersecting and combining solids

Intersect Faces

Intersect Faces draws edges where faces cross, so you can erase the parts you don’t need. Select faces, groups or components, then choose Edit → Intersect Faces, or right-click the selection:

With Model intersects the selection with everything visible in the model, inside or outside the group you’re editing.
With Selection intersects the selected objects only with each other.
With Context intersects the selection with the rest of the group or component you’re editing.

The new edges go into the group or component you’re editing and are selected afterward. Groups and components you intersect with don’t change. Faces that lie in the same plane and overlap each get the other’s outline. Loose geometry you moved onto other loose geometry, like a box set on a slab, is joined into one surface: the slab’s top is split along the box’s footprint.

Solids

A group or component is solid when its faces close it off completely, like a box with all six sides: every edge borders exactly two faces, and it holds no loose edges or nested groups. Select one group or component and open Window → Properties: Solid says whether it is, and Volume shows how much space it encloses. A hollow inside a solid counts as empty space.

Combine Solids

Edit → Combine Solids, also in the right-click menu, combines solid groups and components into new groups. Select them in order: where the order matters, the first one you select is the one that gets cut.

Command	Selection	Result
Union	Two or more solids	One group with everything inside any of them
Subtract	Two solids	The first with the second removed
Intersect	Two or more solids	Only what all of them share
Trim	Two solids	The first with the second removed; the second stays
Split	Two solids	Up to three groups: what each has alone, and what they share
Outer Shell	One or more solids	One group with only their outside, without inner faces or hollows

The new groups are selected, and Undo brings back the originals. Where two solids touch side by side, their flush faces merge into one. Faces keep their paint, including paint applied to the whole group.

If a selected object isn’t a solid, or the result wouldn’t be one, nothing changes and Modlumi tells you why. Models imported from SketchUp sometimes have faces that aren’t quite flat, which can cause this.

Change a solid in place

The bundled Modify Solids plugin, a port of Eneroth Solid Tools, changes a solid without replacing it. The solid keeps its name, paint and position, and every copy of a component changes along with it.

Select the solids to cut or add with and choose Extensions → Modify Solids → Mark Modifiers.
Select the solids to change.
Choose Union, Subtract, Trim or Intersect from the same menu.

Subtract erases the modifiers afterward unless Keep Modifiers is checked; Trim always keeps them. Without marked modifiers, Union and Intersect combine the selected solids into the largest one.