Inset Straight Skeleton

Inset Straight Skeleton

This add-on makes ‘inset polygons’, which you get when you advance the edges of the polygon inwards at a constant rate. Sometimes when you do that, an advancing edge shrinks to nothing, or edges hit an advancing concave corner. The algorithm tries to do the right thing when this happens: one or more new polygons may form at that point, and insetting can continue inside those new polygons.

You can either inset a single polygon or you can treat groups of polygons connected together as a single region, forming a complicated polygon (that may include holes) to be inset as a unit.

In addition to insetting, it is often useful to raise or lower the inset polygon (perpendicular to the inset plane), so a parameter to do that is included also.

Activation

Open Blender and go to Preferences then the Add-ons tab.
Click Mesh then Inset Straight Skeleton to enable the script.

Description

Enter mesh Edit Mode on a mesh object, and select one or more faces.

Scale

Percent

Means that amounts are a percentage of the amount for a full inset.

Absolute

Means that the amounts are in units.

Amount

The distance to move the edges inward.

Height

The distance to move the inset polygons upward.

Region

If checked, treat all selected faces as a region to be inset, otherwise inset each face individually.

Quadrangulate

Todo.

Technical Details

The method used is described on the Straight Skeleton Wikipedia page. Consider this starting polygonal region:

As the edges move inward at a constant speed, two kinds of events can happen. One is that an advancing corner can hit an advancing edge, as seen on the right part of this picture:

When this happens, the inset polygon splits into two.

The other is that an advancing edge can shrink to a point. When the whole process continues until the end, you get something like this:

One cool thing about this algorithm is that if you move upwards or downwards (perpendicular to the inset plane) at a constant speed, you form ‘roofs’ with a fixed pitch.