How do I ...?

sgbotsford

How do I find the barycenter of an arbitrary polygon?

Is there an easy way to construct a guide that is perpendicular to the center of a sloped rectangle?

How do you bisect an angle?

TIG

How do I find the barycenter of an arbitrary polygon?

Find centroid.rb [or my CofG.rb for 3d objects that uses the same technique]

Is there an easy way to construct a guide that is perpendicular to the center of a sloped rectangle?

There are several ways... Here's a basic one...
Draw a guide-line parallel to an edge dragging it away from the edge until it snaps to the mid-point [cyan] of the perpendicular edge.
Select the guide-line and the Rotate Tool, place the cursor on the perpendicular edge and click, holding down the cursor button drag along that edge and the Rotation Tool's gizmo locks so its axis is that edge.
Press the Ctrl-key so it goes into Copy mode
Pick on the rectangle's face and type 90 + <enter>, a new guide-line is added perpendicular to the face and already centered on one edge.
Now activate Move and pick an end of the edge and the mid-point of another edge perpendicular to it,
The guide-line should now move to the center of the rectangle.
Alternatively... make two guide-lines parallel to adjacent edges and offset to the mid-point of the other edge.
These now intersect at the rectangle's center.
You can repeat the rotation trick given above and copy one guide-line so it makes a perpendicular clone, them simply Move that, snapping onto it and then the intersection of the first two guide-lines, which mark the center for you...
Didier Bur's Projection tools also have other ways - e.g. one make the guide-line perpendicular to the face for you and you then need to Move it to the mid-point of one edge and from there across to the mid-point of a perpendicular one...

How do you bisect an angle?

Assuming the angle is unknown...
Draw a guide-line on one of its edges [i.e. parallel to it and on it].
Select it and activate the Rotate Tool, lock its gizmo to the face-plane using the Shift-key and then snap to the apex of the angle.
Press Ctrl-key to active Copy mode.
Pick a point on the original edge, and then a point on the second edge forming the angle.
The guide-line should now form a copy over that second edge.
Before doing anything else type /2 + <enter> and an additional guide-line will be added rotated by exactly half of the angle [note how you can type /3, /4 etc to get finer exact subdivisions of any angle].
Conversely you can 'double' an unknown angle by typing x2 [or 2x] etc...

Wo3Dan

@tig said:

How do I find the barycenter of an arbitrary polygon?

Find centroid.rb [or my CofG.rb for 3d objects that uses the same technique]
The first one is probably GetCentroid.rb by Alex Schreyer.

@unknownuser said:

Is there an easy way to construct a guide that is perpendicular to the center of a sloped rectangle?
Draw a little rectangle ON the sloped face.
Pull this rectangular face out. This will create four edges perpendicular to the larger face.
Delete all edges of the extrusion, all but one of the four edges.
Pull a guide from this edge to whereever (in your case: to the center)

@unknownuser said:

How do you bisect an angle?
One way is to use a two-segments arc from one edge to the other. Look for the magenta chord (equal distances from the apex) and click the third point somewhere on one of the edges, away from the apex.

Wo3Dan

@wo3dan said:

@unknownuser said:

How do you bisect an angle?
.... and click the third point somewhere on one of the edges, away from the apex.
Why should you click the third point on one of the edges?
This is the most general approach on two 3D edges when there is no face between both edges.

Jean Lemire

Hi folks.

For a guide perpendicular to the center of a sloped rectangle, see attached SU file for ideas.

Guide perpendicular to the center of a sloped rectangle.skp