Click to start a polyline at a point; shift + click to continue from that point. Press Z to remove the last point, and press enter to complete the polygon. Space will clear everything.

Middle click (or ctrl/command + click) to set the anchor position.

Key

Grey lines

The polyline

Red point

Anchor/starting point of ray

Blue line

Direction of ray

Cyan point

First intersection of ray and polyline

Yellow point

Mouse position

Orange point

Closest point on polyline to the mouse