3.12 Corners and closed regions

A corner is a coincidence, not a link. Two endpoints within a millimetre of each other are one corner: drag it and every line attached follows. Nothing records that they are joined — snapping makes the coincidence exact on every commit — so there is no connection to create and nothing to go stale after an undo. It is the rule cable trays already live by (§9.1), at drawing precision.

A rectangle’s and a circle’s handles are grips rather than corners: dragging one resizes the shape and keeps it that shape. That is also the one place a click can surprise you — a handle takes the click ahead of the curve it belongs to, and a circle’s grip sits on its rim, so select a circle by clicking the rim somewhere else. Selecting a real corner offers a radius, which rounds it.

A sketch whose two closed outlines are filled, with the region count in the inspector — Figure 3.12 A circle dropped into the traced stage — a trap in the deck — and the drawing now reads **two** closed regions: the deck with a hole in it, and the disc filling the hole. Closed regions fill as you draw, and the inspector counts them with their total area. That number is the thing you are drawing toward: finding out the outline never closed *after* you have put the tools away is finding out too late.

Regions are worked out from the lines, never stored, and two consequences are worth knowing. Regions tile the drawing: a circle inside a rectangle gives you both the plate with a bore in it and the disc filling that bore, so either can be pushed up. And crossing is not joining — two rectangles that merely overlap are two regions, not three. An intersection counts only where the drawing says so with a shared point, which is the same rule as two tray runs that cross without touching.