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Reference

REGION in AutoCAD

How AutoCAD's REGION (REG) turns closed shapes into regions, which loops it finds, why it makes none, and what a region is for. Try it free in your browser.

REGION turns closed shapes into regions: flat areas with no thickness, each bounded by its loops. Select the objects and press Enter; every loop they make becomes a region. A closed object is a loop of its own: a circle, an ellipse, a closed polyline or spline. Open ones — lines, arcs, elliptical arcs, open polylines and splines — are chained end to end, never where they cross, and a chain that does not close is dropped. The command line then counts the loops it found, those it rejected and why, and the regions it created; Enter on nothing says None found..

The objects that went into a region are erased, the others kept, and the regions take the current layer and properties, not the objects'. A loop inside another makes a region of its own, not a hole in it: SUBTRACT takes one from the other. A region draws as its edges and snaps at them; the Properties palette shows its area and perimeter, which AREA, LIST and MASSPROP measure too. UNION, SUBTRACT and INTERSECT combine regions in one plane, EXTRUDE, REVOLVE and PRESSPULL turn them into 3D solids, and EXPLODE gives back their lines, arcs and circles. The whole command is one undo step.

At the command line

The engine draws it and writes its history as you would type it: the prompts in AutoCAD 2027's order, with the same option letters.

Four lines closed end to end and a circle inside them: REGION, with a window round both, makes two regions and erases the objects.
  1. Command: REGION
  2. Select objects:
  3. Specify opposite corner:
  4. Select objects:
  5. 2 loops extracted.
  6. 2 Regions created.
Four lines closed end to end and a circle inside them: REGION, with a window round both, makes two regions and erases the objects.

What it does not do yet

AutoCAD's DELOBJ: the objects that went into a region are always erased here. BOUNDARY, which can make a region from the area round a point, and a region from a hatch's boundary. Editing a region by its grips: they show, but dragging one changes nothing. Its edges as a cutting or boundary edge for TRIM and EXTEND in Quick mode, and HATCH of a region with an elliptical or spline edge. Regions combined with surfaces, which MellowCAD does not have, and the Material row in the Properties palette.

What it does and still lacks, in detail (in English)

Questions

How do I use the REGION command in AutoCAD?

Run REGION (or REG), select the closed shapes or the lines that close one end to end, and press Enter. The session's window round four lines and a circle makes two regions, and the command line counts two loops extracted and two regions created.

Why does REGION not create a region?

The shape is not closed end to end. Lines that cross, or that stop short of one another, close nothing: REGION chains open objects only by their ends, and a gap of a hair is enough to drop the chain. A loop that crosses itself, like a bow tie, is rejected and counted under self intersections. Join the ends with FILLET at radius 0, or TRIM what overshoots, and try again.

How do I make a region with a hole?

Make a region of the outline and one of the hole, then run SUBTRACT: pick the outline, Enter, the hole, Enter. The result is one region, and EXTRUDE turns it into a solid with the hole through it.

How do I find the area of a region?

Select it and read Area in the Properties palette, or run AREA, choose Object and pick it. MASSPROP adds the perimeter, the centroid and the moments of inertia.

Practise it

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