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Circle by 3 points

Circle by 3 points draws the one full circle that passes through three points you click in the sketch editor. It is the way to draw a circle through existing corners when its centre is not known.

What it is

Circle by 3 points draws the circle that passes through three points. Any three points that are not on one straight line have exactly one such circle, so you can draw a circle through corners of the shape in force, or through points of your drawing, without knowing its centre.

The circle is kept as two half circles meeting at its leftmost and rightmost points.

When to use it

Drawing a round shape through three known points, such as the springing points and the crown of an existing arch.
Finding a circle that fits points measured on site.

The steps

Arm Circle by 3 points. In the sketch editor (see Select for the ways in), type C, T, H quickly, or click Circle by 3 points on the editor's toolbar. The hint at the cursor says circle by three points: pick the first point on it.
Click the first point. It snaps to corners, middles, crossings, the dimmed shape in force and the lines of the view. You can also type it as x;y from the editor's origin, the opening's centre at floor level or the plan's origin, and press Enter.
Click the second point. The hint asks for the second point on the circle.
Click the third point. The circle through the three follows the pointer. Click to draw it. Each point can also be typed: x;y, @dx;dy from the point before, or @d<a for a distance at an angle in degrees, in the project's units.
Carry on. The circle is drawn and left selected, and the tool waits for the next one. Escape drops the points given, then goes back to Select.

What it refuses

Three points on one line. No circle passes through them, and the hint says that point makes no shape. The points stay; give another third point.
A bare number. A length typed alone is not taken; type the point as coordinates.

What it writes to the contract

Nothing until you press Done. Each circle is one step inside the editor, and Ctrl+Z steps back through them. The contract changes when you press Done, or Enter with nothing in progress, as one step you can undo: what the lines enclose becomes the window's or door's profile, points [u, v] in metres across the wall and up from the floor, or the pattern area's outline in the room's floorAreas or ceilingAreas, points [x, y] in plan metres.

A circle alone becomes two points, at its far left and far right, each with a bulge of 1, the third number that makes the side to the next point a half circle. A circle through [-1, 0], [0, 1] and [1, 0] drawn as a floor area:

Common questions

Why is my circle refused?

The three points lie on one straight line, and no circle passes through them. Move the third point off that line.

Can I type the points?

Yes. Type each one as x;y, or @dx;dy from the point before, or a distance at an angle such as @0.5<45, and press Enter.

Do the three points become corners of the circle?

No. The circle is kept as two half circles meeting at its leftmost and rightmost points. Your three points lie exactly on it.