Display or derive focii of an ellipse

Is there a way to display or derive the focii of a curve drawn with the ellipse tool?


@dmoyes Hi Dennis,
Select it and type What – Object description shows info:snap00%2011-13-2019

Excellent Fred, thanks.


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Hello - use MarkFoci for this.


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Even better! Thanks Pascal.
Also, is there such a command as an ellipse from 3 points similar to a circle from 3 points? I have a section of an elliptical arc derived from a section and I would like to determine the full ellipse.


I believe that, unlike a circle, there are an infinite number of ellipses that can pass through three points… (correct me if I am wrong) Below is a quickie script that will restore a full ellipse from a partial one (deletes the original).

Edit - this version is a bit nicer, it will let you select near-ellipse curves and has the option to delete or not (option is only visible if no preselection is made).

RestoreEllipse.py (2.0 KB)

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Hello - I believe markFoci will work on an ellipse ‘fragment’, at which point Ellipse > FromFoci and the Near osnap (the the original curve) will get you the full ellipse.


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An ellipse can be determined from four points. http://www.robertobigoni.eu/Matematica/Conics/Ellipse4P/Ellipse4P.html

Yep, that fourth point is the key… Unfortunately, Rhino dos not have a command for that.

4 points isn’t enough to determine a unique ellipse

The link solves with four points only where the axes of the ellipse are aligned with the co-ordinate system axes. You need either a fifth point or the direction of the major axis to determine a unique ellipse.