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As in subject: can I draw in Inkscape a circle from three points, similarly to CAD software?

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  • Hi. Welcome to GDSE. Could you please show an example of what you are trying to achieve? Thanks. Note that most graphic designers (including me) don't use CAD software or know what it's capable of. Thanks. – Billy Kerr May 28 at 14:57
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    @BillyKerr Sorry if I wasn't clear, the issue and solution are graphically shown in the answer from user287001 - I can't upvote it though, I'm a newbie here :D – lmattcris May 28 at 15:14
  • I've given you some upvotes to give you some extra points since you are new. Thanks for clarifying anyway. – Billy Kerr May 28 at 15:27
  • Well, obviously you can draw a three point circle even if the only tools you have are a compass and ruler. In fact i see a lot of mechanical engineers for some reason doing manual three point circles in conjunction to mechanism design even in CAD software. Seemingly nobody mentioned to them that the method they are using i three position fourbar synthesis IS actually the method for find 3 point circles. Incidentally the 2 position method shows how to find all possible 2 point circles. – joojaa May 28 at 16:58
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It's possible:

enter image description here

Draw a path which has three nodes. Apply path effect Ellipse from Points. Make the circle free by applying Path > Object to Path. It becomes the same 4 node path as the usual Bezier curve approximation of circle in vector drawing programs.

It looks circle, but it's a path, it's not SVG circle. If you for some reason need a curve which is internally defined as SVG circle you can draw one over or make Euclidean geometry construction.

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    Ah, fine! It's a bit more complex than in MCAD systems, but the solution is effective, thanks! – lmattcris May 28 at 15:11
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    @lmattcris Note that this will make a circle form Bézier curves and paths, which can only approximate a circle. So don't use Inkscape for this if you want absolute mathematical precision. Inkscape is designed for making pretty pictures, not for building machinery. – Billy Kerr May 28 at 15:23
  • @BillyKerr Yes, I understand the difference. However the great interface of Inkscape helps me create nice documentation that sits between graphic design and MCAD, still not being myself an expert in any of them – lmattcris Jun 1 at 8:07
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Here's another method, using a different path effect.

This method will allow you to move the points afterwards should you so wish. Just note that Inkscape won't create mathematically perfect circles, so this is fine for graphics viewed by humans, not for building machinery.

Anyhoo

  1. Choose the Bézier Curves tool B

  2. In the Tool Controls select the Spiro Path option

enter image description here

  1. Click on three points anywhere on the page, then close the shape by clicking on the first point.

Note: If you want you can also finalize it by doing Path > Object to Path, to convert it to an actual path with Bézier curves.

Here's the result. You can move the Nodes using the Edit Paths by Nodes tool N

enter image description here

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  • Never noticed any undeniably useful applications for the spiro path effect. Thanks for showing one. – user287001 May 28 at 15:57
  • @user287001 - I use it often, not for this though. It's good for getting nice automatic curves, much like Illustrator's Curvature tool. see example – Billy Kerr May 28 at 16:04
  • @user287001 Here's another example just for fun with a clone. – Billy Kerr May 28 at 16:11
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    That's useful. One can search a good form without fixing beforehand more than "it must be made of not too complex arcs". – user287001 May 28 at 16:23
  • If I remember correctly, spiros are based on exponential functions and thus can represent a circle perfectly (since a sine and cosine can be expressed as exponential functions with a complex argument). Of course, Inkscape translates them to Bézier curves before rendering, but I think it does that to explicit circles too. – Wrzlprmft May 29 at 10:20

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