# Drawing an ellipse with given focal points and a point on its circumference using tikz

The following code draws a circle with given center X and a point Z on its circumference using tikz.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{through}

\begin{document}

\begin{tikzpicture}
\coordinate [label=left:$X$] (X) at (0,0);
\coordinate [label=left:$Y$] (Y) at (1,0);
\coordinate [label=left:$Z$] (Z) at (2,3);
\node[draw,circle through=(Z)] at (X) {};
\end{tikzpicture}

\end{document}


How to draw with tikz (or tkz-euclide) an ellipse when the foci are X and Y and a point Z on its circumference are given? In other words how to draw an ellipse with given focal points X and Y which passes through the point Z?

The original post never mentioned anything about scaling. If you do not mind loading xintexpr (which provides high-precision calculations), then here is an alternative that meets your additional scaling request.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{xintexpr}

\begin{document}

\begin{tikzpicture}
\def\Xx{0}\def\Xy{0}
\def\Yx{1}\def\Yy{0}
\def\Zx{2}\def\Zy{3}
\coordinate [label=left:$X$] (X) at (\Xx,\Xy);
\coordinate [label=left:$Y$] (Y) at (\Yx,\Yy);
\coordinate [label=left:$Z$] (Z) at (\Zx,\Zy);
\edef\fixedtotallength{%
\xintthefloatexpr\xintexpr
sqrt((\Zx-\Xx)^2+(\Zy-\Xy)^2)+sqrt((\Zx-\Yx)^2+(\Zy-\Yy)^2)
\relax\relax
}
\xintthefloatexpr\xintexpr
\fixedtotallength/2
\relax\relax
}
\edef\focidistance{%
\xintthefloatexpr\xintexpr
sqrt((\Yx-\Xx)^2+(\Yy-\Xy)^2)
\relax\relax
}
\xintthefloatexpr\xintexpr
sqrt((\fixedtotallength/2)^2-(\focidistance/2)^2)
\relax\relax
}
\pgfmathsetmacro\majoraxisangle{%
atan((\Yy-\Xy)/(\Yx-\Xx))
}
\draw[rotate=\majoraxisangle]
($(X)!0.5!(Y)$) ellipse ({\majoraxisradius} and {\minoraxisradius});
\filldraw[red] (X) circle (2pt) (Y) circle (2pt) (Z) circle (2pt);
\end{tikzpicture}
\begin{tikzpicture}[scale=0.5]
\def\Xx{0}\def\Xy{0}
\def\Yx{3}\def\Yy{2}
\def\Zx{2}\def\Zy{3}
\coordinate [label=left:$X$] (X) at (\Xx,\Xy);
\coordinate [label=left:$Y$] (Y) at (\Yx,\Yy);
\coordinate [label=left:$Z$] (Z) at (\Zx,\Zy);
\edef\fixedtotallength{%
\xintthefloatexpr\xintexpr
sqrt((\Zx-\Xx)^2+(\Zy-\Xy)^2)+sqrt((\Zx-\Yx)^2+(\Zy-\Yy)^2)
\relax\relax
}
\xintthefloatexpr\xintexpr
\fixedtotallength/2
\relax\relax
}
\edef\focidistance{%
\xintthefloatexpr\xintexpr
sqrt((\Yx-\Xx)^2+(\Yy-\Xy)^2)
\relax\relax
}
\xintthefloatexpr\xintexpr
sqrt((\fixedtotallength/2)^2-(\focidistance/2)^2)
\relax\relax
}
\pgfmathsetmacro\majoraxisangle{%
atan((\Yy-\Xy)/(\Yx-\Xx))
}
\draw[rotate=\majoraxisangle]
($(X)!0.5!(Y)$) ellipse ({\majoraxisradius} and {\minoraxisradius});
\filldraw[red] (X) circle (2pt) (Y) circle (2pt) (Z) circle (2pt);
\end{tikzpicture}

\end{document} We can always do the math ourselves. :)

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}

\begin{tikzpicture}
\coordinate [label=left:$X$] (X) at (0,0);
\pgfgetlastxy{\Xx}{\Xy}
\coordinate [label=left:$Y$] (Y) at (3,2);% changed for testing
\pgfgetlastxy{\Yx}{\Yy}
\coordinate [label=left:$Z$] (Z) at (2,3);
\pgfgetlastxy{\Zx}{\Zy}
\pgfmathsetmacro{\fixedtotallength}{%
sqrt((\Zx-\Xx)^2+(\Zy-\Xy)^2)+sqrt((\Zx-\Yx)^2+(\Zy-\Yy)^2)
}
\fixedtotallength/2
}
\pgfmathsetmacro{\focidistance}{%
sqrt((\Yx-\Xx)^2+(\Yy-\Xy)^2)
}
sqrt((\fixedtotallength/2)^2-(\focidistance/2)^2)
}
\pgfmathsetmacro{\majoraxisangle}{%
atan((\Yy-\Xy)/(\Yx-\Xx))
}
\draw[rotate=\majoraxisangle]
($(X)!0.5!(Y)$) ellipse ({\majoraxisradius pt} and {\minoraxisradius pt});
\end{tikzpicture}

\end{document} This will not work if the foci are vertically aligned. But I am sure that you can manage to change the \majoraxisangle calculation to a \minoraxisangle calculation.

• Why when one scales your tikz-picture, the ellipse does not pass through Z? I mean adding [scale=.5] just after \begin{tikzpicture}. – Name Oct 17 '18 at 7:17
• @Name Happy now? ;-) – Ruixi Zhang Oct 19 '18 at 21:55
• Thank you for your detailed answer and fixing the issue with scaling. – Name Oct 20 '18 at 4:46

This is similar in spirit to Ruixi Zhang's answer, computes a bit less auxiliary quantities (all it uses is the fact that the sum of distances of a give point from the foci is a constant) and perhaps more TikZy, i.e. all you need to do is to say

\draw[ellipse through=X and Y and Z];


to draw an ellipse through these points or

\node[elliptical node through=X and Y and Z,draw]{hello};


to draw an elliptical node, where X and Y are the foci and Z is the additional point. However, if you try to pass some, well, let's call them unusual coordinates, there will be a dimension too large error.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{through,calc,shapes.geometric}

\tikzset{ellipse through/.style args={#1 and #2 and #3}{
insert path={let \p1=(#1),\p2=(#2),\p3=(#3),
\n1={veclen(\x1-\x3,\y1-\y3)},
\n2={veclen(\x2-\x3,\y2-\y3)},
\n3={veclen(\x1-\x2,\y1-\y2)},
\n4={sqrt((\n1+\n2)^2-(\n3)^2)/2},
\n5={atan2(\y2-\y1,\x2-\x1)} in
%\pgfextra{\typeout{\n1,\n2,\n3,\n4,\n5}}
($(#1)!0.5!(#2)$)
[rotate around={\n5:($(#1)!0.5!(#2)$)}]circle({(\n1+\n2)/2} and {\n4})
}}}
\tikzset{/tikz/my ellipse a/.store in=\myella,
/tikz/my ellipse b/.store in=\myellb,
/tikz/my ellipse angle/.store in=\myellangle,
set ellipse pars/.code={
\tikzset{my ellipse a={\n6},
my ellipse b={\n4},my ellipse angle=\n5}
},
elliptical node through/.style args={#1 and #2 and #3}{
insert path={let \p1=(#1),\p2=(#2),\p3=(#3),
\n1={veclen(\x1-\x3,\y1-\y3)},
\n2={veclen(\x2-\x3,\y2-\y3)},
\n3={veclen(\x1-\x2,\y1-\y2)},
\n4={sqrt((\n1+\n2)^2-(\n3)^2)/2},
\n5={atan2(\y2-\y1,\x2-\x1)},
\n6={(\n1+\n2)/2} in [set ellipse pars]},
ellipse,
rotate=\myellangle,
minimum width=2*\myella,
minimum height=2*\myellb,
at={($(#1)!0.5!(#2)$)}}}
\begin{document}

\begin{tikzpicture}
\coordinate [label=left:$X$] (X) at (0,0);
\coordinate [label=left:$Y$] (Y) at (2,1);
\coordinate [label=left:$Z$] (Z) at (2,2);
\node[draw,circle through=(Z)] at (X) {};
\draw[ellipse through=X and Y and Z];
\foreach \X in {X,Y,Z}
{\fill (\X) circle (1pt);}
\begin{scope}[xshift=7.5cm]
\coordinate [label=left:$X$] (X) at (0,0);
\coordinate [label=left:$Y$] (Y) at (3,1);
\coordinate [label=left:$Z$] (Z) at (3,0);
\node[draw,circle through=(Z)] at (X) {};
\node[elliptical node through=X and Y and Z,draw]{hello};
\foreach \X in {X,Y,Z}
{\fill (\X) circle (1pt);}
\end{scope}
\end{tikzpicture}
\end{document} • Unfortunately, your solution does not work for the choices X=(0,0), Y=(1,0) and Z=(2,3) given in my original question. The ellipse given by your solution does not pass through Z. – Name Oct 17 '18 at 6:45