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I was trying to draw a part of one ellipse using \pgfpatharcto. However, when I tried to compare with the original complete ellipse, there is a small difference. I attach the code as well as the result.

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}

\draw (2.5cm,0) ellipse (3.5cm and 3.8436cm);

\pgfsetstrokecolor{red}
\pgfmoveto{\pgfpoint{2.841cm}{3.8284cm}}
\pgfpatharcto{3.5cm}{3.8436cm}{0}{0}{0}{\pgfpoint{2.841cm}{-3.8284cm}}\pgfusepath{stroke};

\end{tikzpicture}
\end{document}

enter image description here

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4
  • I wound say this is an rounding error? – user31729 Jan 21 '17 at 13:56
  • @ChristianHupfer I am not quite understand your point. Do you have any suggestions to overcome this issue? – yangyang Jan 21 '17 at 14:13
  • (2.841cm,3.8284) is not on the original ellipse. Try (2.841cm,3.82531415cm) instead. 2.841=3.5 cos(84.4088847) 3.82531415 = 3.8436 sin(84.4088847). – John Kormylo Jan 21 '17 at 16:58
  • @JohnKormylo Yes, you are right. The point is indeed not on the original ellipse. I need to check further. Thanks. – yangyang Jan 21 '17 at 18:36
5

It's also possible to draw twice the ellipse but inside a clipping scope the second time: 

\documentclass[tikz, border=1cm]{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}

\draw (2.5cm,0) ellipse (3.5cm and 3.8436cm);

\begin{scope}
\clip (2.5cm,0)--++(80:3.9) arc(80:-80:3.9)--cycle;
\draw[red] (2.5cm,0) ellipse (3.5cm and 3.8436cm);
\end{scope}

\end{tikzpicture}
\end{document}

enter image description here

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3

You might like to try Metapost to do this. It has a useful subpath syntax that lets you draw segments of a saved path. Compile with lualatex.

enter image description here

\RequirePackage{luatex85}
\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
beginfig(1);
    path e;
    e = fullcircle xscaled 3.5cm yscaled 3.8436cm;
    draw e withpen pencircle scaled 2 withcolor .8 white;
    draw subpath (-2,2) of e withcolor 2/3 red;
endfig;
\end{mplibcode}
\end{document}

Metapost provides a concept of "time" along a path. On a fullcircle path, there are 8 points of time starting with point 0 at "3 o'clock" as it were.

So subpath (0,2) of c would be from 3 o'clock to midday. You can use fractional numbers too: subpath (2.718, 3.1415) of c.

And as shown in my example you can use negative numbers to refer to points "before" point 0.

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