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Possible Duplicate:
Draw arc in tikz when center of circle is specified

an very trivial question but I can't do it! Sorry but I find coordinates confusing.

I would like to draw an arc around a circle; both should have the same center of rotation. And I want to position the arc relative to the circle.

Here is the MWE to correct:

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}

\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\begin{document}
\begin{tikzpicture}
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\draw[thick] (sh1) arc (0:300:2cm);
\end{tikzpicture}
\end{document}

How do I get to have the same center for both shapes, with relative positioning of the arc? Any simple and brilliant idea?

Community
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jejuba
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  • You can replace your \draw ... command with \draw[thick] (sh1) ++(0:2cm) arc (0:300:2cm); to move to a point on the circle before drawing the arc. See Leo Liu's answer to Draw arc in tikz when center of circle is specified – Jake Aug 30 '12 at 11:14
  • Thanks Jake. I did have a look at the post you mention before posting mine but I could not make the code work. I am not familiar with this way of coding. – jejuba Aug 30 '12 at 12:42
  • No problem. It's always a good idea to include the things you've tried and the questions you've already looked at in your question. That way, people will be able to offer more specific advice. – Jake Aug 30 '12 at 12:45

2 Answers2

7

The problem is that for arc you specify the start point of the arc, not the cirle's center. You could define a custom command like this:

The Code

\documentclass{article}
\usepackage{tikz}


\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\def\centerarc[#1] (#2) (#3:#4:#5)% [draw options] (center) (initial angle:final angle:radius)
{ \draw[#1] (#2) ++(#3:#5) arc (#3:#4:#5);
}

\begin{document}
\begin{tikzpicture}
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\centerarc[thick] (sh1) (0:300:2cm)
\end{tikzpicture}
\end{document}

Special thanks to Paul Gaborit and Jake for simplifying this.

The Result

enter image description here

Tom Bombadil
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4

Well, I don't a have brilliant idea ... but note, that arc always starts at given coordinate - in your case at the center of the circle sh1, and its center is shifted by the amount specified in last argument for arc - in your case by 2cm. So, if you shift the start of the arc in the appropriate direction and amount, you will get what you desired (if I understand your problem correctly).

In your case try:

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}

\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\begin{document}
\begin{tikzpicture}
\draw[help lines] (-3,-3) grid + (9,9);
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\draw[thick] ([xshift=20mm] sh1.center) arc (0:300:2cm);
\end{tikzpicture}
\end{document}
Tom Bombadil
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Zarko
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  • Thanks Zarko. It works well. The only (minimal) issue I see is that if I change the angles in the arc, the arc and the circle do not share automatically the same center of rotation. But this can be remedied. Thanks again. – jejuba Aug 30 '12 at 12:52