[Tex/LaTex] Drawing a rectangle along the border of a circle

Question

I would like to draw a rectangle along the border of a circle. The \draw with the \angle operation might do it, but it is rather cumbersome.
I was wondering if there is another more elegant/shorter solution, i.e. a command that draws shapes along other shapes.

Ok sorry for the late update. What I want to do is more something along the following:

A (badly) drawn rectangle following the border of a circle.

Answer 1

Draw Arched "Rectangle" Around Circle:

Below is a macro that draws the desired shape around the circle: \DrawAlong{(Center)}{\Radius}{\Separation}{120}{60} yields the blue shape from 120 to 60 degrees, and \DrawAlong[draw=black,fill=yellow, fill opacity=0.4]{(Center)}{\Radius}{\Separation}{-30}{-60} yields the filled in yellow shape from -30 to -60 degrees:

Further Enhancements

• The code can probably be greatly simplified by using polar coordinates.

Code:

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

\newdimen\XCoord%
\newdimen\YCoord%

\makeatletter
\newlength{\My@OuterArcRadius}
\newlength{\My@InnerArcRadius}
\newlength{\My@OuterXStart}
\newlength{\My@OuterYStart}
\newlength{\My@OuterXEnd}
\newlength{\My@OuterYEnd}
\newlength{\My@InnerXStart}
\newlength{\My@InnerYStart}
\newlength{\My@InnerXEnd}
\newlength{\My@InnerYEnd}
%
\newcommand*{\DrawAlong}[6][]{%
    % [#1] = style (optional)
    % {#2} = center
    % {#3} = radius
    % {#4} = separation
    % {#5} = arch angle start
    % {#6} = arc angle end
    \def\My@center{#2}%
    \def\My@radius{#3}%
    \def\My@separation{#4}%
    \def\My@arcStart{#5}%
    \def\My@arcEnd{#6}%
    %
    \pgfmathsetlength{\My@OuterArcRadius}{\My@radius+\My@separation}
    \pgfmathsetlength{\My@InnerArcRadius}{\My@radius-\My@separation}
    %
    % Extract coordinates of center: (XCoord,YCoord)
    % https://tex.stackexchange.com/questions/33703/extract-x-y-coordinate-of-an-arbitrary-point-in-tikz/33706#33706
    \path \My@center; \pgfgetlastxy{\XCoord}{\YCoord}
    %
    \pgfmathsetlength{\My@OuterXStart}{\XCoord+(\My@OuterArcRadius*cos(\My@arcStart))}
    \pgfmathsetlength{\My@OuterYStart}{\YCoord+(\My@OuterArcRadius*sin(\My@arcStart))}
    \pgfmathsetlength{\My@OuterXEnd}{\XCoord+(\My@OuterArcRadius*cos(\My@arcEnd))}
    \pgfmathsetlength{\My@OuterYEnd}{(\YCoord+\My@OuterArcRadius*sin(\My@arcEnd))}
    %
    \pgfmathsetlength{\My@InnerXStart}{\XCoord+(\My@InnerArcRadius*cos(\My@arcStart))}
    \pgfmathsetlength{\My@InnerYStart}{\YCoord+(\My@InnerArcRadius*sin(\My@arcStart))}
    \pgfmathsetlength{\My@InnerXEnd}{(\XCoord+\My@InnerArcRadius*cos(\My@arcEnd))}
    \pgfmathsetlength{\My@InnerYEnd}{(\YCoord+\My@InnerArcRadius*sin(\My@arcEnd))}
    %
    \draw [ultra thick, blue,#1] 
        (\My@OuterXStart,\My@OuterYStart)
        arc (\My@arcStart:\My@arcEnd:\My@OuterArcRadius)
        -- (\My@InnerXEnd, \My@InnerYEnd)
        arc (\My@arcEnd:\My@arcStart:\My@InnerArcRadius)
        --cycle;
}%
\makeatother

\begin{document}
\newcommand*{\Radius}{2cm}%
\newcommand*{\Separation}{0.2cm}%
%
\begin{tikzpicture}[ultra thick]
  \coordinate (Center) (1cm,3cm);

  \DrawAlong{(Center)}{\Radius}{\Separation}{120}{60}
  \DrawAlong[draw=black,fill=yellow, fill opacity=0.4]{(Center)}{\Radius}{\Separation}{-30}{-60}
  \draw [red ] (Center) circle    (\Radius);
\end{tikzpicture}
\end{document}

---

Draw Rectangle Around Circle:

If you know the radius of the circle, you can simple coordinate calculations to draw the rectangle. The circle radius is specified in \Radius and \Separation defines the separation you want between the circle and rectangle:

Alternatively, you could define a custom shape that has a rectangle around the circle, then simply using it as a node shape you would obtain the desired results:

Code: Known Radius:

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

\newcommand*{\Radius}{2cm}%
\newcommand*{\Separation}{2pt}%

\begin{document}
\begin{tikzpicture}[ultra thick]
  \coordinate (Center) (1cm,3cm);

  \draw [red ] (Center) circle    (\Radius);
  \draw [blue] ($(Center)-(\Radius,\Radius)-(\Separation,\Separation)$) rectangle (\Radius+\Separation,\Radius+\Separation);
\end{tikzpicture}
\end{document}

Code: Node Shape

This is a modified version from How to draw inside a TikZ node, using node style?

\documentclass{article}
\usepackage{tikz}

\makeatletter
\pgfdeclareshape{CircleRectangle}
%
% Rectangle with an inscribed circle. Based on 'circle' shape
%
{%
  % All anchors are taken from the 'circle' shape:
  \inheritsavedanchors[from={circle}]%
  \inheritanchor[from={circle}]{center}%
  \inheritanchor[from={circle}]{mid}%
  \inheritanchor[from={circle}]{base}%
  \inheritanchor[from={circle}]{north}%
  \inheritanchor[from={circle}]{south}%
  \inheritanchor[from={circle}]{west}%
  \inheritanchor[from={circle}]{east}%
  \inheritanchor[from={circle}]{mid west}%
  \inheritanchor[from={circle}]{mid east}%
  \inheritanchor[from={circle}]{base west}%
  \inheritanchor[from={circle}]{base east}%
  \inheritanchor[from={circle}]{north west}%
  \inheritanchor[from={circle}]{south west}%
  \inheritanchor[from={circle}]{north east}%
  \inheritanchor[from={circle}]{south east}%
  \inheritanchorborder[from={circle}]%
  %
  % Only the background path is different
  %
  \backgroundpath{%
    % First the existing 'circle' code:
    \pgfutil@tempdima=\radius%
    \pgfmathsetlength{\pgf@xb}{\pgfkeysvalueof{/pgf/outer xsep}}%
    \pgfmathsetlength{\pgf@yb}{\pgfkeysvalueof{/pgf/outer ysep}}%
    \ifdim\pgf@xb<\pgf@yb%
      \advance\pgfutil@tempdima by-\pgf@yb%
      \pgfutil@tempdimb=\pgfutil@tempdima%
    \else%
      \advance\pgfutil@tempdima by-\pgf@xb%
      \pgfutil@tempdimb=\pgfutil@tempdima%
    \fi%
    \pgfpathcircle{\centerpoint}{\pgfutil@tempdima}%
    %
    % Now the outer rectangle
    \pgfmoveto{\pgfpointadd{\centerpoint}{\pgfpoint{-\pgfutil@tempdima}{-\pgfutil@tempdima}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{\pgfutil@tempdima}{-\pgfutil@tempdima}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{\pgfutil@tempdima}{\pgfutil@tempdima}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{-\pgfutil@tempdima}{\pgfutil@tempdima}}}%
    \pgflineto{\pgfpointadd{\centerpoint}{\pgfpoint{-\pgfutil@tempdima}{-\pgfutil@tempdima}}}%
  }%
}
\makeatother

\begin{document}
\begin{tikzpicture}
  \node [draw=blue, CircleRectangle, inner sep=1pt] at (2,0) {text};
\end{tikzpicture}
\end{document}