How can I add more coordinates to standard TikZ shapes?

Question

Question

I would like to know how to add an arbitrary number of coordinates to the default shapes of TikZ—particularly to the side of a polygon.

Sample

This is similar to https://tex.stackexchange.com/a/47856/13552

Answer 1

Defining new anchors is tricky and basically impossible without looking at the PGF code used to define the shape. But aliasing anchors is quite straightforward (once you have the command to do it):

\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{shapes.geometric}
\makeatletter
\def\pgfaliasanchor#1#2#3{%
  \expandafter\gdef\csname pgf@anchor@#1@#2\endcsname{%
    \pgf@sh@reanchor{\csname pgf@sh@ns@\pgfreferencednodename\endcsname}{#3}%
  }%
}%
\makeatother

\pgfaliasanchor{regular polygon}{peter}{90}
\pgfaliasanchor{regular polygon}{paul}{330}
\pgfaliasanchor{regular polygon}{mary}{south west}
\begin{document}
\begin{tikzpicture}
\node [fill=gray!50, regular polygon, minimum size=1.5in] (shape) at (2,2) {};
\foreach \anchor in {peter, paul, mary}
  \draw [draw=red] (shape.\anchor) 
    circle [radius=1/20] node [above] {\tt\anchor};
\end{tikzpicture}
\end{document}

Answer 2

If you're looking for an arbitrary coordinate around the perimeter you can use (name.number), where name is the shape name and number is an angle in degrees measured from name.east.

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric,plotmarks}
\begin{document}
\begin{tikzpicture}
\node[name=s, shape=regular polygon, inner sep=1cm,draw,color=black!30,thick] {};
\draw[shift=(s.east),blue!70] plot[mark=o] coordinates{(0,0)} {};
  \foreach \anchor in
    {0,10,20,...,350}
\draw[shift=(s.\anchor),red] plot[mark=x] coordinates{(0,0)} {};
\end{tikzpicture}
\end{document}

Edit

When using shape=regular polygon, TikZ names all the sides and corners, starting from the top of the shape (see TikZ 3.0.0 manual Sec. 67.3 Geometric Shapes for details.) In order to specify an arbitrary point along an arbitrary side, you could define a command that places a point some distance between two consecutive corners (i.e., along a side) of the regular polygon. This requires \usetikzlibrary{calc}.

In the example below, I've defined the command mysidemark, which has the following syntax: \mysidemark{<start corner>}{<end corner>}{<fraction of distance along path (between 0 and 1)>}.

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc,shapes.geometric}
\begin{document}
\newcommand{\mysidemark}[3]{%
\draw[red,shift=($(s.corner #1)!#3!(s.corner #2)$)] plot[mark=*] coordinates{(0,0)} {};
}
\begin{tikzpicture}
\node[name=s, shape=regular polygon, regular polygon sides=6, inner sep=1cm,draw,color=black!30,thick] {};
\mysidemark{1}{2}{0}
\mysidemark{1}{2}{0.17}
\mysidemark{1}{2}{0.46}
\mysidemark{2}{3}{0.66}
\mysidemark{2}{3}{0.90}

\begin{scope}[xshift=4cm]
\node[name=s, shape=regular polygon, regular polygon sides=7, inner sep=1cm,draw,color=black!30,thick] {};
\mysidemark{2}{3}{0.07}
\mysidemark{3}{4}{0.25}
\mysidemark{6}{7}{0.77}
\end{scope}
\end{tikzpicture}
\end{document}

Note

This command can be used explicitly, as shown, but more work is necessary to allow it to be used in a \foreach command. See, e.g., \foreach with \newcommand how to use them together?.