Declaring shapes is one of the most useful features of the tikz/pgf library. It can be extended by endless choice and really is a good idea.
The problem with the shapes is the difficulties one has to go through to get it to work.
This is due to the low-level coding that is necessary and essentially a good bookkeeping
of variables is the problem. If one is not used to low-level variable reassignment this
can become a cumbersome and even a task which cannot be completed without coding more than
the basic layer offers.
This is also the problem of your code.
Commenting your code
\pgfdeclareshape{carr}{%
\inheritsavedanchors[from=circle]
\inheritanchor[from=circle]{center}
\inheritanchor[from=circle]{north}
\inheritanchor[from=circle]{south}
\inheritanchor[from=circle]{east}
\inheritanchor[from=circle]{west}
All this is fine, just keep it, you may realise that your east, west, etc. is not
correctly placed. Thus you should add your own.
\backgroundpath{%
% center
\centerpoint
\pgf@xa=\pgf@x
\pgf@ya=\pgf@y
This is also good, you acquire the center coordinates. However, the center coordinates
only have a value if there is text in the node. They are defined as:
\pgf@x=.5\wd\pgfnodeparttextbox
\pgf@y=.5\ht\pgfnodeparttextbox
\advance\pgf@y by-.5\dp\pgfnodeparttextbox
Thus if no text is present they are both 0pt. This tells you why it didn't show if no
node text is supplied.
% west triangle corner
\radius
\pgf@yb=-5\pgf@y
\centerpoint
\advance\pgf@yb by\pgf@y
\pgf@xb=-2\pgf@x
Your problem here is that \radius is actually a dimension. You do not save this value in
any way. You need to use \radius as a regular dimension:
\pgf@x=\radius
This means that what ever you are saving to \pgf@yb you have no idea of what is (or
actually it is \pgf@y from \centerpoint, but that is another matter).
% east triangle corner
\radius
\pgf@yc=-5\pgf@y
\centerpoint
\advance\pgf@yc by\pgf@y
\pgf@xc=4\pgf@x
same problem here. This will not work.
% draw triangle..
\pgfpathmoveto{\pgfpoint{\pgf@xa}{\pgf@ya}}%
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yb}}%
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}%
\pgfpathclose
This is fine
% central circle (from pgflibraryshapes.geometric.code.tex)..
\pgf@process{\radius}
Here is another mistake. Let's go through what \pgf@process does.
Lets consider the following function:
\def\pgfcalc{\pgf@xb=2pt\pgf@x=\pgf@xb}
\pgfcalc will actually define both \pgf@xb and \pgf@x. In many cases you are only
interested in the final result, which is \pgf@x and \pgf@y. In such case one will
invoke \pgfcalc by \pgf@process.
\pgf@xb=1pt
\pgfcalc
% Here \pgf@xb and \pgf@x are set by \pgfcalc
% So \pgf@xb and \pgf@x are both 2pt
\pgf@xb=1pt
\pgf@process{\pgfcalc}
% Here ONLY \pgf@x is set. \pgf@xb is still 1pt
This means that you can only call \pgf@process on a macro and not a dimension (as \radius is). The code you refer to have \radius defined as an anchor which is a macro. You could have done it on \centerpoint if \centerpoint were to change any other than \pgf@x and \pgf@y.
For further information on the purposes see the manual.
Lets move on...
\pgfutil@tempdima=\pgf@x%
\pgfutil@tempdimb=\pgf@y%
\pgfmathsetlength{\pgf@xb}{\pgfkeysvalueof{/pgf/outer xsep}}%
\pgfmathsetlength{\pgf@yb}{\pgfkeysvalueof{/pgf/outer ysep}}%
\advance\pgfutil@tempdima by-\pgf@xb%
\advance\pgfutil@tempdimb by-\pgf@yb%
\pgfpathellipse{\centerpoint}{\pgfqpoint{\pgfutil@tempdimb}{0pt}}{\pgfqpoint{0pt}{\pgfutil@tempdimb}}%
% and lower circles
\pgf@xa=1pt % the radius:should be a parameter
\pgfutil@tempdima=\pgf@xb \advance\pgfutil@tempdima by\pgf@xa
\pgfutil@tempdimb=\pgf@yb \advance\pgfutil@tempdimb by-\pgf@xa
\pgfpathcircle{\pgfpoint{\pgfutil@tempdima}{\pgfutil@tempdimb}}{\pgf@xa}
\pgfutil@tempdima=\pgf@xb \advance\pgfutil@tempdima by-\pgf@xa
\pgfutil@tempdimb=\pgf@yb \advance\pgfutil@tempdimb by-\pgf@xa
\pgfpathcircle{\pgfpoint{\pgfutil@tempdima}{\pgfutil@tempdimb}}{\pgf@xa}
}}
This final part is actually ok, however, it is cumbersome to read and one can easily make
mistakes. I would partition it a bit more.
A fast solution
This solution has been made so that you can work more with it.
There are many options that you need to fine-tune and post-process.
It is up to you to complete with the top circle etc.
\documentclass{article}
\usepackage{tikz}
\makeatletter
\pgfdeclareshape{carr}{%
\inheritsavedanchors[from=circle]
\inheritanchor[from=circle]{center}
\inheritanchor[from=circle]{north}
\inheritanchor[from=circle]{south}
\inheritanchor[from=circle]{east}
\inheritanchor[from=circle]{west}
\backgroundpath{%
% Save radius to x
\pgf@x=\radius
% Radius is also containing the "minimum width" and "minimum height"
% This ensures that even with no text the shape will be drawn.
% Unless of course that min are set to 0pt
% So no need to check for that
% Save radius
\pgfutil@tempdima=\pgf@x%
% west triangle corner "b"
\pgf@xb=-3\pgf@x%
\pgf@yb=-4\pgf@x%
% east triangle corner "c"
\pgf@xc= 3\pgf@x%
\pgf@yc=-4\pgf@x%
% If text is present shift shape to center
% You need to shift more, but to get the idea
\centerpoint
\advance\pgf@xb by\pgf@x
\advance\pgf@yb by\pgf@y
\advance\pgf@xc by\pgf@x
\advance\pgf@yc by\pgf@y
% Save centerpoint in "a" (top triangle point)
\pgf@xa=\pgf@x
\pgf@ya=\pgf@y
% Below are good for debugging purposes.
%\message{^^JTop : \the\pgf@xa,\the\pgf@ya}
%\message{^^JWest: \the\pgf@xb,\the\pgf@yb}
%\message{^^JEast: \the\pgf@xc,\the\pgf@yc}
%\message{^^JCent: \the\pgf@x,\the\pgf@y}
% draw triangle..
\pgfpathmoveto{\pgfpoint{\pgf@xa}{\pgf@ya}}%
\pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yb}}%
\pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}%
\pgfpathclose
% The radius of the small circles
% Read in from option TODO
\pgfutil@tempdimb=3pt
% Move top triangle to head circle
\advance\pgf@ya by.25\pgfutil@tempdimb
% Move west triangle corner to west circle center
\advance\pgf@xb by 1.5\pgfutil@tempdima
\advance\pgf@yb by -\pgfutil@tempdimb
% For handling line thickness if you wish "edge touch" and not "overlap"
%\advance\pgf@yb by -.5\pgflinewidth
% Move east triangle corner to east circle center
\advance\pgf@xc by-1.5\pgfutil@tempdima
\advance\pgf@yc by -\pgfutil@tempdimb
% For handling line thickness if you wish "edge touch" and not "overlap"
%\advance\pgf@yc by -.5\pgflinewidth
% This saves underlying "stuff" when you have the explicit `\pgfqpoint` and is thus a little faster
\edef\pgf@marshal{%
\noexpand\pgfpathcircle{%
\noexpand\pgfqpoint{\the\pgf@xa}{\the\pgf@ya}}
{\the\pgfutil@tempdimb}%
\noexpand\pgfpathcircle{%
\noexpand\pgfqpoint{\the\pgf@xb}{\the\pgf@yb}}
{\the\pgfutil@tempdimb}%
\noexpand\pgfpathcircle{%
\noexpand\pgfqpoint{\the\pgf@xc}{\the\pgf@yc}}
{\the\pgfutil@tempdimb}%
}\pgf@marshal
}}
\makeatother
\begin{document}
\begin{tikzpicture}
\node[shape=carr,draw] at (3,0) {a};
\node[shape=carr,draw] at (5,0) {}; % missing triangle (not anymore)
% Your \draw example will never work! shapes are nodes, you need a node to assign the shape!
\end{tikzpicture}
\end{document}
Here is the end result:
Good luck battling those shapes! :)
