Parametric Pics : Variations of Infinity

Question

Let's argue that I want to draw variations of the infinity symbol (Mostly because I can't draw ducks although it makes for a good MWE). I can't find a simple example that uses pgf/tikz keys to manipulate how a tikz pic is drawn in a concise manner. Looking over my own code base it looks like I'm rather confused about the topic myself.

Requirements :

Let's allow the size of the shape to be altered by the keys width and height and enable some wobble in the lobes with the keys inner targetand outer target. Most importantly we want to select between various calligraphic pens, which must be set by code. This is controlled by selecting left and 'right' for the pen.

Code :

For this example you will need the spath and calligraphy style files, available here, available as the spath3 package and tikz calligraphy libraries.

\documentclass[tikz]{standalone}

\usepackage{spath}
\usepackage{calligraphy}
\usetikzlibrary{calc}

\makeatletter
\tikzset{
 infinity/.is family,
 infinity/.pic = {
  % This is the curve we wish to have drawn
  \calligraphy let \p1=(0,0) in 
     (\p1) .. controls (-\infty@upper*\infty@width, \infty@height) and (-\infty@lower*\infty@width,-\infty@height) .. 
     (\p1) .. controls ( \infty@lower*\infty@width, \infty@height) and ( \infty@upper*\infty@width,-\infty@height) .. cycle;;
  % This curve is simply here as a backup
  \draw[line width=0.1, blue] let \p1=(0,0) in 
     (\p1) .. controls (-\infty@upper*\infty@width, \infty@height) and (-\infty@lower*\infty@width,-\infty@height) .. 
     (\p1) .. controls ( \infty@lower*\infty@width, \infty@height) and ( \infty@upper*\infty@width,-\infty@height) .. cycle;;
 },
 infinity,
 pen/.is choice,
 pen/left/.code={\pen (-.01,.01) -- (.01,-.01);},
 pen/right/.code={\pen (-.01,-.01) -- (.01,.01);},
 pen/.default=left,
 pen/.initial=left,
 width/.store in=\infty@width,
 height/.store in=\infty@height,
 inner target/.store in=\infty@upper,
 outer target/.store in=\infty@lower,
}
\tikzset{
 infinity,
 width        =   1 em,
 height       =   1 em,
 inner target = 0.2 em,
 outer target = 0.8 em,
}
%\tikzset{ % Fails to set the properties before the infinity/.code is executed.
% infinity/.style={width=1em, height=1em, inner target =0.2, outer target =0.8},
%}
\makeatother

\begin{document}
\begin{tikzpicture}
\draw pic {infinity};
\end{tikzpicture}
\begin{tikzpicture}
\draw pic[infinity/pen=left] {infinity};
\end{tikzpicture}
\end{document}

Overview

I am trying to structure the code as follows. Group everything under the infinity family. infinity also acts as the tikz pic whose geometry is controlled by the dimensions \infty@DIMENSION, where DIMENSION is one of width and height, controlling the size, and upper and lower, controlling the wobble. The pen is selected as described above.

Question(s) :

1. How does one set the defaults for the pic setting ? Setting the geometry keys in infinity/.style fails for me. I've therefore defined them the second \tikzset{} call so that the MWE compiles out of the box.

2. I also have to explicitly set the value pen for the calligraphic curve to display. Shouldn't this also be made part of the infinity/.style ?

3. This point relates to 1 and 2 above. Setting the dimensions under infinity/.style fails, as it does not set any of the dimensions before infinity/.code is executed.

   \tikzset{ infinity/.style={...}, }

4. In some other code I have I use the DIMENISON/.get handler instead of 'DIMENSION/.store in' handler. Should I be doing this here ?

5. In the second tikzpicture environment I explicitly set pen using infinity/pen=... is there a way I could simply write pen=... ? Is it then also necessary to specify infinity/.search also=/tikz/ in the first \tikzset where I define infinity ?

Background :

There was a very good answer by Ryan Reich regarding some of this stuff but I can't find it now. I have read through the pgfkeys section of the pgfmanual, to me it reads more like an API document and I find I need an example to connect the dots, hence this question.

Update :

My original intention with this question has been userped by a package update, which is really nice in a "Oh darn" kind of way.

Answer 1

The basic rule with keys (there are exceptions) is that you need to do the following things:

1. Initialise a key,
2. Define a key,
3. Use a key.

On the whole, I find that once I'm in the pgfkeys ecosystem then it is best to stay there. So the values height, width, upper, and lower can be stored in keys rather than needing auxiliary macros.

With the pen, when you use a .code key then that code is executed when it is invoked so it is a little different to storing values for later use. What you wanted was to decide which pen to use and then inside the pic invoke the code necessary to install it. This means separating the setting and getting of that key since you want to set the key in the options to the pic but use it in the actual code. This means it would be easier to use pen to set an option which says which pen to use later on. In fact, you could define the pens at the start of the document and use them in your code.

1. Using infinity/.style means that you are using the infinity key in three different ways: as a pgfkeys family, as a pic, and as a style. This is possibly a little too confusing for the user. Using keys, it's not necessary to also use styles. You can set initial values for the keys with the .initial key handler.

2. By making the pen a value rather than explicit code, we can use the .initial key handler for this as well.

3. The reason infinity/.style doesn't do anything is that you never use it.

4. Using the keys themselves avoids needing the .store in or .get handlers.

5. You can set pen to be an alias to infinity/pen as in the code below.

Here's working code, feel free to ask follow-up questions in the comments:

\documentclass[tikz]{standalone}
%\url{http://tex.stackexchange.com/q/315412/86}

\usetikzlibrary{calc,calligraphy}

\makeatletter
%
% This is a shorthand for getting the value of a relevant key, purely
% to save typing
%
\def\inf@key#1{%
  \pgfkeysvalueof{/tikz/infinity/#1}%
}

\AtBeginDocument{
  %
  % Put these in a box to throw away so that it takes up no space
  %
  \setbox0=\vbox{%
    \definepen[pen name=left] (-.01,.01) -- (.01,-.01);
    \definepen[pen name=right] (-.01,-.01) -- (.01,.01);
  }%
}

\tikzset{
  %
  % So that we can use the pen outside the infinity tree
  %
  pen/.style={
    infinity/pen=#1,
  },
  infinity/.is family,
  infinity/.pic = {
    %
    % The calligraphy/spath3 library currently has some issues with the
    % cycle coordinate.  Oops.
    %
    \calligraphy[pen name=\inf@key{pen}] let \p1=(0,0) in 
     (\p1) .. controls (-\inf@key{inner target}*\inf@key{width}, \inf@key{height}) and (-\inf@key{outer target}*\inf@key{width},-\inf@key{height}) .. 
     (\p1) .. controls ( \inf@key{outer target}*\inf@key{width}, \inf@key{height}) and ( \inf@key{inner target}*\inf@key{width},-\inf@key{height}) .. (\p1);
    % This curve is simply here as a backup
    \draw[line width=0.1, blue] let \p1=(0,0) in 
     (\p1) .. controls (-\inf@key{inner target}*\inf@key{width}, \inf@key{height}) and (-\inf@key{outer target}*\inf@key{width},-\inf@key{height}) .. 
     (\p1) .. controls ( \inf@key{outer target}*\inf@key{width}, \inf@key{height}) and ( \inf@key{inner target}*\inf@key{width},-\inf@key{height}) .. (\p1);
  },
  infinity,
  pen/.initial=left,
  width/.initial=1em,
  height/.initial=1em,
  inner target/.initial=0.2,
  outer target/.initial=0.8
}
\makeatother

\begin{document}
\begin{tikzpicture}
\draw pic[infinity/width=2em] {infinity};
\end{tikzpicture}
\begin{tikzpicture}
\draw pic[infinity/pen=right] {infinity};
\end{tikzpicture}
\begin{tikzpicture}
\draw[pen=right] pic {infinity};
\end{tikzpicture}
\end{document}

Incidentally, the spath library was superseded by the spath3 library which is available from CTAN and available in all good TeX distributions.

Answer 2

Sticking with the older packages I was able to resolve something I was curious about. Namely how does one optionally change the pen when specifying the pic.

Following the advice @Loop Space provided, namely don't mush all of ones keys together under one path, actually use the styles you define, store ones values in the pgf tree and provide a wrapper macro to retrieve them. Results in the following, albeit slightly verbose version of his answer.

\documentclass[tikz]{standalone}

\usepackage{spath}
\usepackage{calligraphy}
\usetikzlibrary{calc}

\makeatletter
\def\inf@key#1{%
  \pgfkeysvalueof{/tikz/infinity dimensions/#1}%
}    
\tikzset{
 % Dimensions
 infinity dimensions/.is family,
 infinity dimensions/width/.initial =1 em,
 infinity dimensions/height/.initial=1 em,
 infinity dimensions/upper target/.initial=0.2,
 infinity dimensions/lower target/.initial=0.8,
 % Options
 infinity options/.is family,
 infinity options/pen/.is choice,
 infinity options/pen/left/.code  = {\pen (-.01, .01) -- ( .01,-.01);},
 infinity options/pen/right/.code = {\pen (-.01,-.01) -- ( .01, .01);},
 infinity options/pen/.default=left,
 % Style
 infinity style/.style={/tikz/infinity options/.cd, pen=#1},
 % Pic/Symbol
 infinity symbol/.pic = {
  % This is the curve we wish to have drawn
  \calligraphy[infinity style=#1] let \p1=(0,0) in 
     (\p1) .. controls (-\inf@key{upper target}*\inf@key{width}, \inf@key{height}) and (-\inf@key{lower target}*\inf@key{width},-\inf@key{height}) .. 
     (\p1) .. controls ( \inf@key{lower target}*\inf@key{width}, \inf@key{height}) and ( \inf@key{upper target}*\inf@key{width},-\inf@key{height}) .. cycle;
  % This curve is simply here as a backup
  \draw[line width=0.1, blue] let \p1=(0,0) in 
     (\p1) .. controls (-\inf@key{upper target}*\inf@key{width}, \inf@key{height}) and (-\inf@key{lower target}*\inf@key{width},-\inf@key{height}) .. 
     (\p1) .. controls ( \inf@key{lower target}*\inf@key{width}, \inf@key{height}) and ( \inf@key{upper target}*\inf@key{width},-\inf@key{height}) .. cycle;
 },
}
\makeatother

\begin{document}
\begin{tikzpicture}
\draw pic {infinity symbol};
\end{tikzpicture}
\begin{tikzpicture}
\draw pic {infinity symbol=right};
\end{tikzpicture}
\end{document}

The difference being that I deliberately kept the .is choice handler, set up the .style and importantly used it. (EZKeys provides an alternate approach). The magic happens with the line(s) infinity style/.style={...} and \calligraphy[infinity style=#1] ... which enable the optional argument specified in \draw pic {infinity symbol= ...}. These act together to passing the argument through to the correct branch/name space within pgfkeys. The \pen ...; code is then called before the calligraphy path is traced, which was not happening in the original question.

I have not managed to resolve the passing of pic actions to infinity dimensions just yet.

It would be beneficial to have a version with all the keys "mushed" together for comparison which is possible as the handlers do not in general clash. It also saves one from naming branches like infinity symbol/options/dimensions, which should really belong in an infinity sub-tree/branch.

This was more of an exercise but it could be useful as it allows 'classes' of pic. As an example if I specified a series of car related pics I could provide a default, say a volla (Beetle), for \draw pic {car}; but allow one to select an alternate \draw pic {car=ALTERNATE}; such as a hotrod, saloon or hatchback.