Modified /tikz/double that doesn't go all the way to the end of lines

Question

I love using the double option to draw a line with a thicker, differently-coloured line underneath it.

However, quite often I have several of these lines meeting at a point. They are all separate lines, but then the thicker parts of the lines overlap the thinner parts and the effect is spoiled.

Ideally I'd like a modified version of the /tikz/double effect, which doesn't draw the thicker line at (say) the beginning and end 5% of the length of the curve. Is this possible? So the thinner, 'internal' line would be drawn as specified, but the thicker, 'underneath' line wouldn't be drawn for the entire length.

Answer 1

Here's an alternative. It uses the same basic idea as Frederic's in that it replaces the doubling by a preaction (doubling really is just a preaction except for a little trickery to handle arrows). But instead of shortening the underlying path, it draws it on a background layer. The method of doing that is to encase the preaction path in a \pgfonlayer ... \endpgfonlayer sandwich. This requires a new key, which I've imaginatively called preaction on background layer. It has to go after the preaction has been defined as it encases the currently defined preaction in the appropriate commands - it can't act presumptively. Although I've put it in to its own style option, you'd want to be able to specify the colour and line width in a similar fashion to the double syntax. That wouldn't be hard to do.

Here's the code (with, for comparison, the original double, Frederic's version, and my version all in a row):

\documentclass{standalone}
\usepackage{tikz}

\pgfdeclarelayer{background}
\pgfsetlayers{background,main}

\makeatletter

\def\pgf@on@bglayer{\pgfonlayer{background}}

\tikzset{
  preaction on background layer/.code={
    \expandafter\def\expandafter\tikz@preactions\expandafter{\expandafter\pgf@on@bglayer\tikz@preactions\endpgfonlayer}
  },
  double behind/.style={
    preaction={
      draw,
      red,
      line width=8pt
    },
    preaction on background layer
  }
}
\makeatother
  \begin{document}
\begin{tikzpicture}
\foreach \ang in {45,90,...,360} {
  \draw[line width=2pt,red,double=black,double distance=4pt] (0,0) -- (\ang:2);
}
\foreach \ang in {45,90,...,360} {
  \draw[black,line width=4pt,preaction={line width=8pt,red,draw,shorten >=2pt,shorten <=2pt}] (5,0) -- ++(\ang:2);
}
\foreach \ang in {45,90,...,360} {
  \draw[double behind,line width=4pt] (10,0) -- ++(\ang:2);
}
\end{tikzpicture}
\end{document}

And the result:

(Incidentally, Frederic's solution would have been my first choice as well, if it weren't for his remark about the curved lines being off.)

Answer 2

An incomplete solution is as follows. It uses a preaction. There is a problem when the curve is not a collection of straight lines, but rather a bezier curve. In this case, the two lines that are drawn (preaction line and regular line) do not pass exactly by the same points (have a close look at the curved part of the picture below). The code is :

\documentclass{minimal}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\draw[red] 
    [preaction={draw,green,shorten >=2pt,shorten <=2pt,ultra thick}]
    (0.5,0.5)--(0,1) -- (0,0) .. controls ++(1,0) and ++(0,-1) .. (1,1);
\end{tikzpicture}
\end{document}

The output is