Draw additional parallel paths in TikZ

Question

How can parallel lines elegantly be added to an existing path? I am looking for a solution or hint to, how a general style can be made, that works with any path.

Example path:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw (0,0) -- (1,1) -| (2,2) -- (3,2) -- (3,1);
\end{tikzpicture}
\end{document}

Same path, with customizable extra lines:

Here I have drawn two extra lines, just to show, what I mean. Ideally a solution should only add one extra line, but somehow be expandable to any number of lines.

The question: How to build parallel paths between nodes?, where my solution is less than elegant, has inspired me to ask this question.

Answer 1

It turns out some of the work is already done, but an important macro \pgfdecoratedangletonextinputsegment is undocumented (and incorrectly defined - possibly why it is undocumented). Here's a not-very-well tested solution for straight lines only with no mid-path moveto commands. I think dealing with curveto and closepath segments will also prove extra-tricky.

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{decorations}

\def\pgfdecoratedcontourdistance{0pt}

\pgfkeys{/pgf/decoration/contour distance/.code={%
    \pgfmathparse{#1}%
    \let\pgfdecoratedcontourdistance=\pgfmathresult}%
}

\pgfdeclaredecoration{contour lineto}{start}
{
    \state{start}[next state=draw, width=0pt]{
        \pgfpathmoveto{\pgfpoint{0pt}{\pgfdecoratedcontourdistance}}%
    }
    \state{draw}[next state=draw, width=\pgfdecoratedinputsegmentlength]{       
        \pgfmathparse{-\pgfdecoratedcontourdistance*cot(-\pgfdecoratedangletonextinputsegment/2+90)}%
        \let\shorten=\pgfmathresult%
        \pgfpathlineto{\pgfpoint{\pgfdecoratedinputsegmentlength+\shorten}{\pgfdecoratedcontourdistance}}%  
    }
    \state{final}{
        \pgfpathlineto{\pgfpoint{\pgfdecoratedinputsegmentlength}{\pgfdecoratedcontourdistance}}%
    }   
}

\begin{document}

\begin{tikzpicture}

\draw [
    postaction={
        decoration={contour lineto, contour distance=-5pt},
        draw=red, dotted, decorate},
    postaction={
        decoration={contour lineto, contour distance=15pt}, 
        draw=blue, dashed, decorate},
    postaction={
        decoration={contour lineto, contour distance=10pt}, 
        draw=green, decorate}
    ] 
    (0, 0) -- (3, 1) -- (4, 4) -- (6, 4) -- 
    (8,-1) -- (2,-2) -- (5, 2) -- (6, 0);

\end{tikzpicture}


\end{document}

Answer 2

I just found the nfold library from 2023 by Jonathan Schulz. It can do exactly that - also for Bézier curves and closed curves.

\documentclass[tikz, border=1cm]{standalone}
\usetikzlibrary{nfold}
\begin{document}
\begin{tikzpicture}
\draw[save path=\savedpath] (0,0) -- (1,1) -| (2,2) -- (3,2) -- (3,1) to[bend left] (2,0) --cycle;
\pgfoffsetpath{\savedpath}{-4pt}
\pgfsetlinewidth{2pt} \color{red} \pgfusepathqstroke
\pgfoffsetpath{\savedpath}{10pt}
\pgfsetdash{{1pt}{1pt}}{0cm} \pgfsetlinewidth{1pt} \color{blue} \pgfusepathqstroke
\end{tikzpicture}
\end{document}

I do not know if this can be made into a high level TikZ \draw.