Surrounding an arbitrary path?

Question

Given an arbitrary path in Latex, e.g. drawn with Tikz, I want to surround this path by a larger path which has a fixed distance to the original path, e.g. 1 cm.

Here is a sketch of it:

Note that this is really only a sketch and in the sketch the distance of the outer path to the inner path is not constant, though it should be. I read the term for my problem would be "offset", yet I could not find out, how I can realize that.

I currently do not have Latex code for this sketch. One can also consider a rectangle surrounded by a larger rectangle for an easy example - the important thing is, that the outer path is generated using a generic code, that can be applied to any path.

The intended use case for this is to draw sewing patterns, the outer path then marks the seam allowance.

Update

Here is a minimal working example for the original path:

\pagestyle{empty}
\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}

% I know this could be drawn with a single
% use of \draw, but in reality my figures
% will be more complicated and will also be created
% using multiple \draw calls.
\draw (0,3) -- (2,7) -- (4,3);
\draw (4,3) -- (3,0) -- (1,0) -- (0,3);

\end{tikzpicture}
\end{document}

This code leads to

How would one add a surrounding path as requested without manually drawing the same figure again, just larger (with manually calculated coordinates) ?

I also like to limit my question a little bit: Arbitrary path is too general, I can't describe it in words, but the things I will be drawing won't be too complicated. A generic way for this provided path example should likely work on other paths I will be drawing.

Answer 1

Here is a solution with clip and double distance (inspired by the comments below cfr's answer):

• Original distances of path are respected (black curve).
• Path with angles and curves are usable.
• May use a list of distances/colors to draw several seam allowances.

The invclip operation comes from How can I invert a 'clip' selection within TikZ?.

Note about line width: Original path is inner clipped so its width is twice narrower than expected. To get all curves with the same width, you must choose a width for seam allowance (line width=1mm in my example) and the double for the original path (line width=2mm in my example).

\documentclass[tikz,border=3cm]{standalone}
\tikzset{%
  draw seam allowance/.style 2 args={
    preaction={line width=1mm,line join=round,double distance=#1*2,draw=#2},
  },
  seam allowances/.style={%
    preaction={clip},
    preaction={draw seam allowance/.list={#1}},
    draw,%fill=white,
  },
  seam allowances/.default={{{2cm}{blue}}},
  invclip/.style={
    clip,
    insert path={
      {[reset cm] (-16000pt,-16000pt)  -| (16000pt,16000pt) -| cycle}
    },
  },
}
\begin{document}
\begin{tikzpicture}
  \def\mypath{
    (0,30) -- (20,50) -- (40,30) to[out=-180,in=130] (30,0)
    -- (10,0) arc(-90:-180:5cm) to[out=90,in=-45] (3,27) -- cycle
  }
  \begin{scope}
    \begin{pgfinterruptboundingbox}
      \path [invclip] \mypath;
    \end{pgfinterruptboundingbox}
    \draw [seam allowances={{4cm}{red},{3cm}{red},{2cm}{blue},{1cm}{blue}},line width=2mm]
    \mypath ;
  \end{scope}
\end{tikzpicture}
\begin{tikzpicture}
  \def\mypath{
    (0,0) -- (-10,10) -- (0,20) -- (10,10) -- cycle
    (0,0) -- (10,-10) -- (0,-20) -- (-10,-10) -- cycle
  }
  \begin{scope}
    \begin{pgfinterruptboundingbox}
      \path [invclip] \mypath;
    \end{pgfinterruptboundingbox}
    \draw [line width=2mm,seam allowances={{4cm}{red},{3cm}{orange},{2cm}{blue},{1cm}{green}}]
    \mypath ;
  \end{scope}
\end{tikzpicture}
\end{document}

Answer 2

Here's an ultra simple solution which just uses double to draw the paths. We define a style seam which really just uses double to draw the line, but with a semantic name. The seam allowance can be set as desired. By default it is set here to 2.5pt.

\tikzset{%
  seam/.style={%
    double distance=\seamallowance,
    draw,
  },
  seam allowance/.store in=\seamallowance,
  seam allowance=2.5pt,
}

Applying this to the example, the result looks like this:

Note that there are gaps where the paths join. Really, you will find it difficult to get neat joins if the paths are not drawn continuously. In this case, you'd want to use cycle.

EDIT

You can't use cycle if you are combining things with plots but it probably won't matter in most cases. Here's an example which begins in the same way as the other cases, but then uses a plot and, finally, it ends at the starting point. The first coordinate plotted is the last given before the plot and the last plotted is the first given at all.

There are tiny gaps here, but does it really matter? I would guess not given the context. Hence, fixing it probably isn't worth the additional hassle and overhead. If the pieces were not to be cut out and pinned to cloth, it would be different and worth the trouble. For this kind of use, however, I'd go with the simpler solution. (The gap is only ever going to be very small, after all.)

While there might be some corner cases in which you might get something odd happening, I think this is probably going to be pretty exceptional given the use case. Generally, clothing is constructed from pieces which are reasonably regular in shape. (People don't want singularities in their bust darts.)

But I don't guarantee any of this, of course. If you end up dressed in rags as a result of my code, you'll just have to take up patchwork.

\documentclass[tikz,border=10pt,multi]{standalone}
\begin{document}
\tikzset{%
  seam/.style={%
    double distance=\seamallowance,
    draw,
  },
  seam allowance/.store in=\seamallowance,
  seam allowance=2.5pt,
}
\begin{tikzpicture}
  \draw [seam] (0,3) -- (2,7) -- (4,3);
  \draw [seam] (4,3) -- (3,0) -- (1,0) -- (0,3);
\end{tikzpicture}
\begin{tikzpicture}
  \draw [seam] (0,3) -- (2,7) -- (4,3) -- (4,3) -- (3,0) -- (1,0) -- cycle;
\end{tikzpicture}
\begin{tikzpicture}
  \draw [seam] (0,3) -- (2,7) -- (4,3) plot [smooth] coordinates { (4,3) (3,-2) (-2,0) (0,3) };
\end{tikzpicture}
\end{document}