Draw only some segments of a path in TikZ?

Question

Is there a good way to create a path in TikZ only some segments of which are stroked? I want to create the whole path in a single command so that it can be filled, but I also want to stroke some parts of the boundary of the filled region, but not all of them. The best solution I've thought of so far is to use edge operations in the middle of the path to do the stroking; for instance here is a green-filled square with one edge stroked:

\path[fill=green] (0,0) -- (1,0) edge (1,1) -- (1,1) -- (0,1) -- cycle;

This is a little better than

\path[fill=green] (0,0) -- (1,0) -- (1,1) -- (0,1) -- cycle;
\draw (1,0) -- (1,1);

but it still requires giving the command that draws the segment from (1,0) to (1,1) twice, which is annoying and error-prone if it's a more complicated command like a Bézier curve, and impossible if it's something like a circular arc that can't be drawn by a edge command (at least, not without writing a custom "to path"). Any suggestions?

Answer 1

I've found a new answer to this using the decorations library. We use the show path construction decoration to split the path up in to sections. We only render the pieces if a certain flag is set. The trick is to figure out a way to input the path in such a way that we can specify the flag switching without significantly disturbing the path. The method I settled on was to use a line of zero length as a toggle.

\documentclass{standalone}
\usepackage{etoolbox}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}

\newbool{partpath}
\pgfkeys{/tikz/partial rendering/.style={
    postaction={
      decorate,
      decoration={
        show path construction,
        moveto code={
          \global\booltrue{partpath}
        },
        lineto code={
          \ifx\tikzinputsegmentfirst\tikzinputsegmentlast
            \ifbool{partpath}{\global\boolfalse{partpath}}{\global\booltrue{partpath}}
          \fi
          \ifbool{partpath}{
            \draw (\tikzinputsegmentfirst) -- (\tikzinputsegmentlast);
          }{}
        },
        curveto code={
          \ifbool{partpath}{
            \draw (\tikzinputsegmentfirst) .. controls (\tikzinputsegmentsupporta) and (\tikzinputsegmentsupportb) .. (\tikzinputsegmentlast);
          }{}
        },
        closepath code={
          \ifbool{partpath}{
            \draw (\tikzinputsegmentfirst) -- (\tikzinputsegmentlast);
          }{}
        }
      }
    }
  }
}

\begin{document}

\begin{tikzpicture}
\path[fill=green,partial rendering]
 (0,0) -- (0,1) -- +(0,0) -- (1,1) -- +(0,0) -- (1,0) -- +(0,0) .. controls +(0,-1) and +(0,-1) .. (0,0);
\end{tikzpicture}

\end{document}

All those -- +(0,0)s are there to toggle the rendering when the path is drawn for the second time. (I tried using empty movetos: just +(0,0), but having those there messed up the fill routine.) Result:

Answer 2

This is a reworking of Jan's code to make it a little more TikZ-like. It still does the same thing: repeats the path, once as a fill and once as a draw, but with certain parts of the path removed for the draw stage. The main modification is that (hopefully) it now looks exactly like a standard TikZ command.

\documentclass{article}

\usepackage{tikz}
\makeatletter
\long\def\my@drawfill#1#2;{%
\@skipfalse
\fill[#1,draw=none] #2;
\@skiptrue
\draw[#1,fill=none] #2;
}

\newif\if@skip

\newcommand{\skipit}[1]{%
\if@skip
\else
#1
\fi
}

\newcommand{\drawfill}[1][]{%
  \my@drawfill{#1}}
\makeatother



\begin{document}
\begin{tikzpicture}
  \drawfill[fill=green,draw=blue,ultra thick] (0,0) -- (0,1) \skipit{--} (1,1) -- (1,0) \skipit{.. controls +(0,-1) and +(0,-1) ..} (0,0);
\end{tikzpicture}
\end{document}

This produces:

I'm sure that there are additional improvements to be made, which is one reason why I'm making this community wiki (the other being that it is really Jan's answer). In particular:

1. I couldn't make the \skipit command take an optional argument and still work. I think that it doesn't get expanded properly before the genuine TikZ commands see it and they don't like having macros as part of their path specification.

2. The assignment of key-value pairs is a bit of a hack: basically, we turn off the drawing on the fill command and turn off the filling on the draw command. It might be useful to be able to specify a set of keys that only get put on the draw command and a set for the fill command, so something like \drawfill[options for both, fillopt={options for fill}, drawopt={options for draw}] but I don't know enough (anything) about pgfkeys so don't have a clue how to go about that.

Answer 3

If I understand correctly, what you want to do is to only have to enter the path once. Would something like this work (it does not follow usual tikz syntax, but that should not be that hard to fix)?

\documentclass[]{article}
\usepackage{tikz}

\def\drawfill#1#2#3{{\def\skip{--}\fill[#1]#3;}{\def\skip{}\draw[#2]#3;}}
\begin{document}
\begin{tikzpicture}
   \drawfill{fill=green}{very thick}{(0,0) -- (0,1) \skip (1,1) -- (1,0)\skip (0,0)}
\end{tikzpicture}
\end{document}

Answer 4

I'd like to be proven wrong, but I think this is impossible without drawing the path twice. Section 12.3.3 of the TikZ manual (v2.10) states:

Note that many options apply only to the path as a whole and cannot be scoped in this way. For example, it is not possible to scope the color of the path. See the explanations in the section on paths for more details.

And section 14.16:

When TikZ encounters and opening or a closing brace ({ or }) at some point where a path operation should come, it will open or close a scope. All options that can be applied “locally” will be scoped inside the scope. For example, if you apply a transformation like [xshift=1cm] inside the scoped area, the shifting only applies to the scope. On the other hand, an option like color=red does not have any effect inside a scope since it can only be applied to the path as a whole.

The problem seems to be that the dash pattern is applied to the path as a whole. Most of the time this is a good thing because it means that dashes work nicely across corners.

So I'd break the this up into several commands:

\begin{tikzpicture}
    \path[fill=green] (0,0) -- (1,0) -- (1,1) -- (0,1) -- cycle;
    \draw (1,1) -- (0,1) -- (0,0) -- (1,0);
    \draw[dashed] (1,0) -- (1,1);
\end{tikzpicture}

For more complicated paths, you might want to store some parts of the path into TeX macros.