How to draw parts of a line dotted with TikZ?

Question

I want to create a straight line between two nodes in a tikzpicture interrupted in the middle by a dotted portion. This should work whatever the orientation of the line.

Here is an expected output:

The best would be to have a line style defined for this instead of each time using clip to remove the straight line and then tweak a dotted one to fit the hole. A minimal code for the solution would look like

\documentclass[a4paper, 12pt]{report}

\usepackage{tikz}

\tikzset{
    middle dotted line/.style={
        thick, 
        % YOUR HELP HERE
    }
}% end of tikzset


\begin{document}

\begin{tikzpicture}
    \node (A) at (0, 3) {};
    \node (B) at (5, 3) {};
    \node (C) at (0, 2) {};
    \node (D) at (5, 0) {};
    \draw[middle dotted line] (A) -- (B);
    \draw[middle dotted line] (C) -- (D);
\end{tikzpicture}

\end{document}

Answer 1

Inspired by Mark Wibrow

\documentclass[tikz, border=5]{standalone}
\usetikzlibrary{decorations.pathreplacing,calc}
\tikzset{%
  middle dotted line/.style={
    decoration={show path construction, 
      lineto code={
          \draw[#1] (\tikzinputsegmentfirst) --($(\tikzinputsegmentfirst)!.3333!(\tikzinputsegmentlast)$);,
          \draw[dotted,#1] ($(\tikzinputsegmentfirst)!.3333!(\tikzinputsegmentlast)$)--($(\tikzinputsegmentfirst)!.6666!(\tikzinputsegmentlast)$);,
          \draw[#1] ($(\tikzinputsegmentfirst)!.6666!(\tikzinputsegmentlast)$)--(\tikzinputsegmentlast);,
      }
    },
    decorate
  },
}
\begin{document}

\begin{tikzpicture}
\draw[middle dotted line] (0,0)--(4,1);
\draw[middle dotted line={line width=1.5pt}] (1,1)--(3,4);
\end{tikzpicture}

\end{document}

Answer 2

Using the spath3 library (at time of writing, needs the development version from github).

\documentclass{article}
%\url{https://tex.stackexchange.com/q/639726/86}
\usepackage{tikz}
\usetikzlibrary{spath3}

\begin{document}

\begin{tikzpicture}
\draw[spath/save=path,ultra thick, red] (0,0) -- +(3,0);
\draw[spath/split at keep start={path}{1/3},spath/use=path];
\draw[spath/split at keep end={path}{2/3},spath/use=path];
\draw[spath/split at keep middle={path}{1/3}{2/3},spath/use=path,dotted];

\draw[spath/save=path,ultra thick, red] (0,-1) .. controls +(6,-1) and +(-6,-1) .. +(3,0);
\draw[spath/split at keep start={path}{1/3},spath/use=path];
\draw[spath/split at keep end={path}{2/3},spath/use=path];
\draw[spath/split at keep middle={path}{1/3}{2/3},spath/use=path,dotted];

\end{tikzpicture}
\end{document}

It works by splitting the path into three components and then rendering each component with a different style. As Henri mentions in the comments, to have a genuine solid and dotted line requires (at least) two draw commands - one for each style. I use one for each segment since combining the first and last segments would actually lead to slightly more complicated code.

As it uses the spath3 library, it works on arbitrary paths.

Here's the result:

Answer 3

Without guru's tricks try this:

\documentclass[tikz, border=1cm]{standalone}
\begin{document}
\begin{tikzpicture}
    \path (0,0) coordinate (A) (3,5) coordinate (B);
    \path (A)--(B) node [inner sep=0pt,pos=.333] (C) {};
    \path (A)--(B) node [inner sep=0pt,pos=.666] (D) {};
    \draw (A)--(C);
    \draw (D)--(B);
    \draw[dotted] (C)--(D);
\end{tikzpicture}
\end{document}

Output:

Answer 4

With the subpath function and time along a path, Asymptote handles this task very easy, for any curve.

unitsize(1cm);

// for curvy curves
path p=(0,-1) .. controls (0,-1)+(6,-1) and (3,-1)+(-6,-1) .. (3,-1);
real t1=.4,t2=.6;   // times on path
path q1=subpath(p,0,t1);
path q2=subpath(p,t1,t2);
path q3=subpath(p,t2,1);
draw(q1^^q3,magenta);
draw(q2,magenta+dotted);

// of course it works for straight lines
path pline=(0,0)--(3,1);
real s1=.3,s2=.7;   // times on path

path q1line=subpath(pline,0,s1);
path q2line=subpath(pline,s1,s2);
path q3line=subpath(pline,s2,1);
draw(q1line^^q3line,blue);
draw(q2line,blue+dotted);

shipout(bbox(5mm,invisible));

Answer 5

A minor variation of Raffaele Santoro's answer using the calc library to shorten the code. It produces the same output.

\documentclass[tikz, border=1cm]{standalone}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
    \path (0,0) coordinate (A) (3,5) coordinate (B);
    \draw (A)--($(A)!1/3!(B)$) ($(A)!2/3!(B)$) -- (B);
    \draw[dotted] ($(A)!1/3!(B)$) -- ($(A)!2/3!(B)$);
\end{tikzpicture}
\end{document}