0

Suppose I have a certain curve, whose "control" points I specify using pre-set coordinates.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,calc,arrows,decorations.pathmorphing,intersections}

\begin{document}
\begin{tikzpicture}
    \coordinate (A) at (0,0);
    \coordinate (B) at (42,0);
    \coordinate (C) at (0,42);
    \draw plot[smooth] coordinates{(A) (B) (C)};
\end{tikzpicture}
\end{document}

Now we can imagine that this is some trajectory and some "object" has been at coordinate (A) at time t = 0 and has arrived at (C) at time t = 1. I would like to know if it is possible to specify a certain spot along this trajectory, saying t = 0.5 for example, that would effectively mean "halfway through" (A) and (B)? I need this functionality so that I can specify a "naturally-looking" shape, and only then draw things on it.

0
1

This can be done using path decorations:

\documentclass[tikz,border=2mm]{standalone}
\usetikzlibrary{decorations.markings}

\begin{document}
\begin{tikzpicture}[
    define coord/.style 2 args={
      decoration={
        markings,
        mark=at position #2 with {\coordinate (#1);},
      },
      postaction=decorate,
    }
  ]
  \coordinate (A) at (0,0);
  \coordinate (B) at (5,0);
  \coordinate (C) at (0,5);
  \draw[blue, define coord={P}{0.2}, define coord={I}{0.5},
              define coord={Q}{0.75}]
    plot[smooth] coordinates { (A) (B) (C) };
  \fill[red] (P) circle[radius=2pt] node[black, below] {At time $0.2$};
  \node[red!80!black, pin=20:$I$] at (I) {middle};
  \node[above right] at (Q) {At $3/4$ of the curve};
\end{tikzpicture}
\end{document}

enter image description here

As you noted, in a given path, the style must be used with increasing coordinates, otherwise the points aren't placed as expected.

2
  • I accept the answer. The only caveat is that the ratios need to be specified in increasing order, otherwise the engine gets somehow confused. – Ilonpilaaja Oct 2 '20 at 6:37
  • @Ilonpilaaja You're right, thanks for pointing it out. I've slightly expanded the example and mentioned the “increasing order” constraint for coordinates. – frougon Oct 2 '20 at 8:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.