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如何在 tikz 中获取任意路径的长度？

tag-icon • tag-icon tikz-pgf
如何在 tikz 中获取任意路径的长度？

假设我在 tikz 中有一个任意路径（如下例所示）。如何获取路径的弧长并将其存储在宏中以供以后使用？

\documentclass{article}

\usepackage{tikz}
\begin{document}

\begin{tikzpicture}
\draw plot[smooth] coordinates {(0,0) (1,3) (4,7) (-2,9)};
\end{tikzpicture}

\end{document}

编辑：

使用@Symbol1 的评论，我能够获得路径长度，但无法将其转移到装饰之外以便在文档的其他地方使用它：

\documentclass{article}   

\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{hobby}
    
\begin{document}

\begin{tikzpicture}
 \draw[decoration={
  markings,
  mark=
  at position 0 cm 
  with
  {
    \draw (0,0) circle (0pt) node[right]{\pgfdecoratedpathlength};
    %\def{\PathLength}{\pgfdecoratedpathlength}
  }
},
postaction=decorate
  ] plot[hobby] coordinates {(0,0) (1,3) (4,7) (-2,9)};
  \end{tikzpicture}

  %The length of the path is $\PathLength$.
\end{document}

答案1

这是使用 的尝试\pgfextra。不确定这是否是最优雅的解决方案。

\documentclass{article}   

\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{hobby}
    
\begin{document}

\begin{tikzpicture}
  \draw[
    decoration={
      markings,
      mark=
        at position 0 cm 
        with
        {
          \draw (0,0) circle (0pt)
            \pgfextra{\xdef\PathLength{\pgfdecoratedpathlength}} 
            node[right] {\pgfdecoratedpathlength};
        }
    },
    postaction=decorate
  ] 
    plot[hobby] coordinates {(0,0) (1,3) (4,7) (-2,9)};
\end{tikzpicture}

The length of the path is \PathLength.
\end{document}

答案2

\documentclass{article}
    \usepackage{tikz}
    \usepgfmodule{decorations}% This is enough. But any decoration library works
\begin{document}

\begin{tikzpicture}
    \makeatletter
    \path plot[smooth] coordinates {(2,2) (1,3) (4,7) (-2,9)}
        \pgfextra{
            The easiest way to smuggle the current softpath
            \pgfsyssoftpath@getcurrentpath\student@softpath
            \global\let\student@softpath = \student@softpath
        }
    ;
    You can backup the soft path if you want to
    \let\teacher@softpath = \student@softpath
    
    Compute the Length
    \pgf@decorate@parsesoftpath\student@softpath\dummy@token@to@hold@parsed@path
    \message{^^J^^J Public Service Announcement ^^J^^J}
    \message{^^J^^J arc length \pgf@decorate@totalpathlength pt ^^J^^J}
    \node[align=center]{arc length\\\pgf@decorate@totalpathlength pt};
    
    Reuse the path
    (student copy is destroyed by parsing, must use teacher copy)
    \pgfsyssoftpath@setcurrentpath\teacher@softpath
    \pgfsyssoftpath@flushcurrentpath
    \pgfsys@stroke
    
    Put the softpath back like nothing happens
    \draw[red,thick,dotted]\pgfextra{\pgfsyssoftpath@setcurrentpath\teacher@softpath};
    
\end{tikzpicture}

\end{document}

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