結上箭頭裝飾的意外行為：出現多個箭頭和間隙

Question

我正在使用knotstikzlibrary 繪製一些結，並想要調整我的結的方向，即添加箭頭。我使用一種稱為“arrowat”的裝飾風格，它在所需位置插入一個箭頭（例如，\draw[arrowat=0.5]將在結果線的中間產生一個箭頭）。定義如下

\tikzset{% 
    arrowat/.style={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\arrow[xshift=2pt]{>}}}}
    }
}

然而，當我嘗試在結上使用這種樣式時，會出現一些問題：在某些位置，一切正常，但如果我嘗試在其他某些位置繪製箭頭，則會出現多個箭頭（以及線條中的間隙）。下面是一個 MWE，顯示了工作正常的情況（左）和工作正常的情況（右）。我的問題是：我該如何解決這個問題，或者更確切地說，防止它發生？

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,knots}

\tikzset{% 
    arrowat/.style={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\arrow[xshift=2pt]{>}}}}
    }
}
\begin{document}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick, arrowat=0.8] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
\end{knot}
\end{tikzpicture}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick, arrowat=0.5] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
    \end{knot}
\end{tikzpicture}

\end{document}

Answer 1

我沒有真正的（即優雅的）解決方案。只是好奇想部分了解發生了什麼並檢查 Ti 的頻率kZ想它位於位置 0.5。在此範例中，這是 10 次，您可以透過取消註解來驗證這一點\typeout{\number\value{arrowcount}}。然後可以添加一個開關，指示真正要在其中的哪一個上繪製箭頭。對應的樣式稱為conditional arrow at.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,knots}
\newcounter{arrowcount}
\tikzset{% 
    arrowat/.style={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\arrow[xshift=2pt]{>}}}}
    },
    conditional arrow at/.style 2 args={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\stepcounter{arrowcount}%
                %\typeout{\number\value{arrowcount}}%
                \ifnum\value{arrowcount}=#2
                \arrow[xshift=2pt]{>}%
                \fi}}}
    },
}
\begin{document}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick, arrowat=0.8] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
\end{knot}
\end{tikzpicture}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick,conditional arrow at={0.5}{6}] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
   \end{knot}
\end{tikzpicture}

\end{document}

使用金鑰時，情況改善了 2 倍（如預期）only when rendering。那你「只有」5 個槽位，每個槽位其實都通往一個箭頭。

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,knots}
\newcounter{arrowcount}
\tikzset{% 
    arrowat/.style={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\arrow[xshift=2pt]{>}}}}
    },
    conditional arrow at/.style 2 args={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\stepcounter{arrowcount}%
                \typeout{\number\value{arrowcount}}%
                \ifnum\value{arrowcount}=#2
                \arrow[xshift=2pt]{>}%
                \fi}}}
    },
}
\begin{document}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick, arrowat=0.8] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
\end{knot}
\end{tikzpicture}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick,only when rendering/.style={conditional arrow at={0.5}{3}}] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
   \end{knot}
\end{tikzpicture}

\end{document}

所以看來問題在於路徑被分解成段。這告訴我們，我們可以將箭頭附加到不可見的路徑上。

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings,knots}
\newcounter{arrowcount}
\tikzset{% 
    arrowat/.style={%
        postaction={decorate,decoration={
                markings,
                mark=at position #1 with {\arrow[xshift=2pt,opacity=1]{>}}}}
    },
}
\begin{document}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \strand [thick, arrowat=0.8] (0,0)
    to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0);
\end{knot}
\end{tikzpicture}

\begin{tikzpicture}
    \begin{knot} [consider self intersections, ignore endpoint intersections=false, clip width=7, flip crossing=3]
    \def\mypath{(0,0) to [out=180, in=270] (-1,1)
    to [out=90, in=180] (1,2.5)
    to [out=0, in=90] (3,1)
    to [out=270, in=0] (2,0)
    to [out=180, in=270] (-0.2,2.5)
    to [out=90, in=180] (1,3.8)
    to [out=0, in=90] (2.2,2.5)
    to [out=270, in=0] (0,0)}
    \draw[thick,opacity=0,arrowat=0.55] \mypath;
    \strand [thick] \mypath;
   \end{knot}
\end{tikzpicture}

\end{document}

這可能是這些提議中最優雅的一個，並且還表明 0.5 處的「真實」箭頭被十字線遮住。