为什么 TikZ 中箭头尖端的弯曲箭头不对称？

Question

人们说一张图片胜过千言万语。

TikZ 绘制的箭头像左图一样倾斜，这有什么充分的理由吗？ 有没有简单的方法可以得到像右图一样好看、对称的箭头，而无需手动定位箭头尖端？

笔记：右图中的线条与箭头的细尖重叠。这不是故意的，但添加shorten >=2pt也会扰乱线条的弯曲。

这是生成上述图像（不带标题）的代码。

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[>=latex]
    \node[draw,circle] at (0,0) (A) {A};
    \node[draw,circle] at (1,0) (B) {B};
    \draw[->] (A) to[out=45, in=135] (B);
    \draw[->] (B) to[out=-135, in=-45] (A);
    \draw[help lines] (0.5,0.5) -- (0.5,-0.5);
\end{tikzpicture}
\begin{tikzpicture}[>=latex]
    \node[draw,circle] at (0,0) (A) {A};
    \node[draw,circle] at (1,0) (B) {B};

    \draw (A) to[out=45, in=135] (B);    % Draw nice line
    \draw[<-] (B.135) -- ++(-0.1,0.066); % Add arrow tip

    \draw (A) to[out=-45, in=-135] (B);  % Draw nice line
    \draw[<-] (A.-45) -- ++(0.1,-0.066); % Add arrow tip

    \draw[help lines] (0.5,0.5) -- (0.5,-0.5);
\end{tikzpicture}

\end{document}

Answer 1

这个选择latex是个坏主意，因为不可能得到正确的结果。箭头的尺寸太大了。看看你的“好”尝试的结果

\begin{tikzpicture}[>=to]
  \node[draw,circle] at (0,0) (A) {A};
  \node[draw,circle] at (1,0) (B) {B};
  \draw[->] (A) to[out=45, in=135] (B);
  \draw[->] (B) to[out=-135, in=-45] (A);
  \draw[help lines] (0.5,0.5) -- (0.5,-0.5);
 \end{tikzpicture}    

在接下来的代码中outer sep=0pt是必要的

 \begin{tikzpicture}[>=to]
   \node[draw,circle,outer sep=0pt] at (0,0) (A) {A};
   \node[draw,circle,outer sep=0pt] at (1,0) (B) {B};
   \draw (A) to[out=45, in=135] (B);
   \draw[<-] (B.135) -- ++(-0.001,0.0009);
   \draw (A) to[out=-45, in=-135] (B);
   \draw[<-] (A.-45) -- ++(0.001,-0.0009);
 \end{tikzpicture}  

Answer 2

一个疯狂的想法：

\usetikzlibrary{decorations.markings}
\begin{tikzpicture}[>=latex, decoration={
      markings, mark=at position 1.0 with {\arrow{>}}}
    ]
    \node[draw,circle] at (0,0) (A) {A};
    \node[draw,circle] at (1,0) (B) {B};
    \draw[postaction={decorate}] (A) to[out=45,   in=135] (B); 
    \draw[postaction={decorate}] (B) to[out=-135, in=-45] (A);
    \draw[help lines] (0.5,0.5) -- (0.5,-0.5);
\end{tikzpicture}

现在箭头的角度是错误的，但也许这比原来的不对称弧更好。

还要注意，不对称或倾斜的明显程度取决于所用箭头的种类。使用默认箭头时，它看起来很完美。使用stealth箭头时，问题会出现，但不如使用latex箭头时那么严重。

更新 如果允许箭头尖端与目标节点稍微重叠，则结果会更好：

\usetikzlibrary{decorations.markings}
\begin{tikzpicture}[>=latex, decoration={
      markings, mark=at position 1 with {\draw[->] (-0.01,0) -- (0.07,0);}}
    ]
    \node[draw,circle] at (0,0) (A) {A};
    \node[draw,circle] at (1,0) (B) {B};
    \draw[postaction={decorate}] (A) to[out=45,   in=135] (B); 
    \draw[postaction={decorate}] (B) to[out=-135, in=-45] (A);
    \draw[help lines] (0.5,0.5) -- (0.5,-0.5);
\end{tikzpicture}