在同一节点之间绘制边，使得没有重叠

Question

如何调整路径\path (a) edge [edge, loop left] node {$1_a$} (a)，使路径的起点和终点都靠近节点，但又不能太靠近以至于节点文本和节点内容重叠。我尝试使用shorten边缘样式，但它似乎只适用于边缘的一端。

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary {calc}
\usetikzlibrary {automata}
\usetikzlibrary {positioning}
\usetikzlibrary {fit}

\begin{document}
\tikzset{edge/.style={
    thin,
    black,
    ->,
    shorten >=8pt,
    shorten <=8pt }
}
\tikzset{node-element/.style={
    inner sep=1.5mm,
    minimum size=1cm,
    }
}

\begin{tikzpicture}[scale=1.5]
    \coordinate (a) at (0,0);
    \coordinate (b) at (2,0);
    \coordinate (c) at (1,2);
    \node [node-element] at (a) {a};
    \node [node-element] at (b) {b};
    \node [node-element] at (c) {c};
    \draw[edge, bend left] (a) to node[above, pos=0.5] {$f$} (b);
    \draw[edge, bend left] (b) to node[below, pos=0.5] {$f^{-1}$} (a);
    \draw[edge] (c) to node[left, pos=0.5] {$h$} (a);
    \draw[edge] (c) to node[right, pos=0.5] {$h^{-1}$} (b);
    \path (a) edge [edge, loop left] node {$1_a$} (a)
          (b) edge [edge, loop right] node {$1_b$} (b);
\end{tikzpicture}
\end{document}

Answer 1

通过将a、定义b为c节点名称而不是零大小坐标来解决。

\documentclass[tikz]{standalone}

\begin{document}
\tikzset{
  edge/.style={
    thin,
    black,
%    ->,
%    shorten >=8pt,
%    shorten <=8pt
  },
  node-element/.style={
%    inner sep=1.5mm,
%    minimum size=1cm,
  }
}

\begin{tikzpicture}[scale=1.5]
    \node [node-element] (a) at (0,0) {a};
    \node [node-element] (b) at (2,0) {b};
    \node [node-element] (c) at (1,2) {c};
    \draw[edge, bend left] (a) to node[above, pos=0.5] {$f$} (b);
    \draw[edge, bend left] (b) to node[below, pos=0.5] {$f^{-1}$} (a);
    \draw[edge] (c) to node[left, pos=0.5] {$h$} (a);
    \draw[edge] (c) to node[right, pos=0.5] {$h^{-1}$} (b);
    \path (a) edge [edge, loop left] node {$1_a$} ()
          (b) edge [edge, loop right] node {$1_b$} ();
\end{tikzpicture}
\end{document}