如何使用相对坐标（而不是硬编码长度）将括号放置在 tikz 中的树下？

Question

我想在树枝下面放置花括号。下面的示例正是这样做的，但标签“Toss 1”和“Toss 2”的放置位置的硬编码值并不理想，因为它们需要在纸张/文本大小发生变化等时进行调整。有没有办法使用相对坐标/节点以更稳健的方式放置两个标签，而不需要硬编码长度（或者至少只有一个这样的长度，即从两个标签到树的垂直距离）？

\documentclass{scrartcl}
\usepackage{tikz}
\usetikzlibrary{positioning, decorations.pathreplacing}
\begin{document}
    \begin{tikzpicture}[line width = 0.25mm, grow=right, sloped,
      level 1/.style={level distance=1.8cm, sibling distance=2.4cm},
      level 2/.style={level distance=2.1cm, sibling distance=1.2cm},
      edge from parent/.style={draw, shorten >=1.4mm, shorten <=1.4mm},
      mynodestyle/.style={inner sep=1mm},
      myleafstyle/.style={inner sep=0mm}
      ]
      \node[mynodestyle] (root) {Coin toss}
      child {
        node[mynodestyle] (toss1) {$T$}
        child {
          node[myleafstyle, coordinate, label={[xshift=-1mm]right: $(T,T)$}] (toss2) {}
          edge from parent
          node[above] {\tiny $1/2$}
        }
        child {
          node[myleafstyle, coordinate, label={[xshift=-1mm]right: $(T,H)$}] {}
          edge from parent
          node[above] {\tiny $1/2$}
        }
        edge from parent
        node[above] {\tiny $1/2$}
      }
      child {
        node[mynodestyle] {$H$}
        child {
          node[myleafstyle, coordinate, label={[xshift=-1mm]right: $(H,T)$}] {}
          edge from parent
          node[above] {\tiny $1/2$}
        }
        child {
          node[myleafstyle, coordinate, label={[xshift=-1mm]right: $(H,H)$}] {}
          edge from parent
          node[above] {\tiny $1/2$}
        }
        edge from parent
        node[above] {\tiny $1/2$}
      };
      % Labels. How to place the curly braces below the branches labeled with 1/2
      %         in terms of nodes/relative coordinates instead of hard-coded lengths?
      % Label "Toss 2"
      \node[coordinate, below right = 6.2mm and 1mm of toss1] (A2) {};% hard-coded
      \node[coordinate, below left  = 2.6mm and 1mm of toss2] (E2) {};% hard-coded
      \draw[decorate, decoration = {brace, mirror, amplitude = 2mm}] (A2)--node[below = 2mm, anchor = north] {Toss 2} (E2);
      % Label "Toss 1"
      \node[coordinate, below right = 18.2mm and -4mm of root] (A1) {};% hard-coded
      \node[coordinate, below left  =  6.2mm and 1mm of toss1] (E1) {};% hard-coded
      \draw[decorate, decoration = {brace, mirror, amplitude = 2mm}] (A1)--node[below = 2mm, anchor = north] {Toss 1} (E1);
    \end{tikzpicture}%
\end{document}

Answer 1

像这样？

与您的 MWE 相比，我添加了calligraphy库（用于更花哨的括号）并重命名了节点。对于括号锚点的位置，使用垂直坐标。

\documentclass{scrartcl}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing,
                    calligraphy}    % new, had to be load after decorations.pathreplacing}
\begin{document}
    \begin{tikzpicture}[
    BC/.style = {decorate,
        decoration={calligraphic brace, amplitude=6pt,
        raise=1mm,
        mirror},% for mirroring of brace
        ultra thick,
        pen colour=red % select color according to your taste
        },
%
grow=right,
level distance=2.2cm,  
level 1/.style = {sibling distance=2.4cm},
level 2/.style = {sibling distance=1.2cm},
edge from parent/.style={draw,thick, shorten >=1.4mm, shorten <=1.4mm},
      N/.style = {inner sep=1pt},
      L/.style = {font=\tiny, above, sloped}
      ]
\node[N] (A) {Coin toss}
    child {node[N] (B) {$T$}
        child {node[N] (C) {$(T,T)$} 
        edge from parent node[L] {$1/2$}
              }
        child {node[N] {$(T,H)$}
        edge from parent node[L] {$1/2$}
              }
        edge from parent node[L] {$1/2$}
      }
    child {node[N] {$H$}
        child {node[N]  {$(H,T)$}
          edge from parent node[L] {$1/2$}
        }
        child {node[N]  {$(H,H)$}
          edge from parent node[L] {$1/2$}
        }
        edge from parent node[L] {$1/2$}
      };
%
\draw[BC] (A.330  |- C.south) -- node[below=8pt] {Toss 1} (B.west |- C.south);
\draw[BC] (B.east |- C.south) -- node[below=8pt] {Toss 2} (C.south west);
    \end{tikzpicture}%
\end{document}

附录： 使用以下forest包可以使树的代码更加简洁、清晰：

\documentclass{scrartcl}
\usepackage{forest}
\usetikzlibrary{decorations.pathreplacing,
                calligraphy}    %had to be loaded after decorations.pathreplacing
\tikzset{
 BC/.style = {% Brace Calligraphic
        decorate,
        decoration={calligraphic brace, amplitude=6pt,
        raise=2pt,
        mirror},% for mirroring of brace
        ultra thick,
        pen colour=red % select color according to your taste
        },
ELS/.style = {%Edge Label Style
              font=\scriptsize, sloped, pos=0.6,
              inner sep=2pt, anchor=#1}
        }
\begin{document}
    \begin{forest}
for tree = {
% tree
    grow = east,
    edge = {thick}, 
   l sep = 12mm,
   s sep =  4mm,
L/.style = {if n=1{edge label={node[ELS=north]{$1/2$}}}
                  {edge label={node[ELS=south]{$1/2$}}}} % since all labels are the same
        }
[Coin toss, name=A
    [$T$, name=B, L
        [{$(T,T)$}, name=C, L]
        [{$(T,H)$},L]
    ]
    [$H$, L
        [{$(H,T)$},L]
        [{$(H,H)$},L]
    ]
]
%
\draw[BC] (A.330  |- C.south) -- node[below=8pt] {Toss 1} (B.west |- C.south);
\draw[BC] (B.east |- C.south) -- node[below=8pt] {Toss 2} (C.south west);
    \end{forest}%
\end{document}

编辑： 如果您希望所有边缘标签都位于边缘上方，则替换

L/.style = {if n=1{edge label={node[ELS=north]{$1/2$}}}
                  {edge label={node[ELS=south]{$1/2$}}}} 

和

L/.style = {edge label={node[font=\scriptsize, sloped, pos=0.6,
                             inner sep=2pt, anchor=south]{$1/2$}}}

并删除了 中的 ELS 样式定义\tikzset。

我更喜欢上图所示的解决方案。