tikz 节点尺寸

Question

如何获取tikz节点尺寸（例如高度和宽度）？我有兴趣使用这些参数来精确计算对象的位置，例如<a.height>在以下示例中使用：

\documentclass{article}
\thispagestyle{empty}
\usepackage{graphicx}
\usepackage{tikz}
\begin{document}
\begin{figure}
  \begin{tikzpicture}
    \node(a) {
      \includegraphics[width=0.8\textwidth]{example-image-a}
    };
    \node(b) at (0, <a.height>*3/4){label};
  \end{tikzpicture}
\end{figure}
\end{document}

Answer 1

两个示例，在不知道节点尺寸的情况下获得所需结果。第二个使用calctikzlibrary

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
  \begin{tikzpicture}
    \node(a) {
      \includegraphics[width=0.8\textwidth]{example-image-a}
    };
    \path (a.center)--(a.north) node[pos=.75, right] {label without calc};
    \node[left] at ($(a.center)!.75!(a.north)$) {label with calc};
  \end{tikzpicture}
\end{document}

更新：

假设我们希望标签label without calc从上例中的实际位置向右移动 2 厘米，向上移动 1 厘米：\path (a.center)--(a.north) node[pos=.75, right] {label without calc}。我们可以使用positioning库将节点相对于路径上的相应点移动。在这种情况下，我们可以说above right=1cm and 2cm但这将使用south west锚点进行放置，并且我们想要west，然后我们在选项anchor=west后添加：above right

\documentclass[border=2mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{calc, positioning}
\begin{document}
  \begin{tikzpicture}
    \node[inner sep=0pt] (a) {
      \includegraphics[width=0.8\textwidth]{example-image-a}
    };
    \path (a.center)--(a.north) node[draw, pos=.75, above right=1cm and 2cm, anchor=west, red] (b) {label without calc};
    \node[left] (c) at ($(a.center)!.75!(a.north)$) {label with calc};

    %Some auxiliary elements.
    \draw[red,<->] (a.center)--(c.east) node[midway, right] {75\%};
    \draw[red,<->] (c.east)--(a.north) node[midway,right] {25\%};
    \fill[red] (c.east) circle (1pt);
    \draw[red,<->] (c.east)-|(b.west)  node[pos=0.25,below] {2 cm} node[pos=0.75,left] {1 cm};
  \end{tikzpicture}
\end{document}