为什么锚点 = 西北不适用于圆形，而适用于矩形？

Question

我想在灰色背景的角落放置一个圆圈，这是代码：

\documentclass[tikz]{standalone}

\usetikzlibrary{calc}

\newlength{\width}
\setlength{\width}{16cm}
\newlength{\height}
\setlength{\height}{10cm}


\begin{document}
\begin{tikzpicture}
\node[fill=gray,minimum width=\width,minimum height=\height] at (0.5\width,0.5\height) {};


\draw[green] (0,0) |- (\width,\height);

\node[circle,anchor=north west,fill=blue,text=white,minimum size=0.2\height,font=\Large,inner sep=0pt,outer sep=0pt] at (0,\height) {T};

\end{tikzpicture}
\end{document}

然而圆圈并不像预期的那样位于左上角：

现在忽略circle并再次尝试：

\node[anchor=north west,fill=blue,text=white,minimum size=0.2\height,font=\Large,inner sep=0pt,outer sep=0pt] at (0,\height) {T};

并且它的工作原理是：

我做错了什么或者这里发生了什么事？

Answer 1

这是因为，您用来定位节点的north west形状的锚点位于形状的边框上。至于形状，锚点也位于形状的边框上，但此处它恰好与形状边界框的左上角重合。circlerectanglenorth west

\documentclass[tikz]{standalone}
\begin{document}

\Huge
\begin{tikzpicture}
\node[name=s, shape=circle, line width=10pt, draw=black!15, fill=yellow!15, inner sep=1cm] {circle\vrule width 1pt height 2cm};
\edef\northwestanchor{north west}
\foreach \anchor/\placement in
    {north west/above left, north/above, north east/above right,
    west/left, center/above, east/right,
    mid west/right, mid/above, mid east/left,
    base west/left, base/below, base east/right,
    south west/below left, south/below, south east/below right,
    text/left, 10/right, 130/above} 
    \draw[shift=(s.\anchor), \ifx\anchor\northwestanchor red\fi] plot[mark=x] coordinates{(0,0)}
    node[\placement] {\scriptsize\texttt{(s.\anchor)}};
\end{tikzpicture}

\end{document}

---

由于您知道节点的高度和宽度，因此您可以将节点向下和向右移动：

\documentclass[tikz]{standalone}
\usetikzlibrary{calc}

\newlength{\width}
\setlength{\width}{16cm}
\newlength{\height}
\setlength{\height}{10cm}

\begin{document}
\begin{tikzpicture}
\node[fill=gray,minimum width=\width,minimum height=\height] at (0.5\width,0.5\height) {};

\draw[green] (0,0) |- (\width,\height);

\node[circle,shift={(0.1\height,-0.1\height)},fill=blue,text=white,minimum size=0.2\height,font=\Large,inner sep=0pt,outer sep=0pt] at (0,\height) {T};

\end{tikzpicture}
\end{document}

Answer 2

尽管贾斯珀·哈比希特已经解释了问题的原因（north west位于边界上），为了获得更灵活的解决方案，您可以定义单独的锚点，就像我在下面的代码中所做的那样corner north west。

我从我的positioning-plus库。其他对角线的锚点也在此处定义。

这是一个通用锚点，意味着每个形状都可以使用它。当然，对于更复杂的形状，这仍然不会返回所需的点。

代码

\documentclass[tikz]{standalone}
\newlength{\width} \setlength{\width}{16cm}
\newlength{\height}\setlength{\height}{10cm}
\makeatletter
\pgfdeclaregenericanchor{corner north west}{%
  \pgf@sh@reanchor{#1}{north}%
  \pgf@ya\pgf@y
  \pgf@process{\pgf@sh@reanchor{#1}{west}}%
  \pgf@y\pgf@ya}
\makeatother

\begin{document}
\begin{tikzpicture}
\node[fill=gray,minimum width=\width,minimum height=\height] at (0.5\width,0.5\height) {};

\draw[green] (0,0) |- (\width,\height);
%                   ......
\node[circle,anchor=corner north west,fill=blue,text=white,
      minimum size=0.2\height,font=\Large,inner sep=0pt,outer sep=0pt] at (0,\height) {T};

\end{tikzpicture}
\end{document}

Answer 3

这将首先绘制圆圈，然后将其他所有东西（背景层）相对于它定位。

\documentclass[tikz]{standalone}

\usetikzlibrary{calc,backgrounds}

\newlength{\width}
\setlength{\width}{16cm}
\newlength{\height}
\setlength{\height}{10cm}


\begin{document}
\begin{tikzpicture}
\node[circle,fill=blue,text=white,minimum size=0.2\height,font=\Large,inner sep=0pt,outer sep=0pt] (T) {T};
\coordinate (origin) at ($(T.north -| T.west) + (0,-\height)$);

\begin{scope}[shift=(origin), on background layer]
  \node[fill=gray,minimum width=\width,minimum height=\height] at (0.5\width,0.5\height) {};

  \draw[green] (0,0) |- (\width,\height);
\end{scope}
\end{tikzpicture}
\end{document}