Latex / Tikz：绘制一条直线的垂直线

Question

也许你可以帮助我，我尝试从点/坐标绘制直线到直线。我用Tikz画画。

      \begin{tikzpicture}
      \coordinate [label=left:$A$] (A) at (-5,-5){};
      \coordinate [label=right:$B$] (B) at (5,-5) {};
      \coordinate [label=right:$C$] (C) at (5,1) {};
      \coordinate [label=left:$D$] (D) at (-5,1) {};

      \draw [thick] (A) -- node[midway] {$\parallel$} (B) -- node[sloped]{$\parallel$} (C) -- (D) -- cycle;

      \coordinate (S1) at ($(D)!0.66!(C)$);
      \coordinate (S2) at ($(A)!0.11!(B)$);
      \draw [very thick] (S1) -- node[above]{x} (S2);
      \draw [red!100, thick] (S1) -- node[above]{T} (A -| B );
      \end{tikzpicture}

This where the red line should go

红线应垂直从坐标（S1）到直线（A - B）。我试着把它画成这样：

     \draw [red!100, thick] (S1) -- node[above]{T} (A -| B );

但他画了一条线来协调A.

谢谢，

Answer 1

您不需要定义新坐标，但可以使用calc库中的投影标识符。

在最后一行你只需要

\draw [red!100, thick] (S1) -- node[left]{T} ($(A)!(S1)!(B)$);

这意味着沿着A-B取S1投射到A-B上的点。

Answer 2

您可以通过为AB上的点定义新坐标（例如S3）来做到这一点：

\begin{tikzpicture}
\coordinate [label=left:$A$] (A) at (-5,-5){};
\coordinate [label=right:$B$] (B) at (5,-5) {};
\coordinate [label=right:$C$] (C) at (5,1) {};
\coordinate [label=left:$D$] (D) at (-5,1) {};

\draw [thick] (A) -- node[midway] {$\parallel$} (B) -- node[sloped]{$\parallel$} (C) -- (D) -- cycle;

\coordinate (S1) at ($(D)!0.66!(C)$);
\coordinate (S2) at ($(A)!0.11!(B)$);
\coordinate (S3) at ($(A)!0.66!(B)$);
\draw [very thick] (S1) -- node[above]{x} (S2);
\draw [red!100, thick] (S1) -- node[left]{T} (S3);
\end{tikzpicture}

Answer 3

你的语法几乎是正确的，但是三通操作员|-和-|从一侧获取x坐标，从另一侧获取y坐标。当您编写A -| B时，您将得到A的y坐标和B的x坐标，但在您的代码中，A和B具有相同的x坐标，因此这将再次为您提供A点。相反，你想要A -| S1，或者等同于S1 |- A。

 \draw [red!100, thick] (S1) --   node[left]{T} (S1 |- A);

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
     \begin{tikzpicture}
      \coordinate [label=left:$A$] (A) at (-5,-5){};
      \coordinate [label=right:$B$] (B) at (5,-5) {};
      \coordinate [label=right:$C$] (C) at (5,1) {};
      \coordinate [label=left:$D$] (D) at (-5,1) {};

      \draw [thick] (A) -- node[midway] {$\parallel$} (B) -- node[sloped]{$\parallel$} (C) -- (D) -- cycle;

      \coordinate (S1) at ($(D)!0.66!(C)$);
      \coordinate (S2) at ($(A)!0.11!(B)$);
      \draw [very thick] (S1) -- node[above]{x} (S2);
      \draw [red!100, thick] (S1) --   node[left]{T} (S1 |- A);
      \end{tikzpicture}
\end{document}