% Copyright 2019 by Till Tantau
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\section{Angle Library}
\label{section-angle}

\begin{tikzlibrary}{angles}
    This library defines pic types for drawing angles.
\end{tikzlibrary}

\begin{pictype}{angle}{\opt{|=|\meta{A}|--|\meta{B}|--|\meta{C}}}
    This pic adds a drawing of an angle to the current path. This ``drawing of
    an angle'' consist of a ``sector'' or ``wedge'' or ``slice'' whose pointed
    end is at point \meta{B} and whose straight sides lie on the lines form
    \meta{B} to \meta{A} and from \meta{B} to \meta{C}. The length of these
    lines is governed by the following key:
    %
    \begin{key}{/tikz/angle radius=\meta{dimension} (initially 5mm)}
        The length of the sides of the angle's wedge:
        %
\begin{codeexample}[preamble={\usetikzlibrary{angles}}]
\tikz \draw (2,0) coordinate (A) -- (0,0) coordinate (B)
         -- (-1,-1) coordinate (C)
           pic [fill=black!50]                      {angle = A--B--C}
           pic [draw,->,red,thick,angle radius=1cm] {angle = C--B--A};
\end{codeexample}
    \end{key}

    The three points \meta{A}, \meta{B}, and \meta{C} \emph{must} be the names
    of nodes or coordinates; you cannot use direct coordinates like ``|(1,1)|''
    here.

    You can leave out the three points, in this case the text |A--B--C| is
    used; so in the above examples we could just have written |{angle}| in the
    first pic.

    Concerning the sector that makes up the drawing of the angle, the angular
    part of this sector is drawn in front of the path if the |draw| option is
    given to the |pic|, while filled sector is drawn behind the |pic|, provided
    an option like |fill| or |shade| is passed to the pic. The following
    example shows the difference:
    %
\begin{codeexample}[preamble={\usetikzlibrary{angles}}]
\tikz \draw [line width=2mm]
      (2,0) coordinate (A) -- (0,0) coordinate (B)
   -- (1,1) coordinate (C)
     pic [draw=blue, fill=blue!50, angle radius=1cm] {angle};
\end{codeexample}

    When |pic text| is set (which you typically do by using the quotes syntax),
    a node will be created whose name is empty (and, thus, inherits the pic's
    name) and which will be at the half-way angle between the lines to \meta{A}
    and \meta{C} and whose distance from \meta{B} is |angle radius| times the
    following factor:
    %
    \begin{key}{/tikz/angle eccentricity=\meta{factor} (initially 0.6)}
\begin{codeexample}[preamble={\usetikzlibrary{angles,quotes}}]
\tikz \draw (2,0) coordinate (A) -- (0,0) coordinate (B)
         -- (1,1) coordinate (C)
  pic ["$\alpha$", draw, ->] {angle};
\end{codeexample}
        %
\begin{codeexample}[preamble={\usetikzlibrary{angles,quotes}}]
\tikz \draw (2,0) coordinate (A) -- (0,0) coordinate (B)
         -- (1,1) coordinate (C)
  pic ["$\alpha$", draw, angle eccentricity=1] {angle};
\end{codeexample}
        %
\begin{codeexample}[preamble={\usetikzlibrary{angles,quotes}}]
\tikz {
  \draw (2,0) coordinate (A) -- (0,0) coordinate (B)
     -- (1,1) coordinate (C)
      pic (alpha) ["$\alpha$", draw] {angle};

  \draw (alpha) circle [radius=5pt];
}
\end{codeexample}
    \end{key}
\end{pictype}

\begin{pictype}{right angle}{\opt{|=|\meta{A}|--|\meta{B}|--|\meta{C}}}
    This pic adds a drawing of a right angle to the current path. It works in
    the same way as |angle| pic.
    %
\begin{codeexample}[preamble={\usetikzlibrary{angles}}]
  \tikz
    \draw (0,0,0) coordinate (O)
      (1,0,0) coordinate (A) -- (O)
      (0,0,1) coordinate (B) -- (O)
      (0,1,0) coordinate (C) -- (O)
      pic [fill=gray,angle radius=4mm] {right angle = A--O--B}
      pic [draw,red,thick,angle eccentricity=.5,pic text=.]
        {right angle = A--O--C};
\end{codeexample}
    %
\end{pictype}


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