Mark 90 degree angle in tikz in german convention

Question

What's the best way to denote a 90 degree angle with german convention like on the following picture in tikz:

Suppose I start with something like this:

\documentclass{article}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
   \coordinate (O) at (0,0); %
   \coordinate (A) at (1,1); %
   \coordinate (B) at (1,-1); %

   \draw (O) -- (A); %
   \draw (O) -- (B); %

\end{tikzpicture}


\end{document}

Answer 1

I have found a solution which uses tkz-euclide and used Herbert's answer to

Defining a macro in LaTeX with an optional parameter in round brackets

to define the optional argument of the macro.

Code

\documentclass{article}


\usepackage{tkz-euclide}
\usetkzobj{all}

%see: http://tex.stackexchange.com/a/12893/4011
\makeatletter
\def\rechterWinkel{\@ifnextchar[\rechterWinkel@i\rechterWinkel@ii}
\def\rechterWinkel@i[#1](#2,#3,#4){%
  \pgfmathsetmacro{\pos@A}{0.5*#1}
  \pgfmathsetmacro{\pos@B}{0.25*#1}
   \tkzMarkAngle[size=\pos@A](#2,#3,#4)
   \tkzLabelAngle[pos=\pos@B](#2,#3,#4){\tikz \fill (0,0) circle (0.6pt);}
}% 
\def\rechterWinkel@ii(#1,#2,#3){%  
\rechterWinkel@i[1](#1,#2,#3)
}% 
\makeatother

\begin{document}

\begin{tikzpicture}
   \coordinate (O) at (0,0); %
   \coordinate (A) at (1,1); %
   \coordinate (B) at (1,-1); %
   \coordinate (C) at (-1,1);

   \draw (O) -- (A); %
   \draw (O) -- (B); %
   \draw (O) -- (C); %

   \rechterWinkel(B,O,A)
   \rechterWinkel[0.5](A,O,C)
\end{tikzpicture}
\end{document}

Output

Answer 2

I'd do it with an arc and a circle.

The command \rechterWinkel takes three arguments:

• the point of the angle,
• the start angle, and
• the arc's radius (you probably want to have same-size right angles, then I suggest to hard-code the radius directly and just use two parameters).

Hard-coded radius

\newcommand*{\rechterWinkelRadius}{.5cm}
\newcommand*{\rechterWinkel}[2]{% #1 = point, #2 = start angle
   \draw[shift={(#2:\rechterWinkelRadius)}] (#1) arc[start angle=#2, delta angle=90, radius = \rechterWinkelRadius];
   \fill[shift={(#2+45:\rechterWinkelRadius/2)}] (#1) circle[radius=1.25\pgflinewidth];
}

Code

\documentclass{article}

\usepackage{tikz}

\newcommand*{\rechterWinkel}[3]{% #1 = point, #2 = start angle, #3 = radius
   \draw[shift={(#2:#3)}] (#1) arc[start angle=#2, delta angle=90, radius = #3];
   \fill[shift={(#2+45:#3/2)}] (#1) circle[radius=1.25\pgflinewidth];
}
\begin{document}

\begin{tikzpicture}
   \coordinate (O) at (0,0); %
   \coordinate (A) at (1,1); %
   \coordinate (B) at (1,-1); %

   \draw (O) -- (A); %
   \draw (O) -- (B); %

   \rechterWinkel{0,0}{-45}{.5}
\end{tikzpicture}
\end{document}

Output