Aligning two semicircles

Question

I'm making \item commands with coloured circles. One needs to be two colours, i.e. two semicircles which form a single circle.

I currently have this, which superimposes both circles:

\documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric}
\newcommand*\reditem{\item[{\tikz[baseline=-3]{\node[shape=circle,draw,fill=red,inner sep=4pt] (char) {};}}]}
\newcommand*\redblueitem{\item[{\tikz[baseline=-3]{
  \node[shape=semicircle,rotate=90,fill=red,inner sep=3pt](char) {}; 
  \node[shape=semicircle,rotate=270,fill=blue,inner sep=3pt] (char) {};}
  }]}

\begin{document}
\begin{frame}
\begin{itemize}
  \reditem test red
  \redblueitem test redblue
\end{itemize}
\end{frame}
\end{document}

How can I adjust this to form a nice two-colour circle?

Answer 1

You need to both align them using their south anchors (which are located in the middle of the straight edge), and set the line width=0pt so there's no gap:

\documentclass[11pt]{article}
\usepackage{tikz}
\usetikzlibrary{shapes.geometric}
\begin{document}
\newcommand*\redblueitem{\item[{\tikz[baseline=-3]{
  \node[shape=semicircle,rotate=90,fill=red,inner sep=3pt, anchor=south, outer sep=0pt](char) {}; 
  \node[shape=semicircle,rotate=270,fill=blue,inner sep=3pt, anchor=south, outer sep=0pt] (char) {};}
  }]}

  \begin{itemize}
  \redblueitem Test
  \end{itemize}
\end{document}

---

A simpler way to achieve this might be to use \paths instead of nodes. Here's one approach in which I've defined a macro that contains the desired circle radius, so you can easily adjust the size of your circles:

\documentclass{beamer}
\usepackage{tikz}

\def\rubensradius{4pt}

\newcommand*\reditem{\item[{\tikz [baseline=-0.8*\rubensradius]{\path [fill=red, draw=blue] (0,0) circle [radius=\rubensradius-0.5\pgflinewidth];}}]}
\newcommand*\redblueitem{\item[{\tikz [baseline=-0.8*\rubensradius]{
    \path [fill=red] (0,\rubensradius) arc [start angle=90, delta angle=180, radius=\rubensradius];
    \path [fill=blue] (0,-\rubensradius) arc [start angle=270, delta angle=180, radius=\rubensradius];
}}]}

\begin{document}
\begin{frame}
\begin{itemize}
  \reditem test red
  \redblueitem test redblue
\end{itemize}
\end{frame}
\end{document}

Answer 2

You can use the calc tikzlibrary:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{shapes,calc}


\newcommand*\redblueitem{\item[{\tikz[baseline=-3]{
\node[shape=semicircle,rotate=90,fill=red,inner sep=3pt] (char) {}; 
\node[shape=semicircle,rotate=270,fill=blue,inner sep=3pt] at ($(char)+(6pt,0)$) {};}
}]}

\begin{document}

\begin{itemize}
\redblueitem something
\end{itemize}

\end{document}

or you can work at the lowest level (PGF):

\documentclass{article}

\usepackage{tikz}

\newcommand*\RBitem{\item[{\tikz[baseline=-3]
\def\r{5pt}
\pgfpathmoveto{\pgfpoint{0pt}{0pt}}
\pgfpathlineto{\pgfpoint{0pt}{\r}}
\pgfpatharc{90}{270}{\r}
\pgfsetfillcolor{red}
\pgfusepath{fill}
\pgfpathmoveto{\pgfpoint{0pt}{0pt}}
\pgfpathlineto{\pgfpoint{0pt}{\r}}
\pgfpatharc{90}{-90}{\r}
\pgfsetfillcolor{blue}
\pgfusepath{fill};}]}

\begin{document}

\begin{itemize}
\RBitem something
\end{itemize}

\end{document}

with the "same" output: