tikz-figures in minipage: Captions at the same height and top alignment

Question

I want to place two tikz pictures next to each other, both of them should be figures I can reference to. Please consider the following MWE:

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document} 
    \begin{figure}
        \centering
        \begin{minipage}{.5\textwidth}
            \centering
            \begin{tikzpicture}
                \draw [fill=black] (0,0) rectangle (1,1);
            \end{tikzpicture}
            \captionof{figure}{a square}
            \label{fig:square}
        \end{minipage}%
        \begin{minipage}{.5\textwidth}
            \centering
            \begin{tikzpicture}
                \draw [fill=black] (0,0) rectangle (1,3);
            \end{tikzpicture}
            \captionof{figure}{a rectangle}
            \label{fig:rect}
        \end{minipage}%
    \end{figure}

    Figures \ref{fig:square} and \ref{fig:rect} demonstrate the difference
    between a square and a rectangle.

\end{document}

which looks like

I need both pictures to be aligned at the top, but the captions to be on the same height as well:

Can you please tell me how those alignments can be realized without loosing the ability to reference the single pictures?

Answer 1

Here is a solution using minipage alignment, baseline option of TikZ picture and the special node current bounding box.

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document} 
\begin{figure}
  \begin{minipage}[t]{.5\linewidth}
    \centering
    \begin{tikzpicture}[baseline=(current bounding box.north)]
      \draw [fill=black] (0,0) rectangle (1,1);
    \end{tikzpicture}
  \end{minipage}%
  \begin{minipage}[t]{.5\linewidth}
    \centering
    \begin{tikzpicture}[baseline=(current bounding box.north)]
      \draw [fill=black] (0,0) rectangle (1,3);
    \end{tikzpicture}
  \end{minipage}
  \begin{minipage}[t]{.5\linewidth}
    \caption{a square}
    \label{fig:square}
  \end{minipage}%
  \begin{minipage}[t]{.5\linewidth}
    \caption{a rectangle}
    \label{fig:rect}
  \end{minipage}
\end{figure}
Figures \ref{fig:square} and \ref{fig:rect} demonstrate the difference
between a square and a rectangle.
\end{document}

To use this code in a macro (as in Gonzalo Medina's answer), you may use the every picture style:

\newcommand\SideBySide[6]{
  \begin{figure}
    \tikzset{every picture/.style={baseline=(current bounding box.north)}}
    \begin{minipage}[t]{.5\linewidth}
      \centering #1%
    \end{minipage}%
    \begin{minipage}[t]{.5\linewidth}
      \centering #4%
    \end{minipage}
    \begin{minipage}[t]{.5\linewidth}
      \caption{#2}
      \label{#3}
    \end{minipage}%
    \begin{minipage}[t]{.5\linewidth}
      \caption{#5}
      \label{#6}
    \end{minipage}
  \end{figure}
}

Syntax:

\SideBySide{<figure1>}{<caption1>}{<label1>}{<figure2>}{<caption2>}{<label2>}

Example:

\SideBySide{
\begin{tikzpicture}
    \draw [fill=black] (0,0) rectangle (1,1);
  \end{tikzpicture}
}{a square}{fig:square}{
\begin{tikzpicture}
    \draw [fill=black] (0,0) rectangle (1,3);
  \end{tikzpicture}
}{a rectangle}{fig:rect}

Answer 2

Use the option [t] as

\begin{minipage}[t]{.5\textwidth}

for minipages.

Also, you have to fake and insert an empty rectangle to have both figures with same height. I don't see another option since they were drawn on independent way.

So I used \path (0,0) rectangle (1,-2); to push the square up.

\documentclass{scrartcl}
\usepackage{tikz}
\begin{document} 
    \begin{figure}
        \centering
        \begin{minipage}[t]{.5\textwidth}   %%%% [t] here
        \centering
        \begin{tikzpicture}
            \draw[fill=black] (0,0) rectangle (1,1);
            \path (0,0) rectangle (1,-2);            %% empty rectangle
        \end{tikzpicture}
        \captionof{figure}{a square}
        \label{fig:square}
    \end{minipage}%
        \begin{minipage}[t]{.5\textwidth}   %%%% [t] here
            \centering
            \begin{tikzpicture}
                \draw [fill=black] (0,0) rectangle (1,3);
            \end{tikzpicture}
            \captionof{figure}{a rectangle}
            \label{fig:rect}
        \end{minipage}%
    \end{figure}

    Figures \ref{fig:square} and \ref{fig:rect} demonstrate the difference
    between a square and a rectangle.

\end{document}

Answer 3

The code:

\documentclass{scrartcl}
\usepackage{tikz}

\newlength\mylen
\newlength\mylena
\newlength\mylenb
\newcommand\SideBySide[6]{%
  \settoheight\mylen{#1}%
  \settoheight\mylena{#1}%
  \settoheight\mylenb{#4}%
  \ifdim\mylenb>\mylena\relax%
    \setlength\mylen{\mylenb}%
  \fi%
  \begin{figure}
  \begin{minipage}[c][\mylen][t]{.5\textwidth}
    \centering#1
  \end{minipage}%
  \begin{minipage}[c][\mylen][t]{.5\textwidth}
    \centering#4
  \end{minipage}\par
  \begin{minipage}[t]{.5\textwidth}
    \caption{#2}
    \label{#3}
  \end{minipage}%
  \begin{minipage}[t]{.5\textwidth}
    \caption{#5}
    \label{#6}
  \end{minipage}%
  \end{figure}%
}

\begin{document} 

\SideBySide{%
\begin{tikzpicture}
    \draw [fill=black] (0,0) rectangle (1,1);
  \end{tikzpicture}%
}{a square}{fig:square}{%
\begin{tikzpicture}
    \draw [fill=black] (0,0) rectangle (1,3);
  \end{tikzpicture}%
}{a rectangle}{fig:rect}

Figures~\ref{fig:square} and~\ref{fig:rect} demonstrate the difference between a square and a rectangle. Figures~\ref{fig:circle} and~\ref{fig:squareb} demonstrate the difference between a square and a circle.


\SideBySide{%
\begin{tikzpicture}
    \draw [fill=cyan] (0,0) circle [radius=2cm];
  \end{tikzpicture}%
}{a circle}{fig:circle}{%
\begin{tikzpicture}
    \draw [fill=orange] (0,0) rectangle (1,1);
  \end{tikzpicture}%
}{another sqaure}{fig:squareb}

\end{document}

Explanation:

The idea is to use two sets of side-by-side minipages; the first one for the figures and the second one, for the captions; all minipages are top aligned.

I defined a command \SideBySide which measures the heights of both images and determines the maximum height; this height is then used as the fixed height for both minipages containing the figures. All calculations are made by the command, so no manual intervention is required. The syntax:

\SideBySide{<figure1>}{<caption1>}{<label1>}{<figure2>}{<caption2>}{<label2>}