Why does '%' sign impact subfigure alignment?

Question

These two LaTeX snippets differ only in the '%' sign at the end of subfigure 1, yet they produce different alignment of subfigures:

Snippet 1 (without '%' sign):

\begin{figure*}
    \centering
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_line.pdf}
        \caption{Subfigure 1}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_region.png}
        \caption{Subfigure 2}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1b_line.pdf}
        \caption{Subfigure 3}
    \end{subfigure}
    \hspace{0mm}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_region.png}
        \caption{Subfigure 4}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1c_line.pdf}
        \caption{Subfigure 5}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1c_region.png}
        \caption{Subfigure 6}
    \end{subfigure}
    % \caption{Stability analysis of example \ref{example:compound}}
    \caption{Hello}
\end{figure*}

Snippet 1 (with '%' sign):

\begin{figure*}
    \centering
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_line.pdf}
        \caption{Subfigure 1}
    \end{subfigure}%
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_region.png}
        \caption{Subfigure 2}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1b_line.pdf}
        \caption{Subfigure 3}
    \end{subfigure}
    \hspace{0mm}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1a_region.png}
        \caption{Subfigure 4}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1c_line.pdf}
        \caption{Subfigure 5}
    \end{subfigure}
    \begin{subfigure}[b]{0.33\textwidth}
        \centering
        \includegraphics[width=\textwidth]{example1c_region.png}
        \caption{Subfigure 6}
    \end{subfigure}
    % \caption{Stability analysis of example \ref{example:compound}}
    \caption{Hello}
\end{figure*}

Why does the '%' sign influence subfigure placement?

Welcome to TeX.se. Although your question is clear (because this is a well-known part of TeX), the code you've given us isn't really helpful: we don't have your images, and you've posted a fragment of code rather than a compilable document. For future questions it's best to post a complete document that others can compile that shows your problem. — Alan Munn, Jul 18 '21 at 13:41

Mico · Answer 1 · 2021-07-18T15:41:08.983

Some general comments:

The 7 \centering instructions per figure* environment don't do anything useful; delete them.
The OP's first figure* environment shows only two, rather than three, subfigures per row. That is because the implicit whitespace after each \end{subfigure} is allowed to persist. That's not the case in the OP's second figure* environment.

Anyway, I think what your float really needs is a handful of well-placed \hfill directives per row, along with a very slight reduction in relative subfigure width, from 0.33\textwidth to 0.32\textwith.

\documentclass[twocolumn,demo]{article} % remove 'demo' option in real document
\usepackage{graphicx,subcaption}
\begin{document}
\begin{figure*}
\captionsetup[subfigure]{skip=0.25\baselineskip}
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1a_line.pdf}
  \caption{Subfigure 1}
\end{subfigure}\hfill
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1a_region.png}
  \caption{Subfigure 2}
\end{subfigure}\hfill
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1b_line.pdf}
  \caption{Subfigure 3}
\end{subfigure} % keep the next line blank to force a line-break

\medskip
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1a_region.png}
  \caption{Subfigure 4}
\end{subfigure}\hfill
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1c_line.pdf}
  \caption{Subfigure 5}
\end{subfigure}\hfill
\begin{subfigure}[b]{0.32\textwidth}
  \includegraphics[width=\textwidth]{example1c_region.png}
  \caption{Subfigure 6}
\end{subfigure}

\caption{Stability analysis}
\label{example:compound}

\end{figure*}
\end{document}

fromthebeeland · Answer 2 · 2021-07-18T18:18:40.710

Try this:

\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}
    \centering
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 1}
    \end{subfigure}
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 2}
    \end{subfigure}
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 3}
    \end{subfigure}
    \vskip\floatsep
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 4}
    \end{subfigure}
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 5}
    \end{subfigure}
    \begin{subfigure}[b]{0.325\textwidth}
        \centering
        \includegraphics[width=\linewidth]{a.png}
        \caption{Subfigure 6}
    \end{subfigure}
    % \caption{Stability analysis of example \ref{example:compound}}
    \caption{Hello}
\end{figure}
\end{document}

You wrote, "It seems like having \textwidth inside each subfigure's width is the issue." That's not the case. — Mico, Jul 18 '21 at 14:49

Why does '%' sign impact subfigure alignment?

2 Answers2

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