\subfloat command from subcaption package doesn't produce bottom margin if used in table

Question

I'm trying to typeset a table of images. If image inserted into cell with \subfloat (subcaption package included), resulting box will be w/o bottom margin, but top, left and right margins will be ok.

Example:

\documentclass[a4paper]{book}
\usepackage[utf8]{inputenc}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}

\chapter{Sample Chapter}

\begin{table}
  \caption{Sample Table}
  \label{tbl:sample}
  \begin{tabular}{|c|c|c|}
    \hline
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image1}} \end{tabular} &
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image1}} \end{tabular} &
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image1}} \end{tabular}
    \\\hline
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}} \end{tabular} &
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}} \end{tabular} &
    \begin{tabular}{c} \subfloat{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}} \end{tabular}
    \\\hline
  \end{tabular}
\end{table}

\end{document}

If subcaption package replaced with subfig the table looks good.

Is there any way to fix such behavior, since I can't use subfig? Or maybe there is another way to do table with images?

Update: Thanks to Gonzalo Medina, I solved original problem. But I stuck with another one with that approach. I'm trying to put titles in separate column:

\documentclass[a4paper]{book}
\usepackage[demo]{graphicx}
\usepackage{caption}

\begin{document}

\begin{table}
  \centering
  \caption{Sample Table}
  \label{tbl:sample1}
  \renewcommand\arraystretch{2}
  \begin{tabular}{|c|c|c|c|}
    \hline\rule{0pt}{\dimexpr0.15\textwidth+1em\relax}

    imeage 1 &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}            

    \\\hline\rule{0pt}{\dimexpr0.15\textwidth+1em\relax}

    imeage 2 &    
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2} &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2} &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}

    \\ \hline
  \end{tabular}
\end{table}

\end{document} 

But text in first column is aligned at bottom of cell. How to make text in title column to be centered?

Answer 1

\subfloat is not a command implemented in the subcaption package, so it won't be recognised by the package. If you want to use the subcaption package, you can use the either the subtable environment or the \subcaptionbox command. If your subimages don't need a caption, you can simply use \includegraphics.

In the following example I used the tabu package to buid the table with the six images. In the first case, the images won't have a caption: in the second case, each image will have its caption:

\documentclass[a4paper]{book}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{tabu}

\begin{document}

\begin{table}
  \centering
  \caption{Sample Table}
  \label{tbl:sample1}
  \tabulinesep =_1em^1em
  \everyrow{\tabucline[0.4pt]-}
  \begin{tabu}{|c|c|c|}
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}            
    \\
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}  &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2} &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}
  \end{tabu}
\end{table}

\begin{table}
  \centering
  \caption{Sample Table}
  \label{tbl:sample2}
  \tabulinesep =_1em^0.5em
  \everyrow{\tabucline[0.4pt]-}
  \begin{tabu}{|c|c|c|}
    \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}}            
    \\
    \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}}  &
     \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}}
  \end{tabu}
\end{table}

\end{document}

Without the tabu package, in order to achieve the same result, some manual adjustments will be needed:

\documentclass[a4paper]{book}
\usepackage[demo]{graphicx}
\usepackage{caption}
\usepackage{subcaption}

\begin{document}

\begin{table}
  \centering
  \caption{Sample Table}
  \label{tbl:sample1}
  \renewcommand\arraystretch{2}
  \begin{tabular}{|c|c|c|}
        \hline\rule{0pt}{\dimexpr0.15\textwidth+1em\relax}
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1} &
    \includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}            
    \\ \hline\rule{0pt}{\dimexpr0.15\textwidth+1em\relax}
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}  &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2} &
    \includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}
    \\ \hline
  \end{tabular}
\end{table}

\begin{table}
  \centering
  \caption{Sample Table}
  \label{tbl:sample2}
  \begin{tabular}{|c|c|c|}
     \hline\rule{0pt}{23pt}
     \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\textwidth,height=0.15\textwidth]{image1}}            
    \\[5.5em] \hline\rule{0pt}{23pt}
    \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}}  &
     \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}} &
    \subcaptionbox{text}{\includegraphics[width=0.15\linewidth,height=0.15\linewidth]{image2}}
   \\[5.5em] \hline
   \end{tabular}
\end{table}

\end{document}