\begin{figure}
     \centering
     \begin{subfigure}[b]{0.5\columnwidth}
         \centering
         \includegraphics[scale=0.25]{figures/case_study/epsilon/676_0.01.png}
         \caption{$\epsilon = 0.01$}
         \label{fig:eps_0.01}
     \end{subfigure}%
     \begin{subfigure}[b]{0.5\columnwidth}
         \centering
         \includegraphics[scale=0.25]{figures/case_study/epsilon/676_0.1.png}
         \caption{$\epsilon = 0.1$}
         \label{fig:eps_0.1}
     \end{subfigure}
     \\
     \begin{subfigure}[b]{0.5\columnwidth}
         \centering
         \includegraphics[scale=0.25]{figures/case_study/epsilon/676_1.png}
         \caption{$\epsilon = 1$}
         \label{fig:eps_1}
     \end{subfigure}%
     \begin{subfigure}[b]{0.5\columnwidth}
         \centering
         \includegraphics[scale=0.25]{figures/case_study/epsilon/676_2.png}
         \caption{$\epsilon = 2$}
         \label{fig:eps_5}
     \end{subfigure}
    \caption{Example of alignment matrix $\pi^u$ on Cell Phones.}
    \label{fig:epsilon_case_study}
    % \vspace*{-0.3cm}
\end{figure}