\begin{figure*}[t]
	\centering
	\setlength{\resLen}{0.275\columnwidth}
	\addtolength{\tabcolsep}{-3pt}
	\begin{tabular}{ccccccc}
		(a) Orig. image & (b1) Ours & (c1) FD & (d1) Abs. diff. & (b2) Ours & (c2) FD & (d2) Abs. diff.
		\\[2pt]
		\multirow{2}{*}[2.6em]{\includegraphics[width=\resLen]{results/grad_vis/fd_ad/original.png}} 
		&
		\multirow{2}{*}[2.6em]{\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv0_ad.png}
				\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv0_ad.png}}
				\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv0_ad.png}}
		\end{overpic}} 
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv0_fd0.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv0_fd0.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv0_fd0.png}}
		\end{overpic}		
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv0_diff0.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv0_diff0.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv0_diff0.png}}
		\end{overpic}
		%\includegraphics[width=\resLen]{results/grad_vis/fd_ad_3/deriv0_diff0.png}
		&
		\multirow{2}{*}[2.6em]{\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv1_ad.png}
				\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv1_ad.png}}
				\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv1_ad.png}}
		\end{overpic}} 
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv1_fd0.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv1_fd0.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv1_fd0.png}}
		\end{overpic}
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv1_diff0.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv1_diff0.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv1_diff0.png}}
		\end{overpic}
		%\includegraphics[width=\resLen]{results/grad_vis/fd_ad_3/deriv1_diff0.png}
		\\
		&
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv0_fd1.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv0_fd1.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv0_fd1.png}}
		\end{overpic} 
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv0_diff1.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv0_diff1.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv0_diff1.png}}
		\end{overpic}
		%\includegraphics[width=\resLen]{results/grad_vis/fd_ad_3/deriv0_diff1.png}
		&
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv1_fd1.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv1_fd1.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv1_fd1.png}}
		\end{overpic}
		&
		\begin{overpic}[width=\resLen]{results/grad_vis/fd_ad_3/marked/deriv1_diff1.png}
			\put(0, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset2/deriv1_diff1.png}}
			\put(75, 0){\includegraphics[width=0.25\resLen]{results/grad_vis/fd_ad_3/inset/deriv1_diff1.png}}
		\end{overpic}
		%\includegraphics[width=\resLen]{results/grad_vis/fd_ad_3/deriv1_diff1.png}
	\end{tabular}
	%
	\caption{\label{fig:validate_v2}
		\textbf{Equal-time comparisons} between derivatives estimated by (b)~our method and (c)~finite difference~(FD) with respect to two distinctive scene parameters: (b1--d1) vertical displacement of the area light and the cube; (b2--d2) horizontal displacement of those objects.
		Pixel intensities of the derivative images are encoded in false colors using the same color map as Figure~\ref{fig:per_component}.
		The FD results on the first row use a larger spacing ($0.1$) and suffer from high bias (see the insets on the bottom left); those on the second row use a smaller spacing ($0.01$) and have high variance while still being biased (as shown by the insets on the bottom right).
		Our technique, in contrast, produces cleaner and unbiased derivative estimations.
	}
\end{figure*}