Move figure labels down

Source Code and Credits: https://github.com/PetarV-/TikZ/blob/master/2D%20Convolution/2d_convolution.tex#L6

My modified code:

begin{figure} [h]
    centering
    begin{tikzpicture}
    matrix (mtr) [matrix of nodes,row sep=-pgflinewidth, nodes={draw}]
    {
        0 & 1 & 1 & |[fill=red!30]| 1 & |[fill=red!30]| 0 & |[fill=red!30]| 0 & 0
        0 & 0 & 1 & |[fill=red!30]| 1 & |[fill=red!30]| 1 & |[fill=red!30]| 0 & 0
        0 & 0 & 0 & |[fill=red!30]| 1 & |[fill=red!30]| 1 & |[fill=red!30]| 1 & 0
        0 & 0 & 0 & 1 & 1 & 0 & 0
        0 & 0 & 1 & 1 & 0 & 0 & 0
        0 & 1 & 1 & 0 & 0 & 0 & 0
        1 & 1 & 0 & 0 & 0 & 0 & 0
    };
    draw[very thick, red] (mtr-1-4.north west) rectangle (mtr-3-6.south east);
    node [below= of mtr-5-4.south] (lm) {$bf I$};
    node[right = 0.2em of mtr] (str) {$*$};
    matrix (K) [right=0.2em of str,matrix of nodes,row sep=-pgflinewidth, nodes={draw, fill=blue!30}]
    {
        1 & 0 & 1 
        0 & 1 & 0 
        1 & 0 & 1 
    };
    node [below = of K-3-2.south] (lk) {$bf K$};
    node [right = 0.2em of K] (eq) {$=$};
    matrix (ret) [right=0.2em of eq,matrix of nodes,row sep=-pgflinewidth, nodes={draw}]
    {
        1 & 4 & 3 & |[fill=yellow!30]| 4 & 1
        1 & 2 & 4 & 3 & 3
        1 & 2 & 3 & 4 & 1
        1 & 3 & 3 & 1 & 1
        3 & 3 & 1 & 1 & 0
    };
    node [below = of ret-4-3.south] (lim) {${bf I} * {bf K}$};
    draw[very thick, orange] (ret-1-4.north west) rectangle (ret-1-4.south east);
    draw[densely dotted, blue, thick] (mtr-1-4.north west) -- (K-1-1.north west);
    draw[densely dotted, blue, thick] (mtr-3-4.south west) -- (K-3-1.south west);
    draw[densely dotted, blue, thick] (mtr-1-6.north east) -- (K-1-3.north east);
    draw[densely dotted, blue, thick] (mtr-3-6.south east) -- (K-3-3.south east);
    draw[densely dotted, orange, thick] (ret-1-4.north west) -- (K-1-1.north west);
    draw[densely dotted, orange, thick] (ret-1-4.south west) -- (K-3-1.south west);
    draw[densely dotted, orange, thick] (ret-1-4.north east) -- (K-1-3.north east);
    draw[densely dotted, orange, thick] (ret-1-4.south east) -- (K-3-3.south east);
    matrix (K) [right=0.2em of str,matrix of nodes,row sep=-pgflinewidth, nodes={draw, fill=blue!10}]
    {
        1 & 0 & 1 
        0 & 1 & 0 
        1 & 0 & 1 
    };
    draw[very thick, blue] (K-1-1.north west) rectangle (K-3-3.south east);
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-1-4.south east) (xx) {scalebox{.5}{$times 1$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-1-5.south east) (xx) {scalebox{.5}{$times 0$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-1-6.south east) (xx) {scalebox{.5}{$times 1$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-2-4.south east) (xx) {scalebox{.5}{$times 0$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-2-5.south east) (xx) {scalebox{.5}{$times 1$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-2-6.south east) (xx) {scalebox{.5}{$times 0$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-3-4.south east) (xx) {scalebox{.5}{$times 1$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-3-5.south east) (xx) {scalebox{.5}{$times 0$}};
    node[anchor=south east, inner sep=0.01em, blue] at (mtr-3-6.south east) (xx) {scalebox{.5}{$times 1$}};
    end{tikzpicture}
    caption{2D convolutional operator where the kernel matrix is moved across the target image and element-wise products are recorded.}
    label{fig:my_label}
end{figure}

Output:

Question: How can I add space/move down the label I under the first block? The label I needs to be moved down. Thank you for your help!