begin{figure}[H]
   begin{tikzpicture}[scale=.5,every node/.style={minimum size=1cm},on grid]

    begin{scope}[
            yshift=-83,every node/.append style={
            yslant=0.5,xslant=-1},yslant=0.5,xslant=-1
            ]


        % opacity to prevent graphical interference
        fill[white,fill opacity=0.9] (0,0) rectangle (6,6);
        draw[step=5mm, black] (0,0) grid (6,6); %defining grids
        draw[step=1mm, red!50,thin] (0.5,0.5) grid (2,2);  %Nested Grid
        draw[black,very thick] (0,0) rectangle (6,6);%marking borders
        fill[red] (1,1) rectangle (1.5,1.5);
        %includegraphics[scale=1,opacity=0.8]{Pics/face_patch}
        %Idem as above, for the n-th grid:
    end{scope}

        begin{scope}[
        yshift=90,every node/.append style={
        yslant=0.5,xslant=-1},yslant=0.5,xslant=-1
                     ]
        fill[white,fill opacity=.9] (0,0) rectangle (5,5);
        draw[step=10mm, black] (1,1) grid (4,4);
        draw[black,very thick] (1,1) rectangle (4,4);
        draw[red,dashed] (0,0) rectangle (5,5);
    end{scope}

        draw[-latex,thick](5.8,-.3)node[right]{$mathsf{Skup svih piksela u } boldsymbol{X}$}
        to[out=180,in=90] (3.9,-1);

    draw[-latex,thick,text=black](5.9,5)node[right]{$mathsf{Skup svih piksela u } boldsymbol{x}_i$}
        to[out=180,in=90] (3.6,5);
     draw[-latex,dashed,text=black](5.7,3)node[right]{$x_i$}
        to[out=180,in=90] (0,5.7);


    draw[-latex,thick,red,text=black](-3.3,-3.2)node[right]{$x_i$}
        to[out=180,in=90] (0,-1.4);

    draw[-latex,dashed,red,text=black](4.3,-1.9)node[right]{$mathsf{Skup svih piksela u } boldsymbol{x}_i$}
        to[out=180,in=90] (1,-1.3);

end{tikzpicture}
end{figure}