Sstrhsrtj

I try to draw a grid on image with cm unit. I know many solution for this topic. My idea right now is try to calculate it as below! I guess should use n1 and n2 as foreach up limit (currently use width and height macro).

documentclass[border=2mm]{standalone}

usepackage{tikz}

usetikzlibrary{shapes,calc}

begin{document}

    begin{tikzpicture}

    defwidth{6}

    defheight{8}

    %tikzstyle{every node}=[ultra thick, draw]

    node[anchor=south west,opacity=0.5,inner sep=0] (image) at (0,0) {includegraphics[width=0.9textwidth]{example-image-a}};

    draw[line width=2pt]

        let 

        p1=(image.south west),

        p2=(image.north east),

        n1={(x2-x1)},

        n2={(y2-y1)}  

        in node[anchor=west] at (0,-2) {Huge n1,n2} 

        foreach col in {0,...,height} {

        foreach row in {0,...,width} {

            (0,col) -- (width,col)

            (row,0) -- (row,height)

        }}

        foreach x in {0,1,...,width} {node[anchor=north] at (x,0) {x}}

        foreach y in {0,1,...,height} {node[anchor=east] at (0,y) {y}}

        ;

    end{tikzpicture}

end{document}

Currently the unit of n1 and n2 is pt, maybe I need to convert it to cm unit but I don't how to do it.

My target is use n1 and n2 as the foreach up limit instead of width and height. so it will match the image exactly!