Btukfyl

In tikzpicture, I can set 1pt as unit but 1px doesn't work! So I guess 1pt equal to 1px. But sounds like almost but not exactly!

This is a example. Original image is 200x150 but the node display is 200.74948ptx150.5621pt!

Should it be exactly 200x150? What's correct way to get 200x150?

documentclass[border=10pt]{standalone}

usepackage{tikz}

usetikzlibrary{calc}

begin{document}

    begin{tikzpicture}[x=1pt,y=1pt]

    %image source: /usr/local/texlive/2017/texmf-dist//latex/mwe/example-image-4x3.png

    node[anchor=south west,inner sep=0,outer sep=0] (image) at (0,0)       

        {includegraphics{example-image-4x3}};

    draw[line width=0.1pt] let 

        p1=(image.south west),

        p2=(image.north east),

        n1={(x2-x1)},

        n2={(y2-y1)}  

        in node[anchor=west] at (0,120) {large n1,n2};

end{tikzpicture}

end{document}

I can not tell you why this happens, but I stumbled over this some time ago, and at least back then I did not know if there is a deeper reason. If you look up the conversion rate in the web, you will find that

1cm = 28.3465pt   (in web).

However, TikZ claims, as shown in the MWE below, it is

1cm = 28.45274pt  (Ti*k*Z).

If you factor in that slight discrepancy, you get, within the rounding errors, a perfect match, as the lowest line in the following output reveals.

documentclass[border=10pt]{standalone}

usepackage{tikz}

usetikzlibrary{calc}

begin{document}

    begin{tikzpicture}[x=1pt,y=1pt]

    %image source: /usr/local/texlive/2017/texmf-dist//latex/mwe/example-image-4x3.png

    node[anchor=south west,inner sep=0,outer sep=0] (image) at (0,0)       

        {includegraphics{example-image-4x3}};

    draw[line width=0.1pt] let 

        p1=(image.south west),

        p2=(image.north east),

        n1={(x2-x1)},

        n2={(y2-y1)}  

        in node[anchor=west] at (0,120) {large n1,n2};

    path let p1=(1cm,0) in node[anchor=west,fill=white] at (0,90) {$x1ne 28.3465pt$};   

    pgfmathsetmacro{myconv}{28.3465pt/28.45274}

    draw[line width=0.1pt] let 

        p1=(image.south west),

        p2=(image.north east),

        n1={(x2-x1)*myconv},

        n2={(y2-y1)*myconv}  

        in node[anchor=west] at (0,40) {large n1,n2};    

end{tikzpicture}

end{document}