Cfrgtkky

How can I highlight text in an image with tikz by using a rectangle and have the rest of the page filled with a grey (or opaque black) tone, see example below (taken from here)?

You can see a screenshot taken from GIMP where I just highlight text using the rectangle selector tool. How can I achieve a similar effect using tikz in xetex (without the yellow dashed lines at the borders)?

My code I have so far is:

begin{tikzpicture}[remember picture, overlay]

        node [anchor=south west,inner sep=0] (A) at (0,0) {includegraphics[width=linewidth, height=paperheight]{image}}{};

        fill [draw=none, fill=black, fill opacity=0.3]

        (0,0) -- 

        (A.north west) -- (A.north east) -- (A.south east) -- 

        (0,0) -- (0,0) -- cycle;

        filldraw [fill=white, draw=white, fill opacity=.3] (3.2,3.2) rectangle (14.6,10.5);

    end{tikzpicture}

The issue I run into with this code is that filling the whole page with an opaque black and filling the rectangle with an opaque white leads to an opaque grey.

Here is a recipe and an application. First the recipe, using this answer.

1. Add a grid on top.


2. Draw the contour you want to highlight.


3. Use the contour in an even odd rule fill (and drop the grid).

On purpose I used a document in which you may not want to use a horizontal box.

documentclass[tikz,border=3.14mm]{standalone}

begin{document}

foreach X in {0,1,2}

{begin{tikzpicture}

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

    {includegraphics[width=0.9textwidth]{Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence,_LOC.jpg}};

    % ^^^ https://upload.wikimedia.org/wikipedia/commons/4/4e/Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence%2C_LOC.jpg

    begin{scope}[x={(image.south east)},y={(image.north west)}]

      path[use as bounding box] (-0.1,-0.1) rectangle (1,1);

       % ^^^ only for animation

      ifnumX<2

        draw[help lines,xstep=.1,ystep=.1] (0,0) grid (1,1);

        foreach x in {0,1,...,9} { node [anchor=north] at (x/10,0) {0.x}; }

        foreach y in {0,1,...,9} { node [anchor=east] at (0,y/10) {0.y}; }

      fi   

      ifnumX=1    

        draw (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle;

      fi

      ifnumX=2

        fill[even odd rule,opacity=0.4]

        (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle

        (0,0) rectangle (1,1);

      fi       

    end{scope} 

end{tikzpicture}}

end{document}

Of course, in the end you drop all the auxiliary stuff to arrive at

documentclass[tikz,border=3.14mm]{standalone}

begin{document}

begin{tikzpicture}

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

    {includegraphics[width=0.9textwidth]{Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence,_LOC.jpg}};

    % ^^^ https://upload.wikimedia.org/wikipedia/commons/4/4e/Dunlap_broadside_copy_of_the_United_States_Declaration_of_Independence%2C_LOC.jpg

    begin{scope}[x={(image.south east)},y={(image.north west)}]

        fill[even odd rule,opacity=0.4]

        (0.11,0.4) -- (0.37,0.404) --  (0.37,0.414) -- (0.11,0.41) -- cycle

        (0,0) rectangle (1,1);

    end{scope} 

end{tikzpicture}

end{document}

Whether or not my choice of the highlighted text has anything to do with a wall at the border to Mexico, I do not want to elaborate on.

Depending on what format the images are, no tikz required:

documentclass{article}

usepackage{graphicx,stackengine,trimclip}

begin{document}

stackinset{l}{150pt}{b}{130pt}{%

  clipbox{150pt 130pt 100pt 50pt}{includegraphics{example-image.jpg}}%

}{%

  includegraphics[decodearray={.3 .5 .3 .5 .3 .5}]{example-image.jpg}%

}

end{document}

Depending on the color palette, the decodearray may need tweaking:

documentclass{article}

usepackage{graphicx,stackengine,trimclip}

begin{document}

stackinset{l}{20pt}{b}{40pt}{%

  clipbox{20pt 40pt 15pt 15pt}{includegraphics{example-grid-100x100bp.png}}%

}{%

  includegraphics[decodearray={.3 .6 .3 .6 .3 .6}]{example-grid-100x100bp.jpg}%

}

end{document}

decodearray will not work for cmyk color schemes, and only a limited number of image formats (jpeg being one).