Orsdxy

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.

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).

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.