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Lets take this string:

boxedincludegraphics[width=0.4textwidth]FIGURE.pdf

that generates this figure:

Now lets say that I need to crop the blank margins of the figure keeping the figure's size.

I though I could do it with something like:

boxedincludegraphics[trim = L B R T, clip, scale=x]FIGURE.pdf

Where L, B, R, and T are respectively the left, bottom, right and top trimming values and x is the scaling value.

The following example should better explain my question:

boxedincludegraphics[width=0.4textwidth]FIGURE.pdfquad %
boxedincludegraphics[scale=0.238]FIGURE.pdf\

boxedincludegraphics[trim = 65 20 150 20, clip, width=0.4textwidth]FIGURE.pdfquad %
boxedincludegraphics[trim = 65 20 150 20, clip, scale=0.238]FIGURE.pdf

The previus code generates the following layout:

As you can see, using scale value instead of width lets me crop the figure keeping its size.

My question is: how can I get the exact (more or less) value of the scale parameter without go by trial and error? (My idea is to write some Emacs Lisp code to change the LaTeX code accordingly.)

I am open to any idea or suggestion.

Note. In most cases I use external software to crop my images (pdfcrop, briss etc.) but, sometimes, as in this case, they don't work so I need to trim manually.

All graphics are written to the PDF with a scale factor. When you specify a size the package just converts it to an appropriate scale factor. You can make Gscale@box remember that scale factor for you (source):

documentclassarticle
usepackagegraphicx

makeatletter
letORG@Gscale@boxGscale@box
longdefGscale@box#1%
 xdefthelastscalefactor#1%
 ORG@Gscale@box#1
makeatother

begindocument

noindent
fboxincludegraphics[width=0.4textwidth]example-imagequad
fboxincludegraphics[scale=thelastscalefactor]example-image\

noindent
fboxincludegraphics[trim = 65 20 150 20, clip, width=0.4textwidth]example-imagequad %
fboxincludegraphics[trim = 65 20 150 20, clip, scale=thelastscalefactor]example-image

enddocument

Output:

You can keep the width= on the graphics and trim/clip separately so not affecting the scaling, see the last example here:

documentclassarticle
usepackagegraphicx,adjustbox
begindocument



includegraphics[width=.4textwidth]example-image

bigskip

includegraphics[clip,trim=1cm .5cm .7cm .1cm, width=.4textwidth]example-image

bigskip

adjustboxclip,trim=1cm .5cm .7cm .1cmincludegraphics[width=.4textwidth]example-image


enddocument

I never liked the idea of scaling and clipping at the same time, not knowing which comes first (keyword order means nothing). This solution clips first then scales by using savebox and adjustbox.

documentclassarticle
usepackagegraphicx
usepackageadjustbox

newsaveboxtempbox

begindocument

noindent
saveboxtempboxincludegraphics[trim = 65 20 150 20, clip]example-image%
fboxadjustboxwidth=0.4textwidthuseboxtempbox

enddocument

I'm not quite sure this is what you are looking for, but here is a way to compute, for instance, 1000 times the desired scale factor, rounded to the nearest integer:

documentclassarticle
usepackagegraphicx

newcountmyscale

begindocument
setbox0=hboxincludegraphicsexample-image%
myscale=numexpr 400*textwidth / wd0 relax
showthemyscale % displays 430 on the terminal

includegraphics[width=0.4textwidth]example-image

includegraphics[scale=0.43]example-image % same width as above

enddocument