[XeTeX] graphic special effects question

[Alan Hoenig]

I hope to be able to produce the following graphic effect from within
XeTeX.  If I include a photo (say) within a TeX document, I'd like to be
able to superpose a smaller square 'filter' over a portion of it so that the
original pic shows through, but appears lighter (or perhaps darker).  That
is, all the features of the original are present and visible but filtered by
the overlay to appear darker or lighter than the surrounding portion of the
photo.  How can I do this?  If there is no XeTeX solution, what is the best
way to proceed.  Thanks for any info!

---Alan Hoenig
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