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Thought this was neat (forgiveness sought if this is a repost)...scroll down to view more photos.
https://interactive.guim.co.uk/embed/2014/apr/image-opacity-slider-master/index.html?ww2-dday
https://interactive.guim.co.uk/embed/2014/apr/image-opacity-slider-master/index.html?ww2-dday