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I have two fairly different image files, except for a single element in the photo. Is there a procedure that can be done in Photoshop (or any other app, for that matter) to extract a mask for that element? When applied, I'd like that mask to remove the background, such that the resulting image (with transparency) can be re-used elsewhere.

Image extraction example

I can arrange one over top of the other, change the upper layer's Blending Mode to Difference, then line up the pieces and see my target appear as a black area, but that doesn't seem to get me all the way there.

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There's nothing that would do that automatically and yield satisfactory results, but you could get most of the way there with a series of channel calculations.

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That linked video was precisely what I needed! That got me 90% of the way there, and a little more editing will clean up the rest. Thanks! – Collin Allen Nov 23 '09 at 18:52

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