Iran's Photoshopped missiles
Clicking on a link from Jens on Twitter, I found myself reading a news report about how Iran fired some missiles and then doctored the images.
What I found funny were the remarks of "photographic experts" stating that the picture "appears" to have been altered, with Gerard Issert, a technician at one of the largest photo laboratories in Paris, telling the paper: "Although the missiles weren't all equidistant from the camera, they're the same size in the picture."
All well and good, but there's a very simple way to prove that the smoke trails were cloned: just fire up Photoshop, blow up the allegedly doctored image (A) by about 500% using nearest-neighbor to preserve the hard edges (C), copy the bit of the smoke trail in the doctored image that looks similar (shown highlighted in C) and paste it over the other smoke trail in difference mode. Et, voilà, you have an almost entirely black rectangle, proving algorithmically that the two areas are identical.
(Just to show you what that section looks like in Difference mode when copied over an area that does contain different content, I also copied it over a piece of the sky in C).
Yay, finally, a use for Difference mode! :)
The emergence of picture B, by the way, was apparently what tipped people off to the forgery.