A sufficiently-careful edit would be undetectable. Slightly adjusting the color value of a single pixel, for example, could probably never be identified as an alteration, because there’s no way to determine that it’s not simply how the original situation was actually photographed.
But in practice, it’s very, *very* difficult to make meaningful changes while perfectly accounting for all the changes necessary to make the new image completely consistent.
Borders can be too sharp, or fail to match up. The gradations of color that result from light and shadow can be inconsistent. One part of the image might no longer seem compatible with other parts — if one eye has a reflection of a light source and the other doesn’t, that’s not really plausible.
All these sorts of subtle incongruities can be sought out and used to make highly educated guesses. It’s not theoretically possible to be truly certain in all cases, but it’s often possible to identify that editing has taken place.
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