Reverse Darkening Face Out Sometimes in documentaries, they have the face darkened out. Is their an effective way to undo this anoynmous effect? Obviously, the curves levels doesn't work as it just shows a bunch of square like patches of dark pixels and no detail as seen. Is their an algorithm that can guess as to how the darkening effect was done?

Below are examples from a documentary film ( It was the only one I could immediately think of as I couldn't find one on gangs / prisoners / murders out of the blue )
If it is done properly, it should be impossible.  If there is some amount of information there, then it might be possible to get a result by cranking up gamma or luminance, but done correctly, the information is set to black pixels for the entire area and there is no difference between pixels over that area to work from.

It is sometimes possible to reverse images that have been pixelated, blurred or distorted (or at least determine if it is the same person) but there is no way to make up information that isn't there.