The hack you're referring to works because the input space is very small and well-defined, and the blur filter that's being applied is perfectly predictable. Th

The hack you're referring to works because the input space is very small and well-defined, and the blur filter that's being applied is perfectly predictable. There's no truly random noise involved. If instead you printed the document out and blurred it with a wet sponge, it would be much harder to reverse, because the real world is much noisier. Similarly, an analog photo at the limit of its resolution is full of real noise that's hard to remove.