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Comment by theoh | Hacker News Reader

Interestingly, some major photo competitions require 'original' RAW files to be submitted along with processed jpegs. The idea is that they can check that the image hasn't been modified in a way that breaks the rules.

Of course, software people know that it would be tedious but technically straightforward to concoct a fake RAW file from another 'raw' format such as a PPM image. The stakes in these competitions can be quite high so it's a bit disconcerting that they rely on a presumption that converting RAW files to editable images is a one-way process.

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No - it's not straightforward. How the light strikes, sensor imperfections, patterns unique to each camera body even, thermal noise. It's very very hard actually. It's one thing to doctor a finished image - it's another, in a forensic level really, to doctor a RAW file. You can make one which on its own is syntactically correct but which does not show all the telltale signs of being shot in actual hardware, no patterns or imperfections etc. This is trivial.

Next level is to make it look plausible on its own, with plausible imperfections. This is a bit more involved and tedious.

The really hard part is to make it look like it came from the exact same individual purported camera body the photographer allegedly used. This is really hard.

An analogy would perhaps be taking a picture of a room:

Now, build another room that would make the same picture. Make sure everything in the room is plausible yet renders exactly the same picture as the first room. (The room is the RAW file.)

Edit: (I'm not saying all the competitions have the time and know how to verify a RAW file.)

Couldn't you just take whatever image you want to convert to a RAW file, make a really large high quality print out of it, and then take a picture of that? I would imagine that with the right lighting setup (and a really high quality printout) it would be pretty hard to tell that it is a picture of a picture.

Nope. Light is a 5D wavefront. Each x,y,z of origin in the real world is actually a sum of photons from many directions, scattering to many other directions. So you have two angular components. At times, phase matters too, and frequency, so each photon needs perhaps 7 parameters to capture its state, (x,y,z,θ,φ,ψ,λ).

On top of that, you have diffraction effects in the optics, chromtic aberration, and even some birefringence effects.

Algorithms exist which can detect if an image has been cropped, and what region of the original it came from. I'll leave as an exercise to the reader how that works :) hint: keyword is "media forensics"

The scenario I'm talking about is not synthesising a RAW file from scratch, but being able to freely edit it and save the result back into a RAW file that looks "original". I don't think you've made any statements that indicate that doing that would be complex.

Again, this is not about faking a RAW file from scratch, just being able to edit it in place.

One potential complication just occurred to me. If, (IFF!) each sensor site has a somewhat unique non-linearity in any way, you can't just go around in the image and change "pixels". You have to adjust them in a way that would not change the patterns normal to that camera for that sensor site and for neighboring sensor sites which could have been affected by the same light.

Oh, that I have too little insight in. I could speculate that going either way. Would be very interesting to know for sure!

Competitions, when an image is contested, sometimes ask for images made before and after the winning image. If it isn't a studio image, the time-series is hard to fake.

A similar misperception is common for PDFs.

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