By robust, I mean resistant to common image transforms (reversal, crop a non-multiple of 8 pixels off and recompress, blur, solarize to 64 levels, then interpolate, and sharpen, change compression levels, optimize for download, scale...
Suppose you are trying to embed ownership/copyright info in an image. How do you do so in a way that survives all of the above singly or in combination? And has sufficient parameters that it is non-obvious that it has been done.
You need to build in a lot of redundancy, and error correction systems. But that is short of workable system.
Ideally such a system is somewhat holographic in that other systems could also put data into an image, without compromising the original data.
Edit: While the use mention is one similar to watermarking, I normally consider a watermark to be visible in the image. Digital rights management has the addition of having various hidden attributes/data.
This came up in a discussion about Facebook putting tracking codes in images. In their case using an uncommonly used metadata field. So take it a step further: Can you put data into an image that is not readily recoverable without knowing a bunch of specific additional information, and that is proof against normal image operations.
Keeping the data secret is easy. Encrypt before encoding the ciphertext into the image. Keeping the data recoverable is harder. I'm inquiring about the latter.