[Lerc]

I have looked to see what existing research there is on doing that but found very little. Obviously the error image is compressible, it has patterns in it that are visible, and it is predominantly greyish. Those are aspects that can be exploited.

Of course the error image will be much larger than the compressed image, but you have moved to the field of lossless compression so the bar is at a different level.

Algorithms designed to compress error maps should work on a different set of assumptions to plain lossless image compression. Error images are noisy but have an overall low magnitude.

I do wonder if there could be a entire field in collaborative image compression where multiple techniques can layer to try and achieve a better result. Algorithms would take an existing image and try and move it closer to the target while using as few bits as possible.