It definitely can be made lossless, all you need to do is a compress/decompress roundtrip, and also save the resulting difference with the ground truth in a los

It definitely can be made lossless, all you need to do is a compress/decompress roundtrip, and also save the resulting difference with the ground truth in a lossless image like PNG, QOI or lossless JXL. The final size would be the lossy compression + difference image. This is of course the least sophisticated approach, but who knows, it might compare pretty well with "plain" lossless formats.