The diff will be low norm (for some suitable norm), but it needn’t be sparse in pixel space. It could be sparse in some other spaces (Eg: wavelet basis), but th

The diff will be low norm (for some suitable norm), but it needn’t be sparse in pixel space. It could be sparse in some other spaces (Eg: wavelet basis), but that’s right where the challenge lies — finding a basis where the data (and/or the residue) to be transmitted is sparse.