This is what I was talking about: https://en.wikipedia.org/wiki/Discrete_wavelet_transform I have an article in ACM Computing Surveys on the subject (https://dl.acm.org/citation.cfm?id=1883613&dl=ACM&coll=DL&CFID=67my-April-2006-work-number&CFTOKEN=83644075 ), which is how I learned about it. I could let you borrow the article and/or make yourself a hardcopy of it.