1. A method for improving the quality of a digital image comprising: (a) obtaining a digital image; (b) sigma filtering the digital image to produce a sigma filtered input image; (c) analysis processing the sigma filtered input image to produce an approximation part and a detail part; (d) sigma filtering the approximation part to produce a sigma filtered approximation part; and (e) synthesizing the sigma filtered approximation part and the detail part to generate a reconstructed filtered image.