1. A method of operating a digital processor to filter noise from an image, said method comprising: filtering data for at least a portion of the image to produce a set of filtered data; compressing the set of filtered data to produce a first set of compressed data; compressing another set of data for the image, to produce a second set of compressed data; and analyzing relative compressibility of at least the first and second sets of compressed data, and selecting at least one threshold value in dependence upon relative compressibility; filtering noise from the image using the at least one threshold value.