1. A method of processing image data for compression, wherein the image data represents an image and includes a plurality of image data values, each image data value representing an appearance characteristic of a pixel of the image, the method comprising: determining a variance between n image data values of the plurality of image data values; comparing the variance to a variance threshold to determine whether the variance meets a preselected condition relative to the variance threshold; and if the variance meets the preselected condition, then changing at least one of the n image data values to reduce the variance.