A method of analyzing image data to quantify prior block-based processing comprises processing a set of pixel values derived from the image data to generate a spatial difference profile along a line perpendicular to assumed block edges, the spatial difference profile representing differences between values of pixels spaced spatially in a direction parallel to said line; summing the spatial difference profile in a direction perpendicular to that line; measuring inter-maxima distances in the spatial difference profile to a fractional precision in pixel spacing units; and aggregating measured inter-maxima distances to determine a block size.