1. A computer-implemented method for analysing an image composed of pixels by estimating a continuous function of a selected pixel attribute, the continuous function being centred on a selected pixel position, the image being analysed for processing of the image, the method comprising: determining a series of nested sets of pixels, each set being centred on a common pixel; estimating a discrete weighting for each set of pixels in accordance with the continuous function; mapping the sets of pixels onto pixels of the image with the common pixel mapped on to the selected pixel position; for each set of pixels, forming a combined value in dependence on the values of the selected pixel attribute for the pixels of the set in the image; estimating multiple groups of discrete weightings, each group of discrete weightings representing a different relationship of weighting with distance from the common pixel; selecting a group of the weightings from the multiple groups of discrete weightings; and estimating the function as a sum of the combined values for each set weighted by the respective discrete weighting from the selected group.