1. A method of segmenting a pixellated image, comprising the steps of: (a) selecting at least one first region from a first reference image; (b) deriving from values of pixels of the at least one first region a first threshold such that a first predetermined proportion of the pixels have values on a first side of the first threshold; (c) forming a difference image as a difference between each pixel of the image and a corresponding pixel of an image of a non-occluded background; and (d) allocating each difference image pixel to at least one first type of region if the value of the difference image pixel is on the first side of the first threshold and the values of more than a first predetermined number of neighbouring difference image pixels are on the first side of the first threshold.