Circular processing of the hue variable - a particular trait of colour image processing

Novel tools for colour image processing are presented. Unlike many magnitudes dealt with in engineering, the hue variable of a colour image is circular and requires a special treatment. Special techniques have been advanced in statistics for the analysis of data from angular variables; likewise in image processing for the processing of the hue variable. We give a definition of the median and of the range of angular data and apply their running versions on images to smooth them and to detect hue edges. We also give definitions of hue morphology; one based on the topological concept of lifting and on grey level morphology; another definition is wholly given in a circular context.