1. A data processing system for performing face detection on a stream of frames of image data, the data processing system comprising: a face detector configured to detect a first face candidate in a first frame by performing face detection within first search tiles defined for the first frame; a colour measurement unit configured to calculate a set of colour parameters including an average colour of the first face candidate expressed according to a predefined colour space, one of the axes of the predefined colour space being substantially oriented in the direction of maximum variation according to a predetermined distribution of skin colour; a transformation unit configured to: transform a second frame into the predefined colour space; and form a skin colour probability map for the second frame by calculating the probability that a given colour is a skin colour from a measure of the colour space distance of that colour from the calculated average colour in the predefined colour space; and a search tile generator configured to generate a plurality of second search tiles based on the skin colour probability map for use by the face detector, the second search tiles defining areas of the second frame within which the face detector is to perform face detection so as to detect one or more second face candidates in the second frame.