1. A method of representing an object appearing in an image or sequence of images, by processing signals corresponding to the image or images, the method comprising: deriving a plurality of sets of co-ordinate values representing the shape of the object; and quantising the co-ordinate values to derive a coded representation of the shape, wherein, for a first set of coordinate values having a value for a given co-ordinate less than the corresponding co-ordinate value in a second set of co-ordinate values, the number of bits allocated to the quantised representation of the given coordinate value for the first set of values is less than the number of bits allocated to the quantised representation of the corresponding co-ordinate value for the second set of values.