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 curvature scale space (CSS) representation of the object outline by smoothing the object outline, deriving an additional parameter reflecting the shape or mass distribution of a smoothed version of the original outline, and associating the CSS representation and said additional parameter as a shape descriptor of the object, wherein said additional parameter corresponds to the circularity of a smoothed version of the original outline.