1. A method for determining a similarity score of a target object with respect to a model object, said target object being in a plane and said model object represented by a model feature vector, the method comprising: generating regions of the plane according to a first mass distribution of the target object and a second mass distribution of a part of said target object, each of said regions having a corresponding mass distribution indicator; calculating a target feature vector for said target object according to at least one of said corresponding mass distribution indicators; and computing said similarity score using said target feature vector and said model feature vector.