Patent US 5,841,909

A set of orthonormal basis functions is determined with respect to a rectangle which includes an arbitrarily shaped image segment. The set of orthonormal basis functions is used to determine a set of orthogonal basis function candidates which are numbered along a zigzag scan. Subsequently, a first orthogonal basis function candidate is extracted and defined as a first orthogonal basis function. Then, a loop for generating a set of orthogonal basis functions is initiated. First, a next basis function candidate is extracted in numbered order. An orthogonal component is determined which is perpendicular to a hyperplane spanned by the orthogonal basis function. Thereafter, a check is made as to whether an absolute value of the orthogonal component exceeds a threshold. If the absolute value does not exceed the threshold, it is disregarded and the operation returns to the beginning of the loop. On the other hand, if the absolute value exceeds the threshold, the orthogonal component is selected and defined as the next orthogonal basis function, after which the operation returns to the beginning of the loop. Alternatively, a set of orthogonal basis functions is generated by successively ascertaining a maximum orthogonal component among a plurality of orthogonal components in place of determining whether the absolute value of the orthogonal component exceeds the threshold.