This work is partially supported by the Foundation for Applied Sciences (STW), the Foundation for Computer Science in the Netherlands (SION) and the Dutch Organization for Scientific Research (NWO).
The line of research described here is initiated during the author's stay at VSSP group of the EE Department of the University of Surrey. Prof. Josef Kittler and colleagues are gratefully acknowledged for their hospitality and for offering a stimulating environment.
Adrian F Clark