The function of a filter is to transform a signal into another one more
suit- able for a given purpose. As such, filters find applications in
telecommunica- tions, radar, sonar, remote sensing, geophysical signal
processing, image pro- cessing, and computer vision. Numerous authors
have considered deterministic and statistical approaches for the study
of passive, active, digital, multidimen- sional, and adaptive filters.
Most of the filters considered were linear although the theory of
nonlinear filters is developing rapidly, as it is evident by the
numerous research papers and a few specialized monographs now available.
Our research interests in this area created opportunity for cooperation
and co- authored publications during the past few years in many
nonlinear filter families described in this book. As a result of this
cooperation and a visit from John Pitas on a research leave at the
University of Toronto in September 1988, the idea for this book was
first conceived. The difficulty in writing such a mono- graph was that
the area seemed fragmented and no general theory was available to
encompass the many different kinds of filters presented in the
literature. However, the similarities of some families of nonlinear
filters and the need for such a monograph providing a broad overview of
the whole area made the pro- ject worthwhile. The result is the book now
in your hands, typeset at the Department of Electrical Engineering of
the University of Toronto during the summer of 1989.