Two-Dimensional Sequency Filters

This paper provides a tutorial exposition of the possible use of sequency filters for the processing of signals that are functions of two space variables. Historically, optical and, more recently, digital techniques have been used for two variable processing of images with the computer providing a domain for other than sinecosine filtering. Sequency filters based on Walsh or similar functions can be implemented with about equal effort for time and space variable signals. They could actually be built with electronic components up to the ultimate case of signals with one time and three space variables. Two kinds of filters have emerged so far: instantaneous filters and sampling filters. In the following sections we briefly discuss other possible transforms for space filters before investigating sequency-oriented filtering techniques. The sequency techniques include instantaneous filters, efficient practical implementation, and some potential circuits for imagery applications.