Document image signal processor having an adaptive threshold

Abstract

A document image signal processor of gray level for use in a facsimile. The document image signal processor establishes a local window and a broad window from document image signals scanned by a CCD image sensor. An average level and a level difference between a maximum and a minimum level of the image signals are extracted from the local window. A maximum threshold of the background of the document image is extracted from the broad window. Thus, the image signals can be binarized with an adaptive threshold. Accordingly, the inventive image signal processor provides improved image, although the document image is collected (scanned) from a material whose brightness distribution is considerably ununiform. The document image signal processor includes a line-memory for reading/writing the document image signal, a local window register for forming a local image window, a broad window register for forming a broad image window register, an address and clock generator for generating address and control clocks, and an adaptive threshold logic unit for producing black/white decision logic of the center pixel.