Patent US 4,326,258

A method and apparatus for reducing the gray scale resolution of a document. The apparatus includes a scanning module for scanning a document along x and y coordinates with regard thereto for generating pixels representing gray scale values for discrete areas of the document along the x and y coordinates, with each pixel having a predetermined number of bits. A high pass filter module is also included for summing the associated pixels within a window to produce a window sum as the window is moved relatively along coordinates corresponding to the x and y coordinates, and the high pass filter module also includes means for comparing a selected pixel within a window with the associated window sum and predetermined criteria and for generating first and second output values in accordance therewith. The first and second output values have a fewer number of bits than the asssociated selected pixel.