1. The method of determining the optimum threshold for thresholding image pixels, comprising the steps of: (a) buffering a block if image pixels in a stream of image pixels, at least one of the pixels in said block of pixels comprising the pixel to be thresholded; (b) identifying maximum and minimum pixels in said block of pixels; (c) differencing said maximum and minimum pixels from one another to provide a difference signal; (d) multiplying said difference signal by a predetermined constant representing a desired change in threshold level to provide a base threshold; and (e) differencing said base threshold from said maximum pixel to provide said optimum threshold.