To provide an improved method using a negative signal in order to realize dot compaction for enhancing sharpness of edge.

A xerographic marking unit comprises a source of pixels representative of digitized image information. A rendering image processor analyzes pixels concerning to the image characteristics and assigns one of at least three values to each pixel based on the results of analysis. Three values are selected from a set of values indicative of positive, negative or neutral for a predetermined exposure bias. The xerographic marking unit further comprises an image generating/outputting unit for converting the value assigned to a pixel into a selective exposing area on a charge holding plane. Consequently, charges on the charge holding plane, e.g. a photon acceptor, includes a positive charge area determined to react on toner electrostatically and a negative charge area determined to react on toner electrostatically.