Method and apparatus for printing digital half-tone images

Abstract

A method of reproducing images formed from binary values of basic pixels arranged in a raster, with an electrophotographic printing unit having an electrically chargeable photo-conductive medium, a selectively activatable light source to discharge of the medium and a developing unit for developing the said medium with toner powder. The method processes the digital image data to compensate for properties of the printing unit by: generating two sub-pixels for each basic pixel, where a first sub-pixel always corresponds in value to the value of the associated basic pixel and a second sub-pixel has a value that depends upon the value of the associated basic pixel and upon the values of basic pixels in a predetermined area around the associated basic pixel; and generating control signals to activate the light source in accordance with each of the generated sub-pixels. Initially, the second pixel is given the same value as that of the associated basic pixel. If, however, this value specifies the activation of the light source and the values of basic pixels in the area around the associated basic pixel indicate that there is a transition in that area between an image portion for exposure and one which is not to be exposed, then the value of the second sub-pixel is converted to the value which does not activate the light source.