1. A processing system for preparing a document image described by image signals at least partially described in terms of c levels, for output to an output device responding to output signals having d levels, to print the document image comprising: an image signal input, receiving image signals described in terms of c levels; halftoning error adder, adding halftoning error from any previous signals to said received image signals to produce modified image signals; an error diffusion circuit, producing for each modified image signal described in terms of c levels, an output signal described in terms of d levels responsive to a thresholding operation using a reference signal, and directing halftoning error signals to the halftoning error adder for addition to at least one adjacent and non-processed image signal in the document image, including: a reference signal generator, producing the reference signal responsive to an initial threshold value, the received image signal and a selected enhancement factor; a depletion zone correction signal generator, producing a correction signal responsive to a previous output signal, and the received image signal, and whether the received image is a light tone, a dark tone or a midtone; correction signal application, applying the depletion zone correction signal to the modified image signal, in order to correct for improper rendition of highlight and shadow areas.