ERROR DIFFUSION HALFTONING WITH HOMOGENEOUS RESPONSE IN HIGH/LOW INTENSITY IMAGE REGIONS

A method and apparatus for quantizing pixels in an image formed by a plurality of pixels, each pixel constituting an image signal representing optical density of the image at a location therein, and having an original optical density value selected from one of a set of `c` original optical density values that has a number of members larger than a desired output set of `d` optical density values, through a process of error diffusion. For an input image so comprised, initially, an image signal is initially modified in accordance with previously determined error. Thereafter, during the quantization step of the process, the threshold signal for quantization determination is dynamically adjusted in accordance with previous output quantization determinations to selectively control likelihood of whether the next pixel will exceed the threshold. The threshold level is selectively modified on a pixel by pixel basis to increase or decrease the likelihood that the next pixel will be quantized to one state or another state. In addition to this control, however,the increase or decrease in the threshold level is based upon a regional input intensity. With the threshold signal set as described, the modified input signal is evaluated, and an output signal that is one of d levels is determined accordingly. Subsequent to quantization, the difference between the determined output signal and the modified input signal is evaluated and stored as error, to be dispersed for addition to a preselected group of unevaluated neighboring signals.