Error diffusion method with symmetric enhancement

Abstract

A processing system for processing electronic images defined in terms of image signals, each image signal representing density at a discrete position within the electronic image, and quantizing image signals defined at 'c' gray levels for use in a device requiring image signals defined at 'd' levels, where 'd' may be less than or equal to 'c', the system including an image input receiving at least a portion of the electronic image as input signals; an error adding circuit, adding error signals determined from any previous thresholding processing to the input signals to generate corrected input signals; a thresholding processor, receiving corrected image signals defined at 'c' levels, and quantizing the corrected image signals to 'd' levels, by comparison to at least one threshold signal and outputting the corrected image signals at 'd' levels as output signals; an image output, outputting output signals defined at 'd' levels; a differencing circuit, generating a difference signal representative of the difference in image density between corresponding corrected input signals and the output signal; an error distribution circuit, receiving the difference signals, and directing weighted portions thereof to the error adding circuit for addition to subsequent image signals in a predetermined spatial relationship to the input signals; and a threshold modulation circuit, varying the threshold signals proportionally to the input image, and recursively varying the threshold signals in response to previous threshold signals.