Colour Plane Synchronization in Colour Error Diffusion

On a bi-level colour device, only eight distinct colours (including black and white) are able to be represented. One common method of mapping image data to such a device is to use error diffusion techniques. These techniques diffuse the error from representing a particular pixel to adjacent unprocessed pixels, with the effect that locally the average pixel value closely approximates that of the original image. However, when applied to colour images, the pattern of dots within the three colour planes lose synchronisation, resulting in coloured patterns within regions of low saturation. This paper demonstrates that this problem may be overcome by modulating the threshold level by image intensity. The results presented show that a threshold deviation on 15% is sufficient to give the desired linear relationship between desynchronisation and saturation, and that the resynchronisation occurs within one or two pixels of an edge between a saturated and unsaturated region.