A method and apparatus perform vector error diffusion and result in reduced graininess of a color halftone image representing a continuous tone image. The method controls the output colors while reducing the visibility of the error diffusion pattern inherent in forming a bilevel image. Specifically, the method involves influencing the dot pattern correlation using vector error diffusion and takes into account the effects caused by the interactions of dot patters for all of the different color components. A distorted color space is used in which colors not favoured, e.g. secondaries, white and black, are further away from most colors in the space than are favoured colors. This can be achieved by changing thresholds used to select the closest color.
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