Color digital halftoning using black and secondary color replacement and color vector dithering

A method of operating a color printer to print a desired image as an array of dots of colored ink at a large number of pixel positions, comprising the steps of:

• receiving information about the desired color image for each pixel position;
• expressing the desired color image information in the form of a color vector for each pixel position, if the information is not already in that form;
• replacing at least part of the black in each color vector with chromatic colors;
• removing a quantity of white to compensate for said black replacement;
• replacing at least part of a secondary color, if present, in each color vector with primary colors, provided that at least a certain further quantity of white remains available after said black replacement;
• removing said further quantity of white to compensate for said secondary-color replacement, if secondary-color replacement has been performed;
• wherein said expressing, replacing and removing steps produce a resulting color vector;
• comparing nested subvectors respectively made up of color components in the resulting color vector, at each pixel position, with a corresponding dither-cell value for that pixel position;
• said nested subvectors being assembled in a sequence comprising black, then a secondary color, then the primary colors in order of increasing lightness, and then white;
• selecting for printing, at the corresponding pixel location, the first component of the color vector which produces a cumulative aggregate greater than said corresponding dither-cell value;
• translating said selected color vector at each pixel location into output color coordinates usable to control the printer;
• and
• applying the output color coordinates to control the printer.