Color images having multiple separations with minimally overlapping halftone dots

An image processing system for a multicolor printer, including a halftone generator that converts contone separation signals to binary separation signals comprising:

• means for receiving a plurality of contone separation signals describing a color image;
• means for storing a halftoning screen, having an m.times.n array of thresholds to be applied to a corresponding m.times.n array of contone image signals describing a discrete area of the image to derive a binary image signal suitable to drive a printer to produce spot or no spot at each of m.times.n locations within the discrete area;
• means for producing first binary image signals describing a black separation, if any, from the contone separation signals, in accordance with the stored halftone screen thresholds, said separation describing a set of locations in the discrete area where black spots will be printed;
• means for producing second binary image signals describing a first non-black color separation, from the contone separation signals, in accordance with the stored halftone screen, said second binary signals producing first non-black color spots at locations where black spots will not be printed;
• means for producing third binary image signals, if any, describing a second non-black color separation, from the contone separation signals, in accordance with the stored halftone screen, said third binary signals producing second non-black color spots at locations where black spots will not be printed and preferentially where first non-black color spots will be not be printed, and overlapping first non-black color spots if required.

A method of preparing a color document defined in terms of contone image signals and multiple separations to binary image signals in multiple separations, using a halftoning process that minimizes overlap of colors within a halftone dot area, the method including the steps of:

• storing a halftone screen cell in a memory device, the halftone cell defining for a discrete area of the image a set of 1 to N incrementally varying thresholds for use in producing binary image signals from the color document contone image signals;
• receiving from a color image source black separation contone image signals, if any, for a discrete area of the image;
• comparing each black separation contone image signal in the discrete area to a member of the set of 1 to N incrementally varying thresholds, and producing black separation binary image signals having a first state upon exceeding the threshold and a second state upon not exceeding the threshold;
• directing the black separation binary image signals to the printer, whereby the printer is directed to print black spots or no black spots at each location within the discrete area;
• receiving first color separation image signals, if any, for the discrete area of the image;
• adding to each first color separation image signals the magnitude of the corresponding black separation image signals to produce an offset first color separation image signal;
• comparing each offset first color separation image signal in the discrete area to a member of the set of 1 to N incrementally varying thresholds, and producing first color separation binary image signals having a first state upon exceeding the threshold and a second state upon not exceeding the threshold;
• logically ANDing the first color binary image signals with inverse black binary image signals, and directing the logical output to the printer, whereby the printer is directed to print first color spots or no first color spots at each location within the discrete area where there is not already a black spot;
• adding to any next color separation image signals the magnitude of the previous color separation image signals, if any;
• comparing each next color separation image signal in the discrete area to a member of the set of 1 to N incrementally varying thresholds, and producing image signals having a first state upon exceeding the threshold level and a second state upon not exceeding the threshold whenever the threshold value is greater than the maximum previous color value;
• logically ANDing the next color binary image signals with inverse previous color binary image signals, and directing the logical output to the printer, whereby the printer is directed to print next color spots or no next color spots at each location in the discrete area where there is not already a color spot;
• and
• determining that there is no location within the discrete area where there is not already a black spot or color spot, and directing the printer to print next color spots or no next color spots as required only at locations where there is a non-black spot.

A method of preparing an electronic color document for printing at a color printer, the documents defined in terms of digital contone image signals and multiple separations to be processed in a predetermined order, and the printer requiring binary printer driver signals in multiple separations, wherein the method minimizes overlap of colorants in the printer reproduction of the color document, including the steps of:

• applying a binarization process to the digital contone image signals of a first separation to produce first separation printer driving signals;
• for each remaining separation, generating a modified image signal from an original separation image signal and a modified image signal of a previously prepared separation;
• applying the binarization process to the modified image signal of a following separation to obtain an intermediate binary signal for the second separation;
• determining a printer driver signal for a first layer of the follow separation as a function of the intermediate binary signal for the following separation and the final binary signal for a previously prepared separation;
• determining if additional layers of colorant are required to obtain the desired color at the printer;
• for a next layer of a color separation, further modifying the modified image signal according to a maximum ink value and a modified image signal of a previously prepared separation;
• applying the binarization process to the further modified signal to determine an intermediate binary value for the next separation layer;
• determining a printer driver signal for the next separation layer as a function of the intermediate binary value for the layer and the final binary value for a previously prepared separation;
• combining the binary signals for all layers of the separation to arrive at the overall binary signal for the separation;
• directing the prepared separations to the color printer for printing.