Apparatus and method for improving black and color separation in halftoned images by printing black dots in a different screen phase

Claim 2

A K separation screen for printing halftone images comprised of three primary colors and black on a print medium, the screen comprising: a square grid comprised of a first plurality of parallel lines spaced a first distance apart and extending in a screen direction and a second plurality of parallel lines spaced the first distance apart and extending in a second direction perpendicular to the screen direction, the first plurality of lines crossing the second plurality of lines to form a plurality of intersections; a plurality of halftone dot printing positions, each of the plurality of printing positions being arranged at one of the plurality of intersections, each of the plurality of printing positions being divided into one of a first printing position type and a second printing position type wherein the first printing position type alternates in both the screen direction and the second direction with the second printing position type; printed halftone dots on the print medium comprised of at least one primary color and white located at ones of the plurality of printing positions having the first printing position type; and printed halftone dots on the print medium comprised of black and white located at ones of the plurality of printing positions having the second printing position type.

Claim 3

A K separation screen according to claim 1 wherein some of the printed halftone dots located at ones of the plurality of printing positions having the first printing position type are comprised of a plurality of primary colors.

Claim 4

A K separation screen according to claim 1 wherein printing positions having a first printing position type are physically offset from printing position having a second printing position type by one-half the screen frequency.

Claim 5

A K separation screen according to claim 1 wherein the screen angle of each of the primary colors is 45°.

Claim 6

A K separation screen according to claim 1 wherein the screen angle of black is 45°.

Claim 7

Apparatus for printing halftone images comprised of three primary input color values and black values, the apparatus comprising: first means for generating a plurality of halftone dot printing positions of a first type, each of the plurality of printing positions of the first type being arranged at one of a plurality of intersections on a square grid comprised of a first plurality of parallel lines spaced a first distance apart and extending in a screen direction and a second plurality of parallel lines spaced the first distance apart and extending in a second direction perpendicular to the screen direction, the first plurality of lines crossing the second plurality of lines to form the intersections; second means for generating a plurality of halftone dot printing positions of a second type, each of the plurality of printing positions of the second type being arranged at one of the plurality of intersections, wherein the first printing position type alternates in both the screen direction and the second direction with the second printing position type; means for printing halftone dots comprised of at least one primary color and white at ones of the plurality of printing positions having the first printing position type; and means for printing halftone dots comprised of black and white located at ones of the plurality of printing positions having the second printing position type.

Claim 8

Apparatus for printing halftone images comprised of three primary input color values and black values, the apparatus comprising: first means for generating a plurality of halftone dot printing positions of a first type, each of the plurality of printing positions of the first type being arranged at one of a plurality of intersections on a square grid comprised of a first plurality of parallel lines spaced a first distance apart and extending in a screen direction and a second plurality of parallel lines spaced the first distance apart and extending in a second direction perpendicular to the screen direction, the first plurality of lines crossing the second plurality of lines to form the intersections; second means for generating a plurality of halftone dot printing positions of a second type, each of the plurality of printing positions of the second type being arranged at one of the plurality of intersections, wherein the first printing position type alternates in both the screen direction and the second direction with the second printing position type; means for printing halftone dots comprised of at least one primary color and white at ones of the plurality of printing positions having the first printing position type; and means for printing halftone dots comprised of black and white located at ones of the plurality of printing positions having the second printing position type.

Claim 9

Apparatus for printing halftone images according to claim 6 wherein the first generating means comprises: means for generating a first clustered dot dither array pattern of threshold values; and means responsive to threshold values in the first dither array pattern and to the three primary color values for generating an output printed dot at the first printing position.

Claim 10

Apparatus for printing halftone images according to claim 7 wherein the second generating means comprises: means for generating a second clustered dot dither array pattern of threshold values; means responsive to the black values for physically offsetting the second dither array pattern relative to the first dither array pattern; and means responsive to threshold values in the second dither array pattern and to the black values for generating an output printed dot at the second printing position.

Claim 11

Apparatus for printing halftone images according to claim 8 wherein the first dither array pattern is identical to the second dither array pattern.

Claim 12

Apparatus for printing halftone images according to claim 8 wherein the first dither array pattern comprises a plurality of identical dither arrays arranged in a rectangular grid pattern.

Claim 13

Apparatus for printing halftone images according to claim 8 wherein the second dither array pattern comprises a plurality of identical dither arrays arranged in a rectangular grid pattern.

Claim 14

Apparatus for printing halftone images according to claim 8 wherein the first generating means comprises: a read-only memory for storing the threshold values; a first address generator responsive to the three primary color values for generating first addresses; and first means responsive to the first addresses for retrieving the threshold values from the read-only memory.

Claim 15

Apparatus for printing halftone images according to claim 8 wherein the second generating means comprises: a second address generator responsive to the black values for generating second addresses; and second means responsive to the second addresses for retrieving the threshold values from the read-only memory.

Claim 16

Apparatus for printing halftone images according to claim 13 wherein the offsetting means comprises means for adding an offset value to the second addresses.

Claim 17

Apparatus for printing halftone images comprised of three primary input color values and black values, the apparatus comprising: a read-only memory for storing a plurality of threshold values comprising a dither array pattern; a first address generator responsive to the three primary color values for generating first addresses; first means responsive to the first addresses for retrieving the threshold values from the read-only memory; a second address generator responsive to the black color values for generating second addresses; second means responsive to the second addresses for retrieving the threshold values from the read-only memory; and means responsive to the black values for adding a predetermined offset value to the second addresses.

Claim 18

Apparatus for printing halftone images comprised of three primary input color values and black values, the apparatus comprising: a read-only memory for storing a plurality of threshold values comprising a dither array pattern; a first address generator responsive to the three primary color values for generating first addresses; first means responsive to the first addresses for retrieving the threshold values from the read-only memory; a second address generator responsive to the black color values for generating second addresses; second means responsive to the second addresses for retrieving the threshold values from the read-only memory; and means responsive to the black values for adding a predetermined offset value to the second addresses.

Claim 19

Apparatus for printing halftone images according to claim 15 wherein the first address generator means comprises: an address generator responsive to the three primary color values and to the black values for generating pixel position signals; and a modulo circuit responsive to the pixel position signals for generating a repetitive set of pixel position addresses.

Claim 22

A method for printing halftone images according to claim 17 wherein step A comprises the steps of: A1. generating a first clustered dot dither array pattern of threshold values; and A2. generating an output printed dot at the first printing position using threshold values in the first dither array pattern and the three primary color values.

Claim 23

A method for printing halftone images according to claim 18 wherein step B comprises the steps of: B1. generating a second clustered dot dither array pattern of threshold values; B2. physically offsetting the second dither array pattern relative to the first dither array pattern in response to the black values; and B3. generating an output printed dot at the second printing position using the threshold values in the second dither array pattern and the black values.

Claim 24

A method for printing halftone images according to claim 19 wherein step B1 comprises the step of: B1A. generating a second dither array pattern identical to the first dither array pattern.

Claim 25

A method for printing halftone images according to claim 19 wherein step A1 comprises the step of: A1A. arranging a plurality of identical dither arrays in a rectangular grid pattern to generate the first dither array pattern.

Claim 26

A method for printing halftone images according to claim 19 wherein step B1 comprises the step of: B1B. arranging a plurality of identical dither arrays in a rectangular grid pattern to generate the second dither array pattern.

Claim 27

A method for printing halftone images according to claim 19 wherein step A1 comprises the steps of: A1B. storing the threshold values in a read-only memory; A1C. generating first addresses in response to the three primary color values; and A1D. retrieving the threshold values from the read-only memory in response to the first addresses.

Claim 28

A method for printing halftone images according to claim 19 wherein step B1 comprises the steps of: B1C. generating second addresses in response to the black values; and B1D. retrieving the threshold values from the read-only memory in response to the second addresses.

Claim 29

A method for printing halftone images according to claim 24 wherein step B2 comprises the step of: B2A. adding an offset value to the second addresses.