Method and apparatus for generating digital halftone images by multi color dithering
First Claim
1. A multi-color dithering method for the reproduction of color input images, the method being characterized by the use of a two-dimensional dither function, the use of an arbitrary number of primary colors and the use of partitions representing relative amounts of primary colors, the method comprising the steps of:
- (a) defining a two dimensional dither function G(x,y);
(b) choosing a set of primary colors {Ci} located within a three-dimensional color working space ABC;
(c) initiating a neighbor finding procedure which renders neighboring primary colors {Cj} of a given input color C in the three-dimensional color space ABC;
(d) computing the primary color of each output image pixel O(x,y) by a set of substeps comprising;
(i) locating in the input color image an input color location (x′
,y′
) associated to the output image pixel O(x,y) and computing its corresponding color C(x′
,y′
);
(ii) finding N neighbor primary colors {Cj} of C in the ABC color space by using the neighbor finding procedure;
(iii) computing the amount Sj(x,y) of every neighbor primary color C1;
(iv) determining the output primary color O(x,y) at output pixel (x,y) by locating at each position (x,y) a partition Pj(x,y) within which the dither function G(x,y) is situated, where upper and lower boundaries of partitions P1, P2, . . . PN are respectively defined by lower boundary 0 and upper boundary S1(x,y), lower boundary S1(x,y) and upper boundary S1(x,y)+S2(x,y), . . . , lower boundary S1(x,y)+S2(x,y)+ . . . +SN-1(x,y) and upper boundary S1(x,y)+S2(x,y)+ . . . +SN(x,y), and by assigning to the output primary color O(x,y) the neighbor primary color Cj associated to partition Pj(x,y); and
(e) outputting the multi-color dithered output image on an output device.
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Abstract
The invention provides a method and apparatus for the reproduction of color images by multi-color dithering. Multi-color dithering is an extension of bi-level dithering for halftoning an input image using as primary colors an arbitrary number of inks. The inks may comprise standard inks or non-standard inks such as non-process color inks, opaque inks, metallic inks, variable color inks and fluorescent inks. Multi-color dithering provides a solution for creating artistic multi-color dithered images, whose screen elements are made of artistic color screen shapes such as micro-letters, symbols and ornaments. When printed at high resolution and at high registration accuracy, multi-color dithering using large dither arrays incorporating artistic dither shapes provides an effective solution for preventing counterfeiting. The use of non-standard inks offers additional protection. Multi-color dithering also offers a solution for printers requiring that all inks are printed side by side, without overlaps.
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Citations
16 Claims
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1. A multi-color dithering method for the reproduction of color input images, the method being characterized by the use of a two-dimensional dither function, the use of an arbitrary number of primary colors and the use of partitions representing relative amounts of primary colors, the method comprising the steps of:
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(a) defining a two dimensional dither function G(x,y); (b) choosing a set of primary colors {Ci} located within a three-dimensional color working space ABC; (c) initiating a neighbor finding procedure which renders neighboring primary colors {Cj} of a given input color C in the three-dimensional color space ABC; (d) computing the primary color of each output image pixel O(x,y) by a set of substeps comprising; (i) locating in the input color image an input color location (x′
,y′
) associated to the output image pixel O(x,y) and computing its corresponding color C(x′
,y′
);(ii) finding N neighbor primary colors {Cj} of C in the ABC color space by using the neighbor finding procedure; (iii) computing the amount Sj(x,y) of every neighbor primary color C1; (iv) determining the output primary color O(x,y) at output pixel (x,y) by locating at each position (x,y) a partition Pj(x,y) within which the dither function G(x,y) is situated, where upper and lower boundaries of partitions P1, P2, . . . PN are respectively defined by lower boundary 0 and upper boundary S1(x,y), lower boundary S1(x,y) and upper boundary S1(x,y)+S2(x,y), . . . , lower boundary S1(x,y)+S2(x,y)+ . . . +SN-1(x,y) and upper boundary S1(x,y)+S2(x,y)+ . . . +SN(x,y), and by assigning to the output primary color O(x,y) the neighbor primary color Cj associated to partition Pj(x,y); and (e) outputting the multi-color dithered output image on an output device. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. An apparatus for color image reproduction by multi-color dithering comprising:
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(a) an input color image storage unit operable for storing an input color image made of input color image pixels of color C(x′
,y′
);(b) a neighborhood calculation unit operable for computing a subset of primary colors {Cj} neighbors of the given input image color C and operable for computing for each neighbor color Cj of the set {Cj} a signal Sj(x,y) giving the relative amount of Cj in respect to input image color C; (c) a dither function storage unit storing the values of a two-dimensional dither function G(x,y); (d) a multi-color dithering unit operable for sweeping through all pixels of the output image (x,y), operable for finding the input color C(x′
,y′
) of corresponding input image pixels, operable for launching the neighborhood calculation unit to compute a subset of primary colors {Cj} neighbors of C(x′
,y′
) as well as their respective signals {Sj(x,y)}, operable for determining a partition Pj(x,y) within which the dither function G(x,y) stored in the dither function storage unit 160 is situated, thereby selecting the primary color Cj(x,y) associated to Pj(x,y) as output color O(x,y);(e) an output multi-color image storage unit operable for storing the selected primary colors O(x,y) generated by the multi-color dithering unit and operable for forwarding the primary colors making up the output multi-color dithered image to an output device; (f) an output device operable for converting the output multi-color dithered image into a visible color image. - View Dependent Claims (14)
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15. A computing system for multi-color dithering comprising
(a) a memory operable for storing an input color image made of input color image pixels C(x′ - y′
), operable for storing the values of a two-dimensional dither function G(x,y), operable for storing the code of a neighborhood calculation software procedure, operable for storing a multi-color dithering software procedure and operable for storing an output multi-color dithered image O(x,y);(b) a central processing unit interacting with said memory and running said neighborhood calculation and multi-color dithering software procedures, operable for computing for each output image pixel (x,y) an output primary color O(x,y) selected from a set of primary colors, operable for storing the output primary color O(x,y) in memory and operable for forwarding the resulting output multi-color dithered image to an output device; (c) an output device operable for converting the output multi-color image into a visible color image, where the multi-color dithering software procedure sweeps through output image pixels, finds for each output image pixel (x,y) a corresponding input image pixel color C(x′
,y′
), launches the neighborhood calculation software procedure which computes a subset of primary colors {Cj} neighbors of C(x′
,y′
) as well as their respective amounts {Sj(x,y)}, determines a partition Pj(x,y) within which the two-dimensional dither function G(x,y) is situated and selects a primary color Cj(x,y) associated to Pj(x,y) as output primary color O(x,y). - View Dependent Claims (16)
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Specification