Color printing yielding a dense black image

Abstract: A dense black image is formed by printing the black ink in combination with at least one other ink without resulting in thick, blurred figure edges. A bitmap of binary information is provided for each of a plurality of color separations, each bitmap indicating desired locations of ink spots for a color separation. Spots are eroded on the boundaries of black image areas (12) in the bitmap to form a modified black image bitmap (14). The modified black image bitmap (14) is printed in ink of at least one color other than black at the same time that the bitmaps for the color separations other than black are printed. The original black image bitmap (12) is then printed in black ink over the printed modified black image bitmap to yield a dense black image.

Description: The present invention relates to color printing in which a color separation for each of a plurality of colors is performed to yield an image and, more particularly, to color printing which yields dense black images. Electronic color printers use a plurality of inks such as cyan, magenta, yellow and black. While the combination of cyan, magenta and yellow should yield black, it is often difficult to achieve the color black which is free of any unwanted color using this combination. Accordingly, the additional use of black ink yields a black which is generally free of unwanted color. The black ink which is used in these electronic color printers is not necessarily very dense. Thus, what should be black ink may, in actuality, be a very dark gray ink or may be a mottled ink. One proposed method to improve the density of the black image when a black ink is used which is not sufficiently dense is to print the image in black ink as well as printing the image using one or more of the other colored inks. In printing the black ink in combination with one or more of the other inks, the black ink removes color from an image that is already dark due to the presence of the other inks. Therefore, more light is absorbed by the image printed in a combination of inks. It is sometimes recommended that the three color process black, i.e., cyan, magenta and yellow inks, be used for light regions and that the black ink only be added as an overprint in a dark shadowed area to improve density. Such printing has drawbacks associated therewith, however. The overprinting of inks may yield ink spots that are larger than a desirable size due to the presence of the additional ink or due to registration problems. In particular, since the color black is often used in images to define fine lines and characters, the overprinting of inks results in the thickening of the fine lines and characters and the blurring of edges of the ink spots. US-A-4,700,399 discloses a color image processing apparatus as described above which has a detector for detecting an edge of an image and a control unit for controlling a black reproduction quantity for an edge according to an output from the detector. For edges, densities of color signals for yellow, magenta, and cyan are reduced in accordance with the edge quantity to increase black density. US-A-4,953,015 discloses a method for printing a color image which makes it possible to obtain black of high density in a printout image. Black ink dots are placed on an image receiving medium so as to produce a black area of an image. A plurality of other inks such as yellow, cyan and magenta are allotted according to a color matrix table and are placed in the form of dots superimposed on at least a part of the black ink. Accordingly, the superimposition of dots can yield larger, blurred spots. A printing method is desirable which yields dense black images having edges which are not thickened or blurred due to additional ink or registration problems. One object of the present invention is to provide a method for printing a dense black image which does not have thickened, blurred edges. According to the present invention, there is provided a printing method for forming a dense black image in color printing, the method comprising the steps of: a) providing an original black image bitmap of binary information for a black image area to indicate desired locations of black ink spots therefor; b) deriving a modified black image bitmap from the original black image bitmap by eroding spots on boundaries of the black image areas in the original black image bitmap, the modified black image bitmap having boundaries within those of the original black image bitmap; c) providing original color image bitmaps of binary information for each color separation different from black; and d) printing the original black image bitmap; characterised in that step d) comprises the steps of: d1) combining the original color image bitmaps of binary information for each color separation different from black with the modified black image bitmap, the combined image bitmap so formed yielding binary information to be printed in colors other than black; d2) printing the combined image bitmap; and d3) printing the original black image bitmap over the combined image bitmap. The invention thus provides a method for printing a dense black image which is not mottled or gray in appearance, and which does not modify the colors in the image. Bitmaps are provided for each of a plurality of color separations, e.g., cyan, magenta, yellow and black separations. An erosion operation is applied to the black image bitmap or a copy thereof such that the boundaries of the regions of the black image bitmap are reduced by one or more pixels. The eroded bitmap is to be printed in a color other than black. The amount of erosion is selected in accordance with expected registration errors. The bitmaps for the color separations other than black and the eroded black image bitmap are combined. Ink is deposited in accordance with the combined bitmaps. Thereafter, black ink is deposited thereover in accordance with the original black image bitmap. The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein: Figure 1 illustrates a bitmap for a black color separation; and Figure 2 illustrates an eroded bitmap for the black color separation of Figure 1 according to the present invention. Referring now to the drawings, and particularly to Figure 1 thereof, a bitmap 10 of binary information for a black color separation is shown. Bitmap 10 illustrates an area of an image which is to be printed in black ink. The area of the image to be printed in black ink has a boundary 12. The remaining portions of the image are to be printed in different ink colors in corresponding color separations. A different color separation is performed for each color of ink used to form the composite image. For exemplary purposes, in accordance with the present invention, four colors of ink, i.e., cyan, magenta, yellow and black inks, are used to form a composite image. Because the black ink is not as dense as a black color obtained by combining the cyan, magenta and yellow inks, the black ink is not used alone when a dense black image is a desired result. Further, the combination of the cyan, magenta and yellow inks also yields a black image which is not as dense as the black image desired. To improve the density of the black image, the present invention provides a bitmap of binary information for each of the cyan, magenta, yellow and black separations. As illustrated in Figure 2, the black bitmap 10 is modified such that the boundaries 12 of black areas are eroded by an amount 16 to form a resultant modified black bitmap 10' having an area bounded by boundary 14, this area being smaller than the area bounded by boundary 12 in the original black bitmap 10. The modified black bitmap 10' yields binary information to be printed in colors other than black. The bitmaps of binary information for each color separation other than the black color separation, which are to be used in superposition with the black to achieve greater density, are combined with the modified black bitmap 10'. Inks are deposited for the colors other than black in accordance with the combined bitmaps. Thereafter, the black bitmap 10 is printed in black ink over the combined bitmaps. The amount of erosion 16 should be selected to compensate for any expected registration errors occurring at the performance of each of the color separations. Thus, even if the color separations are not performed in precise superimposed registration, deposit of the black ink over the printed modified black bitmap would not yield thickened, blurred black images. The eroded black bitmap can be obtained by actually copying the bitmap of binary information for the black image. Alternatively, the eroded black bitmap can be obtained by calculating the value of any spot from the information in the original black bitmap. For example, if the value of a spot at a position (i,j) in the original black bitmap is B(i,j), the value of the spot at position (i,j) in the modified black bitmap is B'(i,j). The value of the spot in the eroded black bitmap can be given by the conjunction of neighbors to the spot of the original black bitmap in accordance with the following equation: B'(i,j) = B(i,j)B(i +1,j)B(i-1,j)B(i,j+1)B(i,j-1) If the value in the original bitmap for the additional color is C(i,j), the value actually used to print the color would be the disjunction C'(i,j) determined in accordance with the following equation: C'(i,j) = C(i,j)B'(i,j) The above-described printing method yields images having dense black areas without yielding thickened, blurred edges. The printing method does not modify the colors in the image since the additional ink is placed beneath the black and acts only to increase the density of the black areas. The erosion step guarantees that the additional ink will lie beneath the black, even when registration errors occur. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the scope of the invention as defined in the following claims.

Claims: A printing method for forming a dense black image in color printing, the method comprising the steps of: a) providing an original black image bitmap (10) of binary information for a black image area to indicate desired locations of black ink spots therefor; b) deriving a modified black image bitmap (10') from the original black image bitmap (10) by eroding spots (16) on boundaries (12) of the black image areas in the original black image bitmap (10), the modified black image bitmap (10') having boundaries (14) within those of the original black image bitmap (10); c) providing original color image bitmaps of binary information for each color separation different from black; and d) printing the original black image bitmap (10); characterised in that step d) comprises the steps of: d1) combining the original color image bitmaps of binary information for each color separation different from black with the modified black image bitmap, the combined image bitmap so formed yielding binary information to be printed in colors other than black; d2) printing the combined image bitmap; and d3) printing the original black image bitmap (10) over the combined image bitmap. The method according to claim 1, including printing the combined image bitmap (10') in ink of at least one of the colors cyan, magenta and yellow. The method according to claim 1, wherein step b) includes calculating the value of each spot in the modified black image bitmap (10') from the original black image bitmap (10) of binary information. The method according to claim 3, wherein the value of each spot in the original black image bitmap is calculated by the conjunction of its neighbors in the original black image bitmap in accordance with the equation: B'(i, j) = B(i, j)B(i+1, j)B(i-1, j)B(i, j+1)B(i, j-1) where B'(i, j) is the value of the spot at position (i, j) in the modified black image bitmap; and B(i, j) is the value of a spot at a position (i, j) in the original black image bitmap. The method according to claim 4, wherein the value of each spot used to print color is determined in accordance with the equation: C'(i, j) = C(i, j)B'(i, j) where C(i, j) is the value in the original color bitmap; and C'(i, j) is its disjunction.

Inventors: HARRINGTON STEVEN J, HARRINGTON, STEVEN J.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1992
Main classification: H04N1
IPC codes: B41J2, H04N1