Abstract:
A method for processing color image data for printing on a color ink jet printer includes reading color image data from a source image, the source image containing color image data of at least a first color area and a second color area. A border region is then identified between the first color area and the second color area. A pixel altering function alters pixels of the source image along the border region between the first color area and the second color area before the source image is converted into a plurality of halftone images. Finally, the halftone images are printed using ink of the first and second colors according to the first and second color areas.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an improved method of processing color image data for maintaining edge quality while printing on a color ink jet printer.  
         [0003]     2. Description of the Prior Art  
         [0004]     Liquid ink printers including inkjet printers deposit black and/or colored liquid inks which tend to spread when the ink is deposited on paper as a drop, spot, or dot. A problem of liquid ink printers is that the liquid inks used have a finite drying time, which tends to be somewhat longer than desired. Bleeding tends to occur when the drops are placed next to each other in a consecutive order or in a cluster of dots within a short time. Bleeding, spreading, and feathering causes print quality degradation including color shift, reduction in edge sharpness, and solid area mottle which includes density variations in said areas due to puddling of inks. Intercolor bleeding occurs when ink from one color area blends into or bleeds with ink from another color area. Intercolor bleeding is often most pronounced where an area of black ink (relatively slow drying) adjoins an area of color ink (relatively fast drying); however, intercolor bleeding can occur at the interface between areas of any color inks having substantially different properties such as dry time or permeability.  
         [0005]     To solve this problem, many solutions have been proposed. In U.S. Pat. No. 6,183,062 entitled “Maintaining Black Edge Quality in Liquid Ink Printing” and assigned to Xerox Corporation, Curtis et al. teach a method for maintaining edge quality between black ink and colored ink, which is incorporated herein by reference.  
         [0006]     Please refer to  FIG. 1 .  FIG. 1  is a flowchart illustrating printing color images according to the prior art. Steps contained in the flowchart will be explained below.  
         [0007]     Step  10 : Start the process for printing a color source image; 
        Step  12 : Perform a color conversion operation on the source image. This conversion typically involves converting red, green, and blue (RGB) colors into cyan, magenta, yellow, and black (CMYK);     Step  14 : Convert the color image into a plurality of halftone images. For example, a color plane is produced for each of the CMYK colors, producing four halftone images;     Step  16 : Pixel altering processing is performed on each of the halftone images;     Step  18 : The altered halftone images are printed; and        
 
         [0012]     Step  20 : End.  
         [0013]     To reduce intercolor bleeding, the prior art carries out a process that operates to detect black/color interfaces where intercolor bleeding is likely to occur and to modify the pixels that are to be printed near the borders of the interfaces. The process comprises three general steps: identifying an interface between a black area and a color area; modifying the pixel pattern in a black border region in the black area; and modifying the pixel pattern in a color border region in the color area. Please refer to  FIG. 2 .  FIG. 2  shows a flowchart illustrating the prior art method for altering pixels in the halftone image for reducing intercolor bleeding.  
         [0014]     Step  16   a  identifies an interface between a black area and a color area. In one embodiment, described in more detail below, step  16   a  collects statistics for pixels within an X X Y window filter to identify an interface and determine if a given pixel is within either border region. However, step  16   a  can use any number of known techniques including, but not limited to, masking, look-up tables, edge detection filters, etc. to identify a black/color interface. A discussion of edge detection filters can be found in U.S. Pat. No. 5,635,967.  
         [0015]     Step  16   b  defines a width N of the black border region near the black/color interface identified in step  16   a . The number of pixels N comprising the black border region should be large enough to reduce intercolor bleeding at the border and small enough to minimize the formation of additional printing artifacts that would likewise reduce image quality. Similarly, step  16   c  defines the width M of the color border region near the interface. As with the selection of black border region, the width M of the color border region should be selected to reduce intercolor bleeding while minimizing the addition of other printing artifacts.  
         [0016]     When defining the width of the border regions consideration may be given to such factors as the position of the border regions, the type of image (e.g., text, line art, graphics, pictorial, etc.), the width of each border, how the pixel pattern within a border will be modified, the print medium used, ink composition, etc. Each of the border regions beneficially are positioned to abut the interface; however, it is understood that the border region need not abut the interface. The total width of the border regions at an interface should be selected to reduce intercolor bleeding at an interface and minimize the formation of additional printing artifacts. Optimum values for border width can be identified through calibration and image analysis studies. The width of the black border is preferably between 0 and 350 μm, and the width of the color border is preferably between 0 and 200 μm. Beneficially, for a 300 dpi ink jet, the width of the N pixel black border is selected from 0 to 4 pixels, and the width M of the color border is defined to be from 0 to 2 pixels.  
         [0017]     Steps  16   d  and  16   e  modify the pixel pattern within the N-pixel black border and M pixel color border regions, respectively. A number of methods exist to modify the pixels or pixel pattern within the border regions. One method of modifying the pixel pattern within a border region replaces selected pixels with a predetermined combination of separation pixels. The replacement operation effectively turns off the separation pixel this is being replaced and turns on the separation pixel(s) replacing it. The replacement of pixels is sometimes referred to as “substitution” or “replacing”. An example of a substitution operation is illustrated in  FIG. 3 . In  FIG. 3 , window  40  shows a 5 X 5 block of composite pixels along a yellow/black interface. Window  42  shows the pixel block of window  40  after a substitution operation wherein within a 2 pixel border (columns  44  and  46 ) every other pixel in the black separation is turned off and replaced with alternating cyan and magenta pixels in the composite image.  
         [0018]     Another method of modifying a pixel pattern removes (turns off) selected pixels in one or more separations from the composite image. This removal of pixels from separations is sometimes referred to as “thinning” or “reducing”.  FIG. 4  illustrates an example of a thinning operation wherein window  50  is a 5 X 5 pixel block of composite pixels along a black/color interface and window  52  shows the same image block after thinning. The thinning operation removes (turns off) all color separation pixels from every other pixel in column  54  and removes yellow separation pixels from every other pixel in column  56 .  
         [0019]     A thinning operation can also be used to reduce the ink coverage in a multiple drop per pixel printer. Briefly, in a multi-drop per pixel printer small ink drops are often used to produce good tone transitions in graphical and pictorial images. However, the size of these drops are not large enough to produce a solid area fill or saturated colors using only one drop per pixel, thereby reducing the color saturation value for that pixel. Thus, the printer typically requires greater than 100% coverage, that is, multiple drops per separation pixel to obtain solid area fill. In  FIG. 5  window  60  illustrates a 5 X 5 pixel area along a black/color interface wherein the black region comprises 150% coverage (i.e., an average of three drops for every two pixels). Window  62  shows the same image area as window  60  after a thinning operation to reduce the drop coverage to approximately 100%, ie., an average of one drop per separation pixel. In window  62 , column  64  illustrates a thinning operation that reduces all two drop pixels to one drop pixels. Columns  66  and  68  illustrate a thinning method that removes approximately half of the two drop pixels.  
         [0020]     As shown in steps  14  and  16  of the flowchart of  FIG. 1 , the prior art method involves first converting the source image into halftone images, and then altering the pixels of the halftone images in order to reduce intercolor bleeding. Unfortunately, if the halftone images have a higher resolution than the source image, many extra calculations and extra memory are required to alter pixels on the halftone images as compared to altering the pixels on the source image.  
       SUMMARY OF INVENTION  
       [0021]     It is therefore a primary objective of the claimed invention to provide a method for processing color image data in order to solve the above-mentioned problems.  
         [0022]     According to the claimed invention, a method for processing color image data for printing on a color ink jet printer is disclosed. The method includes reading color image data from a source image, the source image containing color image data of at least a first color area and a second color area. A border region is then identified between the first color area and the second color area. A pixel altering function alters pixels of the source image along the border region between the first color area and the second color area before the source image is converted into a plurality of halftone images. Finally, the halftone images are printed using ink of the first and second colors according to the first and second color areas.  
         [0023]     It is an advantage of the claimed invention that the method alters the pixels of the source image before converting the source image into a plurality of halftone images. When the resolution of the halftone images is greater than the resolution of the source image, a significant number of calculations and memory are saved by altering the source image directly before the conversion to halftone images.  
         [0024]     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0025]      FIG. 1  is a flowchart illustrating printing color images according to the prior art.  
         [0026]      FIG. 2  shows a flowchart illustrating the prior art method for altering pixels in the halftone image for reducing intercolor bleeding.  
         [0027]      FIG. 3  illustrates an example of a pixel substitution operation.  
         [0028]      FIG. 4  and  FIG. 5  illustrate examples of pixel thinning operations.  
         [0029]      FIG. 6  is a flowchart illustrating printing color images according to the present invention.  
         [0030]      FIG. 7  is a flowchart showing conditions for executing the pixel altering process of the present invention.  
         [0031]      FIG. 8  is a chart comparing memory and calculations required by the present invention method to the prior art method. 
     
    
     DETAILED DESCRIPTION  
       [0032]     Please refer to  FIG. 6 .  FIG. 6  is a flowchart illustrating printing color images according to the present invention. Steps contained in the flowchart will be explained below.  
         [0033]     Step  50 : Start the process for printing a color source image;  
         [0034]     Step  52 : Perform a color conversion operation on the source image. This conversion typically involves converting red, green, and blue (RGB) colors into cyan, magenta, yellow, and black (CMYK). However, the source image can also be converted into other colors such as light cyan, light magenta, orange, and green can also be used;  
         [0035]     Step  54 : Pixel altering processing is performed on the source image;  
         [0036]     Step  56 : Convert the altered source image into a plurality of halftone images. For example, a color plane is produced for each of the CMYK colors, producing four halftone images;  
         [0037]     Step  58 : The halftone images are printed; and  
         [0038]     Step  60 : End.  
         [0039]     As shown in steps  54  and  56  above, the pixel altering for reducing intercolor bleeding is performed on the source image. After the pixel altering process, the source image is then converted into the halftone images. Like the prior art method, the present invention method corrects intercolor bleeding along a border between two different colors of ink. For instance, suppose that black pigment-based ink is used as a first color and either cyan, magenta, or yellow dye-based ink is used as a second color. Since the pigment-based ink and the dye-based ink have different properties, and dry at different rates, the two ink colors may bleed together unless pixel altering processes such as reduction and replacement are used.  
         [0040]     Please refer to  FIG. 7 .  FIG. 7  is a flowchart showing conditions for executing the pixel altering process of the present invention. Steps contained in the flowchart will be explained below.  
         [0041]     Step  100 : Start;  
         [0042]     Step  102 : Input the source image color data for cyan, magenta, yellow, and black (CMYK) colors. Instead, the source image color data can also include color data for light cyan, light magenta, orange, and green colors;  
         [0043]     Step  104 : Calculate a density of black pixels d K  and a density of color pixels d CMY  for the source image;  
         [0044]     Step  106 : Determine if the density of black pixels d K  is greater than the density of color pixels d CMY ; if so, go to step  108 ; if not, go to step  112 ;  
         [0045]     Step  108 : Determine if the density of black pixels d K  is less than the pixel reduction threshold value; if so, go to step  110 ; if not, go to step  114 ;  
         [0046]     Step  110 : A border region between the black pixels and the color pixels is identified. To improve the printing quality at the border region, the pixel altering process is performed, including reduction and/or replacement of pixels;  
         [0047]     Step  112 : Determine if the density of black pixels d K  is less than the pixel replacement threshold value; if so, go to step  110 ; if not, go to step  114 ;  
         [0048]     Step  114 : Output the source image having cyan, magenta, yellow, and black (CMYK) colors; and  
         [0049]     Step  116 : End.  
         [0050]     In step  110 , the pixel altering process of the present invention can use either or both of the pixel reduction and pixel replacement techniques that were illustrated in  FIG. 2  through  FIG. 5 . However, instead of altering pixels of the halftone images, the present invention alters the pixels of the source image. Suppose the variable k is a scalar value equal to the ratio of each linear dimension of the halftone images to the corresponding linear dimension of the source image. When the resolution of the halftone images is greater than the resolution of the source images, the advantages of the present invention method are greatest.  
         [0051]     Since each halftone image is a series of monochromatic dots, each halftone image can use either a bit of data or a byte of data to store information for each dot. While using only one bit per dot for the prior art method provides the most efficient use of memory, extra calculations are required to extract the data stored in bit format.  
         [0052]     Please refer to  FIG. 8 .  FIG. 8  is a chart comparing memory and calculations required by the present invention method to the prior art method. As stated above, k is represents the ratio of linear resolution of the halftone image to the linear resolution of the source image. For example, if the source image has a resolution of 600×600 pixels, and each halftone image has a resolution of 1200×1200 pixels, then k=1200/600=2. Assume that the dimensions of the source image are m×n pixels, where m and n are positive integers. Each of the halftone images would then have dimensions of km×kn. The chart in  FIG. 8  shows the magnitude of the amount of memory used and the number of calculations needed when altering pixels in the halftone images according to the prior art, and compares these quantities to those used when altering pixels in the source image according to the present invention. Other factors that are identical for both the prior art and the present invention are not shown in  FIG. 8  for simplicity.  
         [0053]     The halftone images can use either a whole byte or one bit to store information for each pixel in the halftone images. Both of these cases are shown in  FIG. 8 . First, the prior art method of altering the halftone images stored in the byte format will be compared to the present invention method of altering the source image. Each dimension of the halftone image is larger than that of the source image by the factor k, and this is reflected in  FIG. 8 . The memory used by the halftone image in the byte format is km×kn bytes, whereas the memory used for the source image is only m×n bytes. Likewise, the more pixels there are in an image, the more calculations will be needed for performing the pixel altering processing. Therefore, the number of calculations needed for the halftone image in the byte format is km×kn, while the number of calculations needed for the source image is m×n. From the chart shown in  FIG. 8 , the benefit of the present invention becomes clear. The present invention takes advantage of the fact that the source image has a smaller resolution than the halftone images, and saves both memory and calculations by performing the pixel altering processing on the source image instead of the halftone images.  
         [0054]     For the halftone image stored in the bit format, less memory is needed to store pixel information than with the byte format. Assuming there are eight bits per byte, the bit format uses just one-eighth of the total memory that the byte format uses for storing halftone images. Unfortunately, this memory savings comes with a cost, and the number of calculations required is doubled. The number of pixels remains equal to km×kn, but an additional km×kn number of calculations is needed to handle the overhead of accessing individual bits in memory. Therefore, while it is possible that the prior art method using halftone images stored in bit format may actually save memory as compared to the present invention method, the number of calculations will be far greater.  
         [0055]     As compared to the prior art, the present invention method of altering pixels in the source image saves memory and calculations required for correcting intercolor bleeding. When the factor k is equal to one (when the halftone images have the same resolution as the source image), the present invention method uses the same amount of memory and number of calculations as the prior art method. However, for any values of k greater than 1, the present invention method is more efficient at performing the pixel altering process. Thus, the present invention takes advantage of the relatively smaller resolution of the source image to alter pixels before converting the source image into the halftone images.  
         [0056]     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.