Method and system for enhancing the quality of both color and black and white images produced by ink jet printers.

Description: TECHNICAL FIELD [Detailed description of the invention]

(Industrial application field) The present invention generally relates to recording color and black and white images using digital image processing technology. In particular, the present invention relates to methods and system improvements for improving the quality of such images using state-of-the-art inkjet printers and grayscale technology, ie halftone technology. (Conventional technology) Over the last few years, it has become possible to convert both color and black-and-white images scanned using various image sensors into output ink recordings from various printers such as inkjet printers. An example of such an image conversion system is disclosed in US Pat. No. 4,672,432, which was transferred to Canon Japan, which produces red, blue, and green color pictures obtained by a scanner in cyan and yellow from an inkjet printer. , And can be converted to magenta color output. The content of this US patent is mentioned here for reference. More recently, important developments have been made in the technology of thermal inkjet color printing. As an example of such development, the applicant, Hewlett-Packard Company, introduced a multi-color thermal inkjet printer sold under the trademark "Paint Jet" in 1987. This printer is described in detail in the August 1988 issue of the Hewlett-Packard Journal, Vol. 39, No. 3, which is referred to here for reference. Many trade magazines have recently introduced other new color inkjet printers available, and one example of these publications is the May 31, 1988 Personal Computing Magazine. ) , P. 168 et seq., There is a paper entitled New Printers Banish Blach and White by Mr. WL Rosch. While these color inkjet printers are generally well received and work well in most respects, the images recorded by these printers can compromise the quality of the recorded images. Some showed some unwanted color contrast, i.e. "graininess". There are two ways to reduce this "graininess", or unwanted color contrast. One method utilizes a resolution with a large number of dots per inch (dpi) and the other utilizes grayscale technology. Some of the present inventions fall into the latter category. (Problems to be solved by the invention) Therefore, it is a novel and improved object of the present invention that serves to reduce such unwanted color contrast, or "graininess", of images recorded by inkjet printers, thereby improving the quality of such recorded images. To provide methods and systems. Another object of the present invention is a new and improved method of description type capable of improving the quality of such images without sacrificing resolution and without requiring high resolution. To provide the system. If the resolution is increased, the printing time becomes slower, and it is necessary to increase the nozzles or increase the printing frequency to operate the nozzles, which is inconvenient. Another object of the present invention is a new and improved type of description that can be easily and economically adapted for use in such printers without increasing the dot print density of modern thermal inkjet printers. To provide methods and systems. Another object of the present invention is a novel and improved type described above that can achieve the above object and at the same time minimize the amount of paper wrinkles generated during inkjet printing. To provide methods and systems. (Means to solve the problem) A feature of the present invention is to provide a unique error diffusion and pixel allocation grayscale stage for an electronic system that converts a color image into a hard copy output. The grayscale stage includes, among other things, means for controlling the ejection of ink onto the print medium according to a pixel address sequence controlled by the grayscale numerical values to which the image information is assigned. Another feature of this image conversion system is the ability to adapt ink loaded drops to the print area selected to receive the drops so as to enhance the quality of the print by the inkjet print operation. .. The system includes scanning means for scanning an image to generate digital data representing the image, means for connecting to the scanning means to convert the digital data into grayscale digital information, and the conversion. The digital information is processed by being connected to the means, and the volume of the ink-containing droplets is the maximum allowable volume of the ink droplets of the preselected ink V._maxMeans are included so that it does not exceed. The image conversion system described above is also connected to generate cyan, yellow, magenta, and black pixel information to receive the pixel information and is gray based on the count of ink droplets and the amount of dye added. It features means for assigning scale droplet counts to its pixel information and means for connecting to this assigning means to reduce said droplet counts to a desired lower level number for grayscale. I have. The image conversion system described above also re-reads according to the scan line consisting of colors such as black, cyan, and magenta with high pigmentation, then moderate coloration, and finally low pigmentation. It features means for printing super pixels. In this process, the ink printed with respect to the sequentially printed pixels is distributed to the print medium to ensure the best print quality and minimum color contrast. The above objectives and other features and advantages of the present invention are made possible, among other things, by adding control to the count of ink droplets ejected onto addressable pixels and the corresponding selected amount of dye added. These pixels are defined by the position of multiple rows and columns in the selected grayscale matrix. The count of the droplets and the amount of the corresponding dye added represent digital grayscale information generated by scanning the selected image, and this information is used to use the ink droplets used in the inkjet printer. The count and dye addition amount are selected. The volume of ink droplets for each droplet count and dye addition is calculated, and the sum of the measured ink droplet volumes is the maximum allowable ink droplet volume (predetermined to be received by the selected print medium). V_max) Is compared. The volume of the selected ink droplets is V for a given area of the print medium_maxWrinkles on the paper will be minimized if it does not exceed. These comparisons result in a difference in ink volume or error signal, which is then used to minimize the color contrast at each level of grayscale printed. The count and dye addition amount are selected. V for different types of print media and different inks printed on them to minimize paper wrinkles_maxNeed to be controlled. This is a necessary condition when too much ink is received in a given area, causing uneven stretching or shrinkage of the paper, resulting in a grainy and inhomogeneous paper. However, the present invention is V_maxIt serves to maximize the count of raw droplets within this limit, and thus maximize the count of droplets within a given print surface area, for example 2 x 2 pixels. In addition, this feature has the effect of minimizing color contrast in the printed image. In the preferred embodiment of the present invention, generating pixel information of cyan (C), yellow (Y), magenta (M), and black (K), counting ink droplets, and adding ink droplet dye. New methods are available for minimizing color contrast in print images, including assigning a quantity-based grayscale number to the pixel information. This assigned grayscale number is then a predetermined level of grayscale, depending on the measurement of the number of droplets of ink and the amount of dye added that represent the scanned pixel information of C, Y, M, and K. It is reduced to the number of lower digitals selected in, or the number of lower digitals in the lower level grayscale. Systems and methods according to the invention further include means of assigning each digit in a final adjusted grayscale number to a larger pixel, i.e., a selected pixel within a superpixel. The particular pixel selection process is based on an adjusted number of grayscales. (Example) With reference to Fig. 1, it is now possible to use the scanner 10 to convert a color image to digital grayscale data and then add it to the conversion stage 12 in red-green-blue (RGB) format. There is. As shown in the figure, the RGB output data from the format conversion stage 12 is added to the color conversion stage 14 of cyan-yellow-magenta (CYM), which is the primary color of the subtractive color mixture, by a well-known method. This conversion stage involves removing 100% of the undercolor to obtain black. The chromatic color (black) cannot be easily created by mixing the YMC ink colors, and the ink consumption will increase in the case of such mixing. Therefore, it is desirable to substitute pure black (K) for black made with YMC color. This pure black substitution and generation is known in the art as undercolor correction or undercolor removal (UCR). The use of 100% undercolor removal is intended to minimize ink consumption and improve resolution. The 100% UCR relational expression for colors C, M, Y, and K is K = min (C, M, Y), where new C = CK, new M = MK, new Y = YK. The output of the CYM color conversion stage 14 is a digital data stream added to the error diffusion and pixel allocation stages 16, which further drives the color printer 18, preferably a thermal inkjet color printer. The general functional configuration of the image scanning and playback system of FIG. 1 is generally well known in image processing technology. Regarding the image processing operations and capabilities of such a system, for example, "Introduction to Image" by Mr. BM Dawson published in the March 1987 issue of "BYTE Magazine" on page 169 and after. Introduction To Image Processing It is described in a treatise entitled Algorithems). The first edition of these image processing capabilities and the corresponding system operation was published in December 1986, and Hewlett Packard, Inc., located in Palo Alto, California, USA, owns the copyright. Available in a publication entitled "Designer's Guide to Raster Image Printer Algorithms" by Gary Dispoto et al. These recent publications. For references, both are referred to here for reference. Error diffusion and pixel allocation processing in grayscale image processing operations are generally well known in the art. Error diffusion is a technique used to spread the error between the printable grayscale and the grayscale of the input image data across adjacent pixels. This error diffusion is often performed using a selected one of many well-known algorithms, such as those described in the Dawson paper above. For example, perform this error diffusion using two algorithms well known in the art: either the Floyd and Steinberg 4-point algorithms and the Stuck 12-point algorithm. Can be done. In 1976, "Proceedings of the Society of information" by Robert W. Floyd and Lewis Steinberg. In Volume 17/2 of Display), I published a paper entitled "An Adaptive Algorithm For Special Gray Scale", which included "Error Diffusion". ) There is a description of the algorithm. Also, as detailed in Floyd and Steinberg's papers as described above, error diffusion is the actual grayscale of the pixel values read, followed by the rows and columns of the assigned reference graytable (i). , J) is a technique used to spread the difference from the grayscale to the surrounding pixels of the assigned grayscale pixels. This error diffusion is performed so as to minimize the bleeding of the reproduced image. The above Floyd and Steinberg treatises are referred to here for reference. Other error diffusion methods are found in US Pat. No. 4,680,645, which was assigned to the Applicant and issued to Disport et al., Referenced herein for reference. Figure 2 shows a 16-level gray table, which is based on 2x2 superpixels and is high, medium, and low for each 3 digit grayscale number. It is based on the amount of dye added, and is weighted so that the density ratio of reflectance is 4: 2: 1 as shown in the figure. Therefore, the digit on the left side of each grayscale number in each matrix of the 16-level gray table in Fig. 2 indicates that the amount of dye added for each ink droplet ejected to the pixel is large (H), and each has three. The digit in the center of the grayscale number consisting of digits indicates that the amount of dye added is medium (M), and the digit on the right side of each number indicates that the amount of dye added is small (L). These addition amounts H, M, and L are each selected so that the reflectance ratio on the printed page is 4: 2: 1, as will be apparent to those skilled in the art. The numbers 1, 2, 3 and 4 shown in parentheses in FIG. 2 represent the count of ink droplets, that is, the number of ink droplets for a particular level of dye addition. The combination of this droplet count and the amount of dye added will result in a particular grayscale level assigned to the pixel values actually read by the scanner 10 in FIG. This assigned grayscale value is the number of grayscales in the gray table of FIG. 2 that is closest to the pixel value actually read. The blank parts of the table shown in FIG. 2 without the allocation of the count number of droplets are all (0, 0, 0). Therefore, the number of grayscales consisting of the three digits shown in FIG. 2 is one with any of the values 1, 2, 3, or 4 corresponding to the droplet counts available within the 16 grayscale levels. It consists only of the number of grayscales in a 16-level gray table with the above positive digits. There are practically 256 levels of full grayscale used in the practice of the present invention. This number of 256 is a convenient even number in digital signal processing technology, resulting in a much higher grayscale resolution than is normally required. Only about 50 grayscales between maximum white and maximum black can be resolved by the human eye. Also, the number 256 is a common level of resolution for the scanner 10. Therefore, each of the 16 levels in Figure 2 represents one-sixteenth of the total 256 levels of grayscale. Therefore, the notation (0, 0, 1) at level 1 in the table of FIG. 2 is that the only droplet count and dye addition amount that can be used to achieve level 1 in the gray table is the dye addition amount. Low (L), that is, one drop with a weight of 1, that is, one small drop count. Similarly, the level 2 grayscale is 1 drop (0, 1, 0) with a medium amount of dye added (M) and 2 drops (L) with a low amount of dye added, as shown in level 2 of the table. This can be achieved by using any of (0, 0, 2), and the table can be looked at in the same manner below. However, as will be apparent to those skilled in the art, many other gray levels are available in place of the 16 levels shown in Figure 2. For example, a 32-level gray table is particularly suitable for practicing the present invention, and in the case of the configuration shown in FIG. 2B, it will be used in combination with a reflectance ratio of 8: 3: 1. .. As can be seen from Figure 2A, level 15 of the table lists the exact number of matching grayscales that can be used to achieve the exact level 15, or 15/16, out of a total of 256 levels of grayscale. Absent. Level 15 can be achieved by using the combination of level 14 and error diffusion. The selection of the number of grace cases in FIGS. 2A and 2B for determining and controlling the count of available droplets and the amount of dye added is better understood from the following description of FIGS. 3 and 4. You can do it. With reference to FIGS. 3A and 3B, the data processing system shown in these two related diagrams provides a detailed explanation of the error diffusion and the function of the pixel allocation stage 16 in FIG. This data processing system is composed of several functional blocks including explanations to help the reader's understanding of the present invention. These functional blocks are actually separate means of the computer and are sometimes referred to instead as "stages" or "test stages" when performing YES / NO / tests on specific data in their own right. The test stages in this system are represented by diamond-shaped functional blocks, while the other operating stages that perform specific functional operations based on input data are rectangular. However, to facilitate the description of this data processing system and its two field backloops, each of the functional blocks or stages will now be referred to as a "step" and is generally based on the data manipulated in each of the electronic stages. Represents a functional step to be performed. As shown in step 20, black, cyan, magenta, and yellow pixel values are read by the scanner 10 in FIG. 1 and have rows and columns (i, j) in a 256-level grayscale, respectively. Converted to Y, M, and K digital data. As shown in step 22, for each of the black, cyan, magenta, and yellow gray levels from step 20, the corresponding closest grayscale number in the gray table of Figure 2 is assigned to the actual gray level. The difference from the assigned gray level is spread to the surrounding pixels by the error spread in step 24, as described above. In step 26, for black, cyan, magenta, and yellow, calculate the sum of the drop counts for the minimum number of drop counts available in the 16-level gray table in Figure 2, and step 28 in Figure 3B. In, the sum of these minimum droplet counts is the predetermined maximum acceptable droplet count V, as described above._maxIs compared with. The minimum droplet count summed in step 28 is V_maxIf it exceeds, the drop count is the largest of the drop counts in the groups for colors C, Y, M, and K (D)._max1) The color plane is identified in step 30, and in step 32, the droplet count is decremented to the next lowest value in grayscale, the newly assigned grayscale number and the actual gray scale to be read. The difference from the number of scales is used to perform the corresponding error diffusion in step 34. Then, in step 36, a new total drop count update is made, and then in step 38, V again._maxIs compared with. Here, the total droplet count in step 38 is V._maxIf it is less than, the corresponding accurate digital data is generated in the line 40, which is used to drive the color inkjet printer 18 in FIG. Total drop count in step 38 is still V_maxIf the value is exceeded, the feedback signal generated in the feedback loop 42 is used to re-identify the color plane of the next largest drop count of C, Y, M, or K in step 30, loops 30, 32. , 34, 36, 38, and 42 will be repeated. Here, the total drop count in step 28 is V_maxIf less than, step 44 generates the sum of all maximum droplet counts in the gray table of FIG. 2 for each of the pixel values assigned to K, C, M, and Y. The sum of all K, C, M, and Y droplet counts is then V in step 46._maxIs compared with. The sum of all these drop counts is V_maxIf less than, a signal is generated on line 48 and used to drive the color printer, eliminating the need for further signal processing. However, the sum of all maximum drop counts in step 46 is V_maxIf this is exceeded, the signal output from step 46 to line 50 is added to step 52 for each of the previously summed K, C, M, and Y droplet count maximums, and then Large individual drop count D_max2It is used to identify the color plane of. Therefore, according to the determination in step 54, if there is a grayscale number with the same gray level and a smaller droplet count in the table of FIG. 2, a signal sent from step 54 to line 56 is generated, and in step 58, It is used to reduce the droplet count by advancing to the next lowest maximum number of droplet counts within the same gray level in the gray table of FIG. If the determination in step 54 does not have a lower drop count for the same level of grayscale, then step 5 adds a signal to line 60 to step 62, using said signal in step 62. The drop count is reduced by reducing the gray table level to the next lowest gray level, and the new maximum drop count is selected. Next, the signal on line 64 is added to step 66, error diffusion is performed in step 66, and an error diffusion signal is generated on line 68. The signal of line 68 is added to the next step 70, in which step 70 the total maximum droplet count is updated and fed to the output comparator 72. In this output comparator 72, the updated total drop count is V again._maxCompared to, the newly updated droplet count is still V at this point_maxIt is confirmed whether it is as follows. V_maxIf the following, a signal for driving the color printer is generated on the line 74. However, in step 72, the total drop count is still V._maxIf this is exceeded, the feedback signal on line 76 is returned to the input in step 52 via line 50 for each of the grayscale numbers of the maximum droplet counts of the groups K, C, M, and Y. , Re-identify the color plane of the next highest droplet count at that time. The code below in Table 1 is called "pseudo code" in image processing technology and is a statement for all of the various steps described in relation to Figures 3A and 3B.<img file="JP2980331B2_D0001.tif" /><img file="JP2980331B2_D0002.tif" /><img file="JP2980331B2_D0003.tif" /> An example of the minimum and maximum droplet counts and initial grayscale assignments for C, Y, M, and K above is as follows. The: 256 level grayscale read pixel values are assumed to be K = 20, C = 50, M = 35, and Y = 0. Converting these values to 16 levels of grayscale gives K = 1+ (remainder 4), C = 3 + (remainder 2), M = 2 + (remainder 3), and Y = 0. The above remainder is diffused to surrounding pixels by error diffusion and using well-known Floyd and Steinberg algorithms. This error diffusion is performed on the pixels surrounding the selected specific pixels in the gray levels 1, 3, and 2. According to the table in Figure 2, the minimum drop count for the above 1, 3, 2, and 0 levels in grayscale is Kg._min= 1, Cy_min= 2, Mg_min= 1 and Yg_min= 0, that is, the sum of the minimum droplet counts is 4. The maximum drop count for these same levels in grayscale is Kg_max= 1, Cg_max= 3, Mg_max= 2, and Yg_max= 0, that is, the total maximum droplet count is 6. Therefore, if the maximum acceptable droplet count for each 2x2 superpixel is 8, then the minimum (4) and maximum (6) droplet counts above are both V._maxLess than, then the measurement of the total maximum droplet count in step 46 of FIG. 3B produces an output signal on line 48 for direct control of the color printer 18. Another example of C, Y, M, and K drop counts assumes K = 70, C = 75, M = 35, and Y = 0. Converting these values to 16 levels of grayscale, Kg = 4 has a remainder of Ke = 6; Cg = 4 has a remainder of Ke = 11; Mg = 2 has a remainder of Me = 3; and Yg = 0 has a remainder of Ye = 0. Become. According to the table in Fig. 2, the above Kg_min, Cg_min, Mg_min, And Yg_minThe minimum value of the droplet count for the value of is 1 + 1 + 1 + 0 = 3, while the above Kg_max, Cg_max, Mg_max, And Yg_maxThe maximum value of the droplet count for the value of is 4 + 4 + 2 + 0 = 10. Therefore, V_maxWhen = 8, the signal with the minimum droplet count output in step 26 is V._maxThe output signal of the minimum value of the droplet count is not added from step 28 to step 30, and step 44 is activated. However, the maximum drop count of 10 is V_maxSince it exceeds 8, the activated step 44 described above initiates the maximum droplet counting function described above in connection with FIG. 3B. This operation activates test step 46 and drops count 10 is V._maxIt is asked whether it exceeds 8. Since the answer is "YES", step 52 is activated to identify the color plane of the largest droplet count in this grayscale read with K = 4, i.e. the input data to be added to step 54. Will be done. Since there is a smaller drop count at the same grayscale level as the level for K = 4, and that drop count is K = 3, as shown in the 3/4 column of level 4 in Figure 2A, A "YES" signal is generated on line 56 and added to step 58 to reduce the drop count to K = 3. Step 58 will reduce or decrement the sum of all new drop counts to 9 instead of 10. An update of the sum of the new droplet counts is performed in step 70. At this point, the total sum of 9 for the new drop count is still V_maxFor the question of whether it is greater than 8, the answer is "YES" shown in step 72, so the feedback signal at line 76 is added to step 52, then the color plane with the highest drop count is identified. To. As the numbers in this example show, the answer is C = 4, and this information is added to step 54, using C = 4 to the same gray level as for C = 4 in Figure 2A, C. That is, it is determined whether or not there is a smaller droplet count for cyan. Since the answer is "YES", the signal generated on line 56 is added again to step 58, C = 4 is reduced to C = 3, and it is added to step 70. At step 70, a new sum of all droplet counts of 8 is generated, which is added to step 72 and compared to another new droplet count. New total drop count 8 is V_maxSince it does not exceed 8, a "NO" signal will be generated on line 74 and will be added to the color printer 18 in real time or stored in the printer's storage file for subsequent use. In addition, for the third example of C, Y, M, and K drop counts, the pixel readings are assumed as follows: : K = 85, C = 85, M = 85, Y = 0. Converting these values to 16 levels of grayscale gives Kg = 5 + Ke = 5; Cg = 5 + Ce = 5; Me = 5 + Me = 5; and Yg = 0, Ye = 0. From the table in Figure 2, Kg_min, Cg_min, Mg_min, And Yg_minMinimum value of droplet count = 2 + 2 + 2 + 0 = 6, Kg_max, Cg_max, Mg_max, And Yg_maxFor each of, the maximum drop count = 3 + 3 + 3 + 0 = 9. In addition, for this particular example, V_maxAssuming = 5, the sum of the minimum droplet counts is V_maxA "YES" output signal is generated on line 29 and added to step 30, which activates 32, 34, 36, 38 and feedback loop 42, which form subsequent steps as described above. To. In this loop including steps 30, 32, 34, 36, 38, and feedback line 42, the minimum droplet count selected from the gray table in FIG. 2 is the YES / NO test step 38, as described above. Will be reduced until a "NO" signal is generated and sent to the output line 40. Using a multi-drop color imaging system, such as that described in US Pat. No. 4,680,645, issued to Disport et al., Which ejects droplets on top of single-pixel droplets. At this point, the drop counting and dye addition selection process is complete. However, when a droplet is ejected onto a superpixel, such as a 2x2 superpixel, the droplet is ejected sequentially onto the superpixel, so that the ejected droplet is a quadrant of the desired pixel. Must be assigned to. This operation is described below in relation to FIG. In Fig. 4, there are three lines consisting of "high (H)", "medium (M)", and "low (L)" of the amount of pigment added, and "black", "cyan", "magenta", and "yellow" for this. A matrix of 12 color planes for 2x2 superpixels consisting of 4 columns of color is shown. These 12 superpixels are shown in FIG. 4 as being physically separated, but as will be apparent to those skilled in the art, these superpixels are all from each other during the operation of the print operation described below. They overlap and therefore actually occupy the same space on the print medium. Therefore, the physical separation of these 12 superpixels in Figure 4 is for illustration purposes only. These superpixels are addressed sequentially from left to right, starting at the top or line H, then descending vertically, and progressing to lines M and then L, as is normal for the printed surface area. .. These 12 superpixels represent a time-controlled sequential dot print sequence to minimize color contrast on the printed page. The black, cyan, magenta, and yellow colors are arbitrarily assigned the specified digits 0, 1, 2, and 3, respectively, while the dye additions are high (H), medium (M), and low ( Weighted units 0, 1, and 2 are assigned to the level of L), respectively. Therefore, as the matrix of Figure 4 is printed sequentially from left to right and then scanned vertically downward, the pixels that are individually assigned to receive the sequential droplets of ink move clockwise from pixel to pixel. It is constantly changing. Thus, each pixel printed sequentially (according to its assigned grayscale droplet count) is cycled through C, Y, M, and black colors, in order of high, medium, and low pigment additions. Will be received. The injection sequence of the color printhead of the inkjet printer under control is constantly controlled to print on the next adjacent quadrant of each superpixel, as each superpixel is printed in sequence. As this clockwise (or counterclockwise) clock movement continues, the position of each individual pixel to be printed next rotates with respect to the next superpixel quadrant, thus being scanned 2x. Until all of the two superpixels are printed sequentially, each droplet of ink that is printed sequentially will rotate to the next quadrant next to each superpixel. In this way, the ink is distributed over the entire superpixel matrix in FIG. 4 in a very uniform manner, and this operation minimizes the ink volume per unit print area for the inkjet printing process. In addition, this action minimizes both paper wrinkles and the color contrast or graininess of the printed hardcopy output from the inkjet printer at the same time. For these operations, the color printer or printer storage file 18 is grayscale with final adjustments made from one of the input lines 40, 48, or 74 connected to it, as shown in Figure 3B. It becomes clearer by referring to an example of scale information. As shown in the example of FIG. 4, the "multi" column of the K, C, M, and Y data is "2, 2, 1, 0", and the "medium" of the K, C, M, and Y data. Suppose the column is "2, 2, 0, 1" and the "small" column of the K, C, M, and Y data is "0, 0, 2, 0". Using this grayscale K, C, M, and Y information, the pixel rotation and scanning operations proceed as follows: Superpixel 80 by K = 2 in the "high" dye column. Droplets are ejected onto the quarter circles 76 and 78 of. Then rotate clockwise and C = 2 in the "high" dye column ejects droplets onto the next superpixel 86 and then clockwise adjacent quadrants 82 and 84. Will be done. For M = 1 in the "multi" row, droplets are ejected onto the adjacent quadrant 88 of the next adjacent superpixel 90. Furthermore, for Y = 0, the next superpixel 92 will be skipped. Next, the dye addition amount is addressed in the "medium" column, and K = 2 in that column ejects droplets into the quadrants 94 and 96 of superpixel 98; C = 2 of the next superpixel 104. The droplets are then ejected into the clockwise quadrants 100 and 102; M = 0 skips the next superpixel 106; then Y = 1 ejects droplets into the superpixel 110 quadrant. Be jetted. Subsequently, K = 0 and C = 0 in the "small" column skip the superpixels 112 and 114, respectively; M = 2 in the "small" column causes a quadrant adjacent to the clockwise of the next superpixel 120. Droplets are ejected on 116 and 118, and Y = 0 in the "small" column skips the last pixel 112 in the matrix. Further, the above operation is repeated for the next received grayscale data applied to the printer 18. The above pixel selection process described in connection with FIG. 4 is defined in code format by the following pseudocode for 2x2 superpixel allocation, which defines the data structure, as shown in the table below.<img file="JP2980331B2_D0004.tif" /> The dot allocation method for 2x2 superpixels assigns dot counts from black to cyan, magenta, and yellow in the highest gray-level plane sequence. This dot allocation scheme is repeated for medium and low gray levels as described in the example above. Various modifications can be made to the above embodiments without departing from the scope of the present invention. For example, according to the teachings above, different dye addition ratios of 8: 3: 1 can be used for 32 levels of grayscale and 2x2 superpixels. Furthermore, the present invention can also be used for small drop volumes of different sizes or in multi-drop formatting with only a single pixel print area. However, in the case of a multi-drop printing process, it is necessary to increase the printing frequency as compared with the frequency required in the present invention. Further, the present invention is not limited to use by a thermal inkjet printer, and can be used in various other types of inkjet printers such as heat transfer type or piezoelectric type inkjet printers. The present invention is also Ross Earl Allen (Ross) Of various ink droplet volumes (eg, 160, 80, and 40 picolitres), as described in US Pat. No. 4,746,935, issued to R. Allen) and assigned to the same person. It can also be modified by one of ordinary skill in the art to suit its use. Appropriate modifications of the data processing methodologies shown in Figures 3A and 3B can be used to adapt to the use of multiple droplet sizes with or without multiple gray levels. is there. The priority of selecting the maximum droplet count from the colors (C, Y, M, K) is modified only for the color (C, M, Y) in this set, and is black, that is, the plane of K. It is an advantage that the information of can be left as it is. This is because the black plane contains most of the resolution information, which is the case with 100% UCR where the chromaticity change in the printed image caused by the error diffusion of the color plane is negligible. In addition, visual cognition is more sensitive to resolution than color information. Finally, as a practical matter, the grayscale of yellow is the least important, so it is possible to have yellow as a single dye level from both a hardware and software perspective. (effect) As described above, according to the present invention, the undesired color contrast, or "graininess", of an image recorded by an inkjet printer can be reduced, thereby improving the quality of the recorded image. Further according to the present invention, without sacrificing resolution and without requiring high resolution, thus without increasing the printing time, increasing the number of nozzles, and increasing the printing frequency of the image. The quality can be improved. Further according to the present invention, there is provided an easy, economical, new and improved method and system that can be used in a thermal inkjet printer without increasing the dot print density of the state-of-the-art thermal inkjet printer. Furthermore, the present invention provides new and improved methods and systems capable of minimizing the amount of paper wrinkles that occur during inkjet printing.

[Simple explanation of drawings]

FIG. 1 is a functional block diagram of an image conversion system based on the present invention; Figure 2A is a table showing 16 levels of grayscale based on 2x2 superpixels; Figure 2B is a table showing 32 levels of grayscale, also based on 2x2 superpixels; 3A and 3B are flowcharts showing the data processing methodology and structure used to control the error diffusion and pixel allocation operations shown in FIG. 1, with FIG. 3A showing the first half and FIG. 3B. The figure shows the latter half; Figure 4 is a 3 column, 4 row matrix of 2x2 superpixels to illustrate the ink droplets assigned to each pixel, based on the digit values of each assigned grayscale digital number. 10 ...... Scanner, 12 ...... RGB format conversion stage, 14 ...... CYM color conversion stage, 16 ...... Error diffusion and pixel allocation gradation stage, 18 ...... Color printer