Dither matrix generation method, device, and program

Abstract

To provide a dither matrix generation method, device, and program capable of improving granularity of both monochromatic printing and color printing.SOLUTION: The method for generating dither matrixes used in generating printing data of a plurality of colors by half-toning image data of the colors, generates a first dither matrix used in generating printing data of a first color and a second dither matrix used in generating printing data of a second color different from the first color so as to have correlation to each other.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method, device, and program for generating a dither matrix used in halftone processing. [Background technology]

[0002] 2. Description of the Related Art In conventional printing devices, such as inkjet printing devices that perform printing by ejecting ink from a plurality of arranged nozzles, a blue noise mask is often used to speed up halftone processing.

[0003] A blue noise mask is a dither matrix that removes the low-frequency components to which the human eye is sensitive, and is characterized by excellent dot dispersion and reduced graininess.

[0004] When printing with multiple ink colors, a separate blue noise mask is generally assigned to each color, but when printing a color image, dots can overlap randomly, resulting in increased graininess. In particular, when printing with four colors (CMYK), if C and M dots with similar brightness overlap, the brightness of the dots decreases, resulting in significant graininess.

[0005] Fig. 6 is a schematic diagram for explaining deterioration of graininess. Fig. 6A shows an example of a C dot pattern of a predetermined density, and Fig. 6B shows an M dot pattern of a predetermined density. Fig. 6C shows a dot pattern in which the C dot pattern shown in Fig. 6A and the M dot pattern shown in Fig. 6B are superimposed. Dots OD shown in Fig. 6C indicate dots in which C and M dots are superimposed. When separate blue noise masks are assigned to the C image data and the M image data, graininess deteriorates as shown in Fig. 6C.

[0006] Patent Document 1 proposes a method for reducing the graininess of a color image by determining the arrangement of threshold values ​​of the dither matrices assigned to each color so that the low-frequency components are reduced as a result of combining multiple dither matrices. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-50596 Summary of the Invention [Problem to be solved by the invention]

[0008] However, when a dither matrix is ​​generated with emphasis on the dispersion of the result of combining dot arrays of multiple colors, as in Patent Document 1, graininess during monochromatic printing may worsen compared to when dither matrices are generated independently for each color. FIG. 7 is a schematic diagram illustrating the deterioration of graininess during monochromatic printing using the method of Patent Document 1. FIG. 7A is a diagram illustrating an example of a case where a C dot pattern and an M dot pattern are generated and printed on top of each other using the method of Patent Document 1. The dot OD shown in FIG. 7A represents overlapping C and M dots, and graininess is good. FIG. 7B shows a dot pattern obtained when monochromatic C is printed using the method of Patent Document 1, and FIG. 7C shows a dot pattern obtained when monochromatic M is printed using the method of Patent Document 1. As shown in FIGS. 7B and 7C, there are more gaps (white pixels) between dots, resulting in worsened graininess.

[0009] In view of the above circumstances, an object of the present invention is to provide a dither matrix generation method, device, and program that can improve graininess in both monochrome printing and color printing. [Means for solving the problem]

[0010] The dither matrix generation method of the present invention is a method for generating a dither matrix used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, and generates a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that they are correlated. [Effects of the Invention]

[0011] According to the dither matrix generation method of the present invention, the first dither matrix used when generating print data for a first color and the second dither matrix used when generating print data for a second color are generated so as to have correlation, thereby improving graininess in both monochrome printing and color printing. [Brief explanation of the drawings]

[0012] [Figure 1] A flowchart illustrating an embodiment of a dither matrix generation method of the present invention. [Figure 2] 1. A flowchart illustrating an embodiment of a dither matrix generation method in step S10 of the flowchart shown in FIG. [Figure 3] Schematic diagram for explaining the improvement of graininess according to the present invention. [Figure 4] FIG. 1 is a block diagram showing a schematic configuration of an inkjet printing device that uses a dither matrix generated by an embodiment of the dither matrix generation method of the present invention. [Figure 5] FIG. 5 is a block diagram showing another embodiment of the image processing unit in the inkjet printing device shown in FIG. [Figure 6] Schematic diagram for explaining the deterioration of graininess during color printing using conventional technology [Figure 7] Schematic diagram for explaining the deterioration of graininess during monochrome printing using conventional technology DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the dither matrix generation method of the present invention will be described in detail with reference to the drawings. The dither matrix generation method of this embodiment is a method for generating a dither matrix used when generating print data for driving an inkjet head.

[0014] The dither matrix generation method of this embodiment is a method for generating a dither matrix used when generating multi-color print data for color printing by performing halftone processing on multi-color image data.

[0015] A dither matrix is used when converting multi-tone image data into dot presence / absence image data. A threshold value is assigned to each pixel of the dither matrix. The dither matrix is applied to the image data, and for the pixels at the matching coordinates, the tone value of the image data is compared with the threshold value of the matrix. When the tone value of the image data is larger, dots are formed at those pixels.

[0016] In this embodiment, both the tone value of the image data and the threshold value are 256 tones from 0 to 255 (the larger the value, the darker), and each threshold value is included in the dither matrix by the same number. For example, in a square dither matrix of 256×256 pixels, the threshold values from 0 to 255 are each included in 256 pixels.

[0017] The dither matrix generation method of this embodiment is a method for generating a dither matrix used when generating print data for four-color printing of C (cyan), M (magenta), Y (yellow), and K (black), and is characterized by the method for generating a dither matrix used when generating print data of C and M with the same brightness. In this embodiment, it is assumed that the brightness of the CMYK colors has the relationship of K << C ≒ M << Y. In this embodiment, C corresponds to the first color of the present invention, and M corresponds to the second color of the present invention.

[0018] In this embodiment, the C dither matrix and the M dither matrix are generated so that they have a correlation. Specifically, the M dither matrix is ​​generated using the dot pattern used when generating the C dither matrix, thereby providing a correlation between the C and M dither matrices. By providing a correlation between the C and M dither matrices in this way, it is possible to reduce overlap between C dots and M dots, thereby improving granularity. Note that while this embodiment generates dither matrices for C and M, the present invention may also be applied to generating dither matrices for other combinations of two colors. Furthermore, when performing color printing using multiple colors other than the four colors of C, M, and K, the present invention may also be applied to generating dither matrices for other combinations of two colors.

[0019] Next, a dither matrix generation method according to this embodiment will be described. Fig. 1 is a flowchart for explaining the dither matrix generation method according to this embodiment. Fig. 2 is a flowchart for explaining the dither matrix generation method of S10C in the figure. For ease of understanding, it is assumed here that a dither matrix of 256 pixels (16 pixels x 16 pixels) is generated.

[0020] In the dither matrix generation method of this embodiment, first, a dither matrix for C is generated (S10). For the dither matrix for C, a dither matrix for a blue noise mask is generated using a method similar to that of the related art, and this will be explained with reference to the flowchart shown in FIG.

[0021] First, a random dot pattern of 16 pixels x 16 pixels is generated (S30). At this time, a dot pattern corresponding to, for example, an intermediate density of 128 is generated. In other words, a dot pattern is generated in which 128 of the 256 pixels are "1" and the remaining 128 pixels are "0".

[0022] Next, a process is performed to remove low-frequency components from the initial random dot pattern generated in S30 (S32). Specifically, the initial random dot pattern is subjected to a discrete Fourier transform to convert it into spatial frequency components, and after passing through a high-pass filter, an inverse discrete Fourier transform is performed to exchange the position with the largest difference from the original dot pattern with the position with the smallest difference. This series of steps is repeated multiple times to remove the low-frequency components.

[0023] Next, using the dot pattern generated in S32, dot patterns of each density up to 127 are generated (S34). Specifically, the dot pattern generated in S32 is subjected to a discrete Fourier transform to convert it into spatial frequency components, and after passing through a high-pass filter, an inverse discrete Fourier transform is performed to delete the dot at the position where the difference from the original dot pattern is greatest. This series of steps is repeated multiple times to generate dot patterns of each density up to 127.

[0024] Next, using the dot pattern generated in S32, dot patterns of each density above 129 are generated (S36). Specifically, the dot pattern generated in S32 is subjected to a discrete Fourier transform to convert it into spatial frequency components, and after passing through a high-pass filter, an inverse discrete Fourier transform is performed to add a dot at the position where the difference from the original dot pattern is smallest. This series of steps is repeated multiple times to generate dot patterns of each density above 129.

[0025] Finally, the dot patterns of each density are combined to generate a dither matrix for C (S38). Combining dot patterns of each density involves adding the pixel values ​​of pixels at the same position. For example, if the pixel values ​​of pixels at the same position in two dot patterns of different densities are both "1," the pixel value at that position will be 1 + 1 = 2.

[0026] This concludes the description of the method for generating the C dither matrix.

[0027] Next, returning to the flowchart of Figure 1, a method for generating the M dither matrix will be described. As described above, a dot pattern of a predetermined density N generated when generating the C dither matrix is ​​extracted (S12). The predetermined density is preferably an intermediate density, for example, a density between 100 and 150. In this embodiment, a dot pattern of a density 128 is extracted and is called the first dot pattern.

[0028] Then, the density of the first dot pattern is inverted to generate a second dot pattern (corresponding to the inverted dot pattern of the present invention) (S14). The density of the second dot pattern is calculated by SN, where S is the maximum density. Specifically, if the maximum density is 255 and the density N of the first dot pattern is 128, the density of the second dot pattern is 255-128=127.

[0029] Next, using the second dot pattern generated in S14, dot patterns of each density equal to or less than 126 (0 to (SN-1)) are generated (S16). Specifically, by performing the same process as the process of S32 in FIG. 2 described above, dot patterns of each density equal to or less than 126 are generated.

[0030] Next, using the second dot pattern generated in S14, dot patterns of each density of 128 or more ((S-N+1) to S) are generated (S18). Specifically, by performing the same process as the process of S34 in FIG. 2 described above, dot patterns of each density of 128 or more are generated.

[0031] Finally, the dot patterns of each density are combined to generate a dither matrix of M (S20).

[0032] The dither matrix generation method of this embodiment has been described above.

[0033] According to the dither matrix generation method of this embodiment, the C dither matrix and the M dither matrix are generated so that they have a correlation, so that graininess can be improved in both monochrome printing and color printing.

[0034] Fig. 3 is a schematic diagram for explaining the improvement of graininess according to the present invention. Fig. 3A is a diagram showing an example of a C dot pattern generated by halftoning using the C dither matrix generated in the above embodiment, and Fig. 3B is a diagram showing an example of an M dot pattern generated by halftoning using the M dither matrix generated in the above embodiment. Fig. 3C is a diagram in which the C dot pattern shown in Fig. 3A and the M dot pattern shown in Fig. 3B are superimposed.

[0035] As shown in Figures 3A and 3B, the dither matrix generation method of this embodiment provides excellent dot dispersion and good granularity even in monochrome printing. Furthermore, as shown in Figure 3C, the dither matrix generation method of this embodiment provides excellent granularity in color printing because there is no overlap between C and M dots and white pixels are minimized.

[0036] Furthermore, in the dither matrix generation method of the above embodiment, the M dither matrix is ​​generated using the dot pattern used when generating the C dither matrix, so that the two dither matrices can be correlated using a simpler method.

[0037] Furthermore, in the dither matrix generation method of the above embodiment, a dot pattern of a predetermined density used when generating the C dither matrix is ​​inverted to generate an inverted dot pattern, dots are added to and deleted from the inverted dot pattern to generate dot patterns of other densities, and the dot patterns of each density are combined to generate a second dither matrix, thereby making it possible to further suppress overlapping of C dots and M dots.

[0038] Furthermore, in the dither matrix generation method of the above embodiment, the dot pattern of a predetermined density used when generating the C dither matrix is ​​inverted to generate an inverted dot pattern (second dot pattern), so that dot patterns of each density can be generated more appropriately.

[0039] Furthermore, in the dither matrix generation method of the above embodiment, dots are added and deleted from the inverted dot pattern (second dot pattern) in such a way that the low-frequency components are not increased, thereby making it possible to generate a dither matrix that can further suppress graininess.

[0040] Next, an inkjet printing apparatus that uses a dither matrix generated by the dither matrix generating method of the above embodiment will be described. Fig. 4 is a schematic diagram of an inkjet printing apparatus 1 of this embodiment.

[0041] The inkjet printing device 1 performs printing by ejecting ink onto a sheet-like printing medium such as paper or film based on image data output from a computer or image data output from a document reading device. As shown in Fig. 4, the inkjet printing device 1 includes an image processing unit 10, a head drive control unit 20, an inkjet head unit 30, a conveying unit 40, and a control unit 50.

[0042] Image processing unit 10 receives image data output from a computer or a document reading device and performs various processes on the image data. Image processing unit 10 includes a CPU (Central Processing Unit), a semiconductor memory, etc. The CPU and semiconductor memory of image processing unit 10 may be shared with a control unit 50 (described later), or may be provided separately.

[0043] The image processing unit 10 executes an image processing program stored in advance in a storage medium such as a semiconductor memory or a hard disk, and operates electric circuits to perform processing of each unit, which will be described later.

[0044] As shown in FIG. 4, the image processing unit 10 includes an image data receiving unit 11, a color conversion unit 12, and a halftone processing unit 13.

[0045] The image data receiving unit 11 receives image data in RGB format output from a computer or a document reading device, and outputs the image data to the color conversion unit 12 .

[0046] The color conversion unit 12 converts the image data in the RGB format into image data in the CMYK format, and outputs the converted image data to the halftone processing unit 13 .

[0047] The halftone processing unit 13 performs halftone processing on the image data of each of the C, M, Y, and K output from the color conversion unit 12 to generate binary data or multi-value data.

[0048] The halftone processing unit 13 of this embodiment performs halftone processing using a dither matrix, and a dither matrix generated by the dither matrix generation method described above is stored in advance. That is, the halftone processing unit 13 includes a dither matrix storage unit of the present invention. As a method for halftone processing, a known method is used, for example, by scanning the dither matrix with respect to the image data in the X and Y directions and assigning threshold values.

[0049] The image processing unit 10 is not limited to the above-mentioned processing, but also performs various other known image processing such as gamma correction processing and edge enhancement processing.

[0050] The head drive control unit 20 drives the inkjet head unit 30 based on the print data for each color generated by the halftone processing unit 13 to eject ink from each nozzle of the inkjet head for each color.

[0051] The inkjet head unit 30 is equipped with a plurality of inkjet heads that eject ink of each color of C, M, Y, and K. As described above, each inkjet head is controlled by the head drive control unit 20 based on the print data for that color to eject ink onto the print medium and form a print image on the print medium.

[0052] The transport unit 40 includes a transport mechanism that transports the print medium to the inkjet head unit 30 .

[0053] The control unit 50 includes a CPU, a semiconductor memory, and the like, and controls the entire inkjet printing apparatus 1. The control unit 50 controls the operation of each part of the inkjet printing apparatus 1 by executing a control program stored in advance in a storage medium such as a semiconductor memory or a hard disk, and by operating electric circuits.

[0054] Furthermore, although the inkjet printing device 1 is configured to store a dither matrix in advance in the halftone processing unit 13 of the image processing unit 10, the dither matrix may be generated in the image processing unit 10. Specifically, as shown in Fig. 5, the image processing unit 10 may be provided with a dither matrix generation unit 14 that generates a dither matrix as described above.

[0055] Furthermore, a dither matrix generating device may be configured by providing the dither matrix generating unit and dither matrix storage unit in a computer or the like different from the inkjet printing device 1. In this case, for example, the dither matrix generating program may be installed in a storage medium such as a semiconductor memory or a hard disk, and the dither matrix generating program may be executed by a CPU or the like of the computer, thereby causing the above-described dither matrix generating method to be executed by the computer.

[0056] Note that some or all of the functions executed by the dither matrix generation program may be configured using hardware such as an ASCI (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), or other electrical circuits.

[0057] Furthermore, the present invention is not limited to the above-described embodiments, and the components can be modified and embodied in practice without departing from the spirit of the invention. Furthermore, various inventions can be formed by appropriately combining the multiple components disclosed in the above-described embodiments. For example, all of the components shown in the embodiments can be appropriately combined. Naturally, various modifications and applications are possible within the scope of the invention.

[0058] The present invention further discloses the following supplementary notes.

[0059] (Appendix 1) The dither matrix generation method of the present invention is a method for generating a dither matrix used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, and generates a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that they are correlated.

[0060] (Appendix 2) In the dither matrix generating method described in Supplementary Note 1, the second dither matrix can be generated using the dot pattern used when generating the first dither matrix.

[0061] (Appendix 3) In the dither matrix generation method described in Appendix 2, a dot pattern of a predetermined density used when generating a first dither matrix is ​​inverted to generate an inverted dot pattern, dots are added to and deleted from the inverted dot pattern to generate dot patterns of other densities, and the dot patterns of each density are combined to generate a second dither matrix.

[0062] (Appendix 4) In the dither matrix generating method described in Supplementary Note 3, the predetermined density is preferably an intermediate density.

[0063] (Appendix 5) In the dither matrix generation method described in Supplementary Note 3 or 4, it is preferable that dots are added to and deleted from the inverted dot pattern in such a way that low frequency components do not increase.

[0064] (Appendix 6) The dither matrix generating device of the present invention is a dither matrix generating device used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, and is equipped with a dither matrix generating unit that generates a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that they are correlated, and a dither matrix storage unit that stores the first dither matrix and the second dither matrix.

[0065] (Appendix 7) The dither matrix generation program of the present invention is a dither matrix generation program used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, and causes a computer to execute a step of generating a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that they are correlated. [Explanation of symbols]

[0066] 1. Inkjet printing device 10 Image processing section 11 Image data reception section 12 Color conversion unit 13 Halftone processing section 14 Dither matrix generator 20 Head drive control unit 30 Inkjet head unit 40 Conveying section 50 control section C Cyan dot M Magenta dot OD Overlapping Dots

Claims

1. A method for generating a dither matrix used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, comprising: A dither matrix generation method for generating a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that they are correlated.

2. 2. The dither matrix generating method according to claim 1, wherein the second dither matrix is ​​generated using a dot pattern used in generating the first dither matrix.

3. 2. A dither matrix generating method according to claim 1, wherein a dot pattern of a predetermined density used in generating the first dither matrix is ​​inverted to generate an inverted dot pattern, dots are added to and deleted from the inverted dot pattern to generate dot patterns of other densities, and the dot patterns of each density are synthesized to generate the second dither matrix.

4. 4. The dither matrix generating method according to claim 3, wherein said predetermined density is an intermediate density.

5. 4. The dither matrix generating method according to claim 3, wherein dots are added to and deleted from said reversal dot pattern so as not to increase low frequency components.

6. A dither matrix generating device used when generating print data of multiple colors by performing halftone processing on image data of multiple colors, a dither matrix generating unit that generates a first dither matrix used when generating print data of a first color and a second dither matrix used when generating print data of a second color different from the first color so that the first dither matrix and the second dither matrix have a correlation; a dither matrix generating device comprising a dither matrix storage unit that stores the first dither matrix and the second dither matrix;

7. A dither matrix generation program used when generating multi-color print data by performing halftone processing on multi-color image data, A dither matrix creation program that causes a computer to execute a step of generating a first dither matrix used when generating print data for a first color and a second dither matrix used when generating print data for a second color different from the first color so that they are correlated.

Images