JP2010120185A - Image processing apparatus and image processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing apparatus capable of reducing uneven glossiness on the whole surface of a recording medium and achieving high image quality, and image processing method. <P>SOLUTION: In the case of performing image-processing for an image forming means that forms an image by scanning a recording head including a plurality of recording elements on a recording medium, image data is input, and the recording amount of each of the recording elements with respect to the image data is calculated for each main scanning by the recording head, and an in-scanning pattern is calculated by performing half-tone processing in accordance with the calculated recording amount. Upon the half-tone processing, when the recording amount is larger than a predetermined threshold, dots are arranged on the recording medium in a concentrated manner. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image processing apparatus and an image processing method for an image forming unit including a plurality of recording elements capable of forming dots.

  For example, as an information output device in a word processor, personal computer, facsimile, or the like, there are various recording devices that record information such as desired characters and images on a sheet-like recording medium such as paper and film. Among them, a recording apparatus that forms characters and images on a recording medium by attaching a recording agent to the recording medium has been put into practical use.

  An ink jet recording apparatus, which is one of such recording apparatuses, integrates a plurality of ink discharge ports (nozzles) that can discharge ink of the same color and the same density in order to improve recording speed and improve image quality. Some use an array of nozzles.

  As ink ejected from these nozzle groups, conventionally, dye ink using a dye that is easily dissolved in water as a coloring material has been widely used. The dye ink contains water as a main component, and the color material dissolved in the solvent easily penetrates into the fiber of the recording medium. Accordingly, since the surface shape of the recording medium is easily maintained even after the image is recorded, the gloss of the recorded image is maintained as that of the recording medium itself. For example, if recording is performed using a dye ink on a recording medium having excellent gloss, a recorded image having excellent gloss can be obtained. That is, in an ink jet recording apparatus using dye ink, it is possible to provide gloss of an image by improving the gloss of the recording medium itself.

  However, it is difficult to obtain good light resistance with the dye ink as described above. Since the dye molecules of the color material are easily decomposed by light, the color of the recorded image is faded. Further, the printed matter recorded with the dye ink has low water resistance, and when wetted with water, the dye molecules that have penetrated into the fiber dissolve in the water, so that bleeding tends to occur on the recorded image.

  As a method for solving such a problem inherent to dye ink, there is an ink jet recording apparatus for recording using a pigment ink using a pigment as a color material. Pigment inks exist as particles having a size of several tens of nanometers to several microns in a solvent, and unlike dye inks that exist as molecules, they do not easily penetrate into a recording medium. Accordingly, the water resistance and light resistance are higher than those of the dye ink.

  However, in the pigment ink, uneven glossiness that glossiness varies depending on the density and color of the recorded image depending on the region is likely to occur in the recorded image. One of the causes of uneven glossiness is the property of the pigment ink that the pigment ink is deposited on the recording medium, and the fine shape of the image surface is different between the recorded area and the unrecorded area. Furthermore, the difference in reflectance between the pigment ink and the recording medium is also a cause of uneven gloss. Since the amount of pigment ink applied varies depending on the density and color of the recorded image, the area where the pigment ink covers the recording medium differs, and the difference in the covered area results in a difference in gloss.

  When gloss unevenness occurs due to the above-described causes, a glare area and a matte area are mixed in the same recorded image, and are easily recognized as image defects.

  In addition, gloss unevenness also occurs in an electrophotographic recording apparatus in which toner is fixed on a recording medium and recorded, as in an inkjet recording apparatus using pigment ink.

  As a method for solving the uneven glossiness, there is a method of recording with a colorless transparent ink or a white ink in a recording area not covered with a colored pigment ink (Patent Document 1).

  In addition, a method of recording with a light gray ink, which is an achromatic color and lighter than black, on a recording region not covered with colored pigment ink has been proposed (Patent Document 2).

  Furthermore, the surface of the recorded image is coated (laminated) with a thermoplastic resin to achieve a uniform gloss on the entire surface of the recorded image (Patent Documents 1 and 3), and recording is performed with fine irregularities on the surface of the recorded image. There is also a method (Patent Document 1) that realizes uniform gloss over the entire image surface.

  On the other hand, in an inkjet recording apparatus, the quality of an image to be recorded tends to depend on the performance of the recording head alone. For example, slight errors that occur in the manufacturing process of the recording head, such as variations in the discharge port of the recording head and the discharge heater and piezo elements, affect the discharge amount and direction of the discharged ink. Finally, density unevenness occurs in the formed image, which may cause the image quality to deteriorate.

  Therefore, a multi-pass printing method is known as a countermeasure against such density unevenness (Patent Document 4). According to such a technique, by combining image processing and print control, it is possible to form an image at high speed while suppressing deterioration in image quality due to white streaks and uneven density. The multipass recording method will be described with reference to FIG.

  In the multi-pass printing method, as shown in FIG. 21A, the multi-head 2201 performs three main scans. In addition, as shown in FIG. 21B, the recording scan area in units of 4 pixels, which is half of the 8 pixels in the vertical direction, is completed by two recording scans (passes). In this case, the eight nozzles in the multi-head 2201 are divided into two groups of upper four nozzles (upper nozzle group) and lower four nozzles (lower nozzle group). The dots recorded by one nozzle in one scan are obtained by thinning out image data by about half according to a predetermined image data arrangement, and the remaining half of the dots are formed first in the second scan. By embedding in the image, the recording of the 4-pixel unit area is completed.

  Further, when performing the multi-pass printing method, the first scan and the second scan are made up with each other according to a predetermined arrangement. Usually, as this image data array (thinning mask pattern), as shown in FIG. 22, an array having a staggered pattern for each vertical and horizontal pixel is used. Accordingly, in the unit printing area (in this case, in units of four pixels), printing is completed by the first scan for printing the staggered lattice and the second scan for printing the inverted staggered lattice. FIGS. 22A to 22C show in order of steps how recording of a certain area is completed when the above-described staggered lattice pattern and inverted staggered pattern are used. First, in the first scan, a staggered pattern is recorded using the lower four nozzles (lower nozzle group) as shown in FIG. Next, in the second scan, paper feeding is performed for only four pixels, and an inverted zigzag pattern is recorded as shown in FIG. Thereafter, in the third scan, paper is fed again by 4 pixels, and the staggered pattern is recorded again as shown in FIG.

  If such a multi-pass printing method is carried out, even if the same multi-head as shown in FIG. 21 is used, the effect on the printing area unique to each ejection port is halved. The black stripes and the white stripes are not so noticeable as shown in FIG. Therefore, the density unevenness is considerably reduced.

JP 2002-307755 A JP 2005-96194 A JP 2001-328168 A JP 2002-96455 A

  However, in the method using the colorless and transparent ink as disclosed in Patent Document 1, it is necessary to mount ink unnecessary for color reproduction in the recording apparatus, so that the size and cost of the recording apparatus are problematic. The method using white ink and light gray ink is the same.

  Further, in the method described in Patent Document 2 using light gray ink, an area where ink is not supposed to be applied (for example, an input signal is (R, G, B) = (255, 255) in an 8-bit input device). The light gray ink is applied to the area 255). For this reason, the brightness of the highlight is reduced, and the color reproduction region is reduced.

  Furthermore, as in Patent Documents 1 and 3, the method of laminating or providing fine irregularities on the surface of a recorded image requires a dedicated processing step after a normal recording scan, so that it is different from the recording apparatus. It is necessary to provide a device.

  An object of the present invention is to provide an image processing apparatus, an image processing method, and the like that can reduce the occurrence of uneven gloss on the entire surface of a recording medium and realize high image quality.

  As a result of intensive studies to solve the above problems, the present inventor has come up with various aspects of the invention described below.

  An image processing apparatus according to the present invention is an image processing apparatus that performs image processing for an image forming unit that forms an image by scanning a recording head including a plurality of recording elements on a recording medium. Calculated by the image input means, the recording amount calculation means for scanning for calculating the recording amount for each recording element for each main scan of the recording head, and the recording amount calculation means for scanning. Halftone processing means for calculating an in-scan pattern by halftone processing with respect to the recorded amount, and the halftone processing means is arranged on the recording medium when the recording amount is larger than a predetermined threshold. The dot is concentrated and arranged.

  An image processing method according to the present invention is an image processing method for performing image processing for image forming means for forming an image by scanning a recording head having a plurality of recording elements on a recording medium, and inputting image data An image input step, an intra-scan recording amount calculation step for calculating a recording amount for each recording element for each main scan of the recording head, and an intra-scan recording amount calculation step for the image data. A halftone processing step for calculating an in-scan pattern by halftone processing for a recording amount, and the halftone processing step includes dots on the recording medium when the recording amount is larger than a predetermined threshold value. And a dot placement step for placing the dots in a concentrated manner.

  According to the present invention, since dots are concentrated and arranged according to the recording amount, it is possible to suppress the change in gloss caused by the surface shape of the recorded image and reduce the occurrence of uneven gloss on the entire surface of the recording medium. . As a result, high image quality can be obtained.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a block diagram showing a configuration of an image processing apparatus according to an embodiment of the present invention.

  A printer 2 that functions as an image forming unit is connected to the image processing apparatus 1 through a printer interface or a circuit. The image processing apparatus 1 can be implemented by, for example, a specific program (printer driver or the like) installed in a general personal computer. In that case, each part of the image processing apparatus 1 described below is realized by a computer executing a predetermined program. That is, the image processing apparatus 1 performs image processing for the printer 2 (for image forming means). As another configuration, for example, the printer 2 may include the image processing apparatus 1.

  The image processing apparatus 1 includes an input image buffer 102, a color separation processing unit 103, a color separation lookup table (LUT) 104, a scanning duty setting unit 105, and a scanning duty setting LUT 106. Further, a scanning duty buffer 107, a halftone processing unit 108, a constraint information buffer 109, a halftone image storage buffer 110, and a constraint information calculation unit 111 are also provided.

  Such an image processing apparatus 1 inputs image data to be printed via an image data input terminal 101 (image input means), and an input image buffer 102 stores the input image data. The color separation processing unit 103 separates the input color image into ink colors provided in the printer 2. In this color separation processing, the color separation processing unit 103 refers to a color separation lookup table (LUT) 104. The scan duty setting unit 105 further converts each ink color value separated by the color separation processing unit 103 into each ink color value for each scan (recording amount for each printing element) based on the scan duty setting LUT 106. That is, the scanning DUTY setting unit 105 calculates an ink color value for each recording element for each main scanning of the recording head 201 as an in-scan recording amount calculation unit. Then, the scan duty buffer 107 stores the data.

  The halftone processing unit 108 uses a binary image based on values stored in a constraint information buffer 109 described later, with respect to the multi-gradation (three gradations or more) value of each color obtained by the scanning duty setting unit 105 for each color. The data is converted into data, and the in-scan pattern is calculated.

  The constraint information buffer 109 stores information indicating whether or not dots are likely to be formed at addresses on the recorded image. Note that the constraint information buffer 109 is reserved for each ink color.

  The halftone image storage buffer 110 stores binary image data of each color obtained by the halftone processing unit 108. The constraint information calculation unit 111 performs a predetermined calculation using the binary image data stored in the halftone image storage buffer 110 and the duty data value for each scan stored in the scan duty buffer 107, and stores it in the constraint information buffer 109. Update the information that is being provided.

  The binary image data stored in the halftone image storage buffer 110 is output to the printer 2 via the image data output terminal 112.

  The printer 2 includes a recording head 201, a moving unit 203, a head control unit 204, a conveyance unit 205, an ink color / discharge amount selection unit 206, and a halftone image storage buffer 207.

  As the recording head 201, a thermal transfer method, an electrophotographic method, an ink jet method, or the like can be used, and any of them has a plurality of recording elements (nozzles in the case of an ink jet method).

  The moving unit 203 moves the recording head 201 under the control of the head control unit 204. The transport unit 205 transports the recording medium 202 under the control of the head control unit 204.

  The halftone image storage buffer 207 stores binary image data of each color output from the image processing apparatus 1. The ink color and ejection amount selection unit 206 selects an ink color from the ink colors mounted on the recording head 201 based on the binary image data of each color stored in the halftone image storage buffer 207.

  Such a printer 2 forms binary image data formed by the image processing apparatus 1 on the recording medium by moving the recording head 201 relative to the recording medium 202 vertically and horizontally.

  Here, the recording head 201 will be described. FIG. 2 is a diagram illustrating an exemplary configuration of the recording head 201. In this embodiment, light cyan (Lc) and light magenta (Lm) having a relatively low ink density, in addition to four colors of ink of cyan (C), magenta (M), yellow (Y), and black (K). 6 colors of ink including are mounted on the recording head 201. In FIG. 2, the nozzles are arranged in a line in the paper conveyance direction for the sake of simplicity, but the number and arrangement of the nozzles are not limited to this. For example, nozzle rows having the same color and different discharge amounts may be provided, or a plurality of nozzles having the same discharge amount may be provided, and the nozzles may be arranged in a zigzag manner. In FIG. 2, the ink colors are arranged in a line in the head movement direction, but may be arranged in a line in the paper transport direction.

  Next, the operation of the image processing apparatus 1 of the present embodiment having the above-described functional configuration will be described with reference to the flowchart of FIG.

  First, multi-tone color input image data is input from the image data input terminal 101, and the input image buffer 102 stores the input image data (step S201). It is assumed that the input image data is constructed from three color components of red (R), green (G), and blue (B).

  Next, the color separation processing unit 103 performs color conversion from RGB to CMYK and LcLm ink color planes with reference to the color separation LUT 104 for multi-tone color input image data stored in the input image buffer 102. A decomposition process is performed (step S202). In the present embodiment, each pixel data after color separation processing is handled as 8 bits, but it may be converted with a gradation number higher than that. As described above, the recording head 201 has six types of ink colors. For this reason, RGB color input image data is converted into image data of a total of 6 planes, each of CMYKLcLm planes. That is, image data of 6 types of planes corresponding to 6 types of recording modes is generated in this way.

Here, the color separation processing in step S102 will be described in detail with reference to FIG. FIG. 4 is a diagram showing the contents of color separation processing of the color printer. The color separation processing unit 103 performs conversion shown in the following equations (1) to (6) on the image data (R ′, G ′, B ′) with reference to the color separation LUT 104.
C = C_LUT — 3D (R ′, G ′, B ′) (1)
K = M_LUT — 3D (R ′, G ′, B ′) (2)
Y = Y_LUT — 3D (R ′, G ′, B ′) (3)
K = K_LUT — 3D (R ′, G ′, B ′) (4)
Lc = Lc_LUT — 3D (R ′, G ′, B ′) (5)
Lm = Lm_LUT — 3D (R ′, G ′, B ′) (6)

  Each function defined on the right side of the equations (1) to (6) corresponds to the color separation LUT 104. The color separation LUT 104 determines an output value for each ink color from three input values of red, green, and blue. In this embodiment, since the configuration includes six colors CMYKLcLm, a color separation LUT 104 that obtains six output values from three input values is used.

  In this way, the color separation process in step S102 is performed.

  After step S102, the scan duty setting unit 105 sets the scan number k and the color separation data cutout Y coordinate Ycut (k) (step S103). Note that the initial value of the scan number k is 1, and the scan duty setting unit 105 increments by 1 for each loop. The color separation data cutout Y coordinate Ycut (k) is a color separation data cutout position (nozzle upper end coordinate) at the scan number k.

  Here, as an example, the color separation data cut-out position Y-coordinate Ycut in the case of four-pass printing that includes eight nozzle rows and forms an image in four scans for the same main scanning recording area on the image. A method for setting (k) will be described.

  In general, in the case of four-pass printing, as shown in FIG. 5, when the initial value of the scan number k = 1, image formation is performed on the recording medium 502 using only the lower end quarter of the nozzle 501, and the scan number When k = 2, image formation is performed after feeding paper for 1/4 of the length of the nozzle 501 with respect to the scan number k = 1. Further, at scan number k = 3, image formation is performed after feeding paper for 1/4 of the length of nozzle 501 with respect to scan number k = 2, and thereafter the same paper feed and image formation are repeated to obtain the final output image. Form. Therefore, when the scan number k is 1 (initial value), the color separation data cutout position Ycut (1) is −12.

When the color separation data cut-out position Ycut (k) is generalized, the nozzle row number is Nzzl, the pass number is Pass, and the scan number is k.
Ycut (k) = − Nzzl + (Nzzl / Pass) × k (7)

  In this way, the color separation data cutout position Ycut (k) is set.

  After step S103, the scan duty setting unit 105 refers to the scan duty setting LUT 106, and from the image data of the six types of planes generated by the color separation processing unit 103, a data value (scan duty value) for each scan. Is set (step S104). This data value is the recording amount for each recording element of the recording head 201.

Note that the scanning duty setting LUT 106 is configured as shown in FIG. FIG. 6 is a diagram showing an example of the configuration of the scan duty setting LUT 106 in the case of 8 nozzles and 4 passes. Inflection points P1 to P4 in FIG. 6 are set for every four nozzles, and the duty division ratio for eight nozzles obtained by linearly interpolating each inflection point is the scan duty setting LUT. Here, the numerical values of the inflection points P1 to P4 satisfy the following formula (8).
P1 + P2 + P3 + P4 = 1.0 (8)

  The setting method of the scan duty setting LUT 106 is not limited to this. For example, the inflection point may be set more finely or may be directly specified for each nozzle.

  Then, as shown in FIG. 7, the scan duty setting unit 105 calculates the scan duty from the product of the scan duty setting LUT 106 and the color separation data (image data of six types of planes generated by the color separation processing unit 103). Set. However, when the corresponding nozzle is outside the area of the image Y address, the scanning duty is set to 0. For example, when the scan number k = 1, as shown in FIG. 8, since the image Y address becomes negative at the nozzle row upper end 3/4, 0 is substituted for the scan duty value, which is significant at the nozzle row lower end 1/4. A value is assigned.

  Since the color separation data cutout position Ycut (k) is determined by the scanning number k, when the scanning number k is 1 to 7, the scanning duty value is determined as shown in FIG. Since each scan duty value is determined by the product of the color separation data and the scan duty setting LUT 106, when such a product is acquired while feeding paper, in the area 1 portion, the scan number k is four times of 1 to 4. The total value of one raster formed by scanning becomes equal to the color separation data. Similarly, in the areas 2, 3 and 4, the total value of one raster is equal to the color separation data. However, in the scan duty separation, when the value of the color separation data is smaller than the duty separation threshold D_Th3, image formation is not performed by four scans, and image formation is performed by one to three scans.

Here, details of the scan duty setting in the scan duty setting unit 105 are shown in equations (9) to (15), taking cyan C (X, Y) as an example. C (X, Y) is the scan duty at address (X, Y), and S_LUT (Y) is the set value of the scan duty setting LUT at address Y. D_Th is a duty decomposition threshold set for each scan.
(A) When C (nx, Ycut (k) + ny)> D_Th3 C_d (nx, ny) = C (nx, Ycut (k) + ny) × S_LUT (ny)
(9)
(B) When C (nx, Ycut (k) + ny) ≦ D_Th1, “scan number k = 1, 5,... 4n + 1 (n is an integer of 0 or more)”
C_d (nx, ny) = C (nx, Ycut (k) + ny) (10)
"Scan number other than above"
C_d (nx, ny) = 0 (11)
(C) When C (nx, Ycut (k) + ny) ≦ D_Th2, “scan number k = 1, 5,..., 4n + 1 (n is an integer of 0 or more)”
“Scan number k = 2, 6,..., 4n + 2 (n is an integer of 0 or more)”
C_d (nx, ny) = C (nx, Ycut (k) + ny) / 2 (12)
"Scan number other than above"
C_d (nx, ny) = 0 (13)
(D) When C (nx, Ycut (k) + ny) ≦ D_Th3 “Scan number k = 1, 5,..., 4n + 1 (n is an integer of 0 or more)”
“Scan number k = 2, 6,..., 4n + 2 (n is an integer of 0 or more)”
“Scan number k = 3, 7,..., 4n + 3 (n is an integer of 0 or more)”
C_d (nx, ny) = C (nx, Ycut (k) + ny) / 3 (14)
"Scan number other than above"
C_d (nx, ny) = 0 (15)
Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl.

  Similarly, Lc (X, Y), M (X, y), Lm (X, Y), Y (X, Y), and K (X, y) are also decomposed into scanning duty by the above formula.

  In this specification example, four paths are taken as an example, so exception decomposition of exceptions is set as shown in (9) to (15). However, for example, D_Th1 to D_Th7 are similarly set for eight paths, and duty decomposition of exceptions is performed. Do.

  After step S104, the scan duty buffer 107 stores the scan duty value set by the scan duty setting unit 105 (step S105). For example, as shown in FIG. 10, the scan duty buffer 107 stores, for each color, band-shaped data values corresponding to the number of nozzles in the vertical direction and the image Xsize in the horizontal direction.

  Next, the halftone processing unit 108 converts the total value of the data in the scan duty buffer 107 and the constraint information buffer 109 into a two-level gradation value (binary data) (step S106). A method for such conversion is not particularly limited, but as a method for converting halftone multi-valued input image data into a binary image (or an image having two or more values and a smaller number of gradations than the number of input gradations), For example, R.A. There is an error diffusion method by Floyd et al. This error diffusion method is described in, for example, ““ An adaptive algorithm for spatial gray scale ”, SID International Symposium Digest of Technical Papers, vol 4.3, 1975, pp. 36-37”. In the present embodiment, the halftone processing unit 108 performs halftone processing based on this error diffusion method.

In the present embodiment, the constraint information buffer 109 updates constraint information indicating whether or not a binary image is easily formed at an address on an image to be recorded for each scan number k. However, an initial value of all zeros is substituted at the start of processing for scan number k = 1. That is, the constraint information of each color at the address (X, Y) is C_r (X, Y), Lc_r (X, Y), M_r (X, Y), Lm_r (X, Y), Y_r (X, Y), K_r. Assuming that (X, Y), when the scanning number k = 1, the following expressions (16) to (21) are used.
C_r (nx, ny) = 0 (16)
Lc_r (nx, ny) = 0 (17)
M_r (nx, ny) = 0 (18)
Lm_r (nx, ny) = 0 (19)
Y_r (nx, ny) = 0 (20)
K_r (nx, ny) = 0 (21)
Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl.

  For this reason, when the scan number k ≧ 2, significant constraint information is updated. As the value of the constraint information is smaller, the dot is less likely to be formed at the location, and conversely, the larger the value is, the easier the dot is formed. In addition, the value updated in the constraint information buffer stores an average value of 0 regardless of the scan number. That is, a positive value is stored when a dot is likely to be formed at that location, and a negative value is stored when a dot is difficult to form at that location. Details of the constraint information update method will be described later.

  Here, the operation of the halftone processing unit 108 when performing 4-pass printing will be described in detail.

  Conventionally, in an ink jet recording system, the dot arrangement on a recording medium is often determined by halftone processing so as to increase the dispersibility of the dot arrangement from the viewpoint of image quality problems such as graininess. However, even if a highly dispersive dot arrangement is used in the multi-pass inkjet recording method using pigment ink, the pigment dots 1202 are deposited on the recording medium 1201 in a region where the recording density is high as shown in FIG. , The glossiness will decrease.

  Therefore, in this embodiment, particularly in an area where the recording density is high, the dot arrangement within one scan is concentrated to suppress the change in the surface of the recorded image due to the accumulation of pigment ink, and uneven gloss on the entire recording medium. Reduce the occurrence. In order to concentrate the dot arrangement within the same scan, in this embodiment, when the recording data input to the halftone processing unit 108 is larger than the predetermined threshold Th1, that is, when the recording density is high, error diffusion processing is performed. For example, the error diffusion coefficient is set as shown in FIG.

  In this embodiment, the threshold value Th1 of the recording data serving as a reference for concentrating and arranging dots within the same scan is, for example, a dot density (dot implantation amount) per unit area on the recording medium of 100%. Can be set with recorded data. It is also possible to set the recording density so that the recording medium is hidden by dots. The higher the density of the recording data, the greater the number of dots that can be shot in one scan. Therefore, by setting the threshold value Th1 in this way, dots are concentrated and arranged in one scan. For this reason, it is possible to effectively suppress the change in the surface of the recorded image due to the accumulation of the pigment ink and improve the gloss uniformity.

  When dots are concentrated and arranged in the same scan, adjacent dots are hit on the recording medium at short intervals with respect to a certain dot. Between adjacent dots beaten at short time intervals, the pigment ink is deposited on the recording medium so that one large dot with a uniform surface is formed. Therefore, it is possible to reduce the change in the recorded image surface due to the accumulation of the pigment ink on the recording medium as shown in FIG.

  Therefore, it is desirable to set how much the dots are “concentrated” in the same scan depending on the characteristics of the pigment ink. For example, since ink (pigment) accumulates on an ink with a high penetration speed into a medium and the surface is likely to be rough, it is better to dispose adjacent dots in a shorter time for an ink with a higher penetration speed. That is, by defining the coefficient that controls the degree of “concentration” of dots based on the ink characteristics, it is possible to effectively suppress changes in the surface of the recorded image due to the accumulation of pigment ink.

  In addition, since the accumulation state of the pigment ink changes depending on the characteristics of the recording medium, a method of defining the degree of concentration of the dots based on the characteristics of the recording medium and the pigment ink may be used, but this is not restrictive.

  In FIG. 13, the error diffusion coefficients are assumed to have 12 coefficients K1 to K12. For example, when K1 to K12 are set as shown in FIG. 14, the dots on the recorded image are concentrated.

  Further, when the recording data is equal to or less than the predetermined threshold, the error diffusion coefficient may be set so that the dispersibility of the dot arrangement is high. That is, you may make it arrange | position a dot discretely.

  In order to diffuse and accumulate errors using the error diffusion coefficient, as shown in FIG. 17, four sets of accumulated error line buffers in the halftone processing unit 108 are secured, and the accumulated error line buffers to be used are switched for each scanning number. . For simplicity of explanation, the error diffusion coefficients are four coefficients K1 to K4 shown in FIG.

  Specifically, when “scan number k = 1, 5,..., 4n + 1 (n is an integer of 0 or more)”, (4n + 1) cumulative error line buffer 1702 is used, and “scan number k = 2, 6..., 4n + 2 ”, the (4n + 2) cumulative error line buffer 1703 is used. Further, when (when scan number k = 3, 7,..., 4n + 3), (4n + 3) cumulative error line buffer 1704 is used, and when “scan number k = 4, 8,. The (4n + 4) cumulative error line buffer 1705 is used.

  As shown in FIG. 16, when the error diffusion coefficient has four coefficients K1 to K4, and K1 = 7/16, K2 = 3/16, K3 = 5/16, K4 = 1/16, the accumulated error The line buffers 1702, 1703, 1704, and 1705 are configured as shown in FIGS.

  The cumulative error line buffer 1702 has one recording area E1_0 and storage areas E1_ (x) (x = 1 to W) as many as the number of horizontal pixels W of the input image. The accumulated error line buffers 1703, 1704, and 1705 also have similar recording areas. Further, it is assumed that the accumulated error line buffers 1702, 1703, 1704, and 1705 are all initialized with an initial value 0 only at the start of processing of scan numbers k = 1, 2, 3, and 4.

  In the present embodiment, the above-described four sets of accumulated error line buffers are required for halftone processing of one color. For this reason, a total of 4 × 6 = 24 sets of line buffers are provided in this embodiment that performs processing of six colors.

  As will be described in detail later, the halftone processing unit 108 also includes a cumulative error addition unit 1706, a threshold selection unit 1707, a quantization unit 1708, an error calculation unit 1709, and an error diffusion unit 1710, as shown in FIG. Yes.

  Here, the cyan halftone process in the case of 4-pass printing and the scan number k is 1 will be described with reference to FIGS. FIG. 11 is a flowchart showing the contents of the halftone process.

  First, cyan recording data C_d is input to the halftone processing unit 108 (step S201).

Next, the cumulative error adding unit 1706 adds an error E1_ (x) corresponding to the horizontal pixel position of the input data (step S202). That is, the cumulative error adding unit 1706 performs the calculation represented by the following equation (22), where C_d is the input recording data of the target pixel and C_d ′ is the data after the cumulative error addition.
C_d ′ = C_d + E1_ (x) (22)

Thereafter, the threshold selection unit 1707 selects the threshold T (step S203). In the selection of the threshold value T, for example, the setting represented by the following equation (23) is performed. Here, the value “128” is a value generally used when the input is 0-255.
T = 128 (23)

In order to avoid the dot generation delay, the average quantization error may be reduced so that it is finely set according to the input recording data C_d of the target pixel as represented by the following equation (24). Good.
T = f (C_d) (24)

Further, as expressed by the following equation (25), it may be finely set according to the address (X, Y) in the band.
T = f (X, Y) (25)

Subsequently, the quantization unit 1708 compares the pixel data C_d ′ after the error addition and the threshold T, and determines the binarization result Out_c by the following equations (26) to (29) (step S204).
When C_d ′ <T (26)
Out_c = 0 (27)
When C_d ′ ≧ T (28)
Out_c = 255 (29)

Next, the error calculation unit 1709 calculates the difference Err between the pixel data C_d ′ after the error addition and the output pixel value Out_c in the recording data C_d of the target pixel by the following equation (30) (step S205).
Err (x) = C_d′−Out_c (30)

Thereafter, the error diffusion unit 1710 diffuses the error to the peripheral pixels of the target pixel (step S206). That is, the error diffusion unit 1710 uses the accumulated error line buffer 1702 to perform diffusion processing of the error Err (x) represented by the following equation (31) according to the horizontal pixel position x.
Ec1 (x + 1) ← Ec1 (x + 1) + Err_c (x) × 7/16 (x <W)
Ec1 (x−1) ← Ec1 (x−1) + Err_c (x) × 3/16 (x> 1)
Ec1 (x) ← Ec1_0 + Err_c (x) × 5/16 (1 <x <W)
Ec1 (x) ← Ec1_0 + Err_c (x) × 8/16 (x = 1)
Ec1 (x) ← Ec1_0 + Err_c (x) × 13/16 (x = W)
Ec1_0 ← Err_c (x) × 1/16 (x <W)
Ec1 — 0 ← 0 (x = W) (31)

  In this way, binarization processing for one cyan pixel with scan number k = 1 is performed.

  Then, the processing in steps S201 to S206 is performed for all the pixels in the band (step S207), thereby generating halftone image data in which dots are concentrated and arranged when the recording density in the same scan is high. can do.

  Note that (4n + 2) cumulative error line buffer 1703 is used for scan number k = 2, (4n + 3) cumulative error line buffer 1704 is used for scan number k = 3, and (4n + 4) cumulative error line buffer 1705 is used for scan number k = 4. Halftone processing. In the process of scan number k = 5, the same (4n + 1) cumulative error line buffer 1702 as scan number k = 1 is used as it is without being initialized (all 0s are not substituted). This is because the print error of scan number k = 1 and scan number k = 5 is vertically adjacent, so that the accumulated error is stored in the adjacent lower region. If the accumulated error line buffer is initialized, the error is not stored at the adjacent boundary, and the dot continuity cannot be maintained.

  After step S106, the halftone image storage buffer 110 stores the binary image data after the halftone process (step S107). FIG. 18 is a diagram illustrating a state in which the scan duty value of scan number k = 1 is stored after the halftone process. Binary image data of Nzzl × image Xsize corresponding to the pixel position of the recording data is stored. As can be seen from the configuration of FIG. 18, in the halftone processing in the present embodiment, binary image data is sequentially generated with respect to the number of vertical nozzles and the scanning DUTY buffer 107 data of the horizontal image X size. Accordingly, the halftone image storage buffer 110 is also provided with a memory space for the number of vertical nozzles and the band data of the horizontal image X size.

  With these processes, the halftone process at the scan number k = 1 is completed, and as a result, a binary image formed by scanning the recording head once for each color is stored in the halftone image storage buffer 110 for each color. Will be stored.

  The quantization method in the halftone processing unit is not limited to the error diffusion method, and for example, a blue noise mask type or Bayer type dither method or a density pattern method may be used. Further, a plurality of methods as described above may be used in combination.

  After step S107, band data of vertical: Nzzl and horizontal image: W stored in the halftone image storage buffer 110 is output from the image data output terminal 112 (step S108).

  Next, constraint information is calculated (step S109). Here, in order to simplify the description, the details of the calculation of the cyan constraint information in four-pass printing, the scan number k = 1, and the number of nozzles Nzzl = 16 will be described, with the constraint information calculation block diagram of FIG. 23 and the flowchart of FIG. Will be described with reference to FIG. As described above, the constraint information is information indicating whether or not dots are likely to be shot when determining the dot arrangement of the halftone image at the next scan number k = 2 after the current scan number k = 1. . When the current scan number is k, the next scan number is k + 1.

First, the scan duty filter processing unit 1501 performs filter processing on the cyan scan Duty and C_d in the scan duty buffer 107 with a predetermined filter F_m to calculate C and _df (step S301). FIG. 25 shows a schematic diagram of the filtered data.
C_df = C_d * F_m (32) *: Convolution

  In the present embodiment, the coefficient of F_m is described as an isotropic weighted average filter in which the filter size is 3 × 3 square and the coefficients are almost concentrically arranged as shown in FIG. 26, but is not limited thereto. For example, the filter size may be a square such as 5 × 5, 7 × 7, or 9 × 9, or a rectangle such as 3 × 5, 5 × 7, or 5 × 9, or an anisotropic filter having an elliptic filter coefficient. Alternatively, the filter is not limited to the low-pass characteristic, and may be a band-pass characteristic or a high-pass characteristic filter.

Next, the halftone data filter processing unit 1502 performs a filtering process on Out_c with a predetermined low-pass filter LPF_b (step S302). FIG. 27 shows a schematic diagram of the filtered data.
Out_c_LPF = Out_c * LPF_b (33)

  The coefficients of the LPF_b in this embodiment are also isotropic weighted average filters having a filter size of 3 × 3 squares and coefficients arranged substantially concentrically as shown in FIG. 27 as in F_m, but are not limited thereto. However, LPF_b preferably has a low-pass characteristic. In the present embodiment, F_m and LPF_b are the same, but they may be different coefficients.

Next, the scan duty data shift unit 1504 shifts the data C_df of the scan duty filter processing unit 1501 upward by one paper feed amount LF = Nzzl / Pass = 16/4 = 4 (step S303). FIG. 28 shows a schematic diagram of the shifted data. Assuming that the shifted scanning duty data is C′_df, it is calculated as in the following equation (34). Note that 0 is substituted for the lower four nozzles after the shift.
C′_df (nx, ny) = C_df (nx, ny + LF) (34)
Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl. When “ny + LF ≧ Nzzl”, 0 is substituted. That is, 0 is substituted for the lower end LF nozzle.

Similarly, the halftone data shift unit 1505 shifts the data Out_c_LPF to the paper feed amount (step S304). FIG. 29 shows a schematic diagram of the shifted data. When the shifted data is Out'_c_LPF, the following calculation (35) is performed.
Out′_c_LPF (nx, ny) = Out_c_LPF (nx, ny + LF)
(35)
Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl. When “ny + LF ≧ Nzzl”, 0 is substituted. That is, 0 is substituted for the lower end LF nozzle.

Further, the pre-update constraint information data shift unit 1503 also shifts the pre-update constraint information data C_r onto the paper feed amount (step S305). If the shifted scanning duty data is C′_r, the calculation is performed as in the following Expression (36).
C′_r (nx, ny) = C_r (nx, ny + LF) (36)
Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl. When “ny + LF ≧ Nzzl”, 0 is substituted. That is, 0 is substituted for the lower end LF nozzle.

  As described above, in the constraint information calculation of the next scanning number, the paper feed amount LF is relatively shifted upward. Further, 0 is substituted for the lower end LF nozzle.

  The reason why the buffer data is shifted by the paper feed amount LF in this manner is that the halftone dot arrangement formed by the next scanning number is formed on the recording medium with a relative shift of the paper feed amount LF.

  Further, the subtraction unit 1506 subtracts the data calculated by the halftone data shift unit 1505 from the data calculated by the scanning duty data shift unit 1504 (step S306), and the weight integration unit 1507 performs weighting factor h (real number). (Step S307).

Next, the adding unit 1508 adds the data calculated by the weight integrating unit 1507 and the cyan constraint information shifted by the pre-update constraint information data shift unit 1503 (step S308), and adds this to the post-update constraint information C_r. To do. This post-update constraint information C_r is stored as constraint information for halftone processing after the next scan number k = 2 (or, if the current scan number is k, the next scan number is k + 1).
C_r ← (−Out′_c_LPF + C′_df) × h + C′_r (37)

Here, the equation (37) will be described.
Out′_c_LPF in the equation (37) is data obtained by extracting only a low frequency component from the halftone image of the scan number k using the LPF_b by the halftone data filter processing unit 1502. In Expression (37), by subtracting it from -Out'_c_LPF, the data from which only the low-frequency component is extracted is set to a negative value. This “negative value” effect means that dots are less likely to be shot after the next scan number k + 1. That is, this process has an effect that the low frequency component of the halftone image printed before the scan number k after the next scan number k + 1 has an opposite phase. This completes the constraint information calculation in step S109.

  Next, the calculation result is updated in the constraint information buffer (step S110). This restriction information is held as information for determining dot arrangements after the next scanning number (if the current scanning number is k, the next scanning number is k + 1).

  Next, in the printer 2 that has received the halftone image data, the halftone image storage buffer 207 temporarily stores the image data, and the ink color and ejection amount selection unit 206 selects an ink color that matches the image data. Further, the head control unit 204 starts a printing operation and variation amount detection (step S111). In the printing operation, the recording head 201 drives the nozzles at a constant driving interval while moving from the left to the right with respect to the recording medium 202, and performs a main scanning for recording an image on the recording medium 202, and the main scanning. One sub-scan, which is a scan in a direction perpendicular to the main scan after the end of, is performed.

  Thereafter, it is determined whether or not all scanning has been completed, that is, whether or not all pixels have been processed (step S112). If completed, a series of image forming processes are completed, and if not completed, the process returns to step S103.

  In this way, image processing is performed on the image data input to the image processing apparatus 1.

  In this embodiment, when the same area on the recording medium 202 is scanned using the same or different areas divided by the recording head 201, dots are concentrated in the same area. ing. For this reason, it is possible to suppress the change in gloss caused by the surface shape of the recorded image, reduce the occurrence of uneven gloss on the entire surface of the recording medium 202, and achieve high image quality on the entire surface of the recording medium 202. That is, in this embodiment, in the halftone process for determining the dot arrangement within the same scan, the error diffusion coefficient is set so that the dots are arranged in a concentrated manner when the recording density is greater than a certain threshold value. By concentrating dots in the same area, the change in gloss caused by the surface shape of the recorded image is suppressed, the occurrence of uneven gloss on the entire recording medium is reduced, and the high image quality is achieved on the entire recording medium. It can be realized.

  Note that the error diffusion method is used in the halftone processing of this embodiment, but the dither method may be used as described above. When the dither method is used to concentrate and arrange dots within the same scan, for example, a green noise mask is used as the dither mask. Further, as the halftone processing, error diffusion processing of a feedback system described in “Daniel Leo Lau, Green Noise Digital Halftoning, IEEE International Conference on Image Processing, 1998” may be performed. An example of a circuit that realizes feedback system error diffusion processing is shown in FIG. In this circuit, a quantization unit 2001, an error diffusion coefficient 2002, and a pixel reference mask 2003 are provided. In the feedback system error diffusion processing, the frequency characteristic of the dot arrangement can be controlled to the green noise characteristic by setting the pixel reference coefficient h2004. Then, for example, as shown in FIG. 20, when the pixel reference coefficient h is increased, dots are concentrated and arranged.

  Note that the processing of the above-described embodiment can also be realized by providing a system or apparatus with a storage medium that records software program codes embodying each function. The functions of the above-described embodiments can be realized by the computer (or CPU, MPU) of the system or apparatus reading and executing the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. As a storage medium for supplying such a program code, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, or the like can be used. A CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM, or the like can also be used.

  The functions of the above-described embodiments are not only realized by executing the program code read by the computer. Including the case where the OS (operating system) running on the computer performs part or all of the actual processing based on the instruction of the program code, and the functions of the above-described embodiments are realized by the processing. It is.

  Further, the program code read from the storage medium may be written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer. After that, the CPU of the function expansion board or function expansion unit performs part or all of the actual processing based on the instruction of the program code, and the functions of the above-described embodiments are realized by the processing. Is also included.

  That is, the embodiment of the present invention can be realized by, for example, a computer executing a program. Also, means for supplying a program to a computer, for example, a computer-readable recording medium such as a CD-ROM recording such a program, or a transmission medium such as the Internet for transmitting such a program is also applied as an embodiment of the present invention. Can do. The above-described print processing program can also be applied as an embodiment of the present invention. The above program, recording medium, transmission medium, and program product are included in the scope of the present invention.

1 is a block diagram illustrating a configuration of an image processing apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an exemplary configuration of a recording head 201. 3 is a flowchart showing an operation of the image processing apparatus according to the embodiment of the present invention. It is a figure which shows the content of the color separation process of a color printer. It is a figure which shows the relationship between a scanning number and a nozzle. 6 is a diagram illustrating an example of a scan duty setting LUT 106 in the case of four passes. FIG. FIG. 6 is a diagram illustrating an operation of a scan duty setting unit 105. It is a figure which shows operation | movement in case of scanning number k = 1. It is a figure which shows the scanning duty value in case the scanning number k is 1-7. 5 is a diagram illustrating an example of a scan duty buffer 107. FIG. It is a flowchart which shows the content of a halftone process. It is a figure which shows the cause which glossiness falls. It is a figure which shows the example of an error diffusion coefficient. It is a figure which shows the value of an error diffusion coefficient in case recording density is high. It is a figure which shows the example of a cumulative error line buffer. It is a figure which shows the other example of an error diffusion coefficient. 2 is a diagram illustrating a configuration of a halftone processing unit 108. FIG. It is a figure which shows the halftone image storage buffer 110 in which the scan duty value of the scan number k = 1 was stored after the halftone process. It is a figure which shows the example of the circuit which implement | achieves a feedback type | system | group error diffusion process. It is a figure which shows the relationship between the pixel reference coefficient h and the degree of dot concentration. It is a figure which shows the outline | summary of a multipass recording system. It is a figure which shows the image data arrangement | sequence in a multipass printing system. It is a block diagram which shows the detailed structure of the constraint information calculating part in embodiment of this invention. It is a flowchart which shows the restrictions information calculation process in embodiment of this invention. It is a schematic diagram which shows the scanning duty data after the filter process in the constraint information calculation of the embodiment of the present invention. It is a figure which shows the specific example of the filter in the constraint information calculation of the Example of this invention. It is a schematic diagram which shows the halftone data after the filter process in the restrictions information calculation of embodiment of this invention. It is a schematic diagram which shows the scanning duty data after the shift in the constraint information calculation of the embodiment of the present invention. It is a schematic diagram which shows the halftone data after the shift in the constraint information calculation of the embodiment of the present invention.

Explanation of symbols

1: Image processing device 2: Printer 101: Image data input terminal 102: Input image buffer 103: Color separation processing unit 104: Color separation LUT
105: Scanning duty setting unit 106: Scanning duty setting LUT
107: Scanning DUTY setting buffer 108: Halftone processing unit 109: Constraint information buffer 110: Halftone image storage buffer 111: Constraint information calculation unit 112: Image data output terminal 201: Recording head 202: Recording medium 203: Moving unit 204: Head control unit 205: Conveying unit 206: Ink color and ejection amount selection unit 207: Halftone image storage buffer

Claims (5)

An image processing apparatus that performs image processing for an image forming unit that forms an image by scanning a recording head including a plurality of recording elements on a recording medium,
Image input means for inputting image data;
Intra-scan recording amount calculation means for calculating a recording amount for each recording element for each main scan of the recording head for the image data;
Halftone processing means for calculating an in-scan pattern by halftone processing for the recording amount calculated by the in-scan recording amount calculation means;
Have
The halftone processing means arranges dots in a concentrated manner on the recording medium when the recording amount is larger than a predetermined threshold.   2. The halftone processing unit according to claim 1, wherein, in the halftone processing, when the recording amount is larger than the threshold value, a frequency characteristic of a dot arranged on the recording medium is set as a green noise characteristic. The image processing apparatus according to 1.   The image according to claim 1, wherein the halftone processing unit discretely arranges dots on the recording medium when the recording amount is equal to or less than the threshold value in the halftone processing. Processing equipment. An image processing method for performing image processing for an image forming means for forming an image by scanning a recording head including a plurality of recording elements on a recording medium,
An image input step for inputting image data;
Intra-scan recording amount calculating step for calculating the recording amount for each recording element for each main scan of the recording head for the image data;
A halftone processing step of calculating an in-scan pattern by halftone processing for the recording amount calculated in the in-scan recording amount calculation step;
Have
The image processing method according to claim 1, wherein the halftone processing step includes a dot arrangement step of concentrating and arranging dots on the recording medium when the recording amount is larger than a predetermined threshold value. A program for causing a computer to perform image processing for image forming means for forming an image by scanning a recording head provided with a plurality of recording elements on a recording medium,
In the computer,
An image input step for inputting image data;
Intra-scan recording amount calculating step for calculating the recording amount for each recording element for each main scan of the recording head for the image data;
A halftone processing step of calculating an in-scan pattern by halftone processing for the recording amount calculated in the in-scan recording amount calculation step;
And execute
The halftone processing step includes a dot placement step in which dots are concentratedly placed on the recording medium when the recording amount is larger than a predetermined threshold value.

Images