JP2011031560A - Image forming apparatus and control method for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress unevenness of gloss generated in an image formed with a pigment ink by a simple configuration. <P>SOLUTION: An image processor 102 sends out nozzle data for every scanning of a recording head 1044 to a printer 104 for performing a multi-pass printing with the pigment ink. In this case, since the glossiness of recorded image becomes lower as there are many numbers of passes at image formation, the brightness of input image data is determined in an input signal value determining section 1024. Then, the substantial number of passes is controlled so that all-pass printing may be performed if the brightness is within a predetermined middle brightness range, and a limited-pass printing may be performed with the number of passes smaller than all passes if the brightness is outside the middle brightness range. Thereby, since the glossiness is reduced in the middle brightness region where the glossiness is relatively high while the glossiness is increased in the high/low brightness region where the glossiness is relatively low, the glossiness difference in the whole image can be suppressed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that suppresses uneven glossiness in an image formed of a pigment color material and a control method thereof.

  As an example of an apparatus that performs recording on a recording medium using a recording head that includes a plurality of recording elements, an ink jet recording apparatus that uses a recording head that includes a plurality of ink ejection openings is known. Conventionally, dye inks using dyes that are easily dissolved in water as color materials have been widely used as inks used in ink jet recording apparatuses. The dye ink contains water as a main component, and the color material dissolved in the solvent easily permeates into the fibrous material on the surface of the recording medium. Therefore, 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 the gloss of the recording medium itself. That is, if recording is performed with a dye ink on a recording medium having excellent gloss, a recorded image having excellent gloss can be obtained. Therefore, in an ink jet recording apparatus using a dye ink, it is possible to realize glossiness to an image by improving the glossiness of the recording medium itself.

  On the other hand, there is an increasing demand for improved light resistance and water resistance of printed materials. The dye inks described above are generally known to have low light resistance, and the dye molecules of the color material are easily decomposed by light, so that the color of the recorded image is faded. In addition, it is known that printed matter printed with a dye ink generally has poor water resistance, and when wetted with water, dye molecules that have penetrated into the fiber dissolve in the water, so that bleeding tends to occur in the recorded image.

  In recent years, in order to solve the problems of light resistance and water resistance generated in dye inks, development of pigment inks using pigments as coloring materials has been promoted. Unlike dyes that exist as molecules, pigment inks exist in the solvent as particles of a size of several tens of nanometers to several microns, so when recording using pigment inks, it is difficult to penetrate inside the recording medium, Pigment ink is deposited on the surface of the recording medium. If the deposited pigment ink has irregularities in a fine shape on the image surface on the recording medium, incident light is irregularly reflected and the reflectance is lowered.

  Further, the amount of ink applied varies depending on the density and color of the recorded image to be reproduced. Since the area where ink covers the recording medium is different due to the difference in the amount of ink applied, the reflectance differs between the ink and the recording medium. Therefore, a difference in gloss occurs due to the difference in the area where the ink covers the recording medium.

  For the reasons described above, when recording is performed using pigment ink, a phenomenon called gloss unevenness occurs in which glossiness varies depending on the density and color of a reproduced recorded image. When gloss unevenness occurs, glaring areas and matte areas are mixed in the same recorded image, so that it is easily recognized as an image defect particularly when printing a photographic image. Note that uneven gloss is not a problem limited to an ink jet recording apparatus using pigment ink, and a similar problem occurs in an electrophotographic recording apparatus in which recording is performed by fixing toner on a recording medium.

  As a method for solving the above-mentioned problem, there is a method for recording using a colorless transparent ink or a white ink in an area not covered with colored ink in an apparatus for recording an image using pigment ink. It is known (see, for example, Patent Document 1).

  In addition, in an apparatus for recording an image using pigment ink, a method of recording using an achromatic ink (light gray ink lighter than black ink) in an area not covered with colored ink is known. (For example, refer to Patent Document 2).

  In addition, a recorded image that has been recorded once is coated (laminated) with the surface of a thermoplastic resin or the like to achieve a uniform gloss on the entire surface, or the entire surface is provided with countless fine irregularities. A method for realizing uniform gloss is known (for example, see Patent Document 1).

JP 2002-307755 A JP 2005-96194 A

  However, the method for suppressing the uneven gloss as described above has the following problems.

  For example, in the method of reducing gloss unevenness by using the above-described colorless and transparent ink, it is necessary to mount ink unnecessary for color reproduction in the recording apparatus, which increases the size and cost of the recording apparatus. Similarly, solutions using white ink, light gray ink, and the like also increase the number of inks, so an increase in the size and cost of the recording apparatus cannot be avoided.

  Further, in the method of recording using light gray ink in an area not covered with colored ink, conventional ink is applied with an input signal value of (R, G, B) = (255, 255, 255). Ink is also applied to areas that are not present. Therefore, there is also a problem that the brightness of the bright area (highlight) is lowered and the color reproduction area is reduced.

  In addition, in the method of laminating or providing fine unevenness to the recorded image once recorded, a dedicated post-processing step is required after recording. A processing device must be provided. Although it is possible to integrate the post-processing device with the recording device, in this case as well, the increase in size and cost of the device are problems.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus that suppresses uneven gloss in an image formed by a color material of a pigment and a control method thereof with a simple configuration. And

  As a means for achieving the above object, an image forming apparatus of the present invention comprises the following arrangement.

  That is, an image forming apparatus in which a recording head having a plurality of recording elements forms an image by performing main scanning a plurality of times on the same area on a recording medium, and is an image for inputting multivalued image data A data input means; a pass number setting means for setting the number of passes which is the number of main scans for forming an area corresponding to the image data on the recording medium in accordance with the brightness of the image data; and the recording For each main scanning of the head, print data setting means for setting the print data for each print element in accordance with the number of passes, and N-value conversion processing (N is the N) for the print data set by the print data setting means And N-value conversion means for creating a dot pattern to be formed by applying an integer of 2 or more.

  According to the present invention having the above-described configuration, it is possible to suppress uneven gloss in an image formed with a pigment color material with a simple configuration.

1 is a block diagram showing the configuration of an image forming system in an embodiment according to the present invention. FIG. 3 is a diagram illustrating a configuration example of a recording head in the printer according to the embodiment. The flowchart which shows the image formation process by the multipass in this embodiment, The figure which shows the outline | summary of the path | pass number control in this embodiment, The flowchart which shows the input signal value determination process in this embodiment, The flowchart which shows the setting process of the threshold value for path number control in this embodiment, The conceptual diagram which shows the setting method of the threshold value for path number control in this embodiment, The figure which shows the detail of the input-output data in the color separation process part of this embodiment, The figure which shows the example of a setting of the color separation data cutout coordinate in this embodiment, The figure which shows the outline | summary of the setting method of the recording data in this embodiment, The figure which shows the outline | summary of the setting method of the recording data in this embodiment, The flowchart which shows the halftone process in this embodiment, The block diagram which shows the detailed structure of the halftone process part in this embodiment, It is a figure which shows an example of the error diffusion coefficient in this embodiment.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the accompanying drawings. The configurations shown in the following embodiments are merely examples, and the present invention is not limited to the illustrated configurations.

<First Embodiment>
System Configuration FIG. 1 is a diagram showing a configuration of an image forming system to which the present invention is applied. The image processing apparatus 102 and the printer 104 are connected by a printer interface or a network interface. In FIG. 1, 101 is an input terminal for inputting image data, and 1021 is an input image buffer for storing input image data. Reference numeral 1022 denotes a color separation processing unit that separates an input color image into ink colors of a printer, and refers to a color separation lookup table (hereinafter referred to as a color separation LUT) 1023 when performing color separation processing. Reference numeral 1024 denotes an input signal value determination unit that determines the magnitude relationship between each signal value and a predetermined threshold for each pixel of the image stored in the input image buffer 1021. In a printer 104 to be described later, the recording head 1044 forms an image by a so-called multi-pass method in which the same area on the recording medium is scanned a plurality of times. Hereinafter, the number of times of main scanning is referred to as the number of passes. In this embodiment, multi-pass printing is performed with a predetermined number of fixed passes, but the number of passes that are substantially valid is set based on the determination result in the input signal value determination unit 1024. In the following, the multi-pass printing in the present embodiment will be described as multi-pass printing with fixed 4-pass.

  Further, reference numeral 1025 denotes a recording data setting unit that sets the output data value of each path in the multipath with reference to the determination result in the input signal value determination unit 1024. A recording data storage memory 1027 stores the output data value of each pass set by the recording data setting unit 1025. Reference numeral 1028 denotes a halftone (HT) processing unit that converts multi-tone image data into binary, 1029 denotes an HT image storage memory that stores binary image data after HT processing, and 103 denotes a series of processes. This is an output terminal for outputting the processed image data to the printer 104.

  The printer 104 forms an image on the recording medium 1047 by moving the recording head 1044 in the sub-scanning direction which is the conveyance direction of the recording medium 1047 and the main scanning direction perpendicular to the sub-scanning direction. The printer 104 according to the present embodiment forms an image by a so-called multi-pass method in which the recording head 1044 scans the same area on the recording medium a plurality of times. In addition, pigment ink is ejected from the recording head 1044 to enable formation of a glossy image. As the recording head 1044, a wire dot method, a heat sensitive method, a thermal transfer method, an ink jet method, or the like can be used, and each of them is composed of one or more pixel forming means (nozzles or the like).

  Reference numeral 1045 denotes a moving unit for moving the recording head 1044 and is controlled by the head control unit 1043. A transport unit 1046 transports the recording medium 1047. Further, the ink color and ejection amount selection unit 1042 selects an appropriate ink color from the ink colors mounted on the recording head 1044 according to the output value of the image data from the image processing apparatus 102, and further, the ejection amount. Select.

  FIG. 2 is a diagram illustrating a configuration example of the recording head 1044. In the present embodiment, in addition to the four colors of cyan (C), magenta (M), yellow (Y), and black (K), light cyan (Lc), which has a relatively low ink density compared to cyan and magenta, light Six color pigment inks including magenta (Lm) are mounted on the recording head 1044. As shown in the figure, the recording head 1044 includes a plurality of recording elements (nozzles) for each color.

  2 shows a configuration in which the nozzles of the respective colors are arranged in a line in the paper transport direction for the sake of simplicity of explanation, the number and arrangement of the nozzles are not limited to this example. Any configuration that enables printing can be used. For example, there may be nozzle rows with the same color but different discharge amounts, there may be a plurality of nozzles with the same discharge amount, or the nozzles may be arranged in a zigzag manner. In FIG. 2, the ink color is arranged in a line in the head movement direction, but may be arranged in a line in the paper transport direction.

Image Forming Process Next, the multi-pass image forming process in the image forming system of the present embodiment having the above-described functional configuration will be described with reference to the flowchart of FIG. Hereinafter, in the present embodiment, description will be made on the assumption that multi-pass printing with four passes is performed.

  First, in step S <b> 301, multi-tone color input image data is input from the input terminal 101 and stored in the input image buffer 1021. Here, as the input image data, color image data is constructed by three color components of red (R), green (G), and blue (B).

  In step S <b> 302, the input signal value determination unit 1024 determines the size of each pixel of the image data stored in the input image buffer 1021 from a preset threshold value. This determination result indicates the number of valid passes, and is used as information for setting the output data value of each pass in the recording data setting unit 1025 in the subsequent stage. Although details will be described later in this embodiment, based on the general tendency that the glossiness of a recorded image decreases as the number of passes during image formation increases and decreases as the number of passes increases, the number of recording passes depends on the range of input signal values. It is characterized by controlling. In other words, all pass printing is performed in the intermediate brightness area where the glossiness is relatively high, and the number of recording passes is reduced in the high / low brightness area where the glossiness is relatively low. Suppress. Details of this input signal value determination process will be described later.

  In step S303, the color separation processing unit 1022 performs color separation from multi-tone color input image data stored in the input image buffer 1021 to RGB, CMYK, and LcLm ink color planes using the color separation LUT1023. Processing is performed (S102). The post-color separation data created in this way corresponds to the recording amount of each color in the recording head 1044. In this embodiment, each piece of pixel data after color separation processing is handled as 8 bits, but conversion to a higher number of gradations may be performed. Details of this color separation process will be described later.

  Hereinafter, the processes of S304 to S310 are performed on the post-color separation data created as described above.

  First, in step S304, the recording data setting unit 1025 sets the scan number k and Ycut (k) indicating the Y coordinate as the color separation data cutout position. That is, Ycut (k) is the color separation data cutout position at the scan number k, and corresponds to the nozzle upper end coordinates for the formed image. Details of the setting process of the scan number k and the color separation data cutout position Ycut (k) will be described later.

  In step S305, the recording data setting unit 1025 sets a recording data value indicating the recording amount for each nozzle for each scan based on the determination result in the input signal value determination unit 1024 and the image data of each color plane after color separation. Set. Details of the recording data setting process will be described later. The recording data set in S305 is stored in the recording data storage memory 1027 in S306.

  Next, in S307, the HT processing unit 1028 performs halftone processing for converting the recording data stored in the recording data storage memory 1027 into a smaller number of gradations. Here, the number of gradations of each pixel data of the input image is 8 bits (256 values), and this is converted into 1 bit (binary) by halftone processing. Note that the halftone processing in the present embodiment converts multi-valued input image data into a binary image by a known error diffusion method. Details of the halftone processing in this embodiment will be described later.

  In step S308, the binary image data after halftone processing is stored in the HT image storage memory 1029 as a dot pattern to be formed. In the halftone process of this embodiment, binary image data is sequentially generated for (nozzle number) × (image X size) recording data stored in the recording data storage memory 1027. Accordingly, the HT image storage buffer 1029 similarly secures a memory space for the band data of (number of nozzles) × (image X size).

  As described above, the halftone process at the scan number k = 1 is completed, and as a result, a binary image formed by one head operation is stored in the HT image storage memory 1029.

  In step S <b> 309, (nozzle number) × (image X size) band data stored in the HT image storage memory 1029 is output from the output terminal 103.

  Next, the processing moves to the printer 104 side. First, in step S <b> 310, the printer 104 stores the image data received from the image processing apparatus 102 in the HT image storage memory 1041, and the ink color and discharge amount selection unit 1042 selects an ink color and discharge amount that match the image data. Thus, image formation is started. The image formation in the printer 104 is performed by recording the image on the recording medium by driving each nozzle at a constant driving interval while the recording head 1044 moves from the left to the right with respect to the recording medium.

  In step S311, it is determined whether all scanning has been completed. If completed, a series of image forming processes for multi-tone color input image data is completed. If not completed, the process returns to step S304 to start the process relating to the next scan.

Input signal value determination process (S302)
Hereinafter, the input signal value determination process in the input signal value determination unit 1024 in S302 will be described in detail.

  In general, when recording with a certain amount of ink, if the recording is divided into many passes, the glossiness of the recorded image is relatively low, and if the recording is performed with a small number of passes, the glossiness is relative. Tend to be higher. This is because, in recording divided into many passes, adjacent ink dots on the recording medium are recorded with a time lag, so the fusion of adjacent dots is less than when recording with fewer passes, forming This is because the surface shape of the dots formed is uneven. Accordingly, in the recording divided into many passes, the glossiness is lower than that in the recording with few passes.

  Therefore, in the present embodiment, in order to reduce a phenomenon of so-called gloss unevenness in which glossiness differs for each area of a recorded image, out of all passes (first pass number, four passes in this embodiment) in multipass printing. The number of paths that are substantially valid is determined according to the input signal value. Here, FIG. 4 shows an outline of the path number control in this embodiment. In other words, in the intermediate brightness range where the glossiness is relatively high, printing is performed with the total number of passes (first pass number). On the other hand, in the high lightness region and the low lightness region with relatively low glossiness, recording is performed with a limited number of passes (second pass number, in this embodiment, two passes) smaller than the total number of passes (first pass number). I do. Thereby, the gloss difference between each area | region can be suppressed.

  FIG. 5 shows a flowchart of the input signal value determination process in S302. First, in S1501, the brightness of the pixel currently being processed is compared with a threshold value Th1 for determining a low brightness area. If the brightness of the pixel is equal to or higher than Th1, the process proceeds to S1502, and this time is compared with the threshold value Th2 for determining the high brightness area, and if it is equal to or less than Th2, the all-pass print setting is performed in S1503 because the pixel is within the intermediate brightness area. . On the other hand, if the brightness of the pixel is less than Th1 in S1501, or if the brightness of the pixel is greater than Th2 in S1502, it is determined in S1504 that the pixel corresponds to an area outside the intermediate brightness range, that is, a low brightness area or a high brightness area. Set the limited pass printing. The print settings in S1503 and S1504 are sent to the recording data setting unit 1025 as the determination result in the input signal value determination unit 1024. In step S1505, it is determined whether determination processing has been performed for all pixels in the image data to be formed. If all pixels have been completed, the input signal determination processing is terminated. If there is an unprocessed pixel, processing proceeds to step S1501. Return to start processing the next pixel.

  Here, a method for setting the threshold values Th1 and Th2 will be described with reference to the flowchart of FIG. 6 and the conceptual diagram of FIG.

  First, in S1601, a color chart (patch) adjusted to have a predetermined plurality of brightness values is printed in two passes (two passes) on a predetermined recording medium, and the glossiness of each of the two-pass print patches. Measure L2i. The patch used here is not limited to a patch adjusted to a predetermined lightness, and may be a patch determined according to a predetermined input signal value or a predetermined recording ink amount. 701 shown in FIG. 7 is the glossiness L2i of the two-pass print patch. In step S1602, the same patch as in step S1601 is printed in four passes (four passes), and the glossiness L4i (702) of each four-pass print patch is measured. In step S1603, the average glossiness of the two-pass print patch and the average glossiness of the four-pass print patch are calculated, and the total average glossiness Lave, which is the average, is calculated.

  In step S1604, it is determined whether the difference between the glossiness L2i for 2-pass printing and the glossiness L4i for 4-pass printing is smaller than the total average glossiness Lave for a patch with lightness i. That is, the absolute value of the difference between L2i and Lave is compared with the absolute value of the difference between L4 and Lave. If the former is smaller, the brightness i is set in the two-pass recording area (two-pass setting) in S1605. Otherwise, in S1606, a 4-pass recording area is set (4-pass setting). The above processing is performed until all the brightness values of the patch are completed in S1607. As a result, the number of effective paths at each brightness is set.

  As described above, when the number of effective paths is set for each lightness (input signal value), the lightness for switching from the 2-pass setting to the 4-pass setting and the lightness for switching from the 4-pass setting to the 2-pass setting are acquired. The The brightness as the switching point is determined in S1608 as the threshold value Th1 for determining the low brightness area and the threshold value Th2 for determining the high brightness area. In other words, for lightness lower than Th1, the difference between the total average glossiness is smaller for 2-pass printing than for 4-pass printing, but for lightness between Th1 and Th2, 4-pass printing is the total average over 2-pass printing. The difference from the glossiness is reduced. Further, at a lightness higher than Th2, again, the difference between the total average glossiness is smaller in the 2-pass printing than in the 4-pass printing. Therefore, as shown in FIG. 4, two-pass printing is performed at a lightness lower than Th1, four-pass printing is performed at a lightness between Th1 and Th2, and two-pass printing is performed at a lightness higher than Th2. The number of passes is determined.

  In addition, although the example which switches 2 types of 2 pass printing and 4 pass printing was given here, of course this invention is not this limitation, and it cannot be overemphasized that the same argument holds in the case of n (n> = 2) pass. . In this example, Th1 and Th2 are defined as threshold values. However, a recording pass number table in which the number of pass settings for each lightness is described in advance, and the number of recording passes is determined with reference to the table. Is also possible.

  As described above, in S302, the number of effective paths corresponding to each pixel of the image data stored in the input image buffer 1021 is determined based on the magnitude relationship with the threshold set as described above, and the determination result To the recording data setting unit 1025.

Color separation process (S303)
Hereinafter, the details of the color separation processing in S303 will be described with reference to FIG. FIG. 8 shows details of input / output data in the color separation processing unit 1022. As shown in the figure, the input image data R′G′B ′ is converted into CMYKLcLm data by referring to the color separation LUT1023 as shown in the following equation.

C = C_LUT_3D (R ′, G ′, B ′) (1)
M = 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)
Here, each function defined on the right side of the equations (1) to (6) corresponds to the contents of the color separation LUT 1023. The color separation LUT 1023 determines an output value for each ink color from three input values of red, green, and blue. In the present embodiment, since the configuration includes six colors CMYKLcLm, an LUT configuration that obtains six output values from three input values is obtained.

  With the above processing, the color separation processing in this embodiment is completed.

Scan number and color separation data cut-out position setting process (S304)
Hereinafter, the setting process of the scan number k and the color separation data cutout position Ycut (k) in the recording data setting unit 1025 in S304 will be described in detail. Hereinafter, an example will be described in which the recording head 1044 arranges eight nozzles in a line and forms an image by four scans in the same main scanning recording area on the image. Here, for the sake of simplicity, the description will be made assuming that the paper feed amount LF (k) after scanning at the scanning number k is constant, but it may not be constant in practice. The initial value of the scan number k is 1, and is incremented for each processing loop.

  In general, in the case of four-pass printing, as shown in FIG. 9, at the scanning number k = 1, image formation is performed using only the lower end 1/4 (nozzle numbers n7, n8) of the nozzle 501. Subsequently, at scan number k = 2, after feeding a quarter of the nozzle length with respect to scan number k = 1 (that is, LF (1) = 2), the lower end 1/2 of nozzle 501 (nozzle number). n5, n6, n7, n8) are used to form an image. Further, at scanning number k = 3, after feeding the paper for 1/4 of the nozzle length with respect to scanning number k = 2 (ie, LF (2) = 2), the lower end of nozzle 3/4 (nozzle numbers n3 and n4). , N5, n6, n7, n8). By repeating the image formation and paper feeding as described above, a final output image 502 is formed, and the color separation data cutout position corresponding to the nozzle upper end coordinate at the scan number k = 1 is Ycut (1) = − 6.

  When Yfeed is generalized when the paper feed amount LF (k) is constant, Ycut (k) at the scan number k is given by the following equation when the number of nozzle rows is Nzzl and the number of print passes is Pass. It is done.

Ycut (k) = − Nzzl + (Nzzl / Pass) × k (7)
In the present embodiment, Ycut (k) set according to the scan number and the number of passes as described above is held in the print data setting unit 1025 in advance.

Recording data setting process (S305)
Hereinafter, a recording data setting method in the recording data setting unit 1025 in S305 will be described with reference to FIG. In the present embodiment, for simplicity, an example in which the paper feed amount LF is constant is shown.

  As shown in FIG. 10, a recording data division ratio 802 indicating a ratio of dividing the input data is previously given to each nozzle in the recording head 1044 for each scan. In 802, the horizontal axis represents a recording data division ratio set as a real value that takes a value from 0 to 1 when the input data is 1. The vertical axis represents an address indicating the position of the image in the Y-axis direction, which is the paper feed direction. It should be noted that a known method can be applied to the setting of the print data division ratio for each nozzle in multi-pass printing, and a detailed description thereof will be omitted here.

  In this embodiment, the recording data division ratio 802 is controlled according to the determination result (effective path setting) from the input signal value determination unit 1024 described above. Prior to describing the control of the recording data division ratio in this embodiment, a recording data setting process based on a general recording data division ratio 802 will be described.

  The print data 803 for each nozzle is set by the product of the post-color separation data 801 and the print data division ratio 802. However, as shown in FIG. 11, when the corresponding nozzle is outside the area of the Y address of the image, the recording data is set to 0. For example, when the scan number is k = 1, as shown in FIG. 11, the print data value = 0 is substituted into the nozzle row (upper end 3/4) in which the image Y address is negative, and the nozzle in which the image Y address is positive. Significant values are assigned to the columns (bottom 1/4).

  Since the color separation data cut-out position Ycut (k) is determined by the scan number k, when the scan number k = 1 to 7, the print data is determined as shown in FIG. In FIG. 11, the print data for the nozzle positions for each scan number is shown, and it can be seen that the print data differs for each scan number. Each recording data in FIG. 11 is determined by the product of the color separation data and the recording data division ratio. Accordingly, when taking the product of the print data division ratio while feeding the paper, the print data (1) formed by four scans (4 passes) with the scan number k = 1 to 4 in the area 1 in FIG. The total value of the raster) is the same as the color separation data. Similarly, in the areas 2, 3, and 4, the total value of one raster is the same as the color separation data. That is, the sum of the print data division ratios set for each of the four scans forming a certain area is 1, and the sum of the print data for each nozzle for the same area on the print medium is equal to the data after color separation. .

  The recording data setting process as described above is expressed by the following equation. C (X, Y), Lc (X, Y), M (X, Y), Lm (X, Y), Y (X, Y), and K (X, Y) are addresses (X, Y). ) Shows the color separation data 801 of each color. Also, C_d (X, Y), Lc_d (X, Y), M_d (X, Y), Lm_d (X, Y), Y_d (X, Y), K_d (X, Y) are the addresses (X, Y ) Shows the recording data 803 of each color. The recording data division ratio 802 is assumed to be stored in advance in the recording data setting unit 1025 as an LUT, and S_LUT (Y) indicates the value of the recording data division ratio 802 at address Y. Also, 0 ≦ nx <image X size, 0 ≦ ny <Nzzl (number of nozzle rows: 8 in this case).

C_d (nx, ny) = C (nx, Ycut (k) + ny) × S_LUT (ny) (8)
Lc_d (nx, ny) = Lc (nx, Ycut (k) + ny) × S_LUT (ny) (9)
M_d (nx, ny) = M (nx, Ycut (k) + ny) × S_LUT (ny) (10)
Lm_d (nx, ny) = Lm (nx, Ycut (k) + ny) × S_LUT (ny) (11)
Y_d (nx, ny) = Y (nx, Ycut (k) + ny) × S_LUT (ny) (12)
K_d (nx, ny) = K (nx, Ycut (k) + ny) × S_LUT (ny) (13)
As described above, the print data of each nozzle in multi-pass printing is set based on the print data division ratio 802.

  As described above, in this embodiment, the recording data division ratio 802 is controlled according to the determination result (effective path setting) in the input signal value determination unit 1024. That is, if the determination result in the input signal value determination unit 1024 indicates a 4-pass setting, no control is performed on the recording data division ratio 802. However, when the determination result indicates a 2-pass setting, the recording data division rate divided into four passes is distributed to any two of the four passes, and the division rate into the other two passes Is set to 0. That is, control is performed so that the recording data division ratio for the same region is 1 in total for only two passes. Of course, this control is performed only for the pixels determined to be set for two passes, and the print data division ratio is restored to the original when the processing for the pixels determined to be set for four passes in the same scan is reached. As a result, for the pixel area determined to be set for two passes, the recording head 1044 scans four times, but actual image formation is performed twice (two passes).

  As described above, in S305, the print data of each nozzle is set according to the set number of effective passes.

● Halftone processing (S307)
Hereinafter, the halftone process in the HT processing unit 1028 in S307 will be described in detail with reference to FIGS. Here, in order to simplify the description, a cyan halftone process in four-pass printing and scan number k = 1 will be described as an example. FIG. 13 is a block diagram illustrating a detailed configuration of the HT processing unit 1028, and FIG. 12 is a flowchart illustrating details of halftone processing in the HT processing unit 1028.

  First, in S201 shown in FIG. 12, cyan recording data is input to the HT processing unit 1028. Next, in S202, the accumulated error is added for error diffusion processing. The cumulative error adding process will be described in detail below.

  In this embodiment, it is assumed that there are four coefficients K1 to K4 as error diffusion coefficients for error diffusion processing as shown in FIG. For example, K1 = 7/16, K2 = 3/16, K3 = 5/16, and K4 = 1/16. However, the diffusion coefficient does not need to be fixed as described above, and may be changed according to the gradation, or may be more than the above four coefficients.

  In order to diffuse and accumulate errors using such error diffusion coefficients, the HT processing unit 1028 secures four sets of accumulated error line buffers (2092 to 2095), and switches the accumulated error line buffer to be used for each scan number. The cumulative error line buffer 2902 has one storage area E1_0 and the same number of storage areas E1_ (x) (x = 1 to W) as the number of horizontal pixels of the input image, and 2903, 2904, and 2905 are the same. Storage area. Further, the accumulated error line buffers 2902, 2903, 2904, and 2905 are all initialized with an initial value 0 only at the start of processing of the scan numbers k = 1, 2, 3, and 4, respectively. In the present embodiment, the above-described four sets of accumulated error line buffers are required for halftone processing of one color, and therefore, this requires six lines, that is, a total of 4 × 6 = 24 sets of line buffers. .

  That is, in S202, the error E1_ (x) corresponding to the horizontal pixel position x is added to the recording data by the cumulative error adding unit 2906. That is, if the input recording data of the target pixel is C_d and the data after the accumulated error addition is C_d ′, the following equation is established.

C_d '= C_d + E1_ (x) (14)
In step S <b> 203, the threshold selection unit 2907 selects the threshold T. The threshold value T is set as follows, for example.

T = 128 (15)
Alternatively, in order to avoid dot generation delay, the threshold value T may be finely changed as follows according to the input recording data C_d of the target pixel so that the average quantization error is reduced.

T = f (C_d) (16)
Alternatively, the threshold T may be finely changed as follows according to the address (X, Y) in the band.

T = f (X, Y) (17)
In step S <b> 204, the quantization unit 2908 compares the pixel data C_d ′ after the error addition with the threshold T, and determines Out_c, which is the binarization result of the dots. The rules are as follows:

When C_d '<T,
Out_c = 0 (18)
When C_d '≧ T,
Out_c = 255 (19)
In step S <b> 205, the error calculation unit 2909 calculates a difference Err between the pixel data C_d ′ obtained by adding the error to the recording data C_d of the target pixel and the output pixel value Out_c as follows.

Err (x) = C_d'-Out_c (20)
Next, in S206, the error diffusion unit 2910 diffuses the error. That is, using the (4n + 1) cumulative error line buffer 2902, the error Err (x) is diffused according to the horizontal pixel position x as follows.

E1_ (x + 1) ← E1_ (x + 1) + Err (x) × 7/16 (x <W)
E1_ (x-1) ← E1_ (x-1) + Err (x) × 3/16 (x> 1)
E1_ (x) ← E1_0 + Err (x) × 5/16 (1 <x <W)
E1_ (x) ← E1_0 + Err (x) × 8/16 (x = 1)
E1_ (x) ← E1_0 + Err (x) × 13/16 (x = W)
E1_0 ← Err (x) × 1/16 (x <W)
E1_0 ← 0 (x = W)
···(twenty one)
This completes the binarization (quantization value 0, 255) for one cyan pixel of scan number k = 1.

  Halftone image data can be generated by performing the processes in steps S201 to S206 described above for all the pixels in the band (S207).

  The scanning number k = 1 has been described above. However, for the scanning numbers k = 2 to 4, the halftone process is performed using the cyan accumulated error line buffers 403 to 405. That is, (4n + 2) cumulative error line buffer 2903 is used for scan number k = 2, (4n + 3) cumulative error line buffer 2904 is used for scan number k = 3, and (4n + 4) cumulative error line buffer 2905 is used for scan number k = 4. Then, the halftone process is performed. In the process of scanning number k = 5, the same (4n + 1) cumulative error line buffer 2902 as scanning number k = 1 is used as it is without being initialized (all 0s are not substituted). This is because the print areas of scan number k = 1 and scan number k = 5 are vertically adjacent to each other, and thus the stored accumulated error is applied to the adjacent lower area as it is. If the accumulated error buffer 2902 is initialized with k = 5, the error is not stored at the boundary adjacent to k = 1, and the continuity of dots cannot be maintained.

  Note that the halftone process in the present embodiment is not limited to the above binarization process, but by applying an N-value conversion process (N is an integer of 2 or more) to multi-valued input image data, However, it may be anything that can be converted into an N-value image having a small number of gradations. As the method, instead of the error diffusion method, a blue noise mask type, Bayer type dither method, or density pattern method may be used. Or you may utilize combining these several methods.

Effects of the Present Embodiment As described above, according to the present embodiment, in the pigment ink jet printer of the multi-pass printing method, the substantial number of passes for forming an image is changed according to the value of input multi-value data. Accordingly, uneven glossiness of the formed image can be suppressed.

<Other embodiments>
The present invention can take the form of, for example, a system, apparatus, method, program, or storage medium (recording medium). Specifically, the present invention may be applied to a system composed of a plurality of devices (for example, a host computer, an interface device, an imaging device, a web application, a printer, etc.), or applied to a device composed of a single device. May be.

  In addition, the present invention can also realize processing equivalent to that in the above embodiment using a computer program. In this case, each of the components including FIG. 1 may be functioned by a function or a subroutine executed by the CPU. Further, the computer program is usually stored in a computer-readable storage medium such as a CD-ROM, and is set in a reading device (CD-ROM drive or the like) included in the computer and copied or installed in the system. It becomes executable. Therefore, it is obvious that such a computer readable storage medium is also within the scope of the present invention.

Claims (10)

An image forming apparatus in which a recording head including a plurality of recording elements forms an image by performing a plurality of main scans on the same area on a recording medium,
Image data input means for inputting multivalued image data;
Pass number setting means for setting the number of passes, which is the number of main scans for forming an area corresponding to the image data on the recording medium, according to the brightness of the image data;
Print data setting means for setting print data for each print element according to the number of passes for each main scan of the print head;
N-value conversion means for creating a dot pattern to be formed by applying N-value processing (N is an integer of 2 or more) to the print data set by the print data setting means;
An image forming apparatus comprising:   The pass number setting means sets the first pass number as the pass number when the brightness of the image data is within a predetermined intermediate brightness range, and when the pixel is outside the intermediate brightness range. The image forming apparatus according to claim 1, wherein a second pass number smaller than the first pass number is set.   The intermediate brightness range is measured when a color chart composed of a plurality of brightness values is formed with the first pass number, and when the color chart is set with the second pass number. The image forming apparatus according to claim 2, wherein the image forming apparatus is set based on a gloss level.   The recording data setting means divides the recording amount according to the image data based on the number of passes and a division ratio determined in advance according to the position of the recording element in the recording head. 4. The image forming apparatus according to claim 3, wherein recording data is set for each. The division ratio is determined based on the first number of passes,
5. The recording data setting unit according to claim 4, wherein when the second pass number is set as the pass number, the division ratio is changed based on the second pass number. Image forming apparatus.   The recording data setting unit is configured so that a total of recording data for each recording element with respect to an area corresponding to the image data on the recording medium is equal to a recording amount corresponding to the image data. 6. The image forming apparatus according to claim 4, wherein recording data is set.   The image forming apparatus according to claim 1, wherein the recording head ejects pigment ink. A control method for an image forming apparatus, in which a recording head including a plurality of recording elements forms an image by performing main scanning a plurality of times on the same area on a recording medium,
An image data input step for inputting multi-value image data;
A pass number setting step for setting a pass number that is the number of times of main scanning for forming an area corresponding to the image data on the recording medium in accordance with the brightness of the image data;
A recording data setting step for setting recording data for each recording element according to the number of passes for each main scan of the recording head;
An N-value conversion step for creating a dot pattern to be formed by applying an N-value conversion process (N is an integer of 2 or more) to the print data set in the print data setting step;
A control method for an image forming apparatus, comprising:   A computer program for causing a computer to function as each unit of the image forming apparatus according to claim 1 by being executed by the computer.   A computer-readable storage medium storing the program according to claim 9.

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