JP2010155390A - Image forming apparatus and method for controlling the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a method of exclusively arranging dots having different colors, although effective unless there is no error on a strike position of any dot when a color image is formed by printing by superposing of inks having two or more colors, actually generates an error due to a strike position of a satellite occurs so that color irregularities occur. <P>SOLUTION: A scanning duty setting section 105 calculates out a scanning duty per nozzle on the basis of input image data by each scanning of a recording head. A halftone processing section 108 generates a dot pattern to be formed by applying binarization based on predetermined restriction information to the calculated scanning duty. A restriction information computing section 113 generates restriction information that is referred to by the halftone processing section 108 about a second color to be processed next, on the basis of the scanning duty calculated about a first color, the dot pattern, and an amount of satellites detected by a satellite amount detecting section 207. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and a control method thereof, and in particular, a color image is provided by including a recording head having a plurality of recording elements for discharging a recording agent for each of a plurality of colors and scanning the recording head on a recording medium. The present invention relates to an image forming apparatus and a control method therefor.

  As an image output device such as a word processor, personal computer, or facsimile, a recording device that records information such as desired characters and images on a sheet-like recording medium such as paper or film is generally used. As such a recording apparatus, there are various types of recording methods, and among them, a method of forming an image on a recording medium by attaching a recording agent to the recording medium has been widely put into practical use. As a representative example, an ink jet recording method is known.

  In a recording apparatus to which an ink jet recording method is applied, a nozzle group in which a plurality of ink discharge ports (nozzles) capable of discharging ink of the same color and the same density are arranged in an integrated manner in order to improve recording speed, improve image quality, etc. Is provided. Furthermore, in order to improve image quality, a nozzle group that can eject ink of the same color and different density, or a nozzle group that can eject by changing the ejection amount of ink of the same color and the same density in several stages May be provided. These nozzle groups are provided for a plurality of colors.

  One factor of image quality degradation in such an ink jet recording apparatus is the generation of satellites. A satellite is a minute droplet (dot) formed by dividing an ink droplet ejected from a nozzle into a plurality of ink droplets in addition to a main droplet. Since satellites have very small ink droplets, they are easily affected by air currents during flight, and errors in landing positions are likely to increase, causing image quality deterioration such as density unevenness.

  Such deterioration in image quality due to satellites is particularly noticeable in image formation with a small number of inks used, such as monochrome printing. Therefore, in such a case, a method has been proposed in which the occurrence of satellite landing position errors is suppressed and the image quality deterioration is suppressed by lowering the nozzle drive frequency (see, for example, Patent Document 1). In order to make the influence of satellites unnoticeable, a method using a dot-concentrated recording pattern has been proposed (for example, see Patent Document 2).

  Further, as another factor of image quality deterioration in the ink jet recording apparatus, there is a deterioration in graininess caused by overlapping dots formed by different color inks. In general, an ink jet recording apparatus reproduces various colors of inks of limited colors such as CMYK by changing the recording rate of each color for each region. Therefore, dots of different colors are formed overlapping each other. At this time, if the dot arrangement is determined independently for each color, the overlap of the dots of different colors has a signal on the low frequency component side in the spatial frequency region, and color unevenness occurs and the graininess deteriorates. .

  Therefore, not only optimizing the spatial frequency of the dots of each color so as not to be visually noticeable, but also a method of arranging the dots of different colors exclusively so as not to overlap as much as possible (for example, Patent Documents). 3).

Furthermore, a method for controlling the order and arrangement of dots when forming dots with an image forming apparatus has been proposed (see, for example, Patent Document 4). According to such a technique, by applying an error diffusion method or the like for each scan, it is possible to suppress a decrease in image quality due to density unevenness even when registration of each scan varies. Specifically, main scanning is performed a plurality of times with different nozzle groups on the same main scanning recording area on a predetermined recording medium, and a binary image is formed by the error diffusion method for each main scanning. As described above, when the binary image is generated by performing the error diffusion method for each main scan, the dispersibility of the dot arrangement in the main scan is high and uniform. Therefore, when an image is formed by a plurality of main scans, even if physical registration such as the feeding amount of the recording medium and the position of the recording element fluctuates, the change in graininess is less likely to occur. is there. Furthermore, since there is little correlation in the dot arrangement between a plurality of scans, even if the registration fluctuates, the change in dot coverage with respect to the paper surface is reduced, and density unevenness is considerably mitigated.
JP 2005-238673 A JP 2006-159791 A JP 2003-304408 A JP 2000-103088 A

  However, according to the method of lowering the nozzle driving frequency described in Patent Document 1, it is possible to suppress image quality deterioration due to satellites, but the recording speed is reduced. In recent years, inkjet recording apparatuses have been required to improve the recording speed together with the image quality. Therefore, there is a need for a method that can suppress deterioration in image quality due to satellites without reducing the recording speed.

  Further, according to the method using the dot concentration type recording pattern described in Patent Document 2, since the dots are concentrated, the aggregate of dots is visually noticeable, and the graininess is deteriorated. There was a problem.

  Further, in the method of exclusively arranging different color inks described in Patent Document 3, it is possible to effectively control the overlap between dots if there is no error in the landing positions of the dots. , Improve the image quality. However, in an actual printer, an error occurs in the dot landing position, and in particular, the satellite landing position error is large. Accordingly, there is a problem in that overlapping of dots does not occur as designed and color unevenness occurs.

  Further, according to the method for controlling the dot formation order and arrangement described in Patent Document 4, since the correlation between dot patterns recorded in different main scans is small, the dot patterns are generated by a plurality of scans. There is a problem that low frequency components of an image are emphasized. This low-frequency component is emphasized as the number of scans is increased, and is recognized as graininess that is visually obtrusive.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus having the following functions and a control method thereof. That is, by causing the N-value calculation for the first color image data to reflect the ejection state of the printing element for the second color image data that has already been converted to N-value, the color unevenness caused by the occurrence of satellites is reflected. To achieve high image quality.

  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 that includes a recording head having a plurality of recording elements for discharging a recording agent for each of a plurality of colors and forms a color image by scanning the recording head group on a recording medium. In-scan recording amount calculation means for calculating a recording amount for each recording element in the main scanning of the recording head according to a plurality of colors of image data, and constraints referred to when N-value processing is performed on the image data An N-value conversion process (N is an integer of 2 or more) based on the constraint information is applied to the recording amount calculated by the constraint information generation unit for generating information and the recording amount calculation unit for scanning. N-value conversion means for creating a dot pattern; and discharge state detection means for detecting the discharge state of the recording agent in the recording element. The N-value conversion means is configured to detect the first color in the same main scan. Image de N-value conversion processing is performed prior to the N-value conversion processing on the image data of the second color, and the constraint information creating means is configured to perform the first color conversion on the first color previously performed in the same main scanning. Recording amount calculated by the in-scan recording amount calculation unit, a dot pattern created by the N-value conversion unit, and a discharge state of the recording agent detected by the discharge state detection unit The constraint information for the second color image data is created based on the second color image data.

  For example, the constraint information creation means may be a space of the second color dot pattern created by the N-value quantization means with respect to the first color dot pattern previously created by the N-value quantization means. The constraint information is created so that the phase of the low frequency component in the frequency domain is opposite.

  Further, the constraint information creating means is a space for the second color dot pattern created by the N-valued means with respect to the first color dot pattern previously created by the N-valued means. The constraint information is created so that the phase of the high frequency component in the frequency domain is uncorrelated.

  For example, the constraint information creating means performs the filtering process on each of the recording amount for the first color image data and the dot pattern, thereby obtaining the constraint information for the second color image data. The size of the filter created and applied to each of the recording amount and the dot pattern for the first color image data is set according to the discharge state of the recording agent.

  According to the present invention having the above-described configuration, by reflecting the ejection state of the printing element for the second color image data that has already been N-valued in the N-value conversion operation for the first color image data, Color unevenness due to the occurrence of satellites is reduced, and high image quality is achieved.

  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>
FIG. 1 is a block diagram showing the configuration of the image forming system according to the present embodiment. In FIG. 1, 1 is an image processing apparatus, and 2 is a printer. The image processing apparatus 1 can be implemented by a printer driver installed in a general personal computer, for example. In that case, each part of the image processing apparatus 1 described below is realized by a computer executing a predetermined program. As another configuration, for example, the printer 2 may include the image processing apparatus 1.

  The image processing apparatus 1 and the printer 2 are connected by a printer interface or a circuit. The image processing apparatus 1 inputs image data to be printed from the image data input terminal 101 and stores it in the input image buffer 102. The color separation processing unit 103 separates the input image data into ink colors provided in the printer 2. In this color separation process, a color separation lookup table (LUT) 104 is referred to.

  The scan duty setting unit 105 further converts each ink color value separated by the color separation processing unit 103 into an ink color value of each nozzle for each scan based on the scan duty setting LUT 106, and stores it in the scan duty buffer 107. Store the data. The scanning duty data set here indicates the ink recording amount for each nozzle in each scanning, and therefore the scanning duty setting unit 105 calculates the in-scan recording amount.

  The halftone processing unit 108 uses the binary image based on the values stored in the constraint information buffer 109 described later, with respect to the multi-gradation (three or more gradations) value of each color obtained by the scanning duty setting unit 105 for each color. Convert to data. The binary image obtained here is a formation target in the printer 2.

  The constraint information buffer 109 stores information indicating whether or not dots are likely to be formed at addresses on the recorded image. 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 creates constraint information by a predetermined calculation based on the binary image data stored in the halftone image storage buffer 110 and the duty data for each scan stored in the scan duty buffer 107. The contents of the buffer 109 are updated.

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

  The printer 2 includes a recording head for each of a plurality of colors, and scans the recording head group on the recording medium 202 to form binary image data formed by the image processing apparatus 1 on the recording medium. As the recording head 201, a thermal transfer method, an electrophotographic method, an ink jet method, or the like can be used, and each has one or more 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 under the control of the head control unit 204. The ink color selection unit 206 selects an ink color from a plurality of recording agents mounted on the recording head 201 based on binary image data of each color formed by the image processing apparatus 1.

  Further, the satellite amount detection unit 207 detects the ink ejection state at each nozzle. That is, when ink is ejected from each nozzle, the generation amount of satellites, which are minute droplets generated by dividing from the main droplet, is detected by a built-in sensor. The detected satellite generation amount is transferred to the constraint information calculation unit 111 in the image processing apparatus 1 through the satellite amount input terminal 208.

  The satellite generation amount detected by the satellite amount detection unit 207 is detected as the number of satellites generated and the distribution of the landing positions of the satellites with respect to the landing positions of the main droplets. Specifically, assuming that the landing position of the main droplet is (Mx, My) and the landing position of the satellite is (Sxn, Syn), the following values and combinations thereof are considered as the satellite generation amount. Here, n is an integer from 0 to the number of satellites n_sate, and Σ in the following expression indicates the accumulation from 0 to n_sate.

・ Number of satellite occurrences ・ Ratio of main drop dot diameter (or area) to satellite dot diameter (or area) ・ Square sum of distance between main drop landing position and satellite landing position
Σ {(Sxn−Mx) ² + (Syn−My) ² } (1)
・ Center position of satellite landing position
Σ (Sxn−Mx) ² , Σ (Syn−My) ² (2)
As described above, there are satellite generation amounts represented by one value and those having a value according to the x and y directions. Further, the amount of satellite generated is not limited to the above.

  When the amount of satellite generation differs depending on the ink, the amount of satellite generation of each ink may be passed to the constraint information calculation unit 111, or the maximum value or average value of the satellite generation amount, such as 1 One representative value may be passed to the constraint information calculation unit 111.

  FIG. 2 is a diagram illustrating a configuration example of the recording head 201. In the present embodiment, light cyan (Lc) and light magenta (Lm) having a relatively low ink density are used in addition to cyan (C), magenta (M), yellow (Y), and black (K) inks. The six inks including the same are mounted on the recording head 201.

  2 shows a configuration in which nozzles are arranged in a line in the paper conveyance direction for the sake of simplicity, the number and arrangement of nozzles are not limited to this example. 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 image forming process in 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 input terminal 101 and stored in the input image buffer 102 (S101). Here, as the input image data, color image data is constructed by 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 on the multi-tone color input image data stored in the input image buffer 102 using the color separation LUT 104. A decomposition process is performed (S102). In this embodiment, each pixel data after color separation processing is handled as 8 bits, but conversion to a higher number of gradations may be performed.

  As described above, the recording head 201 in this embodiment has six types of ink colors. Therefore, RGB color input image data is converted to 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 ink colors is generated.

  Hereinafter, the details of the color separation processing in the present embodiment will be described with reference to FIG.

  FIG. 4 shows details of input / output data in the color separation processing unit 103. As shown in the figure, the input image data RGB is converted into CMYKLcLm data according to the following equation with reference to the color separation LUT 104.

C = C_LUT_3D (R, G, B) (3)
K = M_LUT_3D (R, G, B) (4)
Y = Y_LUT_3D (R, G, B) (5)
K = K_LUT_3D (R, G, B) (6)
Lc = Lc_LUT_3D (R, G, B) (7)
Lm = Lm_LUT_3D (R, G, B) (8)
Here, each function defined on the right side of the equations (3) to (8) corresponds to the contents of 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 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.

Scanning Duty Setting Process Returning to FIG. 3, the scanning duty setting unit 105 next sets the scanning number k and Ycut (k) indicating the Y coordinate as the color separation data cutout position (S103). That is, Ycut (k) is the color separation data cutout position at the scan number k and corresponds to the nozzle upper end coordinates. Note that the initial value of the scan number k is 1, and is incremented by 1 for each processing loop.

  Here, as an example of 4-pass printing having 16 nozzle rows and forming an image in four scans for the same main scanning recording area on the image, the color separation data cutout position Y coordinate Ycut is set. A setting method will be described.

  In general, in the case of 4-pass printing, as shown in FIG. 5, with the initial value of the scan number (k = 1), image formation is performed using only the nozzle lower end 1/4, and when the scan number k = 2, scanning is performed. Image formation is performed after feeding the paper for a quarter of the nozzle length for the number k = 1. Further, when the scan number k = 3, the paper is fed by 1/4 of the nozzle length with respect to the scan number k = 2, and then an image is formed. By repeating such image formation and paper feeding, a final output image is formed. Therefore, when the scan number k = 1, the color separation data cutout position Ycut = −12 corresponding to the nozzle upper end coordinates.

  When the above-described color separation data cutout position Ycut (k) is generalized, it is given by the following equation as the number of nozzle rows: Nzzl, the number of passes: Pass, and the scan number: k.

Ycut (k) = − Nzzl + (Nzzl / Pass) × k (9)
When Ycut (k) is set as described above, the scanning duty setting unit 105 next sets a duty value for each scanning based on the scanning duty setting LUT 106 and the image data of each color separation processing plane (S104). .

  According to the scan duty setting LUT 106, in the case of four passes, values as shown in FIG. 6 are given. FIG. 6 shows an example of 16 nozzles and 4 passes, where the vertical axis indicates the nozzle position and the horizontal axis indicates the duty division ratio. According to FIG. 6, inflection points P1 to P4 are set for every four nozzles, and the Duty division ratio for 16 nozzles obtained by linearly interpolating each inflection point is held as the scan duty setting LUT 106. Here, the numerical values of P1 to P4 are set as follows.

P1 + P2 + P3 + P4 = 1.0 (10)
The value held as the scan duty setting LUT 106 is not limited to the above setting method. For example, inflection points may be set finely or may be directly specified for each nozzle.

  The scan duty set in step S104 is set as the product of the scan duty setting LUT 106 and color separation data, as shown in FIG. That is, as shown in the left term of FIG. 7, by multiplying the data after color separation by the Duty division ratio set for each nozzle, the result is shown in the right term of FIG. It is set as the scanning duty for each nozzle. Thereby, at the time of actual scanning, each nozzle forms an image with respect to the target color separation data by ejecting ink corresponding to the scanning duty.

  Here, in the present embodiment, the scanning duty is set to 0 when the corresponding nozzle is outside the area of the image Y address. For example, when the scan number is k = 1, as shown in FIG. 8, since the image Y address becomes negative at the nozzle row upper end 3/4, the scan duty value = 0 is substituted, and the nozzle row lower end 1/4 is significant. A value is assigned.

  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 scan duty is determined as shown in FIG. In FIG. 9, the scan duty with respect to the nozzle position for each scan number is shown, and it can be seen that the scan duty differs for each scan number. Each scan duty in FIG. 9 is determined by the product of the color separation data and the scan duty setting LUT 106. Therefore, when the product with the LUT is taken while feeding paper, the scan number k = 1 to 4 in the area 1 portion. The total value of one raster formed by four scans 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.

  The scan duty value is set for each of C, M, Y, K, Lc, and Lm in the scan duty setting unit 105, and the set scan duty data for each color is stored in the scan duty buffer 107 (S105). ). That is, as shown in FIG. 10, the scan duty buffer 107 stores, for each color, a band-like scan duty data value corresponding to the number of nozzles in the vertical direction and the X size of the image in the horizontal direction.

Halftone Processing Next, in the halftone processing unit 108, the total value of the scan duty data stored in the scan duty buffer 107 and the constraint information data stored in the constraint information buffer 109 is converted into a two-level tone value (2 Halftone processing for conversion into (value data) is performed (S106).

  As shown in FIG. 11, the constraint information buffer 109 also stores a band-like constraint information data value corresponding to the number of nozzles in the vertical direction and the X size of the image in the horizontal direction for each color. The constraint information buffer 109 stores constraint information indicating whether or not a binary image is likely to be formed at an address on the recorded image, and is updated for each scan number k. However, at the start of the process of scan number k = 1, all 0s are substituted as initial values. 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), these are described as follows at the start of scanning number k = 1. Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl (number of nozzle rows: 16 in this case).

C_r (nx, ny) = 0 (11)
Lc_r (nx, ny) = 0 (12)
M_r (nx, ny) = 0 (13)
Lm_r (nx, ny) = 0 (14)
Y_r (nx, ny) = 0 (15)
K_r (nx, ny) = 0 (16)
The smaller the value of the constraint information, the more difficult it is to form a dot at that location. Conversely, the larger the value, the easier the dot is formed. Note that the value stored in the constraint information buffer 109 is controlled so that the average value becomes 0 at any scanning number timing. Specifically, 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 update of the constraint information will be described later.

  In the halftone processing in the present embodiment, for example, a well-known error diffusion method is used as a process for converting multi-value 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). Use. Hereinafter, the halftone process according to the present embodiment will be described in detail with reference to the block diagram of FIG. 12 and the flowchart of FIG. 13, but in order to simplify the description, the cyan half in the 4-pass printing and the scan number k = 1. A tone process will be described as an example. The same processing is performed for other color inks. FIG. 12 is a block diagram showing a detailed configuration of the halftone processing unit 108, and FIG. 13 is a flowchart showing details of the halftone processing in the halftone processing unit 108.

  First, in step S201 shown in FIG. 13, the total of cyan scanning duty and constraint information is input. That is, the constraint information adding unit 401 illustrated in FIG. 12 calculates the total value data Ic of C_d that is cyan scanning duty and cyan constraint information C_r as follows. However, when the scan number k = 1, all the constraint information C_r is 0.

Ic = C_d + C_r (17)
In step S202, the accumulated error adding unit 406 adds the accumulated error Ec1 (x) corresponding to the horizontal pixel position x of the input pixel data to the input data value that is the sum of the scan duty and the constraint information. That is, if the input data of interest is Ic and the data after adding the accumulated error is Ic ′, the following equation is established.

Ic '= Ic + Ec1 (x) (18)
In this embodiment, in order to perform error diffusion and accumulation, the halftone processing unit 108 secures four sets of accumulated error line buffers for cyan (402 to 405 in FIG. 12), and uses accumulated error line buffers to be used. Switch for each scan number. Specifically, n is an integer greater than or equal to 0, and switching is performed as follows.

・ When scan number k = 4n + 1, use cyan (4n + 1) cumulative error line buffer 402 ・ When scan number k = 4n + 2, use cyan (4n + 2) cumulative error line buffer 403 ・ When scan number k = 4n + 3, cyan (4n + 3) cumulative error line buffer 404 is used. When scan number k = 4n + 4, cyan (4n + 4) cumulative error line buffer 405 is used. The four storage areas 1401 to 1404 shown in FIG. That is, there are four sets of “Ec1_0, Ec1 (x)”, “Ec2_0, Ec2 (x)”, “Ec3_0, Ec3 (x)”, and “Ec4_0, Ec4 (x)”. For example, the cyan (4n + 1) cumulative error line buffer 402 has one storage area Ec1_0 and the same number of storage areas Ec1_ (x) (x = 1 to W) as the number of horizontal pixels W of the input image.

Next, in step S203, the threshold selection unit 407 selects the threshold T. For example, a constant value (T = 128) may be used as the threshold value T, or the threshold value T is changed according to C_d as follows so as to reduce the average quantization error in order to avoid dot generation delay. You may do it. T = f (C_d) (19)
Next, in step S204, the quantization unit 408 compares the pixel data Ic ′ 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 Ic '<T Out_c = 0 (20)
When Ic ′ ≧ T, Out_c = 255 (21)
In step S205, the error calculation unit 409 calculates a difference Err_c between the pixel data Ic ′ obtained by adding the error to the target pixel Ic and the output pixel value Out_c as follows.

Err_c (x) = Ic′−Out_c (22)
In step S206, the error diffusion unit 410 diffuses the error generated in step S205 to surrounding pixels. In the present 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 (for example, according to C_d), or may be more than the above four coefficients. good.

  When scan number k = 1, diffusion of miscalculation can be expressed by the following equation.

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)
···(twenty three)
The processing in steps S201 to S206 described above is performed for the addresses (0, 0) to (W-1, Nzzl-1) in the band (S207), so that the dot position of the halftone image data, that is, the dot is turned on. / Off can be determined.

  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. In the process of scan number k = 5, the same cyan (4n + 1) cumulative error line buffer 402 as scan number k = 1 is used as it is without being initialized (all 0s are not substituted). This is because, as shown in FIG. 16, the print areas of scan number k = 1 and scan number k = 5 are adjacent to each other in the vertical direction, so that the stored accumulated error is applied as it is to the adjacent lower area. is there. If the cyan cumulative error line buffer 402 is initialized at k = 5, the error is not stored at the boundary adjacent to k = 1, and the continuity of dots cannot be maintained.

  Returning to FIG. 3, when the halftone process in step S106 is completed as described above, the binary image data after the halftone process is stored in the halftone image storage buffer 110 (S107). Here, FIG. 17 shows a state in which the result of halftone processing of the scan duty of scan number k = 1 is stored in the halftone image storage buffer 110. As shown in the figure, the halftone image storage buffer 110 stores binary image data of (nozzle number: Nzzl) × (image X size: W) corresponding to the pixel position of the scanning duty.

  The above halftone process is similarly applied to inks of colors other than cyan. As a result, a binary image for each ink formed by one head scan is stored in the halftone image storage buffer 110 for each ink color.

  Note that the halftone processing in the present embodiment is not limited to the error diffusion method described above, and may be N-valued by a dither method. In this case, the dither method is applied to the total value data Ic of C_d and cyan constraint information C_r that are cyan scanning Duty.

Constraint information calculation process Next, the constraint information calculation unit 111 calculates constraint information (step S108). As described above, the constraint information in the present embodiment is information indicating whether or not dots are likely to be shot in determining the dot arrangement of the halftone image. For example, assume that halftone processing is performed in the order of cyan as the first color and magenta as the second color. In this case, by calculating constraint information for magenta based on the cyan halftone processing result, it is possible to provide a correlation between the cyan dot pattern and the magenta dot pattern.

  Hereinafter, the constraint information calculation processing according to the present embodiment will be described. However, in order to simplify the description, the constraint information when printing with two colors of cyan and magenta is performed with the scanning number k in the fourteen-pass printing with the number of nozzles of 16. An example of calculation is shown. The same processing can be performed even if the combination of ink colors or the number of inks increases.

  Hereinafter, in the same scan, halftone processing is performed in the order of cyan, which is the first color, and magenta, which is the second color. From the result of cyan halftone processing of scan number k, the magenta half of scan number k An example of calculating constraint information used for tone processing will be shown.

FIG. 18 is a block diagram illustrating a configuration relating to magenta constraint information calculation in the constraint information calculation unit 111, and FIG. 19 is a flowchart illustrating magenta constraint information calculation processing according to the configuration.

  First, in step S301 shown in FIG. 19, the filtering process is performed on the data in the scan duty buffer 107. That is, in the scan duty filter processing unit 501 shown in FIG. 18, C_df is calculated by performing a filter process with a predetermined filter F_c on the C_d that is the cyan scan duty in the scan duty buffer 107 as shown in the following equation. FIG. 20 is a schematic diagram of the scanned scan duty data.

C_df = C_d * F_c (24)
However, * indicates convolution. In the present embodiment, the size (M, N) and the coefficient of the filter F_c are set to values corresponding to the satellite generation amount detected by the satellite amount detection unit 207. . As a satellite generation amount, it is preferable to use a cyan satellite generation amount, but a magenta satellite generation amount may be used. Also, a larger one of the cyan and magenta satellite generation amounts may be used, or a value obtained by some calculation such as an average from the cyan and magenta satellite generation amounts may be used. Details of the filter F_c size and coefficient setting method will be described later. Here, FIG. 20 shows a schematic diagram of a result obtained by applying the filter processing by the filter F_c to the cyan scan Duty of the scan number k = 1 shown in FIG.

  Next, in step S302, the halftone data filter processing unit 502 performs a filter process with a predetermined low-pass filter LPF_c on the Out_c that is the cyan halftone process result of the scan number k as shown in the following equation. Thereby, Out_c_LPF is calculated.

Out_c_LPF = Out_c * LPF_c (25)
The size and coefficient of the filter LPF_c are also set according to the satellite generation amount detected by the satellite amount detection unit 207, similarly to the filter F_c described above. The satellite generation amount is preferably a cyan satellite generation amount. However, the satellite generation amount of other colors or a value obtained by combining the satellite generation amounts of a plurality of colors may be used. The filters F_c and LPF_c may have the same size or coefficient, or may be different from each other. However, the filter LPF_c preferably has a low-pass characteristic. Here, FIG. 21 shows a schematic diagram of a result obtained by performing the filter processing by the low-pass filter LPF_c on the cyan halftone processing result of the scan number k = 1 shown in FIG.

  In step S303, the pre-update constraint information data shift unit 503 moves the constraint information data M_r for magenta at the scan number k-1 by one paper feed amount LF = Nzzl / Pass = 16/4 = 4. Shift to. The shifted scanning duty data M′_r is calculated as follows. Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl.

M′_r (nx, ny) = M_r (nx, ny + LF) (26)
In Expression (26), when ny + LF ≧ Nzzl, M′_r (nx, ny) = 0. That is, 0 is substituted for the lower end LF nozzles after the shift (in this case, 4 nozzles).

  As described above, the reason why the magenta constraint information of the scan number k-1 is shifted by the paper feed amount LF is that the halftone dot arrangement formed by the scan number k-1 and the scan number k is relative on the recording medium. This is because the sheet feed amount is shifted by LF.

  In step S304, the subtraction unit 504 subtracts the data calculated by the halftone data filter processing unit 502 from the data calculated by the scanning duty filter processing unit 501. In step S305, the weight accumulation unit 505 accumulates the weight coefficient h (real number).

  Next, in step S306, the adding unit 506 adds the data calculated by the weight integrating unit 505 and the magenta constraint information of the scan number k-1 shifted by the pre-update constraint information data shift unit 503 to update. Later constraint information M_r is calculated. The following equation shows an arithmetic expression for the updated constraint information M_r calculated in steps S304 and S305.

M_r ← (−Out_c_LPF + C_df) × h + M′_r (27)
The post-update constraint information M_r calculated here is stored in the constraint information buffer 109 as constraint information for magenta of the scan number k.

  As described above, the method for calculating the constraint information for magenta for which halftone processing is performed later from the cyan dot pattern for which halftone processing is performed first in the same scan has been shown.

Cyan Constraint Information Calculation Next, cyan constraint information calculation processing will be described. However, since cyan performs halftone processing before magenta in the same scan on the recording medium, the constraint information calculated here is a constraint used for cyan halftone processing at the next scan number (k + 1). Information.

  FIG. 22 is a block diagram illustrating a configuration related to cyan constraint information calculation in the constraint information calculation unit 111, and FIG. 23 is a flowchart illustrating cyan constraint information calculation processing according to the configuration.

  First, in step S401 shown in FIG. 23, filter processing is performed on the data in the scan duty buffer 107. That is, the scan duty filter processing unit 601 shown in FIG. 22 calculates M_df by performing a filter process with a predetermined filter F_m on M_d that is the magenta scan duty in the scan duty buffer 107 as shown in the following equation.

M_df = M_d * F_m (28)
As in the case of the magenta constraint information calculation described above, values corresponding to the satellite generation amount detected by the satellite amount detection unit 207 are set as the size (M, N) and coefficient of the filter F_m. As the amount of satellite generated, it is preferable to use the amount of satellite generated by magenta, but the amount of satellite generated by cyan may be used. Also, a larger one of the cyan and magenta satellite generation amounts may be used, or a value obtained by some calculation such as an average from the cyan and magenta satellite generation amounts may be used. Details of the filter F_m size and coefficient setting method will be described later.

  In step S <b> 402, the halftone data filter processing unit 602 calculates Out_m_LPF by performing a filter process using a predetermined low-pass filter LPF_m on the Out_m that is the magenta halftone process result as shown in the following equation.

Out_m_LPF = Out_m * LPF_m (29)
Similarly to the filter F_m, the size and coefficient of the filter LPF_m are also set according to the satellite generation amount detected by the satellite amount detection unit 207. The satellite generation amount is preferably the magenta satellite generation amount. However, the satellite generation amount of other colors or a value obtained by combining the satellite generation amounts of a plurality of colors may be used. The filters F_m and LPF_m may have the same size or coefficient, or may be different from each other. However, the filter LPF_m preferably has a low-pass characteristic.

  In step S403, the scanning duty data shift unit 607 shifts the data M_df output from the scanning duty filter processing unit 601 upward by one paper feed amount LF = Nzzl / Pass = 16/4 = 4. . The shifted scanning duty data M′_df is calculated as follows. Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl. 0 is substituted for the lower four nozzles after the shift.

M′_df (nx, ny) = M_df (nx, ny + LF) (30)
In equation (30), when ny + LF ≧ Nzzl, M′_df = 0. That is, 0 is substituted for the lower end LF nozzles after the shift (in this case, 4 nozzles).

  Similarly in step S404, the halftone data shift unit 608 shifts the data Out_m_LPF to the paper feed amount. The shifted halftone data Out'_m_LPF is calculated as follows. Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl.

Out'_m_LPF (nx, ny) = Out_m_LPF (nx, ny + LF) (31)
In Expression (31), as in Expression (30), when ny + LF ≧ Nzzl, Out′_m_LPF (nx, ny) = 0. That is, 0 is substituted for the lower end LF nozzles after the shift (in this case, 4 nozzles).

  In step S405, the pre-update constraint information data shift unit 603 shifts the constraint information data C_r for cyan at the scan number k upward by a single paper feed amount LF = Nzzl / Pass = 16/4 = 4. . The shifted constraint information data C′_r is calculated as follows. Note that 0 ≦ nx <image X size and 0 ≦ ny <Nzzl.

C′_r (nx, ny) = C_r (nx, ny + LF) (32)
Also in the equation (32), when ny + LF ≧ Nzzl, C′_r (nx, ny) = 0. That is, 0 is substituted for the lower end LF nozzles after the shift (in this case, 4 nozzles).

  As described above, the reason why the cyan constraint information of the scan number k is shifted by the paper feed amount LF is that the halftone dot arrangement formed by the scan number k and the halftone dot arrangement of the scan number k + 1 are the recording medium. This is because the sheet is formed with a relative shift by LF.

  In step S406, the subtraction unit 604 subtracts the data calculated by the halftone data shift unit 608 from the data calculated by the scanning duty data shift unit 607. In step S407, the weight accumulation unit 605 accumulates the weight coefficient h (real number).

  In step S408, the adding unit 606 adds the data calculated by the weight integrating unit 605 and the constraint information of cyan of the scanning number k shifted by the constraint information data shift unit 603 before update, Constraint information C_r is calculated. The following equation shows an arithmetic expression for the updated constraint information C_r calculated in steps S406 and S407.

C_r ← (−Out'_m_LPF + M′_df) × h + C′_r (33)
The updated constraint information C_r calculated here is stored in the constraint information buffer 109 as constraint information for cyan of the scan number k + 1.

  The calculation method of the constraint information for cyan used when scanning the next line has been described above.

● Constraint information update processing (details)
Here, the expression (27) indicating the update of the magenta constraint information M_r, which is a feature of the present embodiment, will be described in detail. The following description is similarly applied to the equation (33) indicating the update of the cyan constraint information C_r.

  Out_c_LPF in equation (27) is data obtained by extracting only the low frequency component from the cyan halftone result image Out_c of scan number k using LPF_c in the halftone data filter processing unit 502. In Expression (27), the low frequency component Out_c_LPF in the spatial frequency region of the cyan dot pattern is subtracted from the constraint information before magenta update. As a result, the low-frequency component of the cyan dot pattern is less likely to be hit by dots in magenta. That is, there is an effect that the low frequency component of the magenta dot pattern has an opposite phase to the low frequency component of the halftone image printed at the cyan scan number k.

  Thus, the effect obtained by making only a low frequency component into an opposite phase is demonstrated with reference to FIG.

  FIG. 24 is a diagram schematically showing a change in a printed image with respect to a registration shift (hereinafter referred to as a registration shift) between cyan and magenta inks divided into a high frequency region and a low frequency region. First, consider the high-frequency region of the printed image. When the density distributions of cyan and magenta are in opposite phases (2601), the peaks and valleys of the density distribution are interpolated when there is no registration, and the paper fills the paper uniformly, so that it is difficult to generate a white paper portion. As a result, the density increases, but for example (20 μm in FIG. 24), the cyan and magenta registration shifts tend to cause the density distributions to overlap each other, making it easier to see the white part of the paper and reducing the density. become. In other words, if the density distribution of cyan and magenta in the high-frequency component is reversed in phase on the printed image, the density tolerance against the registration error is lowered. However, when the density distribution becomes uncorrelated with the high frequency component (2602), the density is hardly changed even if a slight misregistration occurs, and the granularity hardly deteriorates.

  On the other hand, in the low-frequency region of the printed image, when the density distribution is in the opposite phase (2603), the visually disturbing low-frequency component is reduced and the deterioration of graininess is suppressed. Furthermore, even if there is a slight registration error, the relationship between the peaks and valleys in the distribution hardly changes, so the concentration tolerance is strong. However, when the density distribution is made uncorrelated with the low-frequency component (2604), the low-frequency component appears on the image regardless of the occurrence of registration, and the graininess deteriorates.

  As described above, according to FIG. 24, in order to have high density unevenness resistance against the registration error on the printed image and to improve the graininess, dots between colors (between cyan and magenta in this embodiment) are used. It can be seen that it is important to realize the following two points in the pattern. That is, 1) Low-frequency components that are visually unsightly are in antiphase. 2) The high frequency components are not anti-correlated and are not correlated.

  In Expression (27), by subtracting Out_c_LPF calculated by the halftone data filter processing unit 502, the phase of the low frequency component in the spatial frequency domain of the cyan and magenta dot patterns is reversed. . Further, by not considering the high frequency component as the constraint information, there is an effect of making the phase of the high frequency component in the spatial frequency region of the dot pattern uncorrelated.

  Further, attention is paid to C_df in the equation (27). C_df is data obtained by filtering the cyan scan duty data at the scan number k by F_c in the scan duty filter processing unit 501. In Expression (27), C_df is added, and the reason is as follows.

  First, the average value in parentheses on the right side of Equation (27) is set to 0, so that the average of the constraint information M_r is always kept at 0, and the scanning duty density and the image density after the halftone process are brought close to each other. It is. This is because if only the data having a negative value in −Out_cLPF is treated as constraint information, the average value becomes small when the constraint information is added to the scan duty, and the number of dots in the halftone process Will decrease. In general, the error diffusion method operates so that the input density and the output density are always the same. Therefore, the constraint information M_r in this embodiment must always be kept at an average of zero. Here, since C_df has the same density as Out_c_LPF, by adding C_df in equation (27), magenta constraint information M_r can always maintain an average of zero.

  Next, the second reason is to control the spatial frequency characteristics of the output image. In the equation (27), the average value in parentheses on the right side must be 0 as described above. However, if LPF_c is applied only to Out_c_LPF, Out_c_LPF and C_df in parentheses differ in terms of spatial frequency, and edge enhancement occurs. In other words, if the blur process is performed only for Out_c and the subtraction is performed without performing the blur process on C_d, the process is equivalent to unsharp masking and edge enhancement occurs. Therefore, edge enhancement can be suppressed by applying the same filter to the scan duty data C_d as the halftone data Out_c.

  However, when the recording medium is uncoated paper such as plain paper or matte paper, the contour of the image tends to be blurred. Therefore, in this case, it is not always necessary to perform the same processing for both C_d and Out_c. For example, the degree of spread of F_c may be narrowed so that edge enhancement is intentionally performed.

  In Expression (27), the strength of the constraint information can be further adjusted by the weighting factor h. For example, if h = 1.0, the constraint information calculated by the filter can be output as it is, and if h = 0.0, an output equivalent to not calculating the constraint information can be obtained. Since the average in parentheses on the right side of Equation (27) is 0, no matter what value is set for h, the average 0 is maintained.

Filter Setting Method A method for setting the size and coefficient of the low-pass filter LPF_c for cyan in Equation (25) and the low-pass filter LPF_m for magenta in Equation (29), which are features of the present embodiment, will be described below. Note that the filters F_c and F_m in the equations (24) and (28) are also set in the same manner as the LPF_c and LPF_m set as follows.

  As described above, by changing the frequency characteristics of LPF_c and LPF_m for extracting low frequency components in the equations (25) and (29), the cyan and magenta dot arrangement in each scan is reversed in phase. It is possible to control the spatial frequency component. That is, when the sizes of LPF_c and LPF_m are large and the pass band is narrow, the band where the dot arrangement of cyan and magenta is uncorrelated is widened, and the robustness against registration is improved, while the graininess is deteriorated. Similarly, when the sizes of LPF_c and LPF_m are small and the pass band is wide, since the band of cyan and magenta dot arrangement has a wide correlation, the robustness against registration is reduced and the graininess is improved.

  In this embodiment, the constraint information calculation unit 111 considers the relationship between the low-pass filter and the dot arrangement of cyan and magenta, and depending on the satellite generation amount detected by the satellite amount detection unit 207, the sizes and coefficients of LPF_c and LPF_m Set.

  First, as the filter size, for example, it is conceivable to increase the filter size as the satellite generation amount increases, and to decrease the filter size as the satellite generation amount decreases.

  Here, as an example of the satellite generation amount, a value that does not consider the satellite generation direction, such as the number of satellite generations, the ratio of the main droplet dot diameter to the satellite dot diameter, and the square sum of the distance between the main droplet and the satellite is used as an example. . In such a case, the vertical size M and the filter N size of the filter are changed according to the amount of satellite generation. The filter may be a square with M = N or a rectangle with M ≠ N. FIG. 25 shows an isotropic load average filter as a low-pass filter example according to the present embodiment, in which M = N = 5 squares and coefficients are arranged almost concentrically.

  In addition, when using satellites that have values in the vertical and horizontal directions, such as the center of gravity of the satellite landing position, the filter vertical size is calculated from the vertical satellite generation amount, and the satellite generation amount in the horizontal direction. To determine the filter horizontal size. For example, as shown in FIG. 26, when satellites are generated spreading in the horizontal direction, the horizontal filter size is made larger than the vertical filter size. An example of the filter in this case is shown in FIG.

  As a method for setting the filter size, as shown in FIG. 28, the relationship between the satellite generation amount and the filter size (M or N) may be set in advance. Of course, this relationship may be set as the LUT. For example, as shown in the following equations (34) and (35), the filter size (M, N) may be set as a monotonically increasing function with the satellite generation amount Sate_amt as a variable.

M = αm · Sate_amt + βm where αm> 0, βm> 0 (34)
N = αn · Sate_amt + βn where αn> 0, βn> 0 (35)
The filter coefficient is not limited to the concentric isotropic load average filter shown in FIG. An elliptic anisotropic filter may be used, and the filter is not limited to a low-pass characteristic, and may be a filter having a band-pass characteristic or a high-pass characteristic. The filter coefficient is appropriately set according to the filter size set according to the satellite generation amount.

  Heretofore, the constraint information calculation processing in the present embodiment has been described in detail.

Image Formation Processing After Constraint Information Calculation Returning to FIG. 3, when the constraint information is calculated as described above in step S108, the constraint information buffer 109 is updated with the constraint information calculation result in step S109.

  Next, in step S110, it is determined whether or not the above-described halftone process in step S106 to the constraint information buffer update process in step S109 have been completed for all ink colors. If not completed, the process is repeated until steps S106 to S109 are completed for all inks.

  When the processing for all ink colors is completed, in step S111, the band corresponding to the number of nozzles (Nzzl) in the vertical direction and the X size (W) in the horizontal direction is stored in the halftone image storage buffer 110. Data is output from the image output terminal 112.

  Next, in step S112, the printer 2 that has received the halftone image data selects an ink color that matches the image data, and starts a printing operation and satellite amount detection.

  As the printing operation here, main scanning is performed once to record an image on the recording medium by driving each nozzle at a constant driving interval while the recording head 201 moves from the left to the right with respect to the recording medium. . Further, when the main scan is completed, a sub-scan that is a scan in a direction perpendicular to the main scan is performed once.

  In the satellite amount detection process, as described above, the number of satellites generated, the position of the center of gravity of the satellites, and the like are detected as satellite generation amounts. The detected satellite generation amount is sent to the constraint information calculation unit 111 in order to determine halftone image data to be recorded in the next scan.

  Then, in step S113, it is determined whether or not all scanning has been completed. If completed, a series of image forming processes are completed, and if not completed, the process returns to step S103. Thus, all of the processing is completed.

  As described above, according to the present embodiment, it is possible to control the correlation of each spatial frequency band for different inks and different dot patterns of main scanning according to the amount of satellite generation. As a result, when the amount of satellite generation is small, that is, when color unevenness is difficult to occur, the dot size between the colors and between main scans of the same color is exclusively arranged by reducing the filter size, and the graininess Can be reduced. In addition, when the amount of satellite generation is large, that is, when color unevenness is likely to occur, by arranging the filter exclusively in the low frequency region by disposing the dot in the low frequency region and by disposing the decorrelation in the high frequency region, Robustness against color unevenness can be improved. Therefore, optimum recording can be performed according to the satellite generation state, that is, the dot landing accuracy.

  In this embodiment, the first color is cyan and the second color is magenta as the halftone processing order. However, this processing order may be fixed, and the amount of satellite generated for each ink. You may change according to. For example, the processing may be performed in order from the smallest satellite. This is because the following problems may occur when ink having a large satellite generation amount is processed first. That is, since the constraint information for the ink to be processed later is calculated using a relatively wide LPF, the frequency region that is uncorrelated in the dot arrangement between these inks becomes wide, and the graininess deteriorates. Resulting in. Therefore, by processing the ink with a small amount of satellite generation first, the LPF for calculating the constraint information can be narrowed with respect to the ink to be processed later, and the graininess can be improved.

<Second Embodiment>
The second embodiment according to the present invention will be described below. In the first embodiment described above, the actual number of satellites generated and the value based on the landing position are treated as satellite generation amounts, and a space for correlating dot patterns between different colors according to the satellite generation amounts. An example of controlling the frequency band is shown. The second embodiment is characterized by controlling the correlation of dot patterns of each main scanning between different colors by not predicting the actual landing position of the satellite but by predicting the occurrence of the satellite. .

  In general, it is known that the landing position of a satellite varies depending on a plurality of factors. The factors include, for example, the flying speed (ink ejection speed) until the ink droplet ejected from the nozzle reaches the paper surface, the head moving speed (carriage speed) during main scanning, and the distance between the nozzle and the paper surface (nozzle / Paper distance), head drive frequency, and the like. Therefore, in the second embodiment, a satellite generation state is predicted from any one of the above four factors or a combination thereof, and the correlation of the dot pattern between colors is controlled.

  FIG. 29 is a block diagram illustrating a configuration of an image forming system according to the second embodiment. FIG. 29 is characterized by including a parameter detection unit 701 that detects parameters contributing to the satellite generation status in place of the satellite amount detection unit 207 shown in FIG. 1 in the first embodiment. The operations in the configuration other than the constraint information calculation unit 111 and the parameter detection unit 701 are the same as those in the first embodiment described above, and thus the description thereof is omitted.

  First, the processing in the parameter detection unit 701 will be described in detail.

  The parameter detection unit 701 obtains a value based on any one of the ink ejection speed, the carriage speed, the nozzle / paper distance, the head driving frequency, or a combination thereof as a parameter contributing to the satellite generation state in the printer 2.

  The parameter detection unit 701 can acquire the design value in the printer 2 as a parameter. For example, when the ink discharge speed is obtained as a parameter, this is a value determined by a physical property value such as the viscosity of each ink or an average discharge amount, and therefore, a design value stored in advance in the printer 2 can be applied. . Similarly, if the parameters are a carriage speed, a nozzle / paper distance, and a head drive frequency, a design value corresponding to a recording mode determined by a combination of a setting value such as image quality priority or speed priority and a paper type may be acquired. .

  In addition to obtaining design values, parameters may be obtained by actually measuring these values using various sensors. For example, using an encoder sensor, the carriage speed can be measured by reading the carriage position from the encoder sensor at equal time intervals. The nozzle / paper distance can be measured by irradiating the paper with laser light and detecting the reflected light.

  In the second embodiment, by obtaining parameters using various sensors in the parameter detection unit 701, it is possible to perform constraint information calculation in consideration of the effect of parameter variation due to the carriage position. For example, when the satellite landing position changes because the carriage speed is not stable at the start or end of scanning, the influence of the change can be added to the calculation of the constraint information. Similarly, with respect to the ejection speed, the influence of the head temperature rising and the ejection speed changing due to repeated ejection of ink during scanning can be added to the calculation of the constraint information. In addition, with respect to the nozzle / paper distance, it is possible to take into account the influence of the change in the nozzle / paper distance due to paper waviness, such as so-called cockling, in the calculation of the constraint information.

  As described above, the parameters acquired by the parameter detection unit 701 are sent to the constraint information calculation unit 111 in the image processing apparatus 1.

  In the constraint information calculation unit 111, constraint information calculation corresponding to the acquired parameter is performed. Also in the second embodiment, similarly to the first embodiment described above, the constraint information is calculated so that any spatial frequency band between different ink and different main scanning dot patterns is uncorrelated. The spatial frequency band to be uncorrelated is arbitrary, but in order to improve the robustness against density unevenness due to registration and reduce graininess, the low frequency band should be correlated and the high frequency band Then, it is desirable to make it uncorrelated. And as the spatial frequency band to be uncorrelated increases, the robustness against color unevenness increases, but the graininess deteriorates. On the other hand, the wider the spatial frequency band with correlation, the lower the robustness, but the better the granularity. In addition, since the calculation method of the constraint information in the second embodiment is the same as that in the first embodiment described above, detailed description thereof is omitted.

  Also in the second embodiment, the LPFs in the equations (25) and (29) are controlled as in the first embodiment described above. That is, when the satellite landing position accuracy is poor, the filter size is increased to increase the robustness against color unevenness. On the other hand, when the satellite landing position accuracy is high, the grain size is improved by reducing the filter size.

  Here, filter control based on the relationship between each parameter and the landing position accuracy of the satellite is shown.

  First, with regard to the ink ejection speed, since the ejection of ink droplets from the nozzles becomes unstable in general, the landing position accuracy of the satellite is deteriorated. Therefore, if the ink discharge speed is high, there is a high possibility that color unevenness will occur due to satellites. Therefore, the robustness against color unevenness is increased by setting the filter size large.

  As the carriage speed increases, the amount of head movement that advances during the period corresponding to the difference between the flight time of the main droplet and the satellite also increases, and at the same time, the self-air flow of the head also increases, so the satellite landing position accuracy deteriorates. . Therefore, the larger the carriage speed, the larger the filter size.

  Further, the nozzle / paper distance corresponds to the flight distance from when the ink lands on the paper to the paper surface. Therefore, the larger the nozzle / paper distance, the larger the filter size.

  Also, as the head drive frequency increases, the ejection becomes unstable, so the satellite landing position accuracy deteriorates. Therefore, the filter size is set larger as the head drive frequency is higher.

  As a filter size setting method in the second embodiment, as shown in FIG. 30, the relationship between the parameter and the filter size (M or N) may be set in advance. Of course, this relationship may be set as the LUT. For example, as shown in the following formulas (36) and (37), the filter size (M, N) may be set as a monotonically increasing function with the parameter Var as a variable.

M = αm · Var + βm where αm> 0, βm> 0 (36)
N = αn · Var + βn where αn> 0, βn> 0 (37)
Further, the filter coefficient may be an elliptical anisotropic filter, or may be a filter having not only a low-pass characteristic but also a band-pass characteristic or a high-pass characteristic.

  Further, the filter coefficient may be a concentric isotropic weighted average filter or an elliptical anisotropic filter, and is not limited to a low-pass characteristic, but a band-pass characteristic or a high-pass characteristic. It may be a filter having. The filter coefficient is appropriately set according to the filter size set according to the satellite generation amount.

  Also in the second embodiment, control information calculation and halftone processing are performed using a filter selected in accordance with the parameters. Since this is the same processing as in the first embodiment described above, detailed description is given here. Description is omitted.

  As described above, according to the second embodiment, the parameter contributing to the satellite amount detected by the printer 2 is reflected in the N-value conversion operation in different inks or in different main scans in the same area on the recording medium. . As a result, it is possible to reduce color unevenness caused by changes in the satellite generation status due to changes in the printer 2 and the recording medium, and to realize image formation with high image quality on the entire surface of the recording medium.

<Other embodiments>
The present invention can be applied to, for example, a so-called full-line type recording apparatus that has a recording head having a length corresponding to the recording width of the recording medium and performs recording by moving the recording medium relative to the recording head.

  In each of the above embodiments, an example in which binarization is performed by the halftone processing unit 108 has been described. However, the present invention is not limited to this example, and an N-binarization process (N is an integer of 2 or more) may be performed. good. Note that N is smaller than the number of gradations of the input image data, and N = 2 when the halftone processing unit 108 performs binarization.

  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 the present invention, a software program for realizing the functions of the above-described embodiments is supplied directly or remotely to a system or apparatus, and the computer of the system or apparatus reads and executes the supplied program code. Is also achieved. The program in this case is a computer-readable program corresponding to the flowchart shown in the drawing in the embodiment.

  Accordingly, since the functions of the present invention are implemented by computer, the program code installed in the computer also implements the present invention. In other words, the present invention includes a computer program itself for realizing the functional processing of the present invention.

  In that case, as long as it has the function of a program, it may be in the form of object code, a program executed by an interpreter, script data supplied to the OS, or the like.

  Recording media for supplying the program include the following media. For example, floppy disk, hard disk, optical disk, magneto-optical disk, MO, CD-ROM, CD-R, CD-RW, magnetic tape, nonvolatile memory card, ROM, DVD (DVD-ROM, DVD- R).

  As a program supply method, the following method is also possible. That is, the browser of the client computer is connected to a homepage on the Internet, and the computer program itself (or a compressed file including an automatic installation function) of the present invention is downloaded to a recording medium such as a hard disk. It can also be realized by dividing the program code constituting the program of the present invention into a plurality of files and downloading each file from a different homepage. That is, a WWW server that allows a plurality of users to download a program file for realizing the functional processing of the present invention on a computer is also included in the present invention.

  In addition, the program of the present invention is encrypted, stored in a storage medium such as a CD-ROM, distributed to users, and key information for decryption is downloaded from a homepage via the Internet to users who have cleared predetermined conditions. It is also possible to make it. That is, the user can execute the encrypted program by using the key information and install it on the computer.

  Further, the functions of the above-described embodiments are realized by the computer executing the read program. Furthermore, based on the instructions of the program, an OS or the like running on the computer performs part or all of the actual processing, and the functions of the above-described embodiments can also be realized by the processing.

  Further, the program read from the recording medium is written in a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, and then executed, so that the program of the above-described embodiment can be obtained. Function is realized. That is, based on the instructions of the program, the CPU provided in the function expansion board or function expansion unit can perform part or all of the actual processing.

1 is a block diagram illustrating a configuration of an image forming system according to an embodiment of the present invention. FIG. 3 is a diagram illustrating a configuration example of a recording head in the printer of the present embodiment. 5 is a flowchart illustrating image forming processing in the present embodiment. It is a figure which shows the detail of the input / output data in the color separation process part of this embodiment. It is a figure which shows the outline | summary of the image formation by 16 nozzles and 4 pass printing in this embodiment. It is a figure which shows an example of the duty division ratio hold | maintained at the scanning duty setting LUT in this embodiment. It is a figure which shows the outline | summary of the setting method of the scanning duty in this embodiment. It is a figure which shows the outline | summary of the setting method of the scanning duty in this embodiment. It is a figure which shows the outline | summary of the setting method of the scanning duty in this embodiment. It is a figure which shows the band structural example of the scanning duty buffer in this embodiment. It is a figure which shows the band structural example of the constraint information buffer in this embodiment. It is a block diagram which shows the detailed structure of the halftone process part in this embodiment. It is a flowchart which shows the halftone process in this embodiment. It is a figure which shows the storage area of each cyan accumulation error line buffer in this embodiment. It is a figure which shows an example of the error diffusion coefficient in this embodiment. It is a figure which shows the mode of the adjoining of the printing area | region in this embodiment. It is a figure which shows the example of storage of the halftone process result in this embodiment. It is a block diagram which shows the detailed structure of the constraint information calculating part of magenta in this embodiment. It is a flowchart which shows the magenta restrictions information calculation process in this embodiment. It is a schematic diagram of the scanning duty data after the filter process in the constraint information calculation of the present embodiment. It is a schematic diagram of the halftone data after the filter process in the constraint information calculation of the present embodiment. It is a block diagram which shows the constraint information calculating part of cyan in this embodiment. It is a flowchart which shows the restriction | limiting information calculation process of cyan in this embodiment. It is a figure which shows the resisting characteristic with respect to the frequency domain of the printed image in this embodiment. It is a figure which shows the specific example of the filter in the constraint information calculation of this embodiment. It is a schematic diagram which shows the example of generation | occurrence | production of the satellite of this embodiment. It is a figure which shows the specific example of the filter in the constraint information calculation of this embodiment. It is a graph which shows the relationship between the filter size and satellite generation amount in this embodiment. It is a block diagram which shows the structure of the image forming system in 2nd Embodiment which concerns on this invention. It is a graph which shows the relationship between the filter size and fluctuation amount in 2nd Embodiment.

Claims (15)

An image forming apparatus comprising a recording head having a plurality of recording elements for discharging a recording agent for each of a plurality of colors, and forming a color image by scanning the recording head group on a recording medium,
In-scan recording amount calculating means for calculating a recording amount for each recording element in the main scanning of the recording head according to the input image data of a plurality of colors;
Constraint information creating means for creating constraint information referred to when N-value processing is performed on image data;
N-value conversion means for applying an N-value conversion process (N is an integer of 2 or more) based on the restriction information to the print amount calculated by the intra-scan print amount calculation means to create a dot pattern to be formed; ,
Discharge state detection means for detecting the discharge state of the recording agent in the recording element,
The N-value conversion unit performs the N-value conversion process on the first color image data before the N-value conversion process on the second color image data in the same main scan,
The constraint information creating means includes a recording amount calculated by the in-scan recording amount calculating means for the first color image data previously converted to N-value in the same main scanning, and the N-value converting means. An image forming apparatus that creates the restriction information for the image data of the second color based on the created dot pattern and the ejection state of the recording agent detected by the ejection state detection unit. .   The constraint information creating means is a spatial frequency region of the second color dot pattern created by the N-valued means for the first color dot pattern previously created by the N-valued means. The image forming apparatus according to claim 1, wherein the constraint information is created so that a phase of a low frequency component in the phase is opposite.   The constraint information creating means is a spatial frequency region of the second color dot pattern created by the N-valued means for the first color dot pattern previously created by the N-valued means. The image forming apparatus according to claim 2, wherein the constraint information is created so that a phase of a high-frequency component in is uncorrelated. The constraint information creating means creates the constraint information for the second color image data by applying a filtering process to each of the recording amount for the first color image data and the dot pattern. ,
The size of a filter applied to each of the recording amount and the dot pattern for the image data of the first color is set according to the discharge state of the recording agent. 2. The image forming apparatus according to item 1.   The ejection state detection means is configured to determine the number of satellites that are minute droplets generated by being divided from the main droplet and the landing position of the satellite when the recording agent is ejected from the recording element. 5. The image forming apparatus according to claim 1, wherein the image forming apparatus is detected as a discharge state.   The ejection state detection means is one or a combination of a recording agent ejection speed from the recording element, a moving speed of the recording head during main scanning, a distance between the recording element and a recording medium, and a driving frequency of the recording head. 5. The image forming apparatus according to claim 1, wherein a value based on the recording medium is detected as a recording agent ejection state. 6.   The image forming apparatus according to claim 6, wherein the discharge state detecting unit detects a set value stored in advance according to a recording agent or an image forming mode as a discharge state of the recording agent.   The image forming apparatus according to claim 1, wherein the N-value conversion unit performs N-value conversion by an error diffusion method or a dither method.   9. The image forming apparatus according to claim 1, wherein N = 2. A control method for an image forming apparatus comprising a recording head having a plurality of recording elements for discharging a recording agent for each of a plurality of colors, and forming a color image by scanning the recording head group on a recording medium,
In-scan recording amount calculating step for calculating a recording amount for each recording element in the main scanning of the recording head according to the input image data of a plurality of colors;
A constraint information creating step for creating constraint information referred to when N-value processing is performed on image data;
An N-value conversion step of creating a dot pattern to be formed by performing an N-value conversion process (N is an integer of 2 or more) based on the restriction information for the print amount calculated in the intra-scan recording amount calculation step; ,
A discharge state detection step of detecting a discharge state of the recording agent in the recording element,
In the N-value conversion step, the N-value conversion processing for the first color image data is performed prior to the N-value conversion processing for the second color image data in the same main scanning.
In the constraint information creation step, the recording amount calculated in the in-scan recording amount calculation step for the first color image data that has been previously converted to N-value in the same main scanning, and the N-value conversion step Forming the restriction information for the image data of the second color based on the dot pattern created in step 1 and the ejection state of the recording agent detected in the ejection state detection step Control method of the device.   In the constraint information creating step, the spatial frequency of the second color dot pattern created in the N-valued step is compared with the first color dot pattern created in the N-valued step previously. The method of controlling an image forming apparatus according to claim 10, wherein the constraint information is created so that a phase of a low frequency component in the region is an opposite phase.   In the constraint information creating step, the spatial frequency of the second color dot pattern created in the N-valued step is compared with the first color dot pattern created in the N-valued step previously. The method of controlling an image forming apparatus according to claim 11, wherein the constraint information is created so that a phase of a high-frequency component in a region is uncorrelated. In the constraint information creation step, the constraint information for the second color image data is obtained by applying a filtering process to each of the recording amount for the first color image data and the dot pattern. Create
The size of a filter applied to each of the recording amount and the dot pattern for the image data of the first color is set according to the discharge state of the recording agent. 2. A method for controlling an image forming apparatus according to item 1.   A program for causing a computer to execute each step in the method of controlling an image forming apparatus according to any one of claims 10 to 13.   A computer-readable storage medium storing the program according to claim 14.