JP6192373B2 - Image processing apparatus, image processing method, and program - Google Patents

Description

  The present invention relates to an image processing apparatus and method for making a gradation lower than the number of gradations of an input image.

  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 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 of such a system, an ink jet recording system is known.

  In such a recording apparatus, halftone processing is required to convert multi-valued input image data into an N-value image representing a dot pattern (N is 2 or more and less than the number of gradations of the input image data). It becomes. In addition, in a recording apparatus using an ink jet recording method, in order to make density unevenness inconspicuous, a multi-pass recording technique that forms an image by recording a color component representing an image multiple times in the same area on the recording medium is adopted. Has been. In order to improve the image quality of an image formed by a plurality of times of recording, there is a halftone processing method that considers the dot formation order and the dot arrangement. In the method described in Patent Document 1, when N-value conversion is performed on image data for each scan by using halftone processing, N-value conversion data (dot pattern) of already generated scan is used. Then, new N-value conversion is performed so that only the low frequency component has an opposite phase. Accordingly, it is possible to suppress the occurrence of fluctuations in density unevenness due to positional deviation due to the low-frequency component having an opposite phase between N-valued data between scans.

  Further, in the method described in Patent Document 2, phase control information is created in advance so that the low-frequency component has an opposite phase between the N-value data corresponding to each scan. By referring to the constraint information, the low frequency component is reversed in phase between N-valued data between scans.

JP 2008-188805 A JP 2008-193266 A

  However, according to the method described in Patent Document 1 described above, when N-value conversion is newly performed, the previously generated N-value data must be held. Therefore, the amount of data transfer is large, and a large amount of bandwidth is used for data communication.

  Further, according to the technique described in Patent Document 2, a band memory for holding phase control information is necessary, and the memory cost is increased.

  Therefore, an object of the present invention is to generate N-valued data for each scan in which low-frequency components are in opposite phases by a simple method without requiring high-cost memory and large-capacity data communication.

In order to solve the above-described problems, the present invention provides an image processing apparatus for forming an image by scanning and scanning a same area on a recording medium a plurality of times for at least one color component representing the image. Generating means for generating scan data corresponding to each scan by dividing the pixel value of each pixel constituting the input image data corresponding to the color component into each scan for each pixel; and a function corresponding to the scan And calculating means for calculating, for each scan, phase control information that expresses the ease of dot formation for each pixel by a positive or negative value, and for the scan data to which the phase control information is added , the phase A halftone processing means for converting the scanning data into halftone image data representing a dot pattern by performing halftone processing using control information, and the scanning Corresponding function is in relation to cancel each other when the superimposed halftone image data for each of the scan, the phase of the dot patterns, each of which represents, characterized in that the opposite phases at a predetermined image frequency band.

  According to the present invention, it is possible to generate N-valued data for each scan in which low-frequency components are in opposite phases by a simple method without requiring high-cost memory and large-capacity data communication.

Block diagram showing the configuration of the image processing apparatus The figure which shows the structural example of the recording head in a printer Flow chart showing image processing The figure which shows the details of the input / output data in the color separation processing section The figure which shows the outline of the image formation by 16 nozzles and 2 pass printing The figure which shows an example of the duty division rate hold | maintained at the scanning duty setting LUT The figure which shows the outline | summary of the setting method of scanning duty The figure which shows the outline | summary of the setting method of scanning duty The figure which shows the outline | summary of the setting method of scanning duty The figure which shows the band structural example of the scanning duty buffer of the scanning number k = 1. The figure which shows the phase control information of 2 paths | paths The figure which shows the area | region division example of the phase control information of 2 paths | paths Block diagram showing the detailed configuration of the halftone processing unit Flow chart showing halftone processing Diagram showing an example of error diffusion coefficient The figure which shows the storage area of each cyan accumulation error line buffer Diagram showing the state of adjacent print area The figure which shows the example of storage of the halftone process result of scanning number k = 1 The figure which shows the phase control information of 4 paths The figure which shows the relationship between the signal A and the signal B

  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, the image forming system includes an image processing apparatus 1 and a printer 2. 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. FIG. 4 is a diagram illustrating the color separation processing unit 103. In the color separation process, a color separation lookup table (LUT) 104 is referred to. Here, color separation is performed on image data corresponding to R (red), G (green), and B (blue). The printer 2 has color materials corresponding to C (cyan), M (magenta), Y (yellow), and K (black). That is, the color separation processing unit 103 outputs image data corresponding to each color of CMYK. In the present embodiment, the image data for each color is 8-bit (256 gradations) data.

  Based on the scan duty setting LUT 106, the scan duty setting unit 105 further converts each image data corresponding to each ink color separated by the color separation processing unit 103 into each ink color value for each scan. In other words, the scan data in the present embodiment indicates the recording ink amount in each scan. The scan duty setting unit 105 stores image data of each scan (hereinafter, scan data) in the scan duty buffer 107.

  The halftone processing unit 108 converts the scan data for each scan corresponding to each color obtained by the scan duty setting unit 105 to N-value using halftone processing. The halftone processing unit 108 in this embodiment performs binarization on image data for each scan. Detailed processing will be described later.

  The phase control information generation unit 109 generates phase control information for each color component. The phase control information is information indicating whether or not dots are easily formed at an address to be recorded. The phase control information generation unit 109 generates phase control information based on an expression representing a function.

  The halftone image storage buffer 110 stores N-value image data for each scan corresponding to each color output from the halftone processing unit 108. The N-value image data stored in the halftone image storage buffer 110 is output from the output terminal 111 to the printer 2.

  The printer 2 forms N-value image data formed by the image processing apparatus 1 on the recording medium by recording and scanning the recording head 201 a plurality of times in the vertical and horizontal directions relative to the recording medium 202. As the recording head 201, a thermal transfer method, an ink jet method, or the like can be used, and each has one or more recording elements. 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 among the ink colors mounted on the recording head 201 based on the N-value image data of each color formed by the image processing apparatus 1.

  FIG. 2 is a diagram illustrating a configuration example of the recording head 201. In this embodiment, as described above, the recording head 201 is mounted on the recording head 201 corresponding to CMYK four color inks. 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. Ink colors of CMYK or higher may be mounted.

  Next, an image forming process executed by the image processing apparatus 1 and the printer 2 of the present embodiment having the above-described functional configuration will be described with reference to a flowchart of FIG.

  First, in step S 101, multi-tone color input image data is input from the input terminal 101 and stored in the input image buffer 102. 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 S102, the color separation processing unit 103 uses the color separation LUT 104 to perform color separation from RGB to CMYK ink colors on multi-tone color input image data stored in the input image buffer 102. Process. In this embodiment, each piece of image data after color separation processing is handled as 8 bits, but conversion to a higher number of gradations may be performed. The input image data R′G′B ′ is converted into CMYK image data (hereinafter referred to as color separation data) with reference to the color separation LUT 104 as shown in the following equation.

C = C_LUT — 3D (R ′, G ′, B ′) (1)
K = M_LUT — 3D (R ′, G ′, B ′) (2)
Y = Y_LUT — 3D (R ′, G ′, B ′) (3)
K = K_LUT — 3D (R ′, G ′, B ′) (4)
Here, each function defined on the right side of the equations (1) to (4) corresponds to the contents of the color separation LUT 104. The color separation LUT 104 determines an output value for each ink color from the three input values of RGB. With the above processing, the color separation processing in this embodiment is completed.

  In step S103, the scan duty setting unit 105 sets the scan number k and Ycut (k) indicating the Y coordinate as the color separation data cutout position. Ycut (k) is the color separation data cutout position at 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, setting the color separation data cutout position Y coordinate Ycut as an example in the case of two-pass printing having 16 nozzle rows and forming an image by two recording scans on the same area on the recording medium. Explain the law.

  In general, in the case of two-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 1/2, and when the scan number k = 2, scanning is performed. Image formation is performed after feeding the paper for a nozzle length of 1/2 for the number k = 1. Therefore, when the scan number k = 1, the color separation data cutout position Ycut corresponding to the nozzle upper end coordinates Ycut = −8.

  When the color separation data cutout position Ycut (k) described above 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 (5)
When Ycut (k) is set as described above, in step S104, the scan duty setting unit 105 next sets a duty value for each scan based on the scan duty setting LUT 106 and the color separation data of each color, and scan data. Is calculated. According to the scan duty setting LUT 106, the sum of the duty values of the respective paths corresponding to the same recording position is set so as to store the color separation data. For example, in the case of two passes, as shown in FIG. 6, the total duty division ratio corresponding to two printing scans is set to 1.0. FIG. 6 shows an example of 16 nozzles and 2 passes, where the vertical axis indicates the nozzle position and the horizontal axis indicates the duty division ratio. According to FIG. 6, the inflection points of P1 and P2 are set for every 8 nozzles, and the duty division ratio for 16 nozzles obtained by linearly interpolating each inflection point is held as the scan duty setting LUT. Here, the numerical values of P1 and P2 are set as follows.

P1 + P2 = 1.0 (6)
The value held as the scan duty setting LUT 106 is not limited to the above setting method. For example, an inflection point may be set finely or may be directly specified for each nozzle.

  The scan data set in step S104 is set as the product of the scan duty setting LUT 106 and the color separation data, as shown in FIG. That is, as shown in the left term of FIG. 7, by multiplying the color separation data by the duty division ratio set for each nozzle, the result is the duty for each nozzle as shown in the right term of FIG. Value is set.

  Here, in this embodiment, when the corresponding nozzle is outside the area of the image Y address, the duty value is set to 0. For example, when the scan number is k = 1, as shown in FIG. 8, the image Y address becomes negative at the nozzle row upper end 1/2, so Duty value = 0 is substituted, and the nozzle row lower end 1/2 is a significant value. Is substituted. Further, since the color separation data cut-out position Ycut (k) is determined by the scan number k, when the scan number k = 1 to 3, the scan data constituted by the duty value is determined as shown in FIG. In FIG. 9, the duty value with respect to the nozzle position for each scanning number is shown, and it can be seen that the duty value is different for each scanning number. In the region 1 in FIG. 9, the sum of the Duty values corresponding to the two scans with the scan numbers k = 1 and 2 is the same as the color separation data. Similarly, in the area 2, the sum of the duty values corresponding to the scan numbers k = 2 and 3 is the same as that of the color separation data.

C_d (nx, ny) = C (nx, Ycut (k) + ny) × S_LUT (ny)
... (7)
Similarly, M (X, Y), Y (X, Y), and K (X, Y) are also decomposed into scanning duty by the above formula.

  The scan data set by the scan duty setting unit 105 as described above is stored in the scan duty buffer 107 in step S105. As shown in FIG. 10, the scan duty buffer 107 stores, for each color, duty values constituting band-shaped scan data corresponding to the number of nozzles in the vertical direction and the X size of the image in the horizontal direction.

  In step S <b> 106, the halftone processing unit 108 uses the scan data stored in the scan duty buffer 107 and the phase control information generated by the phase control information generation unit 109 to convert to half value processing. I do. In this embodiment, halftone processing is performed on 8-bit image data into 1-bit (binary) image data. Details of the phase control information and halftone processing based on the phase control information will be described later. The halftone processing unit 108 generates halftone image data corresponding to each scan. In step S <b> 107, the halftone processing unit 108 stores the generated halftone image data in the halftone image storage buffer 110. Here, FIG. 18 shows a state in which the result of the halftone processing of the scan duty with the 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 scan duty.

  In step S <b> 108, the halftone image data for each band stored in the halftone image storage buffer 110 is output from the image output terminal 111.

  In step S109, the printer 2 that has received the halftone image data selects an ink color that matches the image data, and starts a printing operation. In step S109, while the recording head 201 moves from the left to the right with respect to the recording medium, each nozzle is driven at a fixed driving interval to perform main scanning once to record an image on 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 step S110, 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.

  Here, the phase control information will be described. The phase control information is information for controlling a plurality of dot patterns corresponding to the same region so as to have opposite phases in a predetermined frequency band. As shown in FIGS. 11A and 11B, the phase control information generation unit 109 generates values corresponding to the image X coordinate of the scan data and the nozzle number Nzzl coordinate. In FIG. 11A, the value of the black pixel is high, while the value of the white pixel is low. FIG. 11B is a graph showing the magnitude of the value. “The phase control information generation unit 109 generates phase control information including a value indicating the ease of dot formation for each pixel in the scan data. The value corresponding to each pixel in the phase control information has a value of The larger the pixel is, the more likely it is to be hit.As shown in Fig. 11B, the value corresponding to each pixel is generated so as to change gently as a whole image data. The generation unit 109 determines a value corresponding to each pixel based on the low-frequency signal, since the information generated by the phase control information generation unit illustrated in FIG. In the present embodiment, the value indicated by the phase control information is set so that the average is 0 as a whole. Do If: it is easily formed positive value, a negative value when the hard dots are formed at the relevant location is set.

  The meaning of the reverse phase will be described with reference to FIG. In FIG. 20, signal A and signal B are signals indicating functions that vary periodically. The horizontal axis represents time, and the vertical axis represents the signal value. Here, both the signal A and the signal B are signals represented by the same sine wave, and the phase difference between the two sine waves is π. The antiphase in the two signals means a relationship that cancels each other when a plurality of signals (for example, signal A and signal B) are overlapped. According to FIG. 20, it can be seen that the signal A and the signal B, which are sine waves having a phase difference of π, cancel each other when overlapped. When four signals that cancel each other are generated, if four signals that have a phase difference of π / 2 [rad] that represent functions that fluctuate periodically are used, they cancel each other when they are superimposed. . Thus, in a plurality of signals represented by the same function, by giving a phase difference so as to cancel each other when they are overlapped, the plurality of signals have mutually opposite phases.

That is, the signal A and the signal B having the opposite phases shown in FIG.
Dmn_A (nx, ny) = p · Sin (α · nx) + q · Sin (β · ny)
... (8)
Dmn_B (nx, ny) = p · Sin (α · nx + π) + q · Sin (β · ny + π)
= -P · Sin (α · nx)-q · Sin (β · ny)
... (9)
(Α and β are coefficients that determine the period of the phase control information, and p and q are coefficients that determine the amplitude)
And the phase is different by π (radian unit). On the other hand, when signals having the same phase and the same function are superimposed, the signals do not cancel each other, but rather the signals are emphasized. As described above, in the case of two-pass printing, the value of the phase control information in the region A and the value of the phase control information in the region B are determined based on signals having opposite phases. As shown in FIG. 12, the value of the phase control information corresponding to the same XY address may be the same in the region A and the region B, but the phase control information is generated so as to be in opposite phases as a whole.

Note that, as described above, not only SIN (sine wave) but also an expression that can define an antiphase such as a cosine wave may be used. When the phase control information is expressed as C_r (nx, ny),
C_r (nx, ny) = Dmn_B (nx, ny) (10)
C_r (nx, ny + Nzzl / 2) = Dmn_A (nx, ny) (11)
(Ny <Nzzl / 2)
Here, the periods α and β of the phase control information are preferably changed according to the amount of registration fluctuation (landing position fluctuation) of the printer 2. Even if the phase control information in the region A (corresponding to the first pass) and the region B (corresponding to the second pass) is generated so that the phase control information cancels each other out, the phase control information is actually 2 A positional deviation occurs from the pass. Therefore, it is preferable to generate the phase control information so that the relationship that is substantially in reverse phase is maintained even if the positional deviation occurs. For example, if the fluctuation is large, a signal that is a function of a relatively low frequency (long cycle) is generated, and if the fluctuation is small, a signal that is a function of a relatively high frequency (short cycle) is generated. As a measure of the signal period, it is desirable to generate a signal with a function of a longer period than twice the printer registration fluctuation value. For example, in a printer in which a registration fluctuation of 50 μm (0.05 mm) occurs, a signal having a period of 0.1 mm or more is desirable. It is preferable to generate a signal having a frequency of approximately 10 [cycle / mm] or less. As a result, even when registration occurs, the phase control information for the first pass and the phase control information for the second pass generally maintain an inverse phase relationship. However, since it is only a guide, it may be slightly different from the above conditions. Further, in the expressions (8) and (9), the expression is made by superimposing two sine waves Sin, but it may be expressed by superposing three or more sine waves.

  As described above, from the function of the period according to the registration variation of the printer, a function having an opposite phase relationship is generated as in Expressions (8) and (9). The reason for this is that between the dot pattern (lower nozzle half, area Dmn_A) formed in the first pass and the dot pattern (upper nozzle half, area Dmn_B) formed in the second pass of two-pass printing. This is to make the low frequency component have an opposite phase. The dot pattern corresponding to each pass is obtained by halftoning the scan data to which the phase control information is added. When phase control information having an opposite phase to each other is added to the two scan data, the scan data also have an opposite phase relationship. In this way, by setting the low frequency components in opposite phases in the two dot patterns, good graininess can be ensured regardless of the occurrence of registration shift.

  On the other hand, in the areas Dmn_A and Dmn_B in the equations (8) and (9), signals having a shorter cycle (high frequency) than twice the registration fluctuation value of the printer are not generated. That is, the phase control information has no high frequency component. The reason for this is to make the phase of the high frequency component uncorrelated between the dot pattern formed in the first pass and the dot pattern formed in the second pass. As described above, the dot pattern corresponding to each pass is obtained by performing halftone processing on the scan data to which the phase control information is added. However, since the phase control information does not include a high frequency component, the high frequency component of each scan data does not change even after the phase control information is added. As a result, in the two dot patterns obtained as a result of the halftone process, the characteristics of the original scanning data are retained for the respective high frequency components and are not correlated with each other. Since the high frequency component of the dot pattern corresponding to the first pass and the high frequency component of the dot pattern corresponding to the second pass are random to each other, it is possible to reduce density unevenness due to registration deviation. As described above, the phase control information is information for controlling the two dot patterns so that they are opposite in phase in the lower frequency band and uncorrelated in the higher frequency band.

  Hereinafter, the halftone process performed by the halftone processing unit 108 in step S106 will be described in detail. In the halftone processing in the present embodiment, for example, a known error diffusion method is used as a method of converting multi-value input image data into an N-value image. In the case of binary, a normal error diffusion method may be used, and in the case of N value, a known N value error diffusion method is used. FIG. 13 is a block diagram showing a detailed configuration of the halftone processing unit 108. FIG. 14 shows a flowchart of processing in the halftone processing unit 108. Here, a halftone process corresponding to cyan in two-pass printing and scanning number k = 1 will be described as an example. The halftone processing unit 108 includes an adder 401, a cumulative error adder 204, a threshold selection unit 205, a quantization unit 206, an error calculation unit 207, an error diffusion unit 208, and cumulative error line buffers 202 and 203.

  First, in step S201, the adder 401 adds the input cyan scanning duty and the corresponding phase control information for each pixel to calculate a total. The total value Ic of the duty value (input value) C_d and cyan phase control information C_r constituting the cyan scanning data is calculated as follows.

Ic = C_d + C_r (12)
In step S202, the accumulated error is added for error diffusion processing. 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 input value C_d, or more than the above four coefficients may be provided. In order to diffuse and accumulate errors using such an error diffusion coefficient, the halftone processing unit 108 secures two sets of accumulated error line buffers for cyan (202 to 203), and uses the accumulated error line buffer to be used as a scan number. For example, the switching is performed as follows.

“When scan number k = 1, 3,..., 2n + 1 (n is an integer greater than or equal to 0)” Use cyan (2n + 1) cumulative error line buffer 202 “Scan number k = 2, 4,..., 2n + 2 "When cyan" (2n + 2) cumulative error line buffer 203 is used Note that each of the cyan cumulative error line buffers 202 and 203 is composed of two sets of storage areas indicated by 2021 to 2031 in FIG. That is, there are two sets of “Ec1_0, Ec1 (x)” and “Ec2_0, Ec2 (x)”. For example, the cyan (2n + 1) accumulated error buffer 202 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. The cyan accumulated error line buffers 202 and 203 are all initialized with the initial value 0 only when the processing of the scan numbers k = 1 and 2 is started. For example, at the start of processing for scan number k = 3, (2n + 1) cumulative error buffer 402 is not initialized. In the present embodiment, the above-described two sets of accumulated error line buffers are required for each color, so that it is necessary to prepare four colors for this. That is, a total of 2 × 4 = 8 sets of line buffers are required.

  Here, in order to explain halftone processing for scan number k = 1 as an example, error diffusion processing is performed using cyan (2n + 1) accumulated error buffer 202. In the accumulated error adding unit 204, an error Ec1 (x) corresponding to the pixel position x in the X direction in the input pixel data is added to the sum of the scanning data and the phase control information. That is, if the data after the cumulative error addition is Ic ′ with respect to the input total data Ic, the following equation is established.

Ic ′ = Ic + Ec1 (x) (13)
In step S203, the threshold setting unit 205 sets a threshold T for each pixel. The threshold value T is set as follows, for example. In this embodiment, since one example of binarization is shown, one threshold value is set, but in the case of N-value conversion, (N-1) threshold values are required.

T = 128 (14)
Alternatively, in order to avoid dot generation delay, the threshold value T may be finely changed as follows according to the input value C_d so as to reduce the average quantization error.

T = f (C_d) (15)
Next, in step S204, the quantization unit 206 determines the binarization result Out_c by comparing the pixel data Ic ′ after the error addition with the threshold T. The rules are as follows:

When Ic ′ <T,
Out_c = 0 (16)
When Ic ′ ≧ T,
Out_c = 255 (17)
Next, in step S205, the error calculation unit 207 calculates a difference Err_c between the pixel data Ic ′ to which the error is added and the output pixel value Out_c as shown in Expression (18).

Err_c (x) = Ic′−Out_c (18)
In step S206, the error diffusion unit 208 diffuses the error. That is, using the cyan (2n + 1) accumulated error line buffer 202, the error Err_c (x) is diffused according to the horizontal pixel position x as follows.

Ec1 (x + 1) ← Ec1 (x + 1) + Err_c (x) × 7/16
Ec1 (x−1) ← Ec1 (x−1) + Err_c (x) × 3/16
Ec1 (x) ← Ec1_0 + Err_c (x) × 5/16
Ec1_0 ← Err_c (x) × 1/16
... (19)
This completes N-value conversion (binarization, quantization values 0, 255 in this embodiment) for one cyan pixel of scan number k = 1.

  In step S207, it is determined whether or not the processing in steps S201 to S206 has been performed on the addresses (0, 0) to (W-1, Nzzl-1) in the band, and the processing is performed on all addresses.

  The scan number k = 1 has been described above. However, for the scan number k = 2, the above halftone process is performed using each cyan accumulated error line buffer 203. In the process of scan number k = 3, the same cyan (2n + 1) cumulative error line buffer 202 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. 17, the print areas of scan number k = 1 and scan number k = 3 are adjacent to each other in the vertical direction, so that the stored accumulated error is applied to the adjacent lower area as it is. is there. If the cyan cumulative error line buffer 202 is initialized with k = 3, the error is not stored at the boundary adjacent to k = 1, and the continuity of dots cannot be maintained.

  As described above, according to the present embodiment, in the recording method in which N-value conversion processing is performed for each scan, the output dot arrangement formed in a single main scan has high dispersibility and registration. The following effects are further obtained when suppressing the deterioration of the graininess against the deviation. In the multi-pass recording method, phase control information corresponding to a plurality of passes is generated so that low-frequency components have opposite phases. When the halftone process is performed on the scan data to which the phase control information is added, the dot patterns of each other have an opposite phase in the low frequency band, and the high frequency band becomes random. As a result, good graininess is ensured regardless of whether or not registration deviation occurs. It is possible to reduce density unevenness due to registration deviation by reducing the correlation in the high-frequency component and providing randomness. In particular, in the present embodiment, the phase control information is calculated by an expression representing a function having a periodic variation. As a result, phase control information can be generated by a simple method without requiring data communication requiring a memory or a large bandwidth.

  Note that the method of halftone processing using the phase control information is not limited to the method of the present embodiment. For example, the phase control information represented by the equation may be reflected on the threshold value or the quantization error. In this embodiment, the error diffusion method is used as the halftone process. However, instead of this, the average error minimum method and the dither method may be applied.

  Moreover, although the example in which the phase control information generated by the phase control information generation unit 109 is input to the halftone processing unit 108 has been described, a configuration in which the phase control information is input to the scanning duty calculation unit 105 may be employed.

  Further, in the present embodiment, since the phase control information is added to the scan data, the phase control information indicating that the dot is more easily formed as the value is larger is generated. For example, in the case of adding to the threshold value, it is only necessary to generate phase control information such that the larger the value, the more difficult it is to form dots.

<Modification 1>
In the above-described embodiment, the phase control information generated by the phase control information generation unit 109 is dephased by generating a composite of sine waves that have opposite phases in the region Dmn_A and the region Dmn_B. That is, the phase control information is expressed using trigonometric functions such as a sine wave and a cosine wave. However, other than trigonometric functions can realize anti-phase. For example, dephasing can also be performed by defining an expression of the same random number seed (seed).
The signal shown in FIG. 12 is Dmn_A (nx, ny) = Rnd (nx, ny) (20)
Dmn_B (nx, ny) = − Rnd (nx, ny) (21)
It is also good. Here, Rnd is a uniform random number with the same random number seed.

  In addition, it is possible to express by combining a trigonometric function and a random number, or an antiphase relationship may be generated using a function such as a non-sinusoidal waveform such as a triangular wave, a rectangular wave (square wave), and a sawtooth wave. . That is, the phase control information may be a function that can generate an antiphase relationship between Dmn_A and Dmn_B.

<Modification 2>
In the above-described first embodiment, an example of two paths has been shown. However, for example, even when there are three or more paths, anti-phase conversion is possible. The phase control information generation unit 109 in the case of 2 passes is shown in FIG. 11, but for example in the case of 4 passes, it is shown in FIG.

In the four passes, it is necessary to define four regions Dmn_A to Dmn_D for performing antiphase for each nozzle. The signal of the phase control information generation unit 109 shown in FIG.
Dmn_A (nx, ny) = p · Sin (α · nx) + q · Sin (β · ny) (22)
Dmn_B (nx, ny)
= P · Sin (α · nx + π / 2) + q · Sin (β · ny + π / 2) (23)
Dmn_C (nx, ny)
= P · Sin (α · nx + π) + q · Sin (β · ny + π) (24)
Dmn_D (nx, ny)
= P · Sin (α · nx + 3π / 2) + q · Sin (β · ny + 3π / 2) (25)
By making the phase different from each other by π / 2, the total phase can be reversed.

  Note that the phase control information generation unit 109 can be reversed in phase by appropriately changing the phase setting even with the number of paths other than four paths.

<Other embodiments>
In the above-described embodiment, the function corresponding to each scan (for example, the region A and the region B) has been described by taking a function having a two-dimensional period as an example, but for example, a one-dimensional function having periodicity only in the x direction. This function can be processed in the same way. In this case, the function may be a function in which the phase of each function is shifted according to the y direction.

  In each of the above-described embodiments, an image processing apparatus for forming an image by printing and scanning a plurality of times with respect to the same area on the recording medium for each color component when the image is represented by a plurality of colors has been described. However, the present invention is not limited to this. In the present invention, if at least one color component representing an image is formed by recording and scanning the same area on the recording medium a plurality of times, the above-mentioned various components are focused on that color component. By performing the processing, the above-described effects can be obtained at least for the color component of interest. Needless to say, the present invention can also be applied to the case where an image represented by a single color is formed with one color component. In each of the above-described embodiments, an image is formed by causing a recording head having a plurality of nozzles arranged in a predetermined direction to scan on the recording medium in a direction crossing the nozzle arrangement direction and ejecting ink onto the recording medium. An image processing apparatus using an inkjet recording system has been described. However, for example, the present invention can be applied to 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.

  The present invention can also be realized by supplying a storage medium storing a computer program code of software for realizing the functions of the above-described embodiments to a system or apparatus. In this case, the computer (or CPU or MPU) of the system or apparatus reads out and executes the computer program code stored in the storage medium so that the computer can read the functions of the above-described embodiments.

105 Scan Duty Calculation Unit 108 Halftone Processing Unit 109 Phase Control Information Generation Unit

Claims (8)

An image processing apparatus for forming the image by performing scanning scanning a plurality of times for the same region on a recording medium for a target color component representing an image,
Generating means for generating scan data corresponding to each scan by dividing the pixel value of each pixel constituting the input image data corresponding to the color component of interest into each scan for each pixel;
Calculation means for calculating, for each scan, phase control information that expresses the ease of dot formation for each pixel as a positive or negative value based on the function corresponding to the scan;
Halftone processing means for converting the scan data into halftone image data representing a dot pattern by performing halftone processing on the scan data to which the phase control information is added ,
The functions corresponding to the scans cancel each other when they are overlapped, and the halftone image data for each scan is such that the phase of the dot pattern represented by each is opposite in a predetermined frequency band. Image processing device.   The image processing apparatus according to claim 1, wherein the halftone image data for each scan is uncorrelated in a frequency band in which a phase of a dot pattern represented by each of the halftone image data is higher than the predetermined frequency band.   The image processing apparatus according to claim 1, wherein the function is a trigonometric function.   The image processing apparatus according to claim 1, wherein the function is a function that generates a random number.   2. The target color component corresponds to the single color when the image is a single color, and the target color component corresponds to one color of the plurality of colors when the image is a plurality of colors. 5. The image processing device according to any one of items 4 to 4. The image processing apparatus according to claim 1, wherein the generation unit generates the phase control information using a function having a two-dimensional period. A computer program for causing a computer to function as the image processing apparatus according to any one of claims 1 to 6 . An image processing method for forming the image by recording and scanning a plurality of times with respect to the same region on the recording medium for a target color component representing an image,
By generating the scan data corresponding to each scan by dividing the pixel value of each pixel constituting the input image data corresponding to the color component of interest into each scan for each pixel,
Based on the function corresponding to the scanning, phase control information representing the ease of dot formation for each pixel is calculated for each scanning,
The scan data to which the phase control information has been added is converted to halftone image data representing a dot pattern by performing halftone processing using the phase control information,
The functions corresponding to the scans cancel each other when they are overlapped, and the halftone image data for each scan is such that the phase of the dot pattern represented by each is opposite in a predetermined frequency band. Image processing method.

Images