JP2009089080A - Image processing method, and image processing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing method and an image processing apparatus, capable of being corresponding to a single-path nozzle by achieving high-precision uniformity correction in nozzle recording characteristics while suppressing an operational load. <P>SOLUTION: The image processing method includes the steps of: creating a threshold data correction table formed correspondingly to each of a plurality of recording elements, while specifying a relation between reference threshold data and a corrected threshold data in order to correct recording characteristics of the recording elements; creating a corrected threshold matrix from the corrected threshold data calculated using the threshold data correction table; and converting a multi-gradation input image into an image of a lower gradation than that of the input image using the corrected threshold matrix, wherein a reference threshold matrix in which the number of pixels in a predetermined direction is less than the number of recording elements, is used to create a corrected reference threshold matrix in which the number of pixels in the predetermined direction is more than the number of pixels in the reference threshold matrix. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image processing method and an image processing apparatus, and more particularly to an image processing method and an image processing apparatus capable of realizing highly accurate unevenness correction in the recording characteristics of nozzles and corresponding to a single-pass nozzle.

  Patent Document 1 discloses that input image data is corrected on the basis of measurement results of a displacement amount of a defective nozzle and a dot diameter in order to obtain a high-quality image.

Further, Patent Document 2 discloses that density unevenness is reduced by correcting a threshold value matrix in which gradation threshold values are defined based on errors in droplet ejection characteristics of specified nozzles. .
JP 2004-058282 A JP 2006-263883 A

  However, in Patent Document 1, corrected image data is created by multiplying image data of each pixel of an input image by a density correction coefficient. For this reason, corrected image data must be created every time the image data of each pixel of the input image changes, increasing the computational load.

  Further, in Patent Document 2, the threshold value matrix pixel is multiplied by a uniform correction coefficient corresponding to the droplet ejection error with respect to the threshold value of the pixel corresponding to the nozzle causing the droplet ejection error. There is no disclosure corresponding to nozzles arranged for the width of the recording image, such as single-pass nozzles.

  The present invention has been made in view of such circumstances, an image processing method capable of realizing highly accurate unevenness correction in the recording characteristics of the nozzles while suppressing the calculation load, and corresponding to a single-pass nozzle, and An object is to provide an image processing apparatus.

  In order to achieve the above object, the present invention provides multi-tone input images for image recording using a print head in which a plurality of printing elements are arranged in a predetermined direction, and sets reference threshold data for high and low gray levels for each pixel. In the image processing method for converting to an image having a lower gradation than the input image using a specified reference threshold matrix, the reference threshold data and correction for correcting the reference threshold data to correct the recording characteristics of the recording element A threshold value data correction table creating step for creating a threshold value data correction table that is formed corresponding to each of the plurality of printing elements, and the threshold value data correction table A corrected threshold matrix creating step of creating a corrected threshold matrix composed of the calculated corrected threshold data, and the corrected threshold matrix; An image conversion step of converting the multi-tone input image into an image having a lower tone level than the input image using Rix, and in the corrected threshold value matrix creating step, the number of pixels in the predetermined direction Generating the corrected reference threshold matrix in which the number of pixels in the predetermined direction is larger than the number of pixels in the reference threshold matrix using the reference threshold matrix smaller than the number of the recording elements. And

  According to the present invention, the calculation load is reduced.

  In addition, a corrected threshold value matrix including threshold values that have been optimally corrected in accordance with the recording characteristics of each recording element is created, and highly accurate unevenness correction can be realized.

  In addition, since the corrected reference threshold value matrix in which the number of pixels in the predetermined direction is equal to the number of printing elements is created, it is possible to deal with nozzles arranged in the print image width, such as single-pass nozzles.

  As one aspect of the present invention, the threshold data correction table is updated each time the recording characteristic of the recording element changes, and the corrected threshold value matrix is updated each time the threshold data correction table is updated. It is characterized by.

  According to this aspect, since the corrected threshold value matrix is not updated unless the recording characteristics of the recording element change, high-speed calculation and image processing can be performed.

  As one aspect of the present invention, the corrected threshold value matrix creating step is performed when the input image data is not input and is offline.

  As an aspect of the present invention, in the threshold data correction table creation step, the threshold data correction table is a one-dimensional lookup table.

  As one aspect of the present invention, the threshold value data correction table defines a difference value between the corrected threshold value data and the reference threshold value data.

  According to this aspect, the data amount of the threshold data correction table can be reduced.

  As one aspect of the present invention, the threshold data correction table has a plurality of patterns defined for each image output condition, and in the corrected threshold matrix creation step, the threshold data correction corresponding to the image output condition is performed. The corrected threshold value matrix is formed using a table.

  According to this aspect, it is possible to perform uneven correction with higher accuracy that is optimal for each output condition. In addition, by re-creating the corrected threshold matrix so as to correspond to the output condition, it is possible to perform high-speed and high-precision unevenness correction corresponding to the change of the output condition.

  As one aspect of the present invention, the input image is a single-color image constituting a color image, and the image conversion step adjusts the image readout timing in order to shift the image writing position in accordance with the pitch of the recording elements. It is characterized by doing.

  In order to achieve the above object, the present invention provides a reference threshold value that specifies threshold data for gradations in a multi-tone input image in image recording using a recording head in which a plurality of recording elements are arranged in a predetermined direction. In an image processing apparatus that converts an image having a gradation lower than that of the input image using a matrix, correction for correcting the reference threshold data of the reference threshold matrix and the reference threshold data in order to correct the recording characteristics of the recording element A threshold data correction table creating means for creating a threshold data correction table that is formed corresponding to each of the plurality of printing elements, and a threshold data correction table. A corrected threshold value matrix creating means for creating a corrected threshold value matrix comprising the calculated corrected threshold value data, and the corrected value Image conversion means for converting the multi-gradation input image into an image having a gradation lower than that of the input image using a value matrix, and the corrected threshold value matrix creation means includes pixels in the predetermined direction. Creating the corrected reference threshold matrix in which the number of pixels in the predetermined direction is larger than the number of pixels in the reference threshold matrix using the reference threshold matrix having a number smaller than the number of the recording elements. Features.

  According to the present invention, it is possible to realize highly accurate unevenness correction in the recording characteristics of the nozzle while suppressing the calculation load, and it is possible to correspond to a single-pass nozzle.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Outline of image processing method]
FIG. 1 is a diagram showing an outline of the image processing method of the present invention. As shown in FIG. 1, first, the recording characteristics of each nozzle 51 in an inkjet head 50 (see FIG. 13) in which a plurality of nozzles 51 (see FIG. 13) are arranged in a predetermined direction are measured (step S1).

  Specifically, a test pattern image is formed on the recording medium while relatively moving the inkjet head 50 and the recording medium in the sub-scanning direction. Then, the density of the test pattern image formed on the recording medium is detected. Based on the density of the image thus detected and the density profile stored in advance in the print control memory 152 (see FIG. 12), the recording characteristics for each nozzle 51 are measured. The recording characteristics of the nozzle 51 to be measured include dot position and size (droplet amount).

  Some inkjet heads 50 are capable of drawing a plurality of types of dots, for example, three types of large, medium, and small, and other eight types and sixteen types, depending on the output gradation. In this case, the dot position and size are measured for each dot size used in each gradation value.

  Next, a nozzle gradation correction coefficient is calculated using the measured recording characteristics of the nozzle 51 (step S2). In calculating the nozzle gradation correction coefficient, first, for each gradation value of each nozzle 51, a density correction coefficient di for correcting density unevenness is calculated. Then, using this calculation result, a nozzle gradation correction coefficient for correcting the gradation value of each nozzle 51 is calculated. The nozzle gradation correction coefficient can have not only one for each nozzle 51 but also a plurality.

  Next, using the calculated nozzle gradation correction coefficient, a threshold data correction table that defines the relationship between the reference threshold data of the reference threshold matrix and the corrected threshold data obtained by correcting the reference threshold data is created (step S3). , Threshold data correction table creation step).

  The threshold data correction table is a one-dimensional LUT (lookup table), and is preferably formed corresponding to each of a plurality of nozzles.

  In the threshold data correction table, it is conceivable that the value of the reference threshold data in the reference threshold matrix and the value of the corrected threshold data are defined on a one-to-one basis. In the threshold data correction table, a difference value between the reference threshold data value of the reference threshold matrix and the corrected threshold data value may be defined.

  The threshold data correction table may have a plurality of patterns defined for each image output condition (recording paper condition, drawing mode condition, etc.).

  The nozzle gradation correction coefficient may be obtained by directly measuring the recording characteristics of each gradation value of each nozzle 51 and comparing it with the reference recording characteristics.

  As the reference threshold matrix, a threshold matrix generally used for binarization is used. A distributed dither matrix known as a fat dither matrix or a Bayer dither matrix may also be used. When higher quality is required, a threshold matrix that generates a frequency modulation type dot pattern, which is called an FM screen imitating an error diffusion method dot pattern, may be used.

  In addition, the frequency pattern is a noise pattern that takes into account human visual characteristics, and is called Blue Noise Mask that reproduces the noise frequency characteristic called Blue Noise, and Green Noise Mask that reproduces the noise frequency characteristic called Green Noise. A threshold matrix may be used.

  Next, an unevenness corrected threshold matrix is created from the reference threshold matrix using the created threshold data correction table (step S4, corrected threshold matrix creating step).

  The reference threshold matrix is not limited to binarization in which one threshold exists per pixel, but can also be applied to N-value conversion in which a plurality of thresholds exist per pixel. Further, in an inkjet apparatus that forms a color image, for example, a nonuniformity corrected threshold value matrix that is different for each of C, M, Y, and K is created. In addition, by using a plurality of colors for the same hue or using inks of R, G, B, etc., if there are many types of inkjet heads 50, a corresponding number of reference threshold matrixes are prepared. It is desirable to create an unevenness corrected threshold value matrix.

  A method for creating the unevenness corrected threshold value matrix will be described later.

  In step S3 and step S4, the threshold data correction table is updated every time the recording characteristics of the nozzle change, and at the same time, the unevenness corrected threshold value matrix is also updated.

  The above steps are performed during offline processing in which image data is not input.

  On the other hand, at the time of image output when image data is input, first, when image data is input (step S11), an N-value conversion process for converting a high gradation image into a low gradation image is performed (step S12). , Image conversion step). At this time, N-value conversion processing is performed using the unevenness corrected threshold value matrix generated in step S3, and unevenness corrected N-valued image data is generated. Next, the corrected N-valued image data created in step S12 is converted into head driver data (step S13).

  The above is the outline of the image processing of the present invention.

  On the other hand, FIG. 2 shows an outline of image processing for performing correction processing on input image data as a conventional example. As shown in FIG. 2, first, the recording characteristics of each nozzle are measured (step S21). Next, a nozzle gradation correction coefficient is calculated using the measured recording characteristics of the nozzle (step S22). The above steps are performed during offline processing in which image data is not input, and are common to the image processing of the present invention.

  On the other hand, when the image data is input, when the image data is input (step S31), the difference from the present invention is that the nozzle gradation correction created in step S22 is performed on the input image data. Based on the coefficients, image data with unevenness correction is created (step S32). Next, N-value conversion processing is performed on the created unevenness-corrected image data using a reference threshold matrix (step S33), and the generated unevenness-corrected N-valued image data is converted to head driver data (step S33). Step S34).

  The above is the outline of the image processing for performing the correction processing on the input image data.

  As described above, when correction processing is performed on input image data in the conventional example, correction processing is required every time the input image data changes as described in the background art, and the calculation load is large. turn into.

  In contrast, when correction processing is performed on the reference threshold matrix as in the image processing of the present invention, the same unevenness corrected threshold matrix can be used as long as the recording characteristics of the nozzles 51 do not change. Since there is no need to create a matrix, the calculation burden is reduced.

  3 and 4 are diagrams showing a data flow in the image processing shown in FIG. 1 and FIG.

  As shown in FIG. 3, in the image processing of the present invention (FIG. 1), correction is performed on the reference threshold data T (j, y) of the reference threshold matrix to create unevenness corrected threshold data T ′ (j, y). is doing. Note that (j, y) is an arbitrary address of the pixel in the reference threshold matrix.

  Here, the correction with respect to the reference threshold data T (j, y) in the reference threshold matrix defines the unevenness corrected threshold data T ′ (j, y) for the reference threshold data T (j, y) for each corresponding nozzle. This is performed using a one-dimensional LUT (look-up table). Then, the binarization processing is performed by comparing the value of the input image data G (i, y) and the value of the unevenness corrected threshold data T ′ (j, y) of the unevenness corrected threshold value matrix. .

  On the other hand, as shown in FIG. 4, in the conventional image processing (FIG. 2), the input image data G (i, y) is corrected, and the unevenness corrected image data G ′ (i, y) is obtained. Creating. Here, the correction for the input image data G (i, y) defines the image data G ′ (i, y) that has been subjected to unevenness correction for the image data G (i, y) input for each corresponding nozzle. This is performed using a one-dimensional LUT (lookup table). Then, the binarization processing is performed by comparing the value of the image data G ′ (i, y) after unevenness correction and the value of the reference threshold data T (j, y) of the reference threshold matrix.

  For the one-dimensional LUT (look-up table), the nozzle tone correction coefficients that define the correction values in the image processing of the present invention and the image processing of the conventional example have an inverse function relationship. Therefore, the image data after the binarization process is common in the image processing of the present invention and the image processing of the conventional example.

  Here, according to the conventional image processing (FIG. 2), an operation is required every time the input image data changes, and the operation load increases. On the other hand, according to the image processing of the present invention (FIG. 1), it is not necessary to perform calculation unless the recording characteristics of the nozzles change, and the calculation load is reduced, and unevenness correction has been performed as in the conventional image processing. Image data after binarization processing common to the case of creating image data G ′ (i, y) can be created.

Further, according to the image processing of the present invention (FIG. 1), correction is performed using a correction table in which nozzle gradation correction coefficients for correcting the recording characteristics of each recording element are defined. Threshold data T ′ (j, y) of a non-uniformity corrected threshold matrix composed of thresholds that have been optimally corrected according to the recording characteristics is created, and high-precision unevenness correction can be realized.
[Derivation of density correction coefficient]
The density correction coefficient di is determined by the following equation [Equation 1].

  Here, xi is the landing position of each nozzle with the ideal landing position of the correction target nozzle as the origin. Δvi is a parameter representing the droplet amount error of nozzle i and is defined by Δvi = (Vi / V0) −1. Here, V0 is the ideal (design value) average droplet volume, and Vi is the droplet volume of nozzle i. Also, Π means taking a product in N nozzles used for correction.

  This can be expressed explicitly for the case of N = 3 as follows.

  From the condition of minimizing the low frequency component of the power spectrum of density unevenness, the density correction coefficient of each nozzle can be theoretically derived.

  First, a density profile incorporating the error characteristics of each nozzle is defined as follows:

  The density profile D (x) of the image is the sum of the density profiles printed by each nozzle, and the printing model (density profile printed by one nozzle) represents the printing of the nozzles. The print model is expressed separately as a nozzle output density Di and a standard density profile z (x).

  Strictly speaking, the standard density profile z (x) has a finite spread equal to the dot diameter, but considering the correction of the position error as a problem of density deviation balancing, what is important is the position of the center of gravity of the density profile. The (landing position) and the spread of the density profile is a secondary factor. Therefore, an approximation that replaces the profile with a δ function is reasonable. Assuming such a standard density profile, mathematical handling is facilitated, and an exact solution of the density correction coefficient is obtained.

  FIG. 5A shows a printing model that is realistic, and FIG. 5B shows a δ function type printing model. When approximated by the δ function model, the standard concentration profile is expressed by the following equation.

  In deriving the density correction coefficient, it is considered that the landing position error Δx0 of a specific nozzle (i = 0) is corrected by N peripheral nozzles. Here, the correction target nozzle number is i = 0. Note that peripheral nozzles can also have a predetermined landing position error.

The number (index) of the N nozzles including the correction target nozzle (center nozzle) is expressed by the following equation.

  In this equation, N needs to be an odd number, but it is not necessary to limit N to an odd number when implementing the present invention.

  The initial output density (output density before correction) is expressed by the following equation assuming that only i = 0 has a value.

  When the density correction coefficient is di, the corrected output density Di 'is expressed by the following equation.

  That is, when i = 0, it is represented by the sum of the initial output density value and the correction value (di × Dini), and when i ≠ 0, only the correction value is obtained.

  The landing position xi of each nozzle i is expressed by the following equation.

  When the δ function type printing model is used, the corrected density profile is expressed by the following equation.

  On the other hand, when performing Fourier transform,

  It is expressed. Dini is omitted because it is a common constant.

  Minimizing the visibility of density unevenness is to minimize the low frequency component of the power spectrum of the following equation.

This can be mathematically approximated by the fact that the differential coefficient (first order, second order,...) Of T (f) at f = 0 is zero. Since the number of unknowns di ′ is now N, if the condition for storing the DC component is included, all (N) unknowns di ′ are strictly determined by using the condition that the differential coefficient up to the N−1 order is zero. Determined. In this way, the following correction conditions are determined.

In the δ function model, when each correction condition is developed, it is reduced to N simultaneous equations for Di by easy calculation. Arranging the development of each correction condition gives the following condition group (equation group).

  The meaning of these equations is that the first equation represents preservation of the DC component, and the second equation represents preservation of the center of gravity. The third and subsequent formulas indicate that the N-1th moment in statistics is zero.

  When the conditional expression thus obtained is expressed in matrix form, it can be expressed as follows.

This coefficient matrix A is a so-called Vandermonde type matrix, and its determinant is known to be the following expression using a difference product.

  For this reason, the exact solution of di 'can be obtained using Cramer's formula. The detailed process of the calculation is omitted, but the algebraic calculation shows that the solution is

  Therefore, the density correction coefficient di to be obtained is as follows.

As described above, the exact solution of the density correction coefficient di is derived from the condition that the origin differential coefficient of the power spectrum is zero. As the number N of peripheral nozzles used for correction is increased, the higher-order differential coefficient can be made zero, so that the low frequency energy becomes smaller and the visibility of unevenness is further reduced.

  In this embodiment, the condition for setting the origin differential coefficient to zero is used, but even if it is not completely zero, it is set to a sufficiently small value (for example, 1/10 before correction) compared to the differential coefficient before correction. However, the low frequency component of the power spectrum of density unevenness can be made sufficiently small. That is, in terms of the condition that the low frequency component of the power spectrum is reduced to such an extent that density unevenness is not visually recognized, the range of the value is set in the sense that the origin differential coefficient of the power spectrum is set to a sufficiently small value (approximately 0). To 1/10 or less of the absolute value of the differential coefficient before correction.

[Create corrected threshold value matrix]
FIG. 6 is a schematic diagram of creating a corrected threshold matrix according to the present invention. As shown in FIG. 6, for example, when P <L based on a reference threshold matrix T of P pixels × Q pixels, a nonuniformity corrected threshold matrix T ′ of L pixels × Q pixels is created. Then, by setting the value of L as the value of the number of pixels of the image to be recorded on the recording medium, it is possible to cope with nozzles arranged for the recording image width, such as single-pass nozzles. Therefore, for example, when recording at a resolution of 600 dpi on a recording medium of A3 paper size (short side: 297 mm), nozzles of about 7000 pixels are required, but this can be handled by setting L = about 7000.

  FIG. 6 shows an example in which unevenness correction is performed on the threshold value of a pixel corresponding to an arbitrary i-th nozzle of the head 50 in the unevenness corrected threshold value matrix T ′ shown in FIG. As described above, the unevenness corrected threshold matrix T ′ of L pixels × Q pixels is formed by combining a plurality of reference threshold matrices T of P pixels × Q pixels.

The correction value of the threshold value of the pixel corresponding to the arbitrary i-th nozzle of the head 50 is calculated from the corresponding j columns (j = i mod P) in the reference threshold matrix T of P pixels × Q pixels shown in (c). ) Of pixel threshold data T (j, y) for the corresponding nozzles shown in (b) is subjected to unevenness correction threshold data T ′ (j, y) for threshold data T (j, y) of the reference threshold matrix. The unevenness corrected threshold data T ′ (i, y) = f ⁻¹ (T ₀ (j, y)) corrected by a threshold correction table which is a one-dimensional LUT that defines

  Note that j is a pixel address corresponding to the arrangement direction (main scanning direction) of the nozzles 51 of the inkjet head 50, and y is a value representing a pixel address corresponding to the conveyance direction (sub-scanning direction) of the recording medium. The threshold correction table is formed based on the density correction coefficient. Further, as shown in FIG. 6, the unevenness corrected threshold data T ′ (j, y) with respect to the reference threshold data T (j, y) in the reference threshold matrix has a non-linear relationship.

  Therefore, a description will be given with reference to FIGS. FIG. 7 shows a specific example of creating binarized image data by creating a non-uniformity corrected threshold matrix in the image processing of the present invention. On the other hand, FIG. 7 shows a specific example in which unevenness corrected image data is created and binarized image data is created as conventional image processing.

In the image processing of the present invention, as shown in FIG. 7A, as an example, a plurality of Bayer-type dither matrices of 8 pixels × 8 pixels are similarly used as the reference threshold matrix. Then, as shown in FIG. 7 (b), consider the threshold column _{T 11} in the threshold matrix corresponding to i = 11-th nozzle 51. Here, in the reference threshold matrix, the column j corresponding to the i = 11th nozzle 51 is j = 3 because j = 11 mod 8 = 3. Therefore, the threshold column T ₁₁ in the threshold matrix corresponding to i = 11 th, the reference threshold value matrix j = 3 columns threshold row of corresponds.

Then, from the threshold data correction table F _b ⁻¹ ₁₁ (T) shown in FIG. 7C, unevenness corrected threshold data T ′ corresponding to the threshold data T of the i = 11th column is obtained.

Here, the values of the image data G in the column corresponding to the i = 11th nozzle 51 are all “32”. Therefore, the threshold value T ₁₁ ′ of the corrected threshold matrix is compared with the magnitude of the value of the image data G to create binary image data.

  On the other hand, as an example of conventional image processing, as shown in FIG. 8A, as an example, a Bayer-type dither matrix of 8 pixels × 8 pixels is used as a reference threshold matrix. Then, as shown in FIG. 8B, the input image data G in the column corresponding to the i = 11th nozzle 51 is considered. Here, the values of the input image data G in the column corresponding to the i = 11th nozzle 51 are all “32”.

Then, from the image data correction table F _b11 (G) shown in FIG. 8C, the unevenness corrected image data G ′ (value: corresponding to the image data G (value: 32) in the column corresponding to i = 11th). 26).

Here, in the reference threshold matrix, the column j corresponding to the i = 11th nozzle 51 is j = 3 because j = 11 mod 8 = 3. Therefore, by comparing the magnitudes of the corrected image data G'value with the reference threshold value matrix of j = 3 columns threshold columns T ₃ of, creating a binary image data.

  As shown in FIGS. 7 (c) and 8 (c), the binarized image data is the same both in the case of the conventional image processing and the case of the image processing of the present invention.

In the image processing of the present invention, the image data correction table F _bi (G) (i = 0, 1, 2,...) Corresponding to each nozzle 51 (i = 0, 1, 2,...) As shown in FIG. ..), And the threshold value data correction table F _b ⁻¹ _i (T) (i = 0, 1, 2,...) Corresponding to each nozzle 51 (i = 0, 1, 2,...) Calculated based on. use.

Then, as shown in FIG. 10, from the unevenness corrected threshold data T ′ corresponding to the reference threshold data T obtained from the threshold data correction table F _b ⁻¹ _i (T) (i = 0, 1, 2,...). An example of the unevenness corrected threshold value matrix is shown.

[Effects when outputting color images]
When outputting a color image, it is necessary to increase the image position accuracy (registration accuracy) of each color plate to match each color plate. Even in a high-precision color image output device, the drawing position accuracy may be lowered and the drawing position may be shifted due to various factors such as temperature and humidity, paper deformation, and output device reproducibility. Further, in a normal printing apparatus, it is necessary to finely adjust the output apparatus while looking at a registration accuracy confirmation mark called a register mark when outputting an actual image. At this time, the position accuracy can be finely adjusted by shifting the writing position of the image data in accordance with the nozzle interval that affects the output resolution of the image.

  Here, FIG. 11 is a diagram showing a processing state and a processing flow when the image data writing position is shifted in the image processing of the present invention. As shown in the flowchart of FIG. 11, first, an image read address calculation unit 22 that calculates an address of image data and a threshold read address from an image position adjustment value input unit 20 that adjusts the amount by which the image data write position is shifted. The shift amount of the image data writing position (nozzle position shift amount Δi, pixel position shift amount Δj) is input to the calculation unit 24.

  Then, the image data G reflecting the shift amount of the image data writing position (the shift amount Δi of the nozzle position and the shift amount Δj of the pixel position) is compared with the non-uniformity corrected threshold data T ′ to perform the N-value processing. .

  As described above, since the positional relationship between the image data and the nozzle position changes every time the image data writing position is shifted, in the conventional image processing, it is necessary to create corrected image data each time. However, in the image processing of the present invention, as shown in FIG. 11, it is not necessary to create a corrected threshold matrix again, and an effect that does not require a new calculation can be obtained.

[Overall configuration of image forming apparatus]
FIG. 12 is a schematic block diagram illustrating an example of the overall configuration of an image forming apparatus according to an embodiment of the present invention.

  In FIG. 12, the image forming apparatus 10 according to the present embodiment includes an inkjet head 50, a liquid supply unit 60, a conveyance unit 80, a communication interface 102, a system controller 110, a memory 112 (system control memory), a liquid supply control unit 116, Conveyance control unit 118, print control unit 150, print control memory 152, head driver 154, droplet ejection characteristic measurement unit 162 (ejection characteristic specifying unit), unevenness correction processing unit 164 (threshold correction unit), threshold data correction table creation unit 166, including a non-uniformity corrected threshold value matrix creation unit 168.

  The plurality of inkjet heads 50 eject ink onto a recording medium such as paper. The image forming apparatus 10 includes at least an inkjet head 50 for each ink of C (cyan), M (magenta), Y (yellow), and K (black). In the image forming apparatus 10, light ink droplets such as LC (light cyan) and LM (light magenta) are not essential, but an ink jet head 50 for such light ink may be provided.

  The liquid supply unit 60 supplies ink of each color to each inkjet head 50. The liquid supply unit 60 includes, for example, a conduit from an ink storage unit (not shown) such as an ink cartridge detachably attached to the image forming apparatus 10 to the inkjet head 50, and a pump.

  The transport unit 80 transports the recording medium on a predetermined transport path. For example, it includes a conveyance belt that sucks and places the recording medium, a conveyance roller that drives the conveyance belt, and a conveyance motor that drives the conveyance roller. The conveyance unit 80 relatively moves the recording medium and the inkjet head 50 in the conveyance direction (sub-scanning direction) of the recording medium.

  The communication interface 102 is an image data acquisition unit that acquires image data through communication with the host computer 300. The communication interface 102 uses, for example, a wired communication interface such as USB (Universal Serial Bus), IEEE 1394, Ethernet (registered trademark), or a wireless communication interface. Image data sent from the host computer 300 is taken into the image forming apparatus 10 via the communication interface 102 and temporarily stored in the memory 112.

  Note that the acquisition mode of the image data is not particularly limited to the acquisition mode by communication with the host computer 300. For example, the image data may be acquired by reading the image data from a removable medium such as a memory card or an optical disk.

  The system controller 110 includes a central processing unit (CPU) and its peripheral circuits, and functions as a main control unit that controls the entire image forming apparatus 10 according to a predetermined program, and performs various calculations related to image processing. Functions as an arithmetic unit. The system controller 110 controls each unit of the image forming apparatus 10 such as the communication interface 102, the memory 112, the liquid supply control unit 116, the conveyance control unit 118, and the print control unit 150.

  The memory 112 stores programs executed by the system controller 110 and various data necessary for control. In addition, the memory 112 is used as a temporary storage area for acquired image data and also as a calculation work area.

  The liquid supply control unit 116 performs control to supply ink to the inkjet head 50 by the liquid supply unit 60 in accordance with an instruction from the system controller 110.

  The conveyance control unit 118 is a driver (drive circuit) that drives the conveyance unit 80 in accordance with an instruction from the system controller 110.

  The print control unit 150 generates a dot pattern (binary signal or multilevel signal) from the image data in the memory 112 in accordance with an instruction from the system controller 110, or uses a non-uniformity correction processing unit 164 described later as a reference threshold value. It has a function of creating a non-uniformity corrected threshold value matrix that has been subjected to correction processing on the matrix.

  The print control unit 150 includes a print control memory 152, and image data, parameters, and other data are temporarily stored in the print control memory 152 during image processing in the print control unit 150. In FIG. 12, the print control memory 152 is shown in a mode associated with the print control unit 150, but it can also be used as the system control memory 112. Also possible is an aspect in which the print controller 150 and the system controller 110 are integrated and configured with a single processor.

  Image data (for example, RGB data) input from the host computer 300 via the communication interface 102 is temporarily stored in the memory 112 as described above, and the image data stored in the memory 112 is subjected to a predetermined conversion process. After that, it is sent to the print controller 150 under the control of the system controller 110, and halftoning using the unevenness corrected threshold matrix is performed in the print controller 150, and C (cyan), M (magenta), Y (yellow) , K (black) are converted into dot patterns for each ink color. When the dot pattern is a binary signal, it consists of binary (1 or 0) data indicating whether or not droplets are ejected from the nozzle. When the dot pattern is a multi-value signal, it can represent not only whether or not to eject droplets but also the droplet ejection amount (or dot size). The dot pattern generated using the unevenness corrected threshold value matrix is given to the head driver 154.

  The head driver 154 generates a drive signal for the inkjet head 50 in accordance with the given dot pattern. When a drive signal generated by the head driver 154 is given to the inkjet head 50, a predetermined amount of ink is ejected from the nozzle corresponding to the dot pattern.

  A desired image is formed on the recording medium by ejecting ink from the inkjet head 50 in synchronization with the conveyance speed of the recording medium.

  The droplet ejection characteristic measuring unit 162 identifies the droplet ejection characteristic of the inkjet head 50 by actually measuring the droplet ejection characteristic of the inkjet head 50.

  The specification of the droplet ejection characteristics is not limited to the case of acquiring by actually measuring with the image forming apparatus 10. For example, the droplet ejection characteristics may be specified based on image data read from a recording medium by an external device (for example, a scanner) of the image forming apparatus 10, or the droplet ejection characteristic information itself as an analysis result is used as the host computer 300 or the like. May be obtained from

  The unevenness correction processing unit 164 is a unit that creates the nozzle gradation correction coefficient according to the droplet ejection characteristics of the inkjet head 50.

  The threshold data correction table creation unit 166 is a means for creating a threshold data correction table for each nozzle 51 based on the nozzle gradation correction coefficient created by the unevenness correction processing unit 164.

  The unevenness-corrected threshold value matrix creating unit 168 performs unevenness-corrected threshold values obtained by performing correction processing on the reference threshold value data of the reference threshold value matrix based on the threshold value data correction table for each nozzle 51 created by the threshold value data correction table creating unit 166. A corrected threshold value matrix composed of data is created.

  In the image forming apparatus 10, the unevenness correction processing unit 164 performs unevenness correction processing corresponding to the droplet ejection characteristics of the inkjet head 50 on the corrected threshold matrix in advance, and performs unevenness correction processing in advance during on-demand image formation. A high-quality image is formed at high speed using the corrected threshold matrix.

[Inkjet head structure]
FIG. 13 is a perspective plan view showing a schematic structure of an example of the inkjet head 50.

  In FIG. 13, an inkjet head 50 has a number of nozzles 51 (droplet ejection openings) arranged over a length exceeding at least one side of a maximum size recording paper, and moves relative to a recording medium such as paper. Meanwhile, ink is ejected from the nozzle 51 toward the recording medium.

  The ink jet head 50 includes a plurality of nozzles 51 that eject ink, a pressure chamber 52 that communicates with the nozzles 51 and applies pressure to the ink when ink is ejected from the nozzles 51, and is illustrated in FIG. A plurality of pressure chamber units 54 each including an ink supply port 53 that supplies ink to the pressure chamber 52 from a later-described common liquid chamber is omitted and arranged in a two-dimensional matrix. .

  FIG. 14 is an enlarged view of a part of the inkjet head 50 shown in FIG.

  In FIG. 14, the plurality of nozzles 51 are arranged along the main scanning direction and along a direction that forms a predetermined angle θ with respect to the main scanning direction. That is, the plurality of nozzles 51 are arranged in a so-called two-dimensional matrix.

  Specifically, for example, m nozzles with reference numerals 51-11, 51-21, 51-31,... (Or reference numerals 51-16, 51-26, 51-36,...) Extend along the main scanning direction. It is arranged. Further, for example, n nozzles (in this case, n = 6) 51-11, 51-12, 51-13, 51-14, 51-15, and 51-16 have an angle θ with respect to the main scanning direction. It is arranged along the direction to make.

  A virtual nozzle array (projection nozzle array) formed by projecting all the nozzles 51 onto one line (main scan line) along the main scan direction has a dot pitch P in the main scan direction (that is, ink from the nozzles 51). The nozzles 51 are arranged at substantially the same interval as the interval in the main scanning direction between dots formed on the recording medium by being ejected. In other words, the projection nozzle array is a nozzle array having a pitch substantially the same as the dot pitch P.

  In practice, the interval between the nozzles 51 (nozzle pitch) is n times the dot pitch P (here, n = 6) in the main scanning direction, and in the direction that forms an angle θ with respect to the main scanning direction, D (= P × 1 / cos θ) shown in FIG.

  FIG. 15 is a cross-sectional view taken along the line 15-15 in FIG. 13 and shows one pressure chamber 52 and its periphery. 15 shows an electrical signal from the nozzle 51 for ejecting ink, the pressure chamber 52 communicating with the nozzle 51, the ink supply port 53 through which the ink supplied to the pressure chamber 52 passes, and the head driver 154 in FIG. Individual electrodes 57 to which (drive signals) are applied and piezoelectric elements 58 that generate displacement (distortion) in accordance with the drive signals applied to the individual electrodes 57 are described. 51, a plurality of pressure chambers 52, ink supply ports 53, individual electrodes 57, and piezoelectric elements 58 are provided in a two-dimensional matrix.

  The diaphragm 56 is disposed on the side opposite to the side where the nozzles 51 are disposed across the pressure chamber 52, and is formed as a common plate for the plurality of pressure chambers 52. The diaphragm 56 constitutes the vibration surface of the pressure chamber 52 and vibrates due to the displacement of the piezoelectric element 58, thereby changing the volume of the pressure chamber 52.

  The common liquid chamber 55 is disposed on the side opposite to the side where the pressure chamber 52 is disposed with the vibration plate 56 interposed therebetween, and a plurality of pressure chambers are provided via the ink supply ports 53 formed in the vibration plate 56. Ink is supplied to 52. That is, when the nozzle 51 is viewed below the pressure chamber 52, the common liquid chamber 55 is directly above the plurality of pressure chambers 52 so as to cover all of the plurality of pressure chambers 52. It is formed as a liquid chamber. By such a common liquid chamber 55, the ink is supplied to the pressure chambers 52 arranged in a two-dimensional matrix with a good refill property.

One electrode of the piezoelectric element 58 is constituted by an individual electrode 57, and is connected to the head driver 154 of FIG. 12 via the individual electrode 57. The other electrode of the piezoelectric element 58 is constituted by a diaphragm 56 (common electrode) and is grounded.
The piezoelectric element 58 is made of, for example, piezo, and distortion occurs according to a drive signal given from the head driver 154 via the individual electrode 57. When the piezoelectric element 58 is distorted, the diaphragm 56 vibrates, the volume of the pressure chamber 52 changes, and ink is ejected from the nozzle 51.

  The image forming method and the image forming apparatus of the present invention have been described in detail above. However, the present invention is not limited to the above examples, and various improvements and modifications are made without departing from the gist of the present invention. Of course it is also good.

It is a figure which shows the outline | summary of the image processing of this invention. It is a figure which shows the outline | summary of the image processing of a prior art example. It is a figure which shows the mode of the flow of the data in the image processing of this invention. It is a figure which shows the mode of the flow of the data in the image processing of a prior art example. (A) is a printing model diagram in line with reality, and (b) is a δ function type printing model diagram. It is a schematic diagram of preparation of the corrected threshold value matrix of the present invention. It is a figure which shows the specific example which produces the nonuniformity corrected threshold value matrix and produces binary image data in the image processing of this invention. It is a figure which shows the specific example which produces nonuniformity correction completed image data in the image processing of a prior art example, and produces binarized image data. It is a figure which shows the example of the threshold value data correction table corresponding to each nozzle. It is a figure which shows an example of the nonuniformity corrected threshold value matrix. It is a figure which shows the example at the time of a color image output. 1 is a schematic block diagram illustrating an example of the overall configuration of an image forming apparatus according to an embodiment of the present invention. It is a plane perspective view which shows schematic structure of the example of an inkjet head. It is the figure which expanded a part of inkjet head shown by FIG. FIG. 15 is a cross-sectional view taken along line 15-15 in FIG. 13 and shows one pressure chamber and its periphery.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 50 ... Inkjet head, 51 ... Nozzle, 162 ... Droplet ejection characteristic measurement part, 164 ... Unevenness correction process part, 168 ... Unevenness correction completed threshold value matrix preparation part

Claims (8)

A multi-tone input image in image recording using a print head in which a plurality of printing elements are arranged in a predetermined direction is input to the input image using a reference threshold matrix that defines reference threshold data of high and low gradations for each pixel. In an image processing method for converting to an image with a lower gradation,
In order to correct the recording characteristics of the recording element, it defines the relationship between the reference threshold data and corrected threshold data obtained by correcting the reference threshold data, and is formed corresponding to each of the plurality of recording elements. A threshold data correction table creating step for creating a threshold data correction table;
A corrected threshold matrix creating step of creating a corrected threshold matrix composed of the corrected threshold data calculated using the threshold data correction table;
Converting the multi-tone input image into an image having a lower gradation than the input image using the corrected threshold matrix, and
In the corrected threshold value matrix creating step, the number of pixels in the predetermined direction is larger than the number of pixels in the reference threshold matrix by using the reference threshold matrix in which the number of pixels in the predetermined direction is smaller than the number of recording elements. Creating the corrected reference threshold matrix; and an image processing method comprising: The image processing method according to claim 1,
The threshold data correction table is updated each time the recording characteristics of the recording element change,
The corrected threshold matrix is updated each time the threshold data correction table is updated;
An image processing method characterized by the above. The image processing method according to claim 1 or 2,
The corrected threshold value matrix creating step is performed when the input image data is not input offline;
An image processing method characterized by the above. The image processing method according to any one of claims 1 to 3,
The threshold data correction table is a one-dimensional lookup table;
An image processing method characterized by the above. The image processing method according to any one of claims 1 to 4,
The threshold data correction table defines a difference value between the corrected threshold data and the reference threshold data;
An image processing method characterized by the above. The image processing method according to any one of claims 1 to 5,
The threshold data correction table has a plurality of patterns defined for each image output condition,
In the corrected threshold value matrix creating step, forming the corrected threshold value matrix using the threshold value data correction table corresponding to the output condition of the image;
An image processing method characterized by the above. The image processing method according to any one of claims 1 to 6,
The input image is a single color image constituting a color image,
In the image conversion step, adjusting the image reading timing in order to shift the image writing position in accordance with the pitch of the recording elements;
An image processing method characterized by the above. A multi-tone input image in image recording using a print head in which a plurality of printing elements are arranged in a predetermined direction is converted into a lower rank than the input image using a reference threshold matrix that defines threshold data for high and low gradations. In an image processing apparatus for converting to a tone image,
Defines the relationship between the reference threshold data of the reference threshold matrix and the corrected threshold data obtained by correcting the reference threshold data in order to correct the recording characteristics of the recording elements, and corresponds to each of the plurality of recording elements. Threshold data correction table creating means for creating a threshold data correction table formed
A corrected threshold value matrix creating means for creating a corrected threshold value matrix composed of the corrected threshold value data calculated using the threshold value data correction table;
Image conversion means for converting the multi-tone input image into a lower-tone image than the input image using the corrected threshold matrix;
The corrected threshold value matrix creating means uses the reference threshold value matrix in which the number of pixels in the predetermined direction is smaller than the number of recording elements, and the number of pixels in the predetermined direction is larger than the number of pixels in the reference threshold value matrix. Creating the corrected reference threshold matrix;
An image processing apparatus comprising:

Images