JP2004306552A - Image recording method and image recorder - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image recording method and an image recorder which can obtain an image without density unevenness. <P>SOLUTION: Four kinds of correction patterns are prepared beforehand. The correction patterns are respectively selected for every magnitude of the dot. A correction amount for each gradation value at each nozzle is calculated. The correction amount x is stored in an ink jet printer by making it to correspond to an appearance ratio of each dot for each gradation value 0-255 of the image data. And, when the image is recorded, a nozzle number used in a pixel is discriminated on the basis of positions in the image data in each pixel (S104). The correction amount corresponding to the nozzle number and the gradation value is acquired from a correction amount table. It is added to the input data (S106, S108). A quaternary halftone (S110) is executed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image recording method and an image recording apparatus, and more particularly to an image recording method and an image recording apparatus for recording an image by a recording head capable of forming a plurality of types of dots.
[0002]
[Prior art]
As one of image recording apparatuses for recording a digital color image, a color inkjet printer provided with inks of a plurality of colors has been proposed and widely used for printing an image. Ink jet printers eject ink from a plurality of nozzles in a head while moving a head in which a plurality of nozzles are integrated and arranged in a direction (main scanning direction) orthogonal to a direction in which a printing medium such as paper is fed (sub-scanning direction). An ink dot is formed by landing ink particles on a print medium to record an image. The ink uses black (K), cyan (C), magenta (M), and yellow (Y) as basic colors.
[0003]
In recent years, with this type of ink jet printer, the density of nozzles has been increasing and the number of nozzles in the head has also increased, and in order to obtain good image quality, variations in the size of dots ejected from each nozzle are required. It is desired to suppress. However, in reality, it is difficult to eliminate the variation in the size of the dots ejected from each nozzle from the viewpoint of processing accuracy and cost, and the variation in the size of the dot causes uneven density (see FIG. 14). . FIG. 14 shows an example in which the size of the dots ejected from the nozzles at the top of the head is large and the size of the dots at the bottom of the head is small. I do.
[0004]
As a basic measure against such density unevenness due to dot size variation, in a region where a large dot comes out and the density becomes high, the gradation value of the image data is lowered, and conversely, a small dot comes out. In an area where the density is low, the density unevenness is reduced by increasing the gradation value of the image data.
[0005]
For example, as a technique for reducing density unevenness, techniques described in Patent Documents 1 to 3 and the like have been proposed.
[0006]
In the technique described in Patent Document 1, a plurality of correction tables generated by data reflecting various factors included in a manufacturing process of a print head are prepared, and full-line printing having a memory for storing correction data is performed. A recording pattern is experimentally recorded by using a head, the tendency of the density distribution of the recording pattern image is analyzed, and based on the tendency of the density distribution obtained from the analysis, an optimal correction table is obtained from a plurality of stored correction tables. A correction table is selected, correction data for suppressing density unevenness is generated based on the selected optimum correction table, and the correction data is transmitted to the memory of the recording head. That is, one optimal correction pattern is selected from a plurality of correction patterns in accordance with the characteristics of the recording head. This makes it possible to correct density unevenness.
[0007]
In the technique described in Patent Document 2, the amount of fluctuation of ink ejected from each nozzle of an ink jet printer is stored in advance as a table, and a nozzle for forming a dot at each pixel of an image is specified based on printing conditions. Is stored in advance. Then, with reference to both stored tables, the amount of change in the ink ejection amount is obtained for each pixel. For example, when the ink ejection amount is small, the gradation value of the image data is corrected to a higher value, and then the multi-value conversion is executed. are doing. Thereby, an image can be printed with an appropriate gradation value.
[0008]
Further, according to the technique described in Patent Document 3, a dot recording rate, which is a density at which dots are to be formed, is obtained for each dot of each size from the tone value of a pixel of a document image based on a predetermined correspondence relationship. When the presence or absence of a dot is determined by using the dot recording rate and the variation of the dot is corrected, a correction amount is input for each size of a dot that can be formed, and a predetermined amount is input based on the correction amount. Correct the correspondence of. That is, it has been proposed to have a correction amount for each of a plurality of dot sizes (large, medium, small droplets, etc.), and to correct the dot appearance rate and the gradation value of image data by multiplying the dot recording rate. This makes it possible to adjust the variation of the dots between the nozzles for each color, thereby enabling accurate color expression.
[0009]
[Patent Document 1]
JP-A-10-138509
[Patent Document 2]
JP 2000-25212A
[Patent Document 3]
JP 2001-158085 A
[0010]
[Problems to be solved by the invention]
However, with a binary printer as described in Patent Document 1 or Patent Document 2, it is possible to easily correct the gradation value of image data corresponding to density unevenness. In the case of a printer, for example, if it is possible to distinguish between small droplets / medium droplets / large droplets, the variation (density unevenness) for each nozzle depending on the dot size of no droplet / small droplet / medium droplet / large droplet Is different, there is a problem that it is not known how to change the gradation value of the image data.
[0011]
Further, in the technique described in Patent Literature 3, since the correction is uniformly applied to the nozzles and the variation of the dots for each nozzle is not considered, such a problem cannot be solved.
[0012]
For example, if the density unevenness of small droplets is a pattern in which the upper part of the head becomes darker, and the density unevenness of medium droplets is a pattern in which the central part of the head becomes thinner, even if correction is performed according to the pattern of small droplets, Correction cannot be made for density unevenness such that the central part of the droplet becomes thin, and uneven density remains as the number of medium drops increases.
[0013]
Therefore, it is necessary to perform the correction for each nozzle in consideration of the ratio of each dot of small droplet / medium droplet / large droplet, instead of applying a uniform correction to each nozzle.
[0014]
SUMMARY An advantage of some aspects of the invention is to provide an image recording method and an image recording apparatus capable of obtaining an image without density unevenness.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the invention according to claim 1 provides a method for generating gradations for each pixel so that recording can be performed by a recording head having a plurality of pixel recording units capable of forming dots of a plurality of sizes. An image recording method for converting image data having a value and recording an image with the recording head, wherein a plurality of types of correction patterns are provided for correcting density unevenness expected in the design and manufacturing of the recording head. Then, for each of the plurality of sizes of dots, the correction patterns with the smallest density unevenness are selected one by one, and a correction amount corresponding to each of the pixel recording units is determined based on the selected correction patterns. The image data is calculated and associated with a predetermined dot size appearance rate for each tone value of the image data, and the image data is corrected based on the correction amount associated with the appearance rate.
[0016]
According to the first aspect of the invention, a plurality of types of correction patterns for correcting density unevenness expected in the design and manufacture of the print head are prepared. For example, a plurality of types of correction patterns such as a correction pattern in which the upper side of the image is darker, a correction pattern in which the lower side of the image is darker, a correction pattern in which the upper end and lower end of the image are darker, and a correction pattern in which the center of the image is darker are prepared.
[0017]
Then, for each dot of a plurality of sizes, a correction pattern that minimizes the density unevenness is selected one by one, and a correction amount corresponding to each pixel recording unit is calculated based on the selected correction pattern. It is associated with an appearance rate of a predetermined dot size for each gradation value of image data. The correction amount associated with the appearance rate is stored in, for example, an image recording apparatus having a recording head. Then, the image data is corrected based on the correction amount associated with the appearance rate. That is, the image data reflects the correction amount of the correction pattern for correcting the variation in the dot size of each pixel recording unit.
[0018]
Therefore, instead of applying a uniform correction to each pixel recording unit, the correction can be performed in consideration of a variation in dot size, so that an image without density unevenness can be obtained.
[0019]
As the correction pattern, a solid image is recorded for each dot size, and the correction pattern is selected for each dot size based on the density unevenness of the solid image. It may be. That is, a solid image may be recorded for each dot size, and a correction pattern may be selected for each dot size from the appearance of density unevenness of the solid image. Further, as in the invention according to claim 3, a plurality of prepared solid images in which the density unevenness has been corrected are recorded, and each dot size is selected based on the density unevenness of the solid image. It may be. That is, a solid image corrected by a previously prepared correction pattern may be recorded, and a correction pattern with the least density unevenness may be selected for each dot size.
[0020]
The correction pattern is set in advance such that the difference between the maximum density and the minimum density of the density unevenness is 1.2 to 1.6 as the color difference ΔE as in the invention according to claim 4. The unevenness can be made less noticeable.
[0021]
Further, when there are a plurality of pixel recording units that form one row of the image, that is, when printing is performed in two or more passes, the pixel recording unit that forms the row as described in claim 5 is used. May be calculated as the correction amount of the row. Thus, it is not necessary to check the pixel recording unit corresponding to each pixel, and the number of processing steps can be reduced.
[0022]
The invention according to claim 6, wherein an input means for inputting image data having a gradation value for each pixel, a recording head including a plurality of pixel recording units capable of forming dots of a plurality of types of sizes, The correction amount of each pixel recording unit corresponding to a plurality of types of correction patterns prepared in advance for correcting the density unevenness for each dot size expected in the design and manufacture of the recording head is calculated as the floor of the image data. A storage unit configured to store the dot size in association with an appearance rate of a predetermined dot size for each key value, and input by the input unit based on the correction amount stored in the storage unit in association with the appearance ratio. A correction unit for correcting the image data, and a quantization unit for quantizing the image data corrected by the correction unit to a number of gradations at which the recording head can form dots. .
[0023]
According to the invention described in claim 6, image data having a gradation value for each pixel is input to the input means, and the recording head is provided by a plurality of pixel recording units capable of forming dots of a plurality of sizes. The image is recorded.
[0024]
On the other hand, in the storage means, for example, for the density unevenness appearing on the characteristics of the printhead, a plurality of types of correction patterns for correcting the density unevenness expected in the design and manufacture of the printhead are prepared, and a plurality of types of correction patterns are prepared. For each dot of a size, a correction pattern that minimizes density unevenness is selected one by one, and a correction amount corresponding to each pixel recording unit is calculated based on each correction-pattern of the selection, and a correction amount of the image data is calculated. It is stored in association with the appearance rate of a predetermined dot size for each gradation value.
[0025]
The correction means corrects the image data input by the input means based on the correction amount stored in association with the appearance rate of the dot size stored in the storage means, and the corrected image data is The recording means quantizes the recording head to the number of gradations at which dots can be formed.
[0026]
That is, with this configuration, an image can be recorded by the image recording method according to claim 1, and a uniform correction is not applied to each pixel recording unit, but each pixel recording unit has a different size. Since the correction can be performed in consideration of the variation, an image without density unevenness can be obtained.
[0027]
As the correction pattern, a solid image is recorded for each dot size, and the correction pattern is selected for each dot size based on the density unevenness of the solid image. A pattern may be prepared in advance. That is, a solid image may be recorded for each dot size, and a correction pattern selected for each dot size may be prepared from the appearance of density unevenness of the solid image. According to the present invention, a plurality of solid images prepared by correcting a plurality of types of density unevenness are recorded, and correction is selected for each dot size based on the density unevenness of the solid image. A pattern may be prepared in advance. That is, a solid image corrected by a previously prepared correction pattern may be recorded, and a correction pattern with the least density unevenness selected for each dot size may be prepared.
[0028]
The correction pattern is set in advance such that the difference between the maximum density and the minimum density of the density unevenness is 1.2 to 1.6 as the color difference ΔE, as in the ninth aspect of the present invention. The unevenness can be made less noticeable.
[0029]
Further, when there are a plurality of pixel recording units for forming one row of the image, that is, when printing is performed by two or more passes, the correction unit forms the row. The image data may be corrected by using the average of the correction amounts corresponding to the pixel recording units to be used as the correction amount of the row. Thus, it is not necessary to check the pixel recording unit corresponding to each pixel, and the number of processing steps can be reduced.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. In this embodiment, the present invention is applied to a printing system including an inkjet printer.
[0031]
FIG. 1 is a block diagram showing a schematic configuration of a printing system including an inkjet printer according to an embodiment of the present invention.
[0032]
The input device 20, the output device 21 including the inkjet printer 22, and storage devices and communication devices (not shown) are connected to the computer 10. The input interface (IF) unit 11 and the output IF unit 12 control data exchanged between the various devices described above and the computer 10. Various processes related to printing performed in the computer 10 are managed by the control / calculation unit 13.
[0033]
FIG. 2 is a block diagram illustrating a software configuration of the printing system.
[0034]
When a print command is issued from the application program 41, the printer driver 47 of the computer 10 receives the data, converts the data into a data format that can be output by the inkjet printer 22, and outputs the data.
[0035]
When the printer driver 47 receives data from the application program 41, the first resolution conversion module 42 in the printer driver 47 starts processing. The resolution conversion module plays a role of converting the resolution into a resolution that can be output by the inkjet printer 22. The color conversion module 43 performs color conversion on the data according to the color conversion table 44. The color conversion table 44 is separately created and stored so that the characteristics of the colors created by the image data and the characteristics of the colors expressed by the inkjet printer 22 match. As a specific color configuration, image data including red (R), green (G), and blue (B) is converted from cyan (C), magenta (M), yellow (Y), and black ( It plays a role of converting the data into data matched to a color that can be expressed by the combination of the respective inks of K). The quantization module 45 has a role of quantizing to a quantum number that can be expressed by the inkjet printer 22. The rasterize 46 has a role of replacing the quantized data quantized by the quantization module 45 with data to be transferred to the inkjet printer 22, and transfers the data to the inkjet printer 22 after the replacement.
[0036]
FIG. 3 is a schematic configuration diagram of the inkjet printer 22. The ink jet printer 22 includes a control circuit 31 for performing main control, a drive mechanism for moving the carriage 37 in the axial direction of the platen 39 by the carriage motor 32, and a transport mechanism for transporting the recording medium P by the paper feed motor 38. And a printing mechanism for driving the recording head 40 provided on the carriage 37 to eject ink and form dots.
[0037]
The drive mechanism includes a shaft 36 provided in parallel with the shaft of the platen 39 to support the movement of the carriage 37, a pulley 35 for stretching a drive belt 33 between the carriage motor 32, and the origin of the carriage 37. And a position detection sensor 34 for detecting the position. That is, the belt 33 moves in a direction corresponding to the rotation direction of the carriage motor 32, and accordingly, the carriage 37 and the recording head 40 are guided by the shaft 36, and the installation position direction (forward direction) of the carriage motor 32 in FIG. Or, it is transported in the installation position direction (return direction) of the pulley 35.
[0038]
The printing mechanism includes a cartridge 411K for black ink provided on the carriage 37 and cartridges 412C, 413M, and 414Y for color ink (cyan (C), magenta (M), and yellow (Y)). In addition, the recording head 40 provided on the carriage 37 is provided with a head 41K for black ink and heads 410C, 410M, and 410Y for three colors for each color ink.
[0039]
The control circuit 31 receives information on the recording direction and the ejection amount of each of the CMYK inks from the computer 10 and executes a recording process. The recording medium P is transported in a direction (sub-scanning direction) perpendicular to the transport direction (forward direction, backward direction) of the carriage 37 and the recording head 40, which is the main scanning direction.
[0040]
FIG. 4 is a perspective view of the configuration of the recording head 40 as viewed from obliquely below (the recording medium side). In FIG. 4, the recording head 40 includes a head main body 410, a black ink cartridge 411K installed above the head main body 410, and C, M, and Y color ink cartridges 412C, 413M, and 414Y. In the head body 410, nozzle rows 421K, 422C, 423M, and 424Y corresponding to the colors of the ink in the upper ink cartridge are mounted.
[0041]
Each nozzle row is composed of a plurality of nozzles arranged in the feed direction of the recording medium P, and the nozzle rows 421K, 422C, 423M, and 424Y are arranged in a direction perpendicular to the feed direction of the recording medium P (that is, the carriage 37). In the direction of movement). In FIG. 4, Y (nozzle row 424Y), M (nozzle row 423M), C (nozzle row 422C), and K (nozzle row 421K) are arranged in this order from the left. In FIG. 4, when moving from left to right is the forward direction, and when moving from right to left is the backward direction, the ink lands on the recording medium in the order of KCYM in the forward direction and in the order of YMCK in the backward direction. Will be done. Each nozzle of the nozzle rows 421K, 422C, 423M, and 424Y corresponds to a pixel recording unit of the present invention.
[0042]
In FIG. 4, the arrangement of the nozzle rows is shown for each color. However, the arrangement is not limited to this, and various arrangements such as a staggered arrangement can be applied.
[0043]
Inside the head body 410, ink is supplied from the cartridges 411K, 412C, 413M, and 414Y to the nozzle rows 421K, 422C, 423M, and 424Y. Each of the nozzles constituting the nozzle rows 421K, 422C, 423M, and 424Y has a piezoelectric element.
[0044]
As is well known, the piezoelectric element has a property of changing its shape when a voltage is applied. Therefore, by utilizing this change in shape, the ink of the upper ink cartridge is ejected from the nozzle and the recording medium P Printing (recording) is performed by forming dots on the top. At this time, the size of the dot can be controlled by controlling the shape change of the piezoelectric element.
[0045]
Note that, in the present embodiment, a method including a head that discharges ink using a piezoelectric element is applied, but a recording apparatus that discharges ink by another method may be applied. For example, the present invention can be applied to a recording apparatus that discharges ink by using bubbles generated by applying heat to the vicinity of a nozzle (the size of a dot is controlled by a difference in the amount of applied heat).
[0046]
In the ink jet printer 22 configured as described above, the nozzles vary depending on the dot size. Therefore, in the present embodiment, correction is performed in accordance with the variation in dots before quantization is performed by the quantization module 45.
[0047]
Here, correction of dot variation for each nozzle will be described in detail with an example.
[0048]
In the present embodiment, a description will be given assuming that there is an appearance rate table of no drop / small drop / medium drop / large drop corresponding to the gradation value of image data. That is, in the present embodiment, there is an appearance rate table of the size of each dot corresponding to the gradation values 0 to 255 of the image data.
[0049]
Here, as an example, attention is paid to a gradation value 100 in image data as shown in FIG. The appearance rate of each dot at this time is ₁₀₀ %, Droplet b ₁₀₀ %, Medium drop c ₁₀₀ %, Large drop d ₁₀₀ At%, the correction amount for each dot size is obtained in advance. In this case, the correction amount is 0 × a ₁₀₀ / 100 + α small × b ₁₀₀ / 100 + α medium xc ₁₀₀ / 100 + α large xd ₁₀₀ / 100, added to the tone value 100 of the image data, and then subjected to quantization processing. Note that a ₁₀₀ , B ₁₀₀ , C ₁₀₀ , D ₁₀₀ Represents the appearance rate of each dot when the gradation value is 100, and α small, α medium, and α large represent correction amounts obtained in advance for each dot of small droplet / medium droplet / large droplet.
[0050]
The average may be calculated by weighting image data of several pixels in the main scanning direction. For example, as shown in FIG. 5B, when the tone values of the image adjacent to the image data of tone value 100 are 80 and 200, the image data becomes (80 + 2 × 100 + 200) / 4, The appearance rate of each dot when the tone value is 120 indicates that there is no drop a ₁₂₀ %, Droplet b ₁₂₀ %, Medium drop c ₁₂₀ %, Large drop d ₁₂₀ %, When the correction amount for each dot size is obtained in advance in the same manner as described above, the correction amount is 0 × a ₁₂₀ / 100 + α small × b ₁₂₀ / 100 + α medium xc ₁₂₀ / 100 + α large xd ₁₂₀ / 100, added to the tone value 120 of the image data, and then subjected to quantization processing.
[0051]
However, since it is difficult to check the above-mentioned correction amounts α small, α medium, and α large for each nozzle, several types of correction patterns are prepared in advance based on the characteristics of the recording head 40 expected in design and manufacture, and inspection is performed. The load of the process is reduced.
[0052]
In the present embodiment, as shown in FIGS. 6A to 6D, four types of density unevenness of the recording head 40 (no density unevenness) are generated as shown in FIGS. Include 5 types).
[0053]
That is, when (1) the dots ejected from the nozzles above the recording head 40 appear larger and the lower ones appear smaller, (2) the dots ejected from the nozzles above the recording head 40 appear smaller. (3) When the dots ejected from the nozzles at the ends of the recording head 40 appear larger and the dots ejected from the central portion appear smaller, (3) There are four types of cases where the dots ejected from the nozzles at the ends appear smaller and the dots ejected at the center appear larger.
[0054]
Therefore, in the present embodiment, four types of correction patterns are prepared in advance. The correction pattern is a correction pattern (A) that lowers the gradation value of the upper part of the recording head 40 and increases the gradation value of the lower part, and a correction pattern (B) that raises the gradation value of the upper part of the recording head 40 and lowers the lower part. ), A correction pattern (C) in which the gradation value at the end of the recording head 40 is lowered and the gradation value at the center is increased, and the gradation value at the end of the recording head 40 is increased, A correction pattern (D) for lowering the gradation value is prepared.
[0055]
At the time of correction, one type may be selected from the correction patterns (A) to (D) or, depending on the characteristics of the recording head 40, a plurality of types may be selected and combined.
[0056]
For example, in the case of selecting a correction pattern, in the case of small droplet dots, as shown in FIG. 7A, the dots ejected from the nozzle at the end of the recording head 40 appear larger, and Is smaller, it can be seen from the density unevenness pattern that the density unevenness of the small droplets of the recording head 40 is the above-mentioned pattern (3), so the correction pattern (C) described above is selected. In the case of a medium-drop dot, as shown in FIG. 7B, when the dots ejected from the nozzles on the upper part of the recording head 40 appear larger and the dots ejected on the lower part appear smaller, this density unevenness occurs. It can be seen from the pattern of FIG. 7 that the density unevenness of the middle droplet of the recording head 40 is the above-mentioned pattern (1). Therefore, when the above-described correction pattern (A) is selected and the dot is a large droplet, FIG. As shown in C), when the dots are the same size The, from the pattern of the density unevenness, it selects the correction required because seen no density unevenness of a large drop of the recording head 40. In this way, by selecting a correction pattern, density unevenness can be corrected for each dot size of each nozzle.
[0057]
Next, a method of determining the magnitude of the correction amount of each correction pattern described above will be described.
[0058]
If the correction amount is too small, density unevenness remains even after correction, and if the correction amount is too large, density unevenness opposite to the original will occur after correction, and if the increment of the correction amount is small, prepare The number of correction patterns increases, and a large load is imposed when selecting an optimum correction pattern.
[0059]
Therefore, in the present embodiment, the color difference (density difference) is difficult to understand if the human visual ability is 0.6 to 0.8 or less as ΔE in the magnitude of the correction amount of each correction pattern. Is determined based on ΔE 0.6 to 0.8.
[0060]
For example, when the range of the density difference that is difficult to identify is the color difference ΔE <0.6, the original density unevenness will be described in the + direction (the upper part of the recording head 40 is dark) as shown in FIG. In (A), no correction is necessary because density unevenness is not originally visible.
[0061]
As shown in (B), when the original density unevenness is near the boundary where the correction is required or not, the correction amount is set as large as possible, and the corrected density unevenness is −0.6. In order to satisfy the condition of <ΔE <0.6, the magnitude of the correction amount may be set to 1.2. When the correction amount is fixed to 1.2, the limit of the density unevenness that can be corrected is +1.8 at the maximum as shown in (C), and correction is impossible when the density unevenness is +1.8 or more.
[0062]
Similarly, when the range of the density difference that is difficult to identify is set to the color difference ΔE <0.8, the magnitude of the correction amount may be fixed to 1.6 in consideration.
[0063]
Therefore, in the present embodiment, the size of the correction pattern is fixed in advance, assuming that the variation in the droplet volume of the recording head 40 is within a certain range. Specifically, as shown in FIG. In addition, when a correction pattern is added to a solid image having a uniform density, the density difference between the maximum density and the minimum density, that is, the amplitude of the correction pattern is 1.2 to 1.6 as the above-described color difference ΔE. The correction amount is set to such a magnitude. Note that the correction pattern in FIG. 9 shows an example in which a correction pattern for increasing the density of the upper part of the recording head 40 and decreasing the density of the lower part of the recording head 40 is added.
[0064]
FIG. 10 is a flowchart showing the flow of processing for calculating the correction amount for each dot size according to the above-described method of determining the correction amount of the correction pattern. This process is performed only once at first. For example, it is performed at the time of a manufacturing inspection or the like.
[0065]
First, in step S100, a correction pattern for a small droplet is selected. That is, a solid image having a uniform density is recorded with small droplet dots, and an appropriate correction pattern is selected from the above-described correction patterns (A) to (D) from the density pattern of the solid image.
[0066]
Subsequently, in step S12, a correction amount for each nozzle is calculated. That is, the correction amount α having such a magnitude that the color difference ΔE becomes 1.2 to 1.6 is calculated.
[0067]
Similarly, in step S14, a correction pattern for a medium drop is selected, in step S16, the correction amount α for each nozzle of the medium drop is calculated, in step S18, a correction pattern for a large drop is selected, and in step S20, , A large correction amount α for each nozzle of the large droplet is calculated.
[0068]
The correction amount may be selected by recording a solid image and selecting a correction pattern that minimizes density unevenness, or a solid image to which the above-described correction patterns (A) to (D) have been added. May be recorded and a correction pattern with the least density unevenness may be selected.
[0069]
Then, in step S22, the correction amount x for each gradation value of 0 to 255 for each nozzle is set to 0 × a using a table describing the appearance ratio of each dot for each gradation value of the image data. ₀ ~ ₂₅₅ / 100 + α small × b ₀ ~ ₂₅₅ / 100 + α medium xc ₀ ~ ₂₅₅ / 100 + α large xd ₀ ~ ₂₅₅ / 100. Note that a ₀ ~ ₂₅₅ , B ₀ ~ ₂₅₅ , C ₀ ~ ₂₅₅ , D ₀ ~ ₂₅₅ Represents the appearance rate of each dot at each gradation value of 0 to 255.
[0070]
Next, in step S24, the calculated correction amount x for each gradation value of 0 to 255 for each nozzle is stored in a memory or the like of the inkjet printer 22 as a correction amount table. That is, the correction amount x is stored in the inkjet printer 22 as an LUT (lookup table) in association with a predetermined appearance rate of each dot for each of the gradation values 0 to 255 of the image data. Step S24 corresponds to the storage unit of the present invention.
[0071]
In the present embodiment, as described above, the correction amount for each nozzle is calculated in the order of small droplet, medium droplet, and large droplet. However, the order in which the correction amount for each nozzle is calculated is not limited to this. is not.
[0072]
Subsequently, a process of creating multi-valued quantized data for printing by the inkjet printer 22 in which the correction amount x (correction amount table) for each gradation value of 0 to 255 in each nozzle is stored in advance will be described. I do. FIG. 11 is a flowchart illustrating an example of a quantization data creation processing routine, which is a process performed by the control / calculation unit 13 in the computer 10 illustrated in FIG. 1 executing the software illustrated in FIG. .
[0073]
First, when the quantization data creation process is started, the original data is input to the control / calculation unit 13 in step S50. The original data has 256 tones from 0 to 255 for each color of each pixel R, G, B. The resolution of this original data is arbitrary. Step S50 corresponds to an input unit of the present invention.
[0074]
In step S52, the control / calculation unit 13 converts the resolution of the input original data into a resolution for printing by the inkjet printer 22. In this resolution conversion processing, printing may be performed without performing this processing depending on the user's request or the resolution of the input data.
[0075]
Next, in step S54, the control / calculation unit 13 performs a color conversion process. The color conversion process is performed using a color conversion table in which a combination of data of three colors of R, G, and B according to the characteristics of C, M, Y, and K is stored in advance.
[0076]
Generally, the number of gradations that can be expressed by a printer is smaller than the number of gradations of the original data. For example, the number of gradations of original data represented by a bitmap (Bmp) file is 256 gradations for each color of RGB, and the number of gradations that can be expressed by a printer is at most 10 gradations. Often less.
[0077]
Therefore, in step S56, the control / calculation unit 13 performs a quantization process on the data subjected to the color conversion process to reduce the number of gradations to the number of gradations that can be expressed by the printer. Here, it is assumed that quantization is performed from the 256 gradation levels of the original data to four gradation levels that can be expressed by the inkjet printer 22. In the present embodiment, “dot non-formation”, “small droplet formation”, “medium droplet formation”, and “large droplet formation” are described. The number of gradations is not limited to four, but may be three, five, or more.
[0078]
Then, the control / calculation unit 13 performs the quantization process for each color, determines whether or not the quantization process has been completed for all the colors, and when the quantization process has been completed for all the colors, performs the quantization data creation process. End the routine.
[0079]
Next, the above-described quantization processing will be described in detail.
[0080]
FIG. 12 is a flowchart illustrating a subroutine of a quantization process performed by the control / calculation unit 13.
[0081]
First, in step S100, a correction amount table previously stored in the inkjet printer 22 is acquired as described above, and in step S102, the process waits until the acquisition of the correction amount table ends.
[0082]
In step S104, for each pixel, the nozzle number used for that pixel is determined based on the position in the image data. For example, in one pass, all pixels in one row are hit by the same nozzle, so that the nozzle number can be determined only by the position information in the sub-scanning direction. In the case of two or more passes, the nozzle to be used differs depending on the position in the main scanning direction in one row, so that the nozzle number at that pixel can be determined from the position information in the main scanning direction and the sub-scanning direction.
[0083]
Subsequently, in step S106, the data of the gradation value of the pixel is read, and the process proceeds to step S108, where the correction amount corresponding to the nozzle number and the gradation value is obtained from the correction amount table and added to the input data. Thereby, it is possible to correct the variation of the nozzle for each dot size. Steps S100 to S108 correspond to the correcting means of the present invention.
[0084]
Then, in step S110, a quaternary halftone process is performed on the corrected image data. For example, since a quaternary halftone (quantization) process can be performed by using a general dither method or an error diffusion method, detailed description will be omitted. Step S110 corresponds to the quantization means of the present invention.
[0085]
In step S112, it is determined whether or not the above processing has been completed for all pixels, and the processing from step S100 is repeated until the determination is affirmed.
[0086]
That is, in the present embodiment, as described above, a correction pattern with the least density unevenness is selected for each dot size from several types of correction patterns predetermined so as to have a density difference that is difficult to understand due to human visual ability. Then, the image data is corrected based on this. Since the quantization is performed after the correction amount of the correction pattern for correcting the variation in dot size of each nozzle is reflected, the correction can be performed in consideration of the variation in dot size of each nozzle, and the density unevenness can be corrected. Image can be obtained.
[0087]
Further, in the present embodiment, there are four kinds of predetermined correction patterns (excluding the density non-uniformity), so that the number of steps in the manufacturing inspection process can be reduced.
[0088]
In the above-described embodiment, in the case of two or more passes as shown in FIG. 13, since it is known which nozzle is used to print each pixel, it is possible to correct each pixel as described above. When creating the correction amount table (step S22), instead of calculating the correction amount for each gradation value of 0 to 255 for each pixel, the correction amount for each gradation value of 0 to 255 is calculated for each row. Thus, density unevenness can be corrected. FIG. 13 shows two passes as an example.
[0089]
That is, assuming that the nozzle number constituting a certain row is # i # j, the correction amount at a certain gradation value k is ai, aj in the nozzle number # i # j, it is composed of #i and #j. The correction amount when the gradation value of the image data is k in the row can be (ai + aj) / 2. This does not depend on whether the pixel is printed with the nozzle #i or the nozzle #j.
[0090]
As described above, in the case of two or more passes, the correction amount is obtained in units of rows, not in units of nozzles, so that it is not necessary to check the nozzle number of each pixel, and the number of processing steps required for correcting uneven density can be reduced. it can.
[0091]
【The invention's effect】
As described above, according to the present invention, a plurality of types of correction patterns for correcting density unevenness expected in the design and manufacture of a recording head are prepared, and the highest density pattern is provided for each of the plurality of types of dots. The correction patterns that reduce unevenness are selected one by one, and a correction amount corresponding to each pixel recording unit is calculated based on each selected correction pattern, and a predetermined dot is set for each gradation value of image data. By correcting the image data based on the correction amount associated with the appearance rate in association with the appearance rate of the size of each pixel, instead of applying a uniform correction to each pixel recording unit, the correction is performed for each dot size. Since the correction can be performed in consideration of the variation of the image, there is an effect that an image without density unevenness can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a schematic configuration of a printing system including an inkjet printer according to an embodiment of the present invention.
FIG. 2 is a block diagram showing a software configuration of a printing system according to the embodiment of the present invention.
FIG. 3 is a diagram illustrating a schematic configuration of an ink jet printer.
FIG. 4 is a perspective view of the configuration of the recording head as viewed obliquely from below (the recording medium side).
FIG. 5 is a diagram illustrating an example of dot variation correction for each nozzle.
FIG. 6 is a diagram illustrating an example of types of density unevenness.
FIG. 7 is a diagram illustrating an example of variation for each dot size.
FIG. 8 is a diagram for explaining the magnitude of a correction amount.
FIG. 9 is a diagram illustrating an example in which a correction pattern is added to a solid image having a uniform density.
FIG. 10 is a flowchart illustrating a flow of processing for calculating a correction amount for each dot size.
FIG. 11 is a flowchart illustrating an example of a quantization data creation processing routine.
FIG. 12 is a flowchart illustrating a subroutine of a quantization process.
FIG. 13 is a diagram for explaining a method for calculating a density unevenness correction amount for two or more passes.
FIG. 14 is a diagram illustrating an example of density unevenness caused by variation in dot size.
[Explanation of symbols]
22 Inkjet Printer
40 recording head
45 Quantization module
47 Printer Driver
421K, 422C, 423M, 424Y nozzle row

Claims (10)

Image data having a gradation value for each pixel is converted so that recording can be performed by a recording head having a plurality of pixel recording units capable of forming dots of a plurality of sizes, and an image is formed by the recording head. An image recording method for recording,
Prepare a plurality of types of correction patterns for correcting the density unevenness expected in the design and manufacturing of the recording head,
For each of the plurality of types of dot sizes, the correction pattern that minimizes the density unevenness is selected one by one, and a correction amount corresponding to each of the pixel recording units is calculated based on the selected correction pattern. Associated with the appearance rate of a predetermined dot size for each gradation value of the image data,
An image recording method, wherein the image data is corrected based on the correction amount associated with the appearance rate. 2. The image recording method according to claim 1, wherein the correction pattern records a solid image for each dot size, and selects each of the dot sizes based on density unevenness of the solid image. . 2. The correction pattern according to claim 1, wherein a plurality of prepared solid images in which density unevenness has been corrected are recorded, and each of the dots is selected for each dot size based on the density unevenness of the solid image. 3. Image recording method. 4. The correction pattern according to claim 1, wherein the correction pattern is set in advance so that a difference between a maximum density and a minimum density of the density unevenness is 1.2 to 1.6 as a color difference ΔE. The image recording method according to the above section. The method according to claim 1, wherein when there are a plurality of pixel recording units forming one row of the image, an average of correction amounts corresponding to the pixel recording units forming the row is calculated as a correction amount of the row. The image recording method according to claim 1. An input unit for inputting image data having a gradation value for each pixel, and a recording head including a plurality of pixel recording units capable of forming dots of a plurality of sizes,
The correction amount of each pixel recording unit corresponding to a plurality of types of correction patterns prepared in advance for correcting the density unevenness for each dot size expected in the design and manufacture of the recording head is calculated based on the image data. Storage means for storing in association with an appearance rate of a predetermined dot size for each gradation value,
Correction means for correcting the image data input by the input means based on the correction amount stored in the storage means in association with the appearance rate;
Quantizing means for quantizing the image data corrected by the correction means to a number of gradations at which the recording head can form dots,
An image recording device comprising: The said correction pattern is a solid image recorded for each dot size, and a correction pattern selected for each dot size is prepared in advance based on the density unevenness of the solid image. 7. The image recording device according to 6. The correction pattern is characterized in that a solid image in which a plurality of types of density unevenness prepared in advance are corrected is recorded, and a correction pattern selected for each dot size based on the density unevenness of the solid image is prepared in advance. The image recording apparatus according to claim 6, wherein: 9. The correction pattern according to claim 6, wherein the correction pattern is set in advance so that the difference between the maximum density and the minimum density of the density unevenness is 1.2 to 1.6 as the color difference ΔE. An image recording apparatus according to the item. When there are a plurality of the pixel recording units forming one row of the image, the correction unit uses the average of the correction amounts corresponding to the pixel recording units forming the row as an amount of correction of the row to obtain the image data. The image recording apparatus according to claim 6, wherein the correction is performed.

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