Patent Grant
7327503
1. Field of the Invention
The present invention relates to an image correction method for correcting a nonejection state, which is an inherent characteristic of each recording head of an inkjet recording system that ejects ink dots onto a recording medium to form an image thereon.
2. Description of the Related Art
Along with the popularization of copying machines, information processing equipment such as word processors and computers, and communication equipment, digital image-recording apparatus using inkjet recording heads have come into widespread use as image-forming (recording) apparatuses for the aforesaid equipment. Also, recent enhancements in image quality and colorization of visual information in the information processing equipment and communication equipment has necessitated concomitant enhancements in image quality and colorization in recording apparatuses.
In such a recording apparatus, for miniaturizing and speeding up the forming of a pixel, a plural-recording-elements integrated recording head (also referred to as a multi-head) is used, in which plural ink nozzles and ink paths are integrated in high density. Furthermore, for colorization, the apparatus generally has plural multi-heads corresponding to respective colors of cyan, magenta, yellow, and black. Using this structure, technology has strived to output high grade images at high speed and at low cost. In one method to increase speed, a one-pass high-speed method, in which the length of the multi-head is about the width of a recording medium, is coming into use.
For example, in transverse-feed page printers for A-4 size paper, the length of the multi-head is about 30 cm, and 7000 nozzles or more are required to achieve 600 dpi images. It is extremely difficult to manufacture such multi-heads having such a large number of nozzles without some defects. In addition, the nozzles will not necessarily have the same performance characteristics. Furthermore, some nozzles become incapable of ejection after being used. Therefore, it is worth noting head shading techniques for correcting density nonuniformity due to ejection-amount nonuniformity and deviations in landing position (kink), as well as nonejecting-nozzle correction (nonejection complementary) techniques for performing complementary processing on a nonejecting nozzle to enable even a multi-head with defects to be used.
Generally in head shading techniques, the density is measured for every nozzle and the input-image data is then corrected for the measured result. For example, if the ejection amount of one nozzle is reduced for some reason so as to reduce the density corresponding to that nozzle, this technique corrects the input image data so that a gradation value corresponding to the affected nozzle is increased so as to yield uniform density throughout the printed images.
The nonejection complementary technique, described in another U.S. patent application, (U.S. Ser. No. 845,498) assigned to the same assignee as this application, sets forth other methods for collecting nozzle output variations. If one nozzle for cyan is nonejecting, for example, methods for compensating for this ink shortage include (i) substituting with the ejection of nozzles on both sides of the nonejecting nozzle (adjacent complementation), (ii) complementing the nonejecting nozzle with an ink dot of another color, such as black, (different-color complementing), and (iii) distributing the data corresponding to the nonejecting nozzle to nozzles at both ends of the head.
The above-mentioned patent application is especially effective in a recording apparatus using a full-line head, which corresponds to those heads that span the entire width of the recording sheet.
With respect to the different-color complementing described above, a method has been proposed for determining the amount of the different-color ink to be complementarily ejected, which uses pixel-image density data (a gradation value) determined as a function of the number of successive nonejecting nozzles.
However, the different color complemented result often may vary from that anticipated, depending on the ejection condition of the adjacent nozzles. For example, when the amount of the ink ejected from the adjacent nozzles on both sides is large so as to increase the size of an ink dot, if the amount of different-color complementing ink is not reduced from the determined standard amount (hereinafter the amount of the complementing is referred to as a “reference different-color complementing amount”), the resultant complementing may become conspicuous due to the effect of the large number of ink dots adjacent to the nonejecting nozzle. That is, it is necessary to determine the amount of the different-color complementing by measuring the degree of the effect on the vicinity. This situation is shown in
Solid lines in
When the ejection by the nozzles on both sides of the nonejecting nozzle is the same in dot diameter and dot density as that in the other nozzles, and only the landing position of the ejection is shifted in the nozzle-line direction (Y kink), the appearance is slightly different from the above-mentioned case in which the dot diameter is changed. Solid lines in
From these factors, the ejecting conditions of nozzles in the vicinity of the nonejecting nozzle, specifically dot density, dot diameter, and kink, can be comprehended, and then, if there are no fluctuations in the dot density and dot diameter, the complementing may be performed with the reference different-color complementing amount. However, if there are fluctuations in the dot density and dot diameter, the complementing must be performed with an amount increased or decreased from the reference different-color complementing amount by referring to the density of the nonejecting nozzle portion.
However, typical reading devices (scanner) scarcely read dot density and existence of an ink dot of approximately 60 μm; and as for the kink, although a kind of smaller kinks approximately several dozen μm can be recognized, especially those of several μm, cannot be recognized by the scanner.
It is not cost-effective to perform the correction with a high-efficiency scanner capable of reading the density, size, and position of an ink dot of several μm.
The present invention can provide an image correction method for correcting a nonejecting nozzle without using a high-efficiency scanner.
In the present invention, a pattern for reading an ejecting state of a head is recorded and analyzed so as to determine the presence of a nonejecting nozzle while density distribution data corresponding to each nozzle is obtained so as to determine a complementary table for each nozzle so as to perform different-color complementing with reference to the density distribution in the nonejecting nozzle.
Moreover, a suitable arithmetic calculation is performed on the density distribution data corresponding to each nozzle so as to determine a complementary table for each nozzle to perform the different-color complementing.
Specifically, an arithmetic calculation is performed on the density distribution data corresponding to each nozzle, and if the resultant value of the calculation on a nonejecting nozzle is larger than the reference set value, a complementary table is set so that the different-color complementary amount is larger than the value shown in the reference different-color complementary table. However, if the resultant value is smaller than the reference set value, a complementary table is set so that the different-color complementary amount is smaller than the value shown in the reference different-color complementary table.
According to one aspect of the present invention, an image correction method for an inkjet recording apparatus for recording images by ejecting ink on a recording medium using a recording head having a plurality of nozzles for ejecting ink arranged on the recording head includes the steps of outputting a pattern for measuring recording characteristics of the recording head, determining a nonejecting nozzle from the plurality of nozzles and obtaining a density distribution corresponding to each nozzle based on the measured density of the output pattern, determining a complementary table for each nozzle for complementing with a color different from the color corresponding to the nonejecting nozzle by comparing the obtained density distribution corresponding to the nonejecting nozzle with a reference preset value and converting image data corresponding to the nonejecting nozzle into different-color image data for ejection by another nozzle using determined complementary table. The reference preset value is a value of the density distribution corresponding to the nonejecting nozzle in a state that sizes and density of ink drops ejected from nozzles in the vicinity of the nonejecting nozzle are constant and there is no deviation in a landing position. One of a table and a function showing a complementary amount with the different color in the state for each gradation value of input images is prepared for each number of consecutive nonejecting nozzles as a reference different-color complementary table. From a magnitude relation between density distribution in a portion of a target nonejecting nozzle and the reference preset value for each number of consecutive nonejecting nozzles, a different-color complementary table for each nozzle is determined by referring to the reference different-color complementary table for each number of consecutive nonejecting nozzles.
According to another aspect of the present invention, an image correction method for an inkjet recording apparatus for recording images by ejecting ink on a recording medium using a recording head having a plurality of nozzles for ejecting ink arranged on the recording head includes the steps of outputting a pattern for measuring recording characteristics of the recording head, determining a nonejecting nozzle from the plurality of nozzles and obtaining a density distribution corresponding to each nozzle based on the measured density of the output pattern, performing a predetermined arithmetic calculation on the obtained density distribution, determining a complementary table for each nozzle for complementing with a color different from the color corresponding to the nonejecting nozzle by comparing the calculated density distribution corresponding to the nonejecting nozzle with a reference preset value and converting image data corresponding to the nonejecting nozzle into different-color image data for ejection by another nozzle using the determined complementary table. The reference preset value is a value of the density distribution corresponding to the nonejecting nozzle in a state that sizes and density of ink drops ejected from nozzles in the vicinity of the nonejecting nozzle are constant and there is no deviation in a landing position. One of a table and a function showing a complementary amount with the different color in the state for each gradation value of input images is prepared for each number of consecutive nonejecting nozzles as a reference different-color complementary table. From a magnitude relation between density distribution corresponding to a target nonejecting nozzle and the reference preset value for each number of consecutive nonejecting nozzles, a different-color complementary table for each nozzle is determined by referring to the reference different-color complementary table for each number of consecutive nonejecting nozzles.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.
Embodiments according to the present invention will be described below.
According to the present invention, a pattern for reading an ejecting state of a head is recorded and measured so as to determine the presence of a nonejecting nozzle, while density distribution, corresponding to each nozzle, is obtained so as to determine a complementary table for each nozzle so as to perform different-color complementing for the nonejecting nozzle. Such different-color complementing may preferably include inks of different color as well as inks of similar color, but different density.
Moreover, a suitable arithmetic calculation is performed on the density distribution corresponding to each nozzle so as to determine a complementary table for each nozzle to perform the different-color complementing.
Specifically, if the density distribution corresponding to each nozzle or the result of a suitable arithmetic calculation performed on the density distribution is larger than the reference set value, a complementary table is set so that the different-color complementary amount is larger than the value shown in the reference different-color complementary table. However, if the result is smaller than the reference set value, a complementary table is set so that the different-color complementary amount is smaller than the value shown in the reference different-color complementary table.
According to this specific technique, reference set values for each of 1, 2, and 3 successive nonejecting nozzles are compared with density distribution of a target nozzle, or a calculated value thereof, so as to obtain a relative number of successive nonejecting nozzles from the results, so that a complementary table for the relative number of successive nonejecting nozzles is prepared by referring to the reference different-color complementary tables for 1, 2, or 3 successive nonejecting nozzles, with suitable interpolation. The interpolation is not specifically limited, so that generally used methods such as linear interpolation or spline-curve interpolation may be used.
The above-mentioned arithmetic calculation is to calculate the density distribution corresponding to each nozzle in units of several pixels or in consideration of visual characteristics, specifically, there are averaging processing and weighted averaging processing in units of 2 to 7 pixels on 50 μm to 300 μm and 600 dpi basis. More preferable calculations include convolution integration using a VTF (visual transfer function) representing visual characteristics and convolution integration using a PSF (point spread function). These latter methods are more preferred because the visual characteristics are reflected therein. In addition, mathematically, the above-mentioned convolution integration is interchangeable with the inverse Fourier transformed value of the product of the Fourier transformed density distribution and the Fourier transformed VTF or PSF, so that any one of the methods may be used. The VTF and PSF are given by the following equations.
VTF:
Wherein vl: distance of distinct vision (mm) u: number of waves (1/mm)
PSF:
ae−2(x/δ)2
Wherein x: distance of distinct vision (mm) σ: dispersion (mm) a: normalization constant
The distance of distinct vision (vl) in the VTF represents the distance between a recording medium and the observer's eyes, which is typically set to be 200 to 400 mm. Also, when f=5.45 or less, density comparison in separated portions is not performed, and the VTF is set to be 1.
On the other hand, the dispersion σ in the PSF indicates the degree of broadening in the Gaussian function. Although it is not interchangeable with the vl, in view of the degree of spatial effect, a vl of 200 to 400 mm substantially corresponds to a σ of 0.085 to 0.19 mm (2 to 4.5 pixels on 600 dpi basis), so that when the PSF is used, values within the above-mentioned range may be preferable. In addition, frequency response characteristics of the VTF and PSF are shown in
Next, an overview of the present invention will be described with reference to the drawings.
As described above, the solid lines of
The broken lines of
In determining the complementary amount, the above-mentioned reference set value indicates the density distribution in the nonejecting nozzle, or the result of the operation thereof, when the density and size of the dot recorded by the nozzles in the vicinity of the nonejecting nozzle are constant and, moreover, when there is no deviation in the landing position (kink). This situation corresponds to results (A1) through (A3) in
Along with different-color complementing, same-color complementing may be performed using an adjacent nozzle, so that more efficient complementing can be performed. In this case, the reference different-color complementary table needs to be reset as a different-color complementary table after the adjacent complementing is performed with the same color.
Furthermore, the information for each nozzle obtained by the arithmetic calculation may be used as a correction parameter for correcting density nonuniformity (shading correction); if higher spatial-frequency response is desired, a parameter for shading correction may also be calculated by performing a separate arithmetic calculation.
The pattern used for checking ejection conditions of the head is a pattern such as a nonejection-detection pattern, in which lines recorded by one nozzle are step-wise arranged, and a staggered pattern with a recording duty factor of 50%; however, it is not limited to these patterns, and may be any pattern as long as nonejection of a nozzle and density distribution for each nozzle can be checked. Also, patterns with several kinds of recording duty factors may be used so as to obtain density distribution for each nozzle. Using the patterns with plural recording duty factors enables the head shading to be performed in more detail.
The reading the pattern for checking ejection conditions is performed using a commonplace scanner. To obtain optimum results, the optical resolution of such scanners is preferably at least the same as that of the recording head. If the resolution of the reading optical system is excessively low, precise feedback cannot be achieved because the read data is not as precise. Also, the reading system may be mounted on the printer online or offline, so that it is not specifically limited.
The data read with the scanner is correlated with each nozzle and the nonejection and density distribution are detected therefrom so as to perform arithmetic calculations, such as averaging and convolution integration on the density distribution. At this time, for the nozzle determined to be nonejecting, a different-color complementary amount is determined by comparing the result calculated for the position corresponding to the nozzle with the pre-set value. The result of this operation may also be used for shading correction. In general, shading data is represented as a rate of deviation from the average density during the recording of an even pattern, so that the above-mentioned result of the operation is also used when the shading data is calculated. On the basis of the shading data for each nozzle obtained in such a manner, shading correction may be performed using a γ conversion table and gray-scale conversion function.
After performing the nonejection correction and shading correction in such a manner, either binarization or multi-level coding is performed thereon so as to actually record images by converting the data into bit map data. The above-mentioned binarization or multi-level coding is not specifically limited; however, in order to eliminate unevenness between nozzles, an error diffusion method having comparatively high frequency response may be preferable.
Embodiments according to the present invention will be described below with reference to the drawings.
According to a first embodiment, gray-scale images are output using a side-shooter type thermal inkjet recording head. The resolution (nozzle density) of the recording head is 600 dpi, and the head has a length of about 303 mm with 7168 nozzles arranged thereon. The amount of ink to be ejected (ejection amount) from each nozzle is designed to be about 8 pl.
A printer having the four longitudinal multi-heads for cyan C, magenta M, yellow Y, and black K is experimentally manufactured so as to output images. The resolution of the output image is 600×600 dpi, and a one-pass recording system is adopted, in which a recording medium passes relative to the head fixed within the printer.
Various additives for the ink C, M, Y, and K are controlled so as to substantially equalize their physical properties, namely, viscosity: 1.8 cps, and surface tension: 39 dyn/cm. The driving conditions of the head are frequency: 8 kHz, voltage: 10 V, and applied pulse width: 0.8 μs. By driving under these conditions, an approximately 8 pl ink droplet is ejected at a speed of about 15 m/s.
When printing images, first, a nonejecting-nozzle detection pattern 100 and a shading pattern 101 shown in
These patterns are read with a scanner with an optical resolution of 1200 dpi so as to detect nonejecting nozzles and measure density distribution. Specific methods for detecting nonejecting nozzles and measuring density distribution are shown as follows. Each marker 102 is provided for specifying a particular nozzle number, and the plural markers are arranged at intervals of 512 nozzles, i.e., 14 markers in total. The image data read with the scanner is separated into each color and converted into a gray scale for each color, which reflects color density. From the gray scale data, the position of the marker is read. In order to correlate this data into the data correlated with the nozzle position, rotation and enlarging or contracting are appropriately performed so as to correspond to the pixels equivalent to 600 dpi.
The detection of the nonejecting nozzle is performed using the nonejecting-nozzle detection pattern 100 after performing the suitable rotation and enlarging or contracting as described above. From each row of the pattern, a section equivalent to 7168×50 pixels is isolated, and furthermore, three pixels in the vicinity of a target position to be positioned by nature are to be a decision part. If the density of this decision part is substantially the same as that of a nonrecorded portion, the corresponding nozzle is determined to be nonejecting.
As for the density distribution for each nozzle, the central section of the shading pattern 101 with a recording duty factor of 50%, which is equivalent to 7168×400 pixels, is isolated, and 400 pixels for each nozzle are averaged to have the density distribution.
According to the embodiment, the convolution integration is performed on the density distribution using the PSF with a dispersion of 127 μm, which is equivalent to 600 dpi, 3 pixels. Part of the result (equivalent to 200 pixels) is shown in
Various kinds of correction processing are performed in the image-correction section 23 by referring to data stored in the data storage 22. Such correction processing will be described with reference to the flow in
Also, the calculated value of the nozzle portion, shown in (B) of
After correcting the entire image data in such a manner, in the image-processing section 3, the binarization is performed so as to prepare the bit map data. According to the embodiment, the binarization is performed using a general error diffusion method. The bit map data are further fed to the head driver 4 so as to output corrected images.
The images obtained in such a manner are excellent with inconspicuous streaks of nonejecting portions.
In a second embodiment, images are corrected and output according to a similar method as the first embodiment; however, the convolution integration uses the VTF at the distance of distinct vision vl=250 mm, and shading corrections are additionally prepared. The embodiment will be described centering on these points.
According to the second embodiment, the same pattern as that of the first embodiment is recorded so as to determine a nonejecting nozzle and to obtain density distribution for each nozzle. The result at this point is the same as in the first embodiment. An arithmetic calculation is then performed on the density distribution using the above-mentioned VTF formula. At this time, with the inverse Fourier transformed VTF and the density distribution, the arithmetic calculation of convolution integration is performed. The data for shading correction is then prepared as a rate of the weighted-average value of the density distribution for three pixels of each nozzle in the average value for all the nozzles other than the nonejecting nozzles. Part of the result is shown in
The reference set values for the 1 to 3 successive nonejecting nozzles are 90, 61, and 32, respectively. According to this embodiment, the relationship between the number of successive nonejecting nozzles and the reference set value is approximated by a cubic curve (
After correcting the entire image data in such a manner, the binarization is performed in the same way as in the first embodiment so as to prepare the bit map data, thereby outputting corrected images.
The images obtained in such a manner are excellent with inconspicuous streaks from nonejecting portions.
As described above, according to the present invention, a pattern for reading an ejecting state of a head is measured and recorded so as to determine the presence of a nonejecting nozzle by the result while density distribution corresponding to each nozzle is obtained. Based on the density distribution, or the result of a suitable arithmetic calculation performed on the density distribution, a complementary amount to perform the different-color complementing is determined, so that image defects, which cannot be corrected by a conventional method, are reduced. Also, as a result, there is an advantage that a number of manufactured heads that are actually usable is increased.
While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
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