WO2018207535A1

WO2018207535A1 - Resolving power measurement method and device, program, and printing device

Info

Publication number: WO2018207535A1
Application number: PCT/JP2018/015043
Authority: WO
Inventors: 完司永島
Original assignee: 富士フイルム株式会社
Priority date: 2017-05-12
Filing date: 2018-04-10
Publication date: 2018-11-15
Also published as: JPWO2018207535A1; JP6761543B2

Abstract

Provided are a resolving power measurement method and device, program, and printing device that make it possible to calculate a contrast transfer function using a relatively simple algorithm even if a chart has a missing line. In the present invention, a chart including dark and light stripes having concentration profiles that change in rectangular shapes at a prescribed interval is read by an optical reader. For each pixel in a range for calculation in image data for the read image, the interpixel contrasts between the pixel and each pixel removed from the pixel by 1 to d pixels in the stripe interval direction are calculated. The contrast transfer function for each stripe position in the dark and light stripes is calculated from the maximum values from among the contrast values calculated for each pixel.

Description

Resolution measuring method and apparatus, program, and printing apparatus

The present invention relates to a resolution measuring method and apparatus, a program, and a printing apparatus, and more particularly, to a data processing technique suitable for measuring a contrast transfer function (CTF) of an optical reading apparatus.

As one of the methods for measuring the resolving power of the optical reader, data of the read image obtained by reading a CTF measurement chart including a rectangular wave pattern in which the density end face changes into a rectangular shape such as black and white stripes by the optical reader. A method for analyzing CTF characteristics is known (Patent Document 1 and Patent Document 2).

Generally, when the CTF value is obtained by processing the read image data of the CTF measurement chart, the maximum value and the minimum value (that is, black and white peak and bottom) of the read value are obtained from the read image data of the CTF measurement chart. The CTF value is calculated using the Michelson contrast definition formula shown below.

CTF = (Max−Min) / (Max + Min) Equation [1]
In Expression [1], Max is the maximum value of the reading value, and Min is the minimum value of the reading value.

JP 2007-259360 A JP 2007-158846 A

When reading a chart using an optical reader having a reading resolution sufficiently higher than the fineness of the chart, using a chart free of defects in black and white stripes, such as a dedicated CTF measurement chart prepared for measuring CTF However, the CTF calculation process is not difficult to the extent that it is necessary to search for the maximum value and the minimum value of the read value from the read image data.

However, for example, when inspecting the performance state of an inline scanner mounted on an inkjet printing apparatus in the market, in order to measure CTF from a chart printed using the printing function of the inkjet printing apparatus, the following problem 1 As shown in (3) to (3), there are problems different from those of normal CTF measurement.

Problem 1: There are undischargeable nozzles that do not discharge among a plurality of nozzles in an ink jet head, and line missing of a chart may occur. In addition to a case where a part of the line is missing, a part of the line may be missing, such as a thin line or a shortened line.

Problem 2: When the line width or drawing resolution of a chart drawn using an inkjet printing apparatus is close to the resolution of the reading pixel (reading resolution) in the in-line scanner, the read data depends on the phase of the reading position of the chart. Change periodically. Specifically, when the line width is close to the size of the reading pixel, the line is read as a thin line when the line straddles two reading pixels. If the CTF value is directly calculated using such a reading value, the CTF value becomes small and an accurate CTF value cannot be obtained. A phenomenon in which the read data periodically changes depending on the phase of the positional relationship between the stripe position and the read pixel in the chart is called a moire phenomenon.

Problem 3: In the ink jet printing apparatus, a nozzle inspection chart for inspecting the ejection state of each nozzle of the ink jet head is drawn during execution of a job, before the start of a job, or between jobs. From the drawing result of the nozzle test chart, it is possible to detect ejection failure such as undischarge, ejection bending, or drop volume abnormality. The nozzle inspection chart is, for example, a so-called 1-on-N-off type ladder chart, and includes a line pattern drawn individually by each nozzle. For example, in a nozzle inspection chart drawn by a single-pass inkjet printing apparatus, a line drawn by a nozzle adjacent to the main scanning direction that is the nozzle arrangement direction of the line-type inkjet head is shifted by one pixel in the main scanning direction. It is drawn adjacent. When such a nozzle inspection chart is to be used as a CTF measurement chart, a line drawn by adjacent nozzles is close to the line (a joint part of steps of a line pattern drawn in a staircase pattern) ), Since the line width is in a wide state, the read value by the in-line scanner becomes a value affected by the line width. Therefore, in order to obtain an accurate CTF value, it is desirable to exclude the influence of the read data on the portion where the line is close (joint portion). If the calculation process is advanced while excluding some specific data from the read image, the algorithm becomes complicated.

The above-mentioned problems 1 to 3 are remarkable problems for the inline scanner, but are common to various optical reading apparatuses including not only the inline scanner but also an offline scanner. Further, Problem 1 is a problem regarding the ink jet printing apparatus. However, even in an image forming method other than the ink jet method, the chart line may be lost due to some factor. Therefore, there is a problem similar to the problem 1 for various types of printing apparatuses.

The present invention has been made in view of such circumstances, and a resolution measuring method capable of solving at least one of the plurality of problems described above by a relatively simple algorithm and calculating an accurate CTF, and An object is to provide an apparatus, a program, and a printing apparatus.

In order to solve the problem, the following aspects of the invention are provided.

The resolution measurement method according to aspect 1 is a read image that obtains image data of a read image obtained by reading a chart including light and shade fringes in which a density cross section changes into a rectangular shape at a specific interval at least in part. Between the data acquisition step and the pixels that belong to at least a part of the calculation target range in the image data of the read image, and pixels that are separated by a number of pixels that is greater than or equal to 1 and less than or equal to the specific number of pixels in the grayscale stripe interval A pixel-to-pixel contrast calculation step for calculating the contrast between pixels and a resolving power calculation for calculating a contrast transfer function at each stripe position of the gray stripes from the maximum contrast value calculated for each pixel by the pixel-to-pixel contrast calculation step. The specific pixel number is a pixel number corresponding to the specific interval of the gray stripes in the image data of the read image. A pixel number that is defined as the number of pixels that satisfies the condition below and satisfying one or more, d is the specific pixel number, and represents the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range of the image data of the read image and n, the value of pixel _{X n} represented by the pixel number n X (n), the value of a pixel _{X n + m} apart m pixels in the spacing direction of the streaks from the pixel _{X n} and X (n + m), m is 1 When taking an integer value satisfying ≦ m ≦ d or −d ≦ m ≦ −1, the inter-pixel contrast calculating step has the following formula:
CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
Accordingly, a process for calculating the contrast between the pixels and a process for obtaining the maximum value among the CTFs (X _n , X _{n + m} ) obtained for different m with respect to the pixel X _n are included.

According to the aspect 1, the CTF can be obtained by a simple algorithm that combines the calculation of the contrast between pixels and the maximum value search. Further, according to the aspect 1, even when a line defect occurs in the chart, the contrast calculated for the pixel corresponding to the position of the line defect is a low value. It is excluded and does not remain in the calculation result. Therefore, the process according to the aspect 1 includes a process for dealing with line defects.

Aspect 2 is the resolving power measurement method according to Aspect 1, in the inter-pixel contrast calculation step, for all m satisfying 1 ≦ m ≦ d or −d ≦ m ≦ −1 with respect to the pixel _Xn . CTF _(X _{n, X n + m)} is calculated, and, of all the _{_{m, CTF (X n, X}} n + m) the maximum value of, in resolution measurement method for performing processing for the contrast value of the pixel in the pixel _{X n} is there.

Aspect 3 is the resolving power measurement method according to Aspect 2, and the resolving power calculation step sets a group of pixels equal to or greater than the number of fringe interval pixels for pixels arranged in the interval direction of light and shade fringes, This is a resolution measurement method including a process in which the maximum value among the contrast values of the pixels calculated for the pixels is set as the contrast value at each pixel position belonging to the group.

Embodiment 4, a resolution measuring method aspect 2 or aspect 3, the pixel resolution calculation step, the relative pixel X _n, a plurality of pixels arranged in a row in a line parallel direction streaks are calculated respectively Is the resolution measurement method including the process of setting the maximum value among the contrast values in the contrast value at the position of the pixel column to which the pixel _Xn belongs.

Aspect 5 is the resolution measurement method according to any one of aspects 2 to 4, wherein the chart includes a plurality of gray stripes in which two or more gray stripes are arranged in a line direction parallel to the gray stripes, The plurality of gray stripes have the same stripe interval, and the positions of the stripes are shifted from each other by a specific distance in the interval direction of the gray stripes, and the resolution calculation step includes image data of the read image of the chart including the plurality of gray stripes From the pixel contrast values calculated for a plurality of pixels arranged in a line in a direction parallel to the line direction with respect to the pixel _Xn , the maximum value is determined at the position of the pixel column to which the pixel _Xn belongs. It is a resolution measuring method including the process which makes it a contrast value.

Aspect 6 is the resolution measurement method according to aspect 5, wherein the specific distance in the interval direction of the gray stripes is a positive integer of the number of pixels in the stripe interval, and the number of gray stripes arranged in the line direction is (stripes). This is a resolution measurement method that is a positive integer multiple of (number of spaced pixels / specific distance).

Embodiment 7, a resolution measuring method of any one of Embodiments 6 from embodiment 3, the resolution calculating step, with respect to pixel X _n, a plurality of pixels arranged in a row in a line parallel direction streaks This is a resolution measurement method including a process of excluding, as an abnormal value, a value that is greater than a specified ratio among the calculated contrast values of pixels in comparison with the average contrast value of pixels in the line direction.

Aspect 8 is the resolving power measurement method according to any one of Aspects 3 to 6, wherein the resolving power calculation step is performed for a plurality of pixels arranged in a line in a line direction parallel to the gray stripes with respect to pixel _Xn . Among the calculated contrast values of pixels, a value greater than a specified ratio compared to the average contrast value of pixels in the line direction is defined as an abnormal value, and the contrast value of a pixel indicating an abnormal value is within a range of the specified number of surrounding pixels. Is a resolution measurement method including a process of replacing the average value of the contrast values of pixels not corresponding to an abnormal value.

Aspect 9 is the resolving power measurement method according to any one of aspects 1 to 8, wherein the interval between the gray stripes is P millimeters, and the interval between the reading pixels of the optical reading device on the chart is S millimeters. In the resolution measurement method, ≧ 2 × S and the specific pixel number is the smallest integer d that satisfies d ≧ P / (2 × S).

Aspect 10 is the method of measuring a resolving power according to any one of aspects 1 to 8, wherein the interval between the light and shade stripes is P millimeters, and the interval between the reading pixels of the optical reading device on the chart is S millimeters. In the resolution measurement method, ≧ S and the specific number of pixels is a minimum integer d that satisfies d ≧ P / S.

Aspect 11 is the resolving power measurement method according to any one of aspects 1 to 8, where P is a millimeter stripe interval, and S millimeters is an interval between read pixels of the optical reading device on the chart. Resolution measuring method in which ≧ 2 × S and the number of specific pixels is an integer d equal to or larger than a minimum integer satisfying d ≧ P / (2 × S) and less than or equal to a minimum integer satisfying d ≧ P / S It is.

Aspect 12 is the resolution measurement method according to any one of aspects 1 to 11, wherein the chart includes a plurality of patterns of gray stripes at different specific intervals.

Aspect 13 is the resolving power measurement method according to any one of aspects 1 to 12, wherein the direction of the gray stripes included in the chart is the first direction, the second direction intersecting the first direction, or the first direction. And a resolution measuring method that is both in the second direction.

Aspect 14 is the resolution measurement method according to any one of aspects 1 to 13, wherein the focus state of the optical reader is determined from the calculation result of the contrast transfer function calculated for the pixels in the entire reading range of the optical reader. It is a resolution measuring method including a process.

Determining the focus state means determining whether the calculation result of the contrast transfer function is within a predetermined range defined by the optical reader, and determining whether the focus state of the optical reader is normal. To do.

Aspect 15 is the resolution measurement method according to any one of aspects 1 to 14, and the chart is a resolution measurement method in which a shading pattern is printed on a recording medium using a printing apparatus.

Aspect 16 is the resolution measuring method according to any one of aspects 1 to 15, and includes a chart reading step of reading a chart using an optical reading device.

Aspect 17 is the resolving power measurement method according to Aspect 16, wherein the optical system of the optical reader includes an imaging lens having a single optical axis or a lens array having a plurality of optical axes.

Aspect 18 is the resolution measuring method according to any one of Aspects 1 to 17, and includes a chart printing step of printing a chart including light and shade stripes using a printing apparatus.

Aspect 19 is the resolution measurement method according to Aspect 18, wherein the printing apparatus includes an optical reading device, and the chart is printed by the printing device, and the chart is read by the optical reading device in the printing device. Is the method.

Aspect 20 is the resolving power measuring method according to Aspect 18 or Aspect 19, wherein the printing apparatus is a resolving power measuring method which is an ink jet printing apparatus.

Aspect 21 is the resolving power measurement method according to Aspect 20, wherein each line of light and shade stripes is recorded by droplet ejection from a single nozzle in the ink jet head.

Aspect 22 is the resolution measuring method according to

Aspect

20 or 21, wherein the optical reader is an inline scanner mounted on an ink jet printing apparatus.

The resolution measuring device according to aspect 23 obtains image data of a read image obtained by reading a chart including light and shade stripes in which a density cross section changes into a rectangular shape at a specific interval at least in part by an optical reading device. Between the data acquisition unit and the pixels that belong to at least a part of the calculation target range of the image data of the read image, and pixels that are separated by a number of pixels that is 1 or more and a specific number of pixels or less in the interval direction of the gray stripes An inter-pixel contrast calculation unit that calculates the contrast between pixels, and a resolving power calculation that calculates the contrast transfer function at each stripe position of the gray stripes from the maximum contrast value calculated for each pixel by the inter-pixel contrast calculation unit The specific pixel number is less than or equal to the stripe interval pixel number, which is the number of pixels corresponding to the specific interval of the gray stripes in the image data of the read image It is defined as the number of pixels satisfying one or more conditions, the specific pixel number is d, and the pixel number indicating the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range in the image data of the read image is n , the value of a pixel _{X n} represented by the pixel number n X (n), the value of a pixel _{X n + m} apart m pixels in the spacing direction of the streaks from the pixel _{X n} and X (n + m), m is 1 ≦ m When taking an integer value satisfying ≦ d or −d ≦ m ≦ −1, the inter-pixel contrast calculation unit has the following formula:
CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
Thus, the resolution measurement apparatus performs the processing for calculating the contrast between the pixels and the processing for obtaining the maximum value among CTFs (X _n , X _{n + m} ) obtained for different m for the pixel X _n .

In the resolution measuring device according to the aspect 23, the same matters as the specific matters specified in the aspects 2 to 22 can be appropriately combined. In that case, the process or step (step) specified in the invention of the resolving power measurement method can be grasped as an element of a processing unit or a functional unit serving as a means responsible for the corresponding processing or operation.

The program according to the aspect 24 has the resolution of the optical reader based on the image data of the read image obtained by reading the chart including at least a density stripe whose density cross section changes to a rectangular shape at a specific interval by the optical reader. A program for causing a computer to execute data processing to be measured, the computer having a read image data acquisition step of acquiring data of a read image of a chart, and each of the image data of the read image belonging to at least a part of a calculation target range A pixel-to-pixel contrast calculating step for calculating a contrast between pixels with respect to a pixel from a pixel separated by a number of pixels equal to or greater than 1 and a specific number of pixels in the grayscale stripe interval direction, and a pixel-by-pixel contrast calculating step From the maximum value of the contrast calculated in the And a resolution calculation step for calculating the image, wherein the specific pixel number is equal to or less than the number of fringe interval pixels, which is the number of pixels corresponding to the specific interval of the gray stripes in the image data of the read image. It is defined as the number of pixels, and the specific pixel number is represented by d, the pixel number representing the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range in the image data of the read image is represented by n, and the pixel number n X (n) the value of a pixel _{X n} is the value of pixel _{X n + m} apart m pixels in the spacing direction of the streaks from the pixel _{X n} and X (n + m), m is 1 ≦ m ≦ d, or - When taking an integer value satisfying d ≦ m ≦ −1, the inter-pixel contrast calculating step is performed by the following equation:
CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
The program includes a process for calculating the contrast between the pixels and a process for obtaining the maximum value among CTFs (X _n , X _{n + m} ) obtained for different m with respect to the pixel X _n .

In the program according to aspect 24, the same matters as the specific matters specified in aspects 2 to 22 can be appropriately combined. Note that a non-transitory tangible medium including a computer-readable recording medium in which the program is recorded is included in an aspect of the present invention.

The printing apparatus according to aspect 25 includes an image forming unit that forms an image on a recording medium, an optical reading device that optically reads an image formed on the recording medium using the image forming unit, and an optical reading device that reads the image. And a data processing device that performs arithmetic processing on the image data of the read image that has been read, and the image forming unit prints a chart including light and shade stripes in which the density cross section changes into a rectangular shape at a specific interval, at least partially, and optical The reading device reads a chart, the data processing device acquires a read image data acquisition unit that acquires image data of a read image of the chart read by the optical reading device, and at least a part of calculation objects of the image data of the read image A pixel that calculates the contrast between pixels with respect to each pixel belonging to the range from a pixel that is 1 or more and not more than a specific number of pixels in the interval direction of the gray stripes A contrast calculation unit, and a resolution calculation unit that calculates a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel by the inter-pixel contrast calculation unit. Is defined as the number of pixels satisfying one or more of the stripe interval pixel number, which is the number of pixels corresponding to the specific interval of the gray stripes in the image data of the read image, and the specific pixel number is d and the image data of the read image Among these, at least a part of the calculation target range, the pixel number indicating the position in the interval direction of the gray stripes is n, the value of the pixel _Xn represented by the pixel number n is X (n), and the gray level from the pixel _Xn When the value of a pixel X _{n + m} separated by m pixels in the stripe interval direction is X (n + m), and m is an integer value satisfying 1 ≦ m ≦ d or −d ≦ m ≦ −1, Contrast calculator The following equation,
CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
Thus, the printing apparatus performs the processing for calculating the contrast between the pixels and the processing for obtaining the maximum value among the CTFs (X _n , X _{n + m} ) obtained for different m with respect to the pixel X _n .

In the printing apparatus according to aspect 25, the same matters as the specific matters specified in aspects 2 to 22 can be appropriately combined. In that case, the process or step (step) specified in the invention of the resolving power measurement method can be grasped as an element of a processing unit or a functional unit serving as a means responsible for the corresponding processing or operation.

Aspect 26 is the printing apparatus according to aspect 25, wherein the image forming unit includes a line-type inkjet head configured by connecting a plurality of head modules, and the data processing device transmits the contrast transmitted by the resolving power calculation unit. This is a printing apparatus that performs processing for detecting local non-uniformity in the ink discharge amount of the head module using a function.

In addition, as another aspect, the following invention aspect is disclosed.

An image of a read image obtained by reading a chart including at least a density stripe whose density cross section changes into a rectangular shape at a specific interval by an optical reader, which measures the resolution of the optical reader. The data is acquired, and for each pixel belonging to at least a part of the calculation target range of the acquired read image data, a pixel that is 1 or more and a specific pixel number or less apart in the interval direction of the gray stripes And having at least one processor for calculating a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel. The number is defined as the number of pixels satisfying one or more of the stripe interval pixel number which is the number of pixels corresponding to the specific interval of the gray stripes in the image data of the read image. Is, a specific number of pixels d, pixels represented the pixel number n, the pixel number n representing the position of the spacing direction streaks of pixels belonging to at least part of the calculation target range of the image data of the read image X the value of _n X _(n), the value of a pixel _{X n + m} apart m pixels in the spacing direction of the streaks from the pixel _{X n} and X (n + m), m is 1 ≦ m ≦ d, or, -d ≦ m ≦ When taking an integer value satisfying -1, the processor:
CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
Thus, the resolution measurement apparatus performs the processing for calculating the contrast between the pixels and the processing for obtaining the maximum value among CTFs (X _n , X _{n + m} ) obtained for different m for the pixel X _n .

According to the present invention, the CTF can be obtained with a relatively simple algorithm even when a line defect in the chart occurs.

FIG. 1 is a diagram illustrating an example of a chart image displayed on a chart used for CTF inspection. FIG. 2 is a schematic diagram of digital image data of a read image obtained by reading a chart using an optical reading device. FIG. 3 is obtained by adding an identification code for identifying the position of each pixel to a part of the pixels of the digital image data shown in FIG. FIG. 4 is a schematic diagram of the digital image data of 3 rows and 15 columns shown in FIG. FIG. 5 is a diagram illustrating a C position obtained by shifting the position of the image data to the left by two pixels with respect to the A position illustrated in FIG. FIG. 6 is a diagram illustrating a D position in which the position of the image data is shifted to the left by 3 pixels with respect to the A position illustrated in FIG. 4. FIG. 7 is a diagram illustrating an E position obtained by shifting the position of the image data to the left by 4 pixels with respect to the A position illustrated in FIG. FIG. 8 is a diagram illustrating an F position obtained by shifting the position of the image data to the right by one pixel with respect to the A position illustrated in FIG. FIG. 9 is a diagram illustrating a G position obtained by shifting the position of the image data to the right by two pixels with respect to the A position illustrated in FIG. FIG. 10 is a diagram illustrating an H position obtained by shifting the position of the image data to the right by 3 pixels with respect to the A position illustrated in FIG. FIG. 11 is a diagram illustrating an I position obtained by shifting the position of the image data to the right by 4 pixels with respect to the A position illustrated in FIG. FIG. 12 is a diagram illustrating an example of a strip-shaped matrix partitioned by the search width. FIG. 13 is a diagram illustrating an example of a chart image including stepped black and white stripes. FIG. 14 is a diagram illustrating an example of a pixel group in the read image data illustrated in FIG. 3. FIG. 15 is a diagram illustrating an example of the read pixels arranged in a line in the column direction parallel to the chart line in the read image data illustrated in FIG. 3. FIG. 16 is a flowchart illustrating an overall process flow related to the CTF measurement process in the inkjet printing apparatus according to the embodiment. FIG. 17 is a flowchart showing the contents of print image data generation processing. FIG. 18 is a flowchart showing the processing contents of the printing process. FIG. 19 is a flowchart showing the processing contents of the reading processing step. FIG. 20 is a flowchart showing the contents of the CTF calculation process. FIG. 21 is a flowchart showing the contents of the contrast calculation process. FIG. 22 is a flowchart showing the contents of the contrast calculation process. FIG. 23 is an example of an image of light and shade stripes as a chart image. FIG. 24 is a chart showing an example of the data array IM (L, C) of the read image. FIG. 25 is a chart showing an example of the CTF midway calculation array CTFT (L, C). FIG. 26 is a chart showing an example of the CTF calculation array CTF (BC). FIG. 27 is a diagram illustrating an example of a rectangular range in the CTF midway calculation array. FIG. 28 is a block diagram illustrating functions of a data processing apparatus that performs CTF calculation according to the resolution measurement method according to the embodiment. FIG. 29 is a side view illustrating the configuration of the inkjet printing apparatus according to the embodiment. FIG. 30 is a block diagram illustrating a main configuration of a control system of the inkjet printing apparatus. FIG. 31 is a graph illustrating an example of a calculation result of CTF.

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

<Explanation of terms>
Terms used in the present disclosure will be described.

“Resolution” refers to the closest distance at which two close points to be read can be reproduced and read as two points in an optical reader using an optical system.

“Resolving power” refers to the reciprocal of the resolution distance. The resolution is the degree of ability to correctly read the degree of contrast (contrast) of images of various finenesses by quantifying the extent that two points can be reproduced and read as two points for each distance of the two points. .

“CTF” is an abbreviation for Contrast Transfer Function and refers to a contrast transfer function. CTF is one of indices indicating the resolution. CTF is the resolving power measured with a rectangular bright and dark pattern having a specific frequency. If the brightness level can be read correctly, CTF = 1 is set, and if it is not read at all, CTF = 0 is set. In addition, what was measured with the sinusoidal light / dark pattern is a modulation transfer function (MTF).

The definition formula by Michelson contrast for CTF is as shown in Formula [1].

The “light / dark stripe” is a pattern in which the density cross section changes into a rectangular shape, and corresponds to a rectangular light / dark pattern having a specific frequency. “Monochrome stripe” is an example of light and shade stripes. The arrangement direction of the stripes in the light and shade stripes is called the stripe interval direction, and the direction parallel to the stripes is called the line direction.

“Chart” is an object to be read on which a chart image including at least light and shade stripes is displayed.

“Resolution” is the reciprocal of the interval between dots in the printer. In the reading side scanner (optical reading device), it is the reciprocal of the reading pixel interval. When the subject (reading object) and the scanner are relatively stationary, in a scanner using a lens and an image sensor, the resolution is determined from the imaging magnification of the lens and the pixel interval of the image sensor. The resolution of the scanner is called “reading resolution” or scanner resolution. The resolution of dots drawn by the printer is called “drawing resolution”. The resolution is usually represented by the number of dots per inch (dpi: dot per inch) for both the drawing resolution and the reading resolution. One inch is 25.4 millimeters. “Scanner” corresponds to “optical reading device”. “Printer” corresponds to “printing apparatus”. An imaging sensor is synonymous with a reading sensor.

<< Outline of Resolution Measurement Method According to Embodiment >>
FIG. 1 is a diagram illustrating an example of a chart image displayed on a chart used for CTF measurement. A chart 10 shown in FIG. 1 is a black and white stripe chart, and includes a vertical stripe pattern in which black vertical lines are arranged at regular intervals in the horizontal direction. FIG. 1 shows an enlarged part of the black and white stripes displayed on the chart 10. Note that the actual chart further spreads in the vertical and horizontal directions of FIG. 1, a larger number of vertical lines are arranged in the horizontal direction, and the vertical lines extend further in the vertical direction of FIG. 1.

The white portion between the black line 12 and the line 12 in the chart image is called a white portion 14. The white and black stripes of the chart 10 have the same white and black intervals, and the density cross section changes to a rectangular shape at specific intervals. That is, the white width Ww of the white portion 14 is equal to the black width Wb that is the thickness (line width) of the line 12. The interval Pb between the

lines

12 and 12 is equal to the interval Pw between the white portion 14 and the white portion 14. The interval Pb corresponds to the repetition period of the line 12. The line 12 may be referred to as “black line” or “stripes”. Note that a chart including black and white stripes having different white widths Ww and black widths Wb is not limited to a chart including black and white stripes having the same white width Ww and black width Wb.

FIG. 2 is a schematic diagram of digital image data of a read image obtained by reading a chart using an optical reading device. A chart for CTF measurement is read by an optical reader, and image information is digitized into digital image data. FIG. 2 shows a part of the image data of the read image 20 obtained by reading the chart image shown in FIG. Note that the image data of the read image obtained by reading the actual chart has a larger number of pixels in both the vertical direction and the horizontal direction. The image data of the read image 20 may be referred to as “read data”.

In FIG. 2, a small square 22 displayed at the upper left indicates the size of one pixel of the digital image data. The size of one pixel of the digital image data corresponds to the size of the reading pixel of the optical reading device. That is, the reading of the chart 10 is performed using an optical reading device of a reading pixel having a width smaller than the line width of the black and white stripes. The white portion of the chart 10 is brightly read, and the read value is a relatively large value. The black portion of the chart 10 is read darkly, and the read value is a relatively small value. The read value is a pixel value in the read image data. The pixel value is synonymous with “pixel signal value” or “pixel value”.

Due to the characteristics of the optical system of the optical reading device, the black and white stripes of the chart 10 are read with a slight blur and the contrast is lowered, so in FIG. 2, the pixels near the black line position center 24 of the chart are displayed darkly. Pixels around the part position 26 are displayed in a slightly light gray color. Here, in order to simplify the description, an example is shown in which the line interval is an integral multiple of the read pixel interval. The example of FIG. 2 is an example in which the line interval is four times the read pixel interval, that is, the line interval is 4 pixels when the line interval is expressed by the number of read pixels. When the line interval is not an integral multiple of the read pixel interval, a moiré in which the phase relationship between the line position and the read pixel position is shifted and the density of the read image changes in a complicated manner occurs. If the line interval is an integral multiple of the read pixel interval, the phase of the line position and the read pixel position are aligned, and as shown in FIG. 2, for each pixel position in the line arrangement direction, black, The read image changes in light and shade with a constant repeating pattern of gray, white, gray, black, gray, white. A method for dealing with moire will be described later.

FIG. 3 shows a part of the pixels of the digital image data shown in FIG. 2 with an identification code for identifying the position of each pixel. FIG. 3 shows an example in which each pixel belonging to a part of the pixel range of the digital image data shown in FIG. 2 is associated with an identification code in which alphabet symbols and numbers are combined so that each position can be recognized. That is, the pixel position is represented by a combination of a symbol and a number corresponding to the column and row, such as “a1”. The alphabetical symbol of the identification code represents the position of “column”, and the number represents the position of “row”.

In the case of FIG. 3, for example, the center of one of the vertical lines of the chart is read by the pixels b1, b2, and b3. Therefore, the positions of the pixels b1, b2, and b3 are displayed darkly. In the case of this example, since the line interval is 4 pixels when expressed in terms of the number of read pixels, the line positions arrive at every 4 pixels arranged in the column direction.

The read data shown in FIG. 3 is digital image data including pixels of 3 rows and 15 columns from w1 to k3. In the following description, digital image data is schematically represented using an identification code indicating the position of each pixel, and a CTF calculation procedure according to the resolution measurement method of the present disclosure will be described.

<< Outline of CTF Calculation Procedure According to Embodiment >>
In this example, a 3 × 7 square range from a1 to g3 in FIG. 3 is referred to as “A position”, and this “A position” will be described. Like the pixel group indicated as the A position, a target pixel range that is a calculation target of CTF calculation is referred to as a “calculation target range”.

FIG. 4 is a schematic diagram of the digital image data of 3 rows and 15 columns shown in FIG. In the upper part of FIG. 4, a range of “A position” is displayed by enclosing a 3 × 7 square range from a1 to g3 with a frame line. Each position obtained by shifting the position of the image data by one pixel to the left in FIG. 4 with respect to the A position is defined as a B position, a C position, a D position, and an E position. Further, positions where the position of the image data is shifted by one pixel to the right in FIG. 4 with respect to the A position are defined as an F position, a G position, an H position, and an I position.

The lower part of FIG. 4 shows the B position where the position of the image data is shifted by one pixel to the left of the figure with respect to the A position. That is, the A position is shifted by one pixel to the left in FIG.

FIG. 5 shows a position C in which the position of the image data is shifted by 2 pixels to the left of the figure with respect to the position A. That is, the A position is shifted to the left in FIG. Similarly, the A position is shifted to the left by 3 pixels to obtain the D position. Further, the A position is shifted to the left by 4 pixels to be the E position.

FIG. 6 shows a D position in which the position of the image data is shifted to the left by 3 pixels with respect to the A position.

FIG. 7 shows an E position obtained by shifting the position of the image data to the left by 4 pixels with respect to the A position.

Similarly, for the shift of the image data in the right direction, the A position shown in the upper part of FIG. 4 is shifted by one pixel to the right to the F position, two pixels to the G position, three pixels to the H position, and four pixels. Each position is determined by shifting.

FIG. 8 shows an F position obtained by shifting the position of the image data to the right by one pixel with respect to the A position.

FIG. 9 shows the G position where the position of the image data is shifted to the right by 2 pixels with respect to the A position.

FIG. 10 shows an H position in which the position of the image data is shifted to the right by 3 pixels with respect to the A position.

FIG. 11 shows an I position obtained by shifting the position of the image data to the right by 4 pixels with respect to the A position.

If the image data is shifted to the left from the A position shown in FIG. 4, a position where there is no image data is formed on the right side of the image data, and if the image data is shifted to the right from the A position, a pixel is displayed on the left side of the image data. Since there is a position where there is no value, the data shortage position is indicated by “u” in each of FIGS. The handling of the singular part where the pixel value is insufficient will be described later.

In the CTF calculation method according to this embodiment, the position of image data is shifted by one pixel to the left in FIG. 4 from the position A, and the position of image data is 1 on the right in FIG. The contrast between pixels is calculated for each combination of pixels at the same row position and column position at the F position to I position shifted by pixels.

The contrast between pixels is a contrast evaluation value calculated using a value of two pixels according to a calculation formula according to the definition formula of CTF. The contrast between pixels is called “inter-pixel CTF”.

<CTF calculation comparison range>
The range of pixels to be compared when obtaining the inter-pixel CTF is referred to as a CTF calculation comparison range. The CTF calculation comparison range is a range up to “+ d” pixels on the right side and “−d” pixels on the left side, where d is the inter-pixel distance with the pixel farthest among the pixels to be compared. d indicates the maximum pixel shift amount, and is a positive integer. The examples shown in FIGS. 4 to 11 are for d = 4. Preferred conditions for d will be described later. d corresponds to the specific number of pixels.

<Calculation of inter-pixel CTF>
The inter-pixel CTF is calculated according to the following equation [2].

CTF (X _n , X _{n + m} ) = {X (n + m) −X (n)} / {X (n + m) + X (n)} Formula [2]
X (n) is the value of pixel _{X n.}

X (n + m) is the value of pixel _{X n + m} apart m pixels in the spacing direction of the streaks from the pixel _{X n.}

N is a pixel number representing the position in the interval direction of the gray stripes of each pixel belonging to the calculation target range.

M takes an integer value satisfying 1 ≦ m ≦ d or −d ≦ m ≦ −1.

When the imaging sensor of the optical reader is a monochrome sensor, each of X (n) and X (n + m) is a single value (a value of one channel not including color information).

When the imaging sensor of the optical reading device is a color sensor, each of X (n) and X (n + m) has values of R, G, and B channels, for example. Therefore, CTF (X _n , _{Xn + m} ) is calculated for each of R, G, and B.

The identification code of each pixel described in the example of FIG. 3 corresponds to the pixel number n. The pixel identification code is also used as a notation of the pixel value represented by the identification code. More specifically, the expression [2] is applied to the example of FIG. 3, for example, the following calculation is performed on the pixel a1 shown in FIG. 3 by combining the A position and the B position.
CTF (a1, b1) = (b1-a1) / (b1 + a1) Formula [3]
On the right side of Equation [3], the value of the pixel a1 is represented as a1, and the value of the pixel b1 is represented as b1. The pixel identification code is also used as a notation of the pixel value represented by the identification code.

Further, the following calculation is performed depending on the combination of the A position and the C position.
CTF (a1, c1) = (c1-a1) / (c1 + a1) Formula [4]
Similarly, the following calculation is performed from the D position to the I position.
CTF (a1, d1) = (d1-a1) / (d1 + a1) Formula [5]
CTF (a1, e1) = (e1-a1) / (e1 + a1) Equation [6]
CTF (a1, z1) = (z1-a1) / (z1 + a1) Equation [7]
CTF (a1, y1) = (y1-a1) / (y1 + a1) Formula [8]
CTF (a1, x1) = (x1-a1) / (x1 + a1) Equation [9]
CTF (a1, w1) = (w1-a1) / (w1 + a1) Formula [10]
In this example, there are a total of 8 shift positions (B position to I position), 4 positions on the left and right with respect to the A position. The inter-pixel CTF is calculated for each of the 8 pixels in total.

That is, the same calculation as in the equations [3] to [10] described for the pixel a1 is performed for all the pixels a1 to g3.

<Maximum search for inter-pixel CTF and determination of contrast for each pixel>
According to the equation [2], CTF (X _n , X _{n + m} ) is calculated for all m satisfying 1 ≦ m ≦ d or −d ≦ m ≦ −1 for each pixel X _n , The maximum value among the CTFs (X _n , X _{n + m} ) of _m is defined as the contrast for the pixel X _n , CTF (X _n ). For example, for the pixel a1, the maximum value among the calculation results CTF (a1, b1) to CTF (a1, w1) of the eight inter-pixel CTFs calculated by the equations [3] to [10] The contrast for a1 is set, and the maximum value is CTF (a1). Similar calculation is performed for each of the pixels a2 and a3. Further, the same calculation is performed from the b column to the g column that is the calculation target range. Thereby, the contrast for each pixel from the pixel a1 to the pixel g3 belonging to the calculation target range, that is, CTF (a1) to CTF (g3) can be obtained. The contrast with respect to the pixel _Xn is expressed as CTF ( _Xn ). In this way, CTF (X _n ) specified for each pixel is referred to as a “pixel CTF value”.

Furthermore, among CTF (a1), CTF (a2), and CTF (a3), which are the CTF values of the pixels a1 to a3 belonging to the same vertical column (a column), the maximum value is the CTF value of the a column. And CTF (a). Similarly, the maximum CTF value of each pixel is obtained for each column from the b column to the g column, and is set as CTF (b) to CTF (g) of each column. Thus, the CTF values at the respective column positions from the a column to the g column can be obtained.

<About the line direction of the chart>
Since the chart 10 for measuring CTF illustrated in FIG. 1 is a chart including a vertical line pattern, the comparison target pixel when calculating the CTF value of each pixel is a horizontal direction orthogonal to the line of the chart 10. The comparison range when obtaining the maximum value from the calculation result of the CTF value of each pixel is a pixel in the vertical direction parallel to the line.

On the other hand, when calculating the CTF for the horizontal chart, the vertical / horizontal relationship described above changes by 90 degrees. That is, if the CTF measurement chart is a horizontal chart, pixels in the vertical direction perpendicular to the chart line are used as comparison target pixels when calculating the CTF value of each pixel. The comparison range when obtaining the maximum value from the calculation result of the CTF value is a horizontal pixel parallel to the line.

<About positive and negative signs in CTF calculation>
According to the CTF calculation method according to the present disclosure, the calculation result of the CTF value of each pixel does not become negative. When calculating a value using a formula such as Equation [2], in general, there is a constraint that the number to be subtracted is larger than the subtracted number so that the numerator of the fractional part on the right side does not become negative. Provide. However, the CTF calculation method according to the present disclosure does not require special consideration. It is because the image is dark at the position of each line of the chart (for example, b1), so the read data becomes a relatively small value (for example, the relationship between b1 and d1), and the numerator on the right side of Equation [2] is positive Because.

On the other hand, at the position between the chart lines (for example, d1), the numerator on the right side of the formula [2] is negative (for example, in the case of d1 and b1). However, CTF (b1, d1) for obtaining the inter-pixel CTF of d1 with respect to b1 from the calculation formula of Equation [2], and CTF (d1, b1) for obtaining the inter-pixel CTF of b1 with respect to d1 Is CTF (b1, d1) = − CTF (d1, b1), and there is a calculation result of the same magnitude in absolute value.

In other words, the calculation of the inter-pixel CTF always involves the replacement of the calculation target between the pair of pixels used for the calculation. Therefore, if the inter-pixel CTF is calculated for each pixel belonging to the calculation target range, the absolute value is the same. And negative values occur in pairs.

After calculating the CTF value of each pixel for all the pixels belonging to the calculation target range, in the subsequent calculation, the maximum value among the CTF values of each pixel in the pixel range within the line interval of the chart is determined, and this CTF maximum value Is a CTF value in the vicinity of the line, so that the calculation result of the CTF maximum value is not affected even if the negative sign is not taken into consideration. That is, when the maximum CTF value within the line interval of the chart is obtained, the negative value is ignored, and eventually the CTF value near the line becomes a positive value as defined in Equation [1].

<Search width for searching the maximum value>
At the stage of calculation for obtaining the CTF value of each pixel belonging to the calculation target range or the stage of calculation for obtaining the CTF value of each column, a combination of black and white Max and Min as defined in Equation [1] is used. There is no guarantee that it will be calculated. The portion calculated from the combination of Max and Min has the maximum CTF value.

For example, if the chart stripe interval is not an integer multiple of the read pixel interval, the phase relationship between the line position and the read pixel position gradually shifts, and at some line, the line position and The position of the read pixel most closely matches. At the place where the position of the line and the position of the read pixel are the same, the read value becomes the minimum value.

That is, if the CTF is calculated in a certain wide range in the fringe spacing direction, a combination of CTF values close to Max and Min can be obtained even when the phase relationship is slightly shifted.

For example, if the CTF value of each pixel is calculated for all the pixels within the pixel range of the strip matrix shown below, a portion calculated from a combination close to Max and Min is included somewhere in it. Find it with a maximum search.

The search width that is the search range of the maximum value search is determined based on the period of black and white stripes on the chart image. The period of black and white stripes is a repeated spatial period of stripes, and is synonymous with the interval of stripes arranged at regular intervals. For example, if the black and white period is 4 pixels as in the example shown in FIGS. 1 to 3, the search width is 4 pixels. For the position A described with reference to FIG. 4, a matrix is vertically divided for each search width, and the maximum value of the CTF value of the pixel in the strip matrix is obtained. The maximum value thus obtained is the CTF of the vertical line within the strip width.

FIG. 12 is an example of a strip-like matrix delimited by the search width. Here, a striped matrix of 3 rows and 4 columns from b columns to e columns divided by a search width of 4 pixels is shown. The maximum CTF value is searched from the pixel range of the strip-like matrix shown in FIG. The maximum CTF value obtained in this way is the CTF at the position of the strip matrix. When the CTF value of each column is obtained, the maximum value among CTF (b) to CTF (e) is the CTF value representing the position of the strip matrix.

<In case of missing line on the chart>
For example, when a chart of light and shade stripes is printed using an ink jet printing apparatus, if there are non-ejection nozzles in the ink jet head, a part of the light and shade stripes is lost. In this case, in the read image, the missing portion becomes white, and the CTF value of the pixel corresponding to the missing line position = 0. That is, since the CTF value of the pixel that has read the position where the line is missing is 0, it is excluded (ignored) in the search for the maximum value.

<Process to reduce the influence of measurement noise>
In the above striped matrix, the following processing may be performed for the purpose of reducing the influence of measurement noise. That is, for a pixel from which a line image of black and white stripes as shown in FIG. 1 is read, if the length of the strip matrix is 3 rows, it is considered that these 3 rows read the same line, and is larger than the maximum value in the strip matrix. These three data are averaged, and the average value is used as the CTF value of the column. However, if non-ejection occurs in the middle of line drawing, the CTF value of the pixel that has read the missing portion of the line due to non-ejection is CTF = 0. Data shall not be included.

<Moire countermeasures>
In general, the interval between the stripes of the chart is not an integral multiple of the interval between the read pixels, and moire is caused in the read image of the chart due to the relationship between the interval between the chart lines and the reading resolution of the optical reading device. For example, when a shading chart printed by a printing function of an inkjet printing apparatus equipped with an inline scanner is read by an inline scanner, moire occurs in the read image due to the relationship between the drawing resolution of the chart and the reading resolution of the inline scanner. In calculating the CTF, it is preferable to perform any one of the following three countermeasure processes in order to eliminate the influence of the moire.

Moire countermeasure processing 1: Since the moire cycle is known in advance, a positive integer multiple of the moire cycle is set as the CTF calculation cycle. Further, in the column direction, the number of read data pixels close to the print interval for which CTF is to be calculated is obtained, and the maximum value is obtained from the CTF calculation result of the pixel range of the distance defined by the read data pixel number. Note that Pb> Pr, and the moire interval is 2 × Pb × Pr / (Pb−Pr) where Pb is the interval between the light and shade stripes on the print and Pr is the interval between the pixels of the read image.

The CTF calculation period is the width per section for calculating the CTF value. For example, if the moire period is 10 mm, the CTF calculation period is set to 10 mm. Thereby, CTF can be calculated | required with the fineness of the range where the period of a moire goes around. If the CTF is calculated at a calculation cycle that is a positive integer multiple of the moire cycle, the information on the maximum moire shading can be extracted.

As described above, according to the CTF calculation method according to the present disclosure, the read image data of the chart is shifted in the line interval direction, and the difference and sum of the values are calculated between the corresponding pixels. By combining a simple calculation process of dividing the image and a simple process of finding the maximum value from the calculation result, a contrast data group for each pixel in a pixel range of approximately the same size as the read image can be obtained. it can. Since this data group includes the influence of moiré, the maximum value (value with the smallest influence of moiré) is searched for every section of the pixel range in the moire cycle that has been grasped in advance, and the maximum value is found in the corresponding section. The representative value of.

Moiré countermeasure processing 2: An average value is obtained by excluding extremely distant values (abnormal values) from CTF values of each pixel which is a data group of contrast of each pixel.

Moiré countermeasure process 3: An average value is obtained using only data within “average value ± 2σ” known as a statistical calculation method among the CTF values of each pixel which is a data group of contrast of each pixel. σ is a standard deviation.

By performing any one of the above moire countermeasure processes 1 to 3, it is possible to eliminate the influence of moire and obtain the CTF over the reading width.

<Use of charts with stair-like black and white stripes>
FIG. 13 is an example of a chart image including stepped black and white stripes. A chart image 30 shown in FIG. 13 includes a plurality of black and

white stripes

31, 32, and 33 in which vertical lines 12 are arranged at a constant interval Pb in the horizontal direction. Here, an example of three-stage black and white stripes in which three black and

white stripes

31, 32, and 33 are arranged in a stepwise manner in the line direction is shown. The line direction is a direction parallel to the line of the black and white stripes, and corresponds to the vertical direction in the case of the black and white stripes of the vertical stripes illustrated in FIG.

In each of the black and

white stripes

31, 32, 33, the stripe interval Pb and the interval Pw of the white portion 14 are equal. Similarly to the example described in FIG. 1, the black width Wb and the white width Ww are equal in each of the black and

white stripes

31, 32, and 33.

In FIG. 13, the second black and white stripe 32 is arranged at a position shifted by a specified distance r in the stripe interval direction with respect to the position of the first black and white stripe 31 displayed at the top. The The third-stage black and white stripes 33 are arranged at positions shifted by a specified distance r in the stripe interval direction with respect to the position of the second-stage black and white stripes 32. The specified distance r is an example of a “specific distance”. The “interval direction” is a direction in which stripes are arranged at a constant interval Pb. In the case of this example, it indicates a direction (lateral direction in FIG. 13) orthogonal to the line direction of the line 12. In FIG. 13, three steps of black and white stripes are illustrated, but stepped black and white stripes having two or more steps and an arbitrary number of steps can be used.

Although the example of three stages is shown in FIG. 13, the specified distance r in the stripe interval direction is a positive integer of the stripe interval Pb, and the number of monochrome stripes arranged in the line direction, that is, the number of stages, is ( It is desirable to be a positive integer multiple of the interval Pb / the specified distance r).

The chart image shown in FIG. 13 has a specified distance = 1 and a black-and-white stripe interval Pb = 4, so the number of black-and-white stripes in the line direction (number of stages) = (4/1) = 4 is desirable. The unit of the specified distance r and the unit of the black-and-white stripe interval Pb is millimeters. The specified distance and the black-and-white stripe interval Pb may be expressed by the number of pixels in units of the size of one read pixel.

A chart including such stepped black and white stripes is read by an optical reading device, and the CTF calculation method according to the present embodiment is applied from the obtained read image data to calculate the CTF value of each pixel. For the CTF value calculation result of each pixel thus obtained, the maximum value among the calculation results of the CTF values of the pixels arranged in a line in a direction substantially parallel to the chart line is the CTF value of the pixel _Xn. can do.

In general, the interval between the stripes of the chart is not an integral multiple of the interval between the reading pixels. From the relationship between the interval between the stripes of the chart and the reading resolution of the optical reading device, the position of the line and the position of the reading pixel are Thus, data with the phase relationship shifted can be obtained. When a chart including stepped black and white stripes as shown in FIG. 13 is read, there is a place where the center of the line of some stage and the center of the read pixel are more coincident among the plurality of stages of black and white stripes. Probability of existence increases.

Therefore, by applying the CTF calculation method of the present embodiment using a chart including stepped black and white stripes, even when the size of the read pixel of the optical reader is close to the interval between the stripes of the chart. A value close to the actual CTF can be obtained.

Each line of the black and white stripes shown in FIG. 13 can be recorded one by one for each nozzle of the inkjet head. That is, each line of black and white stripes can be recorded by a single nozzle.

FIG. 13 shows an example in which the black and white stripes 31 to 33 at each stage are connected in contact with each other in the line direction. May be.

<Preparation for CTF calculation and handling of singular parts at both ends>
With respect to a _n to g _n (n = 1, 2, 3) at the A position shown in FIG. 4, the B position to the I position have pixels in the same column and row even after the pixels are shifted. On the other hand, among the image data, the pixel data of the corresponding positions indicated by “u” from the B position to the I position are lost in the four pixel columns of the 1st to 4th lines and the 12th to 15th lines of the A position depending on the direction of shifting the pixels. . Therefore, since the inter-pixel CTF cannot be calculated as it is, the four columns at both ends are excluded from the calculation, or the data of the average value of the pixels in the peripheral range is applied to a position where there is no pixel data as a result of shifting the pixels. Then, the inter-pixel CTF is calculated.

For example, “u” in the first row shown in the rightmost column at position B shown in FIG. 4 contains an average value of h1 to k1. The average value of h2 to k2 is entered in “u” in the second row shown in the rightmost column at position B shown in FIG. The average value of h3 to k3 is entered in “u” in the third row shown in the rightmost column at position B shown in FIG. As for “u” shown in the rightmost two columns at the C position shown in FIG. 5, the average value of h1 to k1 is entered in “u” in the first row in both columns. Hereinafter, since it is the same, description is abbreviate | omitted.

The average value of the pixel values of the peripheral pixels is an intermediate brightness value because the data of the dark portion and the light portion of the gray stripe image is averaged as an image. Since the value is “gray” for black and white stripes, the calculation result of the inter-pixel contrast is a relatively small value. That is, when the method of applying the average value data of the peripheral pixels to the pixel position of the singular part where the pixel data disappears by shifting the pixel, the calculation of the inter-pixel CTF using the pixels to which the average value is applied Since the result does not become the maximum value, it does not remain in the calculation result of the CTF value of each pixel and does not affect the calculation result for obtaining the maximum value.

If the exception process of excluding the singular part at the end where the pixel data is lost by shifting the pixel is excluded from the calculation process, the calculation algorithm becomes complicated.

On the other hand, the method of applying the average value data of the peripheral pixels to the singular part is more preferable because it does not require exception processing and can simplify the calculation algorithm.

<Processing taking into account both ends of the chart and missing lines>
In a black and white striped chart, the white background is widened at a portion where there is no line, such as the outside of the line at both ends and a portion where the line is missing in the chart, so that the black and white contrast increases. Therefore, a pixel that has been calculated with a high resolving power specifically with respect to the surrounding pixels is suspected of missing lines due to non-ejection or an influence outside the chart, and excludes the data of the pixel, or is not specific The average value of CTF values of surrounding pixels is used.

<Example of pixel group settings>
Not only the grouping like the strip matrix described with reference to FIG. 12, but the calculation result of the CTF value of each pixel belonging to the calculation target range, the stripe interval in the arrangement direction (interval direction) of black and white stripe lines. A group of pixels equal to or more than the number of pixels (number of fringe spacing pixels) is set, and for each group, the maximum value among the calculation results in the group is determined as the CTF at each pixel position belonging to the pixel group. Also good.

FIG. 14 is an example of a pixel group in the read image data shown in FIG. FIG. 14 shows an example in which a range of five pixels a1 to e1 is set as one group. Of the CTF values of each pixel in this group, the maximum value, for example, CTF (b1) may be used as the CTF value of each pixel of a1 to e1.

<Example of handling pixels lined up in the line direction of the chart>
A chart at each pixel position with respect to the calculation result of the CTF value of each pixel belonging to the calculation target range or the calculation result of the CTF value of each pixel obtained by setting the pixel group described with reference to FIG. Among the calculation results of the read pixels arranged in a line in a direction substantially parallel to the line, the maximum value may be determined as the CTF at each pixel position belonging to the pixel line position of that line.

FIG. 15 shows an example of the read pixels arranged in a line in the column direction parallel to the chart line in the read image data shown in FIG. FIG. 15 shows b pixel columns in which the pixels b1 to b3 are arranged in the vertical direction. Of the CTF values of the pixels b1 to b3, the maximum value, for example, CTF (b2) may be used as the CTF value of each of the pixels b1 to b3.

In this method, when the staircase-shaped black and white stripes described in FIG. 13 are used, the pixel value fluctuates due to the influence of measurement noise, or the read pixel of the chart is slightly in the direction of the black and white stripe line. This is useful when you are leaning to

<Exclusion processing of abnormal values>
In the processing described with reference to FIGS. 13 and 14, the CTF (X _n ) value of a certain pixel X _n in the CTF value data group of each pixel is at least compared with the CTF values of the pixels before and after that. When the ratio is larger than the predetermined ratio, a process of excluding the CTF value of the pixel _Xn as an abnormal value may be performed. The “CTF value of the pixels before and after that” may be, for example, an average value of CTF (X _n ) of pixels arranged in the line direction, that is, an average value of contrast of pixels in the line direction. For example, the predetermined ratio can be 10%. The predetermined ratio set in advance corresponds to the “specified ratio”.

Further, instead of the process of excluding as an abnormal value, a process of replacing the CTF value of the pixel _Xn with the average value of the CTF values of pixels that are not abnormal values out of the predetermined number of pixels before and after the pixel _Xn may be performed. For example, the predetermined number of pixels may be 10 pixels. The range of the predetermined number of pixels before and after corresponds to the “range of the predetermined number of surrounding pixels”.

<About optical reader>
The optical system of the optical reader may be an imaging lens having a single optical axis or a lens array having a plurality of optical axes. As a lens array having a plurality of optical axes, for example, a SELFOC (registered trademark) lens array can be used. Further, an optical reading device that reads a wider range by using a plurality of optical reading devices using an imaging lens having a single optical axis may be used.

As the imaging sensor of the optical reader, a two-dimensional array sensor such as a CCD (Charge Coupled Device) image sensor or a CMOS (complementary metal oxide semiconductor) image sensor, or a one-dimensional line sensor can be used. An optical reader using a one-dimensional line sensor includes a scanning device that relatively moves the chart and the optical system of the optical reader to acquire a two-dimensional image of the chart. The scanning device may be a transport device that transports the chart to the optical reading device.

<Parameters related to CTF calculation>
The main parameters and preferable conditions related to the CTF calculation method of the present disclosure will be described.

・ Distance between shading stripes of the chart: P (mm) where P> 0.

· Spacing of read pixels on the chart: S (mm) where S> 0.

When calculating contrast of light and dark stripes from the chart reading data with pixels separated in the stripe arrangement direction (interval direction) for each pixel, the distance from the most distant pixel: d (pixel)
Here, since d ≧ 1 and the number of pixels, it is a positive integer.

In the following, P, S, and d may be described with their units omitted for ease of viewing.

Assuming that S> P with respect to the relationship between P and S, a plurality of light and shade stripes are included in one read pixel including a part of the stripes, so S <P must be satisfied.
Therefore,
P / S ≧ 1 Formula [11]
It is.
On the other hand, from the sampling theorem, in order to read periodic fringes, at least 2 × S ≦ P is required. Therefore,
P / S ≧ 2 Formula [12]
It is.

Combining equation [11] and equation [12] results in:
P / S ≧ 2 Formula [13]
It is.

Reading by the optical reader needs to satisfy the equation [13]. However, considering the cost, in practice, the condition of the equation [13] is greatly exceeded, and P / S >> 1. S cannot be reduced.

On the other hand, since P is defined from the spatial frequency for which the resolution is to be measured, it cannot be increased freely.

Therefore, as a result, S which satisfies Equation [13] and can obtain the resolution necessary for the optical reader is selected as much as possible.

<About desirable value of “d” as specific pixel number>
Assuming an ideal state where gray stripes are read at a sufficiently high resolution (S is very small. S << P, that is, P / S >> 1), each pixel of the read image is arranged in the stripe arrangement direction ( (Interval direction), the interval between the central pixel position of the dark stripe that is the darkest read value and the central pixel position of the thin stripe that is the thinnest read value is P / 2 (mm). Expressed as a number
P / (2 × S) (pixel) Equation [14]
It becomes.

From the definition of CTF calculation (formula [1]), the CTF can be obtained by using the darkest read value and the thinnest read value in the pixel range of the number of read pixels.

In the CTF calculation method of the present disclosure, for each pixel of the read image data, attention is paid to “one pixel” (hereinafter referred to as “pixel α”), and this is referred to as “another pixel” (hereinafter referred to as “pixel β”). The contrast is calculated with the above to obtain the contrast of light and shade stripes. “Another pixel” refers to a pixel that is a predetermined number of pixels away (d) from the adjacent pixel in the stripe arrangement direction.

Therefore, the following condition 1 is necessary in this calculation.

Condition 1: It is necessary that a pixel including the center position of the dark stripe and a pixel including the center position of the thin stripe are included between the pixel α and the pixel β. Note that the interval between the pixel including the center position of the dark stripe and the pixel including the center position of the thin stripe is expressed by Equation [14].

Also, in the CTF calculation method of the present disclosure, it is considered that d is made as small as possible in order to reduce the calculation amount. The following condition 2 is added from the viewpoint of reducing the amount of calculation.

Condition 2: When the pixel α is a pixel including the center position of the dark stripe, a pixel including the center position of the thin stripe adjacent to the dark stripe is included in the predetermined pixel number d.

Here, since the distance between the pixel including the center position of the dark stripe and the pixel including the center position of the thin stripe is P / (2 × S) from the equation [14], d = P / (2 × S) However, since d is a positive integer, “the smallest integer d that satisfies d ≧ P / (2 × S)” is an appropriate value (method 1). In the example of FIGS. 1 to 3, since P = 4 and S = 1, d = 2.

In this calculation, the pixel α is described as “when the pixel includes the center position of the dark stripe”, but in the calculation using the pixel when the pixel α is not, the calculation result of the CTF calculation method of the present disclosure is Since it does not become a large value, it is excluded by the processing by the pixel group setting described in FIG.

However, when the pixel α is a pixel including the center position of the thin stripe, the calculation result is the same as the pixel including the center position of the dark stripe except that the sign of the calculation result is different. The processing by the pixel group setting described in FIG. 14 is not affected.

The above describes the case where reading is ideal (when obtaining from the lens design value), but in an actual optical reader, the design aberration of the imaging lens, manufacturing error, or adjustment error, An image height error (generated due to distortion and / or distortion) occurs due to a temperature change in the installation environment of the apparatus, and the interval S between the read pixels is different in each place within the reading range. Considering this, d is obtained using a value where S becomes the smallest due to an image height error. In this case, since S is the smallest, d is the largest (the number of pixels is increased).

Thus, how to obtain the smallest S including the actual error may be actually obtained as a characteristic measurement for each optical reading device and obtained from the formula of “Method 1” (Method 2). ).

Further, it may be obtained from the smallest S that can be considered from the design aberration magnitude, the manufacturing error, or the specification value of the adjustment error (Method 3).

Furthermore, since the image height error of an optical reading apparatus used for inspection as described in the present embodiment is usually relatively small, a simple method for setting d slightly larger is also used. Measurement costs can be reduced with methods that do not rely on actual measurements.

The following method can be considered as a simple method.

Since the position of the read pixel with respect to the gray stripe, that is, the relationship between the stripe and the phase of the pixel is arbitrary, in order to satisfy “condition 1”, the length of the d pixel (d × S (mm))
d × S ≧ P Formula [15]
If so, one or more periods of gray stripes can be included between the pixel α and the pixel β that is d (pixels) away. Here, in the equation [15], even when d × S = P, there is d + 1 (pixel) between the pixel α and the pixel β that is d (pixel) away from the pixel α. be able to.

On the other hand, if d is further increased, stripes away from the pixel α are included, such as the second and third periods of the light and shade stripes, so that the adjacent dark portions and the light and dark stripes are thin. In order to obtain the CTF from the reading value of the portion, it is inappropriate that d is too large. Since Equation [15] can be written as d ≧ P / S, d may be “select the smallest integer among d satisfying d ≧ P / S” from Method [15] (Method 4). ).

In the example of FIGS. 1 to 3, since P = 4 and S = 1, d = 4.

When this “Method 4” is compared with “Method 1”, the local image height error is set not to exceed twice at most. Further, it is set so that the gray stripes at the distant positions are not subject to calculation as lens design values for the pixel α.

From the above, for simplicity, it is desirable to set d between “Method 1” and “Method 4”. That is, the integer d is equal to or larger than the minimum integer satisfying d ≧ P / (2 × S) and equal to or smaller than the minimum integer satisfying d ≧ P / S. Further, in view of the calculation amount, it is preferable to be closer to the smaller “method 1” in this range.

When d of this “Method 4” is used and when there is a chart line defect due to non-ejection in the ink jet printing apparatus, the thin part of the light and dark stripes is read more thinly in the line defect part. As a result, the value obtained using the missing pixel is an abnormal value calculated with a high CTF.

On the other hand, since the CTF of the optical reader does not change rapidly locally, it is desirable to combine an abnormal value exclusion process with the obtained CTF.

<< Example of CTF Measurement Processing Flow in Inkjet Printing Apparatus According to Embodiment >>
FIG. 16 is a flowchart illustrating an overall process flow related to the CTF measurement process in the inkjet printing apparatus according to the embodiment. The ink jet printing apparatus according to the embodiment includes an inline scanner, and a printing process and a reading process after printing are executed as a series of operations.

The CTF measurement method in the inkjet printing apparatus of this example includes a print image data generation step (step S1), a print processing step (step S2), a reading processing step (step S3) executed in parallel with step S2, and a CTF. Calculation processing step (step S4).

<S1: Print image data generation process>
In step S1, the inkjet printing apparatus generates print image data for printing the CTF measurement chart.

FIG. 17 is a flowchart showing the contents of print image data generation processing. The print image data generation process shown in FIG. 17 is executed by the control device of the inkjet printing apparatus.

In step S11, the control device acquires chart size information. The chart size information includes information on the main scanning direction size and the sub scanning direction size of the chart image. The chart size information may be specified in advance by a program or may be input via a user interface. The chart size information may be expressed in units of millimeters or may be expressed in units of pixels based on the recording resolution.

In step S12, the control device acquires chart arrangement coordinate information. The chart arrangement coordinate information includes coordinate information for specifying the position of each of the light and dark stripes that make up the chart image.

The chart arrangement coordinate information includes information defining the position, length, and thickness of each stripe. The chart arrangement coordinate information may be designated in advance by a program, or may be input via a user interface. The coordinate axes of the chart arrangement coordinate information may be expressed in units of millimeters or may be expressed in units of pixels based on the recording resolution.

In step S13, the control device acquires chart shading stripe interval information. The chart shading stripe interval information may be specified in advance by a program, or may be input via a user interface. Further, the chart shading stripe interval may be calculated from the information of the chart arrangement coordinates acquired in step S12.

In step S14, the control device generates print image data for chart printing. Based on the information acquired in steps S11 to S13, the control device generates print image data for chart printing for printing the CTF measurement chart defined in the information.

In step S15, the control device stores the print image data. The control device performs processing for storing the print image data for chart printing generated in step S14 in a data storage unit such as a memory.

When the print image data is saved in step S15, the flowchart of FIG. 17 is terminated and the process returns to the main flow of FIG.

<S2: Printing process>
The ink jet printing apparatus performs printing based on the print image data generated in the print image data generation process in step S1.

FIG. 18 is a flowchart showing the processing contents of the printing processing step (S2).

In step S21, the control device sets printing conditions. The printing conditions include paper type, number of printed sheets, printing mode, and the like.

In step S22, the control device reads the print image data. The control device reads the print image data saved in step S15 in FIG. 17 from the data storage unit.

In step S23, the control device waits for a page sync input. The page sync is a synchronization signal that controls the start timing of printing for one page on the recording medium. For example, the page sync is generated based on an encoder signal output from a rotary encoder that detects a rotation angle of a drum that conveys a recording medium. Alternatively, the page sync may be generated based on a paper leading edge detection signal output from a sensor that detects the leading edge position of the recording medium.

After receiving the page sync, the control device executes printing in step S24. The control device executes printing according to the print image data read in step S22.

In step S25, the control device determines whether or not to complete printing. If printing is to be continued, the process returns to step S21, and the processes in steps S21 to S24 are repeated. If the control device determines in step S25 that printing is to be completed, the flowchart of FIG. 18 is terminated and the process returns to the main flow of FIG. The printing process (S2) is an example of a chart printing process.

<Step S3: Read Processing Step>
The ink jet printing apparatus reads the chart printed in the printing process (step S2) with an inline scanner.

FIG. 19 is a flowchart showing the processing contents of the reading processing step (step S3). In step S31, the control device calculates reading range coordinates to be read using an inline scanner in the recording area of the recording medium. The control device calculates reading range coordinates based on the information of the chart arrangement position coordinates.

In step S32, the control device waits for a page sync input. After receiving the page sync, the control device reads the chart image after printing in step S33. When receiving the read image data from the inline scanner, the control device performs shading correction processing on the read data in step S34.

In step S35, the control device stores the read image data after the shading correction. The read image data is stored in a data storage unit such as an internal memory and / or an external storage device of the control device.

Step S2 and step S3 are parallel processes, and the processes are performed in parallel. Therefore, in step S36, the control device determines whether printing has been completed. If printing continues, the process returns to step S31, and the processes of steps S31 to S36 are repeated.

If the control device determines in step S36 that printing has been completed, the flowchart of FIG. 19 is terminated and the process returns to the main flow of FIG.

Note that a part or all of the processing in the reading processing step may be executed inside the optical reading device. The reading process (step S3) is an example of a chart reading process.

<Step S4: CTF calculation processing step>
The control device performs a CTF calculation process after the reading process step (step S3).

FIG. 20 is a flowchart showing the contents of the CTF calculation process.

In step S41, the control device reads the read image. The control device reads the read image data saved in step S35 of FIG. 19 from the data storage unit. Step S41 is an example of a read image data acquisition process.

In step S42 of FIG. 20, the control device performs contrast calculation processing using the read image data that has been read. The content of the contrast calculation process will be described later.

In step S43, the control device stores the data of the CTF calculation result calculated in the contrast calculation processing step (step S42). The data of the CTF calculation result is stored in a data storage unit such as an internal memory and / or an external storage device of the control device.

In step S44, the control device determines whether or not processing of all images has been completed. If an unprocessed image remains, the control device returns to step S41 and repeats the processing of steps S41 to S44.

When the control device determines in step S44 that the processing of all images has been completed, the flowchart of FIG. 20 is terminated and the process returns to the main flow of FIG.

<Step S42: Contrast Calculation Processing Step>
21 and 22 are flowcharts showing the contents of the contrast calculation process.

In step S51, the control device determines the pixel interval d. The pixel interval d is a pixel interval that is a combination of the most distant pixels when calculating contrast between two pixels distant in the stripe arrangement direction. The pixel interval d is referred to as a comparison pixel maximum interval.

In step S52, the control device sets the row index LI to the initial value LI = 1.

In step S53, the control device sets the column index CI to the initial value CI = 1 + d.

In step S54, the control device sets the calculation target pixel value AP to AP = IM (LI, CI). Here, IM (LI, CI) represents the read data of the pixel position represented by the row index LI and the column index CI.

FIG. 23 is an example of an image of light and shade stripes as a chart image. A data array obtained by reading the vertical stripes shown in FIG. 23 is assumed to be IM (L, C). L is an index representing a position in the row direction, and C is an index representing a position in the column direction. The position of the pixel of the read image is specified by (L, C).

FIG. 24 is a chart showing an example of the data array IM (L, C) of the read image. FIG. 24 shows an example of a data array including pixels of 1 to C columns in the column direction and 1 to L rows of pixels in the row direction.

In step S55 of FIG. 21, the control device adds OP = IM (LI, CL-d) to IM (LI, CL-1) and OP = IM (LI, CI + 1) to IM ( LI, CL + d), substituting the values of these 2 × d pixels, respectively,
(OP-AP) / (OP + AP)
And the maximum value among the calculation results of each pixel (LI, CI) is stored in CTFT (LI, CI). Step S55 is an example of a contrast calculation process between pixels.

FIG. 25 is an example of a CTF midway calculation array CTFT (L, C) that stores the CTF midway calculation result shown in step S55.

In step S56 of FIG. 21, the control device determines whether or not the column index CI satisfies CI = Cd.

If it is determined in step S56 that CI <C−d, the process proceeds to step S57, the column index CI is incremented by “+1”, the process returns to step S54, and the processes of steps S54 to S56 are repeated.

If it is determined in step S56 that CI = C−d is satisfied, the process proceeds to step S58 to determine whether or not the row index satisfies LI = L.

If it is determined in step S58 that LI <L, the process proceeds to step S59, the row index LI is incremented by "+1", the process returns to step S53, and the processes in steps S53 to S58 are repeated.

If it is determined in step S58 that LI = L is satisfied, the process proceeds to step S61 in FIG.

In step S61, the control device sets the index BCI of the column of the CTF calculation array to the initial value BCI = 1.

FIG. 26 is a chart showing an example of the CTF calculation array CTF (BC).

The CTF calculation array CTF (BC) is a data array including pixels of 1 to BC columns in the column direction. BC represents a value obtained by discarding the decimal part of (L-2 × d) / d. BCI is an index indicating a pixel position in the range of 1 to BC.

In step S62 in FIG. 22, the control device excludes an abnormal value from the CTFT value in the rectangular range of CTFT (1, BCI × d + 1) to CTFT (L, BCI × d + d) and has a value near 0. After excluding (for example, 0.05 or less), an average value of three values from the larger value is obtained and held in CTF (BC). Step S62 is an example of a resolving power calculation step.

FIG. 27 shows an example of a “rectangular range” in the CTF midway calculation array. The gray areas shown in FIG. 27 and the filled areas (1, 3) to (L, 4) are examples of “rectangular areas” when d = 2 and BCI = 1. Here, a rectangular range of two columns is shown, but the rectangular range can be the range of the strip matrix described with reference to FIG.

In step S63 in FIG. 22, the control device determines whether or not BCI = BC is satisfied. When it is determined in step S63 that BCI <BC, the control device proceeds to step S64. In step S64, the control device increments the column index BCI by "+1", returns to step S62, and repeats the processing of steps S62 to S63.

If it is determined in step S63 that BCI = BC is satisfied, the CTF calculation result is obtained, and the flowchart of FIG. 22 is terminated and the process returns to the flow of FIG.

The CTF calculation result obtained by the flowchart of FIG. 22 is stored in step S43 of FIG.

<Data processor configuration>
FIG. 28 is a block diagram illustrating functions of a data processing apparatus that performs CTF calculation according to the resolution measurement method according to the embodiment. The function of the data processing apparatus 110 shown in FIG. 28 can be incorporated in the control device of the ink jet printing apparatus. Each function of the data processing device 110 can be realized by computer hardware and software. The resolution evaluation method according to the present embodiment can be understood as a calculation method for measuring the CTF of the optical reader. In the resolution evaluation method according to the embodiment, the chart 102 is used to measure CTF, the optical reading device 100 that reads the chart 102, and the data processing device 110 that performs arithmetic processing on the read image data read by the optical reading device 100. And are used.

The optical reader 100 optically reads the black and white shading chart 102 and generates image data of the read image. In other words, the optical reading device 100 reads at least a portion of the chart 102 where the gray stripes are displayed, and generates the read gray stripe image information. The gray stripe image information read by the optical reading device 100 is digitized and input to the data processing device 110. “To be digitized” means that the image information is converted into digital image data. The data processing device 110 acquires image data of a read image of the chart 102 read by the optical reading device 100 and performs data processing.

The data processing device 110 includes a data acquisition unit 112, a data storage unit 114, a contrast calculation processing unit 116, and a data output unit 118. Further, an input device 120 and a display device 122 are connected to the data processing device 110.

The data acquisition unit 112 is an interface that acquires image data of a read image of the chart 102 read via the optical reading device 100. The data acquisition unit 112 may be configured by any one or a combination of a data input terminal, a communication interface, and a media interface. The data acquisition unit 112 is an example of a read image data acquisition unit.

The data storage unit 114 includes a storage device such as a memory and / or a hard disk. The data storage unit 114 includes a read image storage unit 132, an inter-pixel CTF storage unit 134, and a CTF calculation result storage unit 136. The read image storage unit 132 stores the data array IM (L, C) of the read image described with reference to FIG. The inter-pixel CTF storage unit 134 stores the CTF intermediate calculation array CTFT (L, C) described in FIG. The CTF calculation result storage unit 136 stores the CTF calculation array CTF (BC) described in FIG. Each of the read image storage unit 132, the inter-pixel CTF storage unit 134, and the CTF calculation result storage unit 136 may be a storage area in one storage device, or may be a different storage device. .

The contrast calculation processing unit 116 executes the contrast calculation processing described with reference to FIG. The contrast calculation processing unit 116 includes a pixel interval determination unit 142, an inter-pixel CTF calculation unit 144, and a CTF calculation unit 146.

The pixel interval determination unit 142 determines the pixel interval d with the pixel farthest from the pixels to be compared when obtaining the inter-pixel CTF. The pixel interval determination unit 142 determines an appropriate d based on the stripe interval of the chart. d may be defined in advance by a program, or may be determined based on information appropriately designated by the user through the input device 120.

The inter-pixel CTF calculation unit 144 executes the process of step S55 in FIG. The inter-pixel CTF calculation unit 144 is an example of an inter-pixel contrast calculation unit. The calculation result of the inter-pixel CTF calculation unit 144 is stored in the inter-pixel CTF storage unit 134.

The CTF calculation unit 146 executes the process of step S62 in FIG. The CTF calculator 146 is an example of a resolving power calculator. The calculation result of the CTF calculation unit 146 is stored in the CTF calculation result storage unit 136.

The data output unit 118 is an interface that outputs each data stored in the data storage unit 114 to a processing unit inside the data processing device 110 or to the outside of the device.

The input device 120 can employ various input devices such as a keyboard, a mouse, a touch panel, and a trackball, and may be an appropriate combination thereof. As the display device 122, various display devices such as a liquid crystal display can be employed. The input device 120 and the display device 122 function as a user interface. The user can set various parameters and input and edit various information using the input device 120 while confirming the content displayed on the screen of the display device 122.

In addition, the display device 122 functions as a measurement result information providing unit that provides the CTF measurement result to the user. For example, the display device 122 displays CTF measurement result information and / or evaluation result information. The data processing device 110 is an example of a resolution measuring device.

The data processing device 110 has a function of inputting information on the stripe interval of light and shade stripes from the outside, or a function of detecting a predetermined interval from the read image. The data processing device 110 converts the acquired value of the stripe interval into the number of pixels in the read image of light and shade stripes. The fringe interval expressed by the number of pixels in the read image is called the fringe interval pixel number.

When the data processing device 110 calculates the contrast of light and shade stripes with respect to each pixel from the read image data in the stripe arrangement direction, the pixel that is the furthest away pixel in the stripe arrangement direction Specifies the inter-distance (maximum comparison pixel interval). The comparison pixel maximum interval is represented by the number of pixels, and is set to a value that is 1 or more and less than or equal to the stripe interval pixel number. The comparison pixel maximum interval is a specific number of pixels, and this is d [pixel].

In accordance with the calculation formula (formula [2]) described above, the data processing apparatus 110 is separated in the stripe interval direction by the number m of pixels not less than 1 and not more than d with respect to at least some pixels _Xn of the read image. to calculate the inter-pixel CTF between the pixel, the contrast value of each pixel X _n from the maximum value of those (CTF (X _n)) and was determined and calculated for each pixel X _n CTF (X _n) The CTF at each fringe position is calculated from the maximum value. The data processing device 110 is an example of a resolution measuring device. Note that “1 or more and d or less” means one or more and a specific number of pixels or less.

《Examples of using CTF measurement results》
The CTF is calculated for the pixels in the entire reading range of the optical reading device 100 by the above-described method, and based on the calculation result, it is determined whether the CTF is within a predetermined range defined by the optical reading device. It is possible to determine whether or not the focus state is normal. The data processing apparatus 110 performs a process of determining the focus state of the optical reading apparatus 100 from the CTF calculation result, and causes the display apparatus 122 to display the determination result.

<< Device configuration example of inkjet printing apparatus >>
FIG. 29 is a side view illustrating the configuration of the inkjet printing apparatus 201 according to the embodiment. The ink jet printing apparatus 201 is an example of a printing apparatus. The ink jet printing apparatus 201 includes a paper feed unit 210, a treatment liquid application unit 220, a treatment liquid drying unit 230, a drawing unit 240, an ink drying unit 250, and an accumulation unit 260.

The paper feeding unit 210 automatically feeds the paper P one by one. The paper feeding unit 210 includes a paper feeding device 212, a feeder board 214, and a paper feeding drum 216. The type of the paper P is not particularly limited. For example, printing paper mainly composed of cellulose, such as high-quality paper, coated paper, and art paper, can be used. The paper P corresponds to one form of a medium on which an image is recorded. The paper P is placed on the paper feed table 212A in a bundled state in which a large number of sheets are stacked.

The sheet feeding device 212 takes out the bundled sheets P set on the sheet feeding table 212A one by one from the top and feeds them to the feeder board 214. The feeder board 214 conveys the paper P received from the paper feeding device 212 to the paper feeding drum 216.

The paper supply drum 216 receives the paper P fed from the feeder board 214 and conveys the received paper P to the processing liquid application unit 220.

The processing liquid application unit 220 applies the processing liquid to the paper P. The treatment liquid is a liquid having a function of aggregating, insolubilizing or increasing the viscosity of the color material component in the ink. The treatment liquid application unit 220 includes a treatment liquid application drum 222 and a treatment liquid application device 224.

The processing liquid application drum 222 receives the paper P from the paper supply drum 216 and transfers the received paper P to the processing liquid drying unit 230. The treatment liquid coating drum 222 includes a gripper 223 on the peripheral surface, and the gripper 223 grips and rotates the leading end portion of the paper P, so that the paper P is wound around the peripheral surface and conveyed.

The processing liquid coating device 224 applies the processing liquid to the paper P conveyed by the processing liquid coating drum 222. The treatment liquid is applied with a roller.

The processing liquid drying unit 230 performs a drying process on the paper P coated with the processing liquid. The processing liquid drying unit 230 includes a processing liquid drying drum 232 and a hot air blower 234. The processing liquid drying drum 232 receives the paper P from the processing liquid coating drum 222 and transfers the received paper P to the drawing unit 240. The treatment liquid drying drum 232 includes a gripper 233 on the peripheral surface. The treatment liquid drying drum 232 conveys the paper P by gripping and rotating the leading end of the paper P with the gripper 233.

The hot air blower 234 is installed inside the processing liquid drying drum 232. The hot air blower 234 blows hot air onto the paper P conveyed by the processing liquid drying drum 232 to dry the processing liquid.

The drawing unit 240 includes a drawing drum 242, a head unit 244, and an inline scanner 248. The drawing drum 242 receives the paper P from the treatment liquid drying drum 232 and transfers the received paper P to the ink drying unit 250. The drawing drum 242 includes a gripper 243 on the circumferential surface, and grips and rotates the leading edge of the paper P with the gripper 243, whereby the paper P is wound around the circumferential surface and conveyed. The drawing drum 242 includes a suction mechanism (not shown), and transports the paper P wound around the peripheral surface while attracting the peripheral surface to the peripheral surface. A negative pressure is used for the adsorption. The drawing drum 242 has a large number of suction holes on the peripheral surface, and sucks the paper P onto the peripheral surface by suction from the inside through the suction holes.

The head unit 244 includes inkjet heads 246C, 246M, 246Y, and 246K. The inkjet head 246C is a recording head that discharges cyan (C) ink droplets. The ink-jet head 246M is a recording head that ejects magenta (M) ink droplets. The inkjet head 246Y is a recording head that discharges yellow (Y) ink droplets. The ink jet head 246K is a recording head that ejects black (K) ink droplets. Ink is supplied to each of the inkjet heads 246C, 246M, 246Y, and 246K from an ink tank (not shown) that is an ink supply source of a corresponding color via a pipe path (not shown).

Each of the inkjet heads 246 </ b> C, 246 </ b> M, 246 </ b> Y, and 246 </ b> K is configured by a line head corresponding to the paper width, and each nozzle surface is disposed to face the peripheral surface of the drawing drum 242. The paper width here refers to the paper width in a direction orthogonal to the conveyance direction of the paper P. The inkjet heads 246C, 246M, 246Y, and 246K are arranged at a constant interval along the conveyance path of the paper P by the drawing drum 242.

Although not shown in the drawing, a plurality of nozzles serving as ink ejection openings are two-dimensionally arranged on the nozzle surfaces of the inkjet heads 246C, 246M, 246Y, and 246K. “Nozzle surface” refers to an ejection surface on which nozzles are formed, and is synonymous with terms such as “ink ejection surface” or “nozzle formation surface”. A nozzle arrangement of a plurality of nozzles arranged two-dimensionally is called a “two-dimensional nozzle arrangement”.

Each of the inkjet heads 246C, 246M, 246Y, and 246K can be configured by connecting a plurality of head modules in the paper width direction. Each of the inkjet heads 246C, 246M, 246Y, and 246K has a nozzle row that can record an image with a specified recording resolution in one scan of the entire recording area of the paper P in the paper width direction orthogonal to the transport direction of the paper P. A full line type recording head. A full-line type recording head is also called a page wide head. The specified recording resolution may be a recording resolution determined in advance by the ink jet printing apparatus 201, or may be a recording resolution set by user selection or by automatic selection by a program corresponding to the printing mode. Also good. The recording resolution can be set to 1200 dpi, for example. The paper width direction perpendicular to the paper P transport direction may be referred to as the nozzle row direction of the line head, and the paper P transport direction may be referred to as the nozzle row vertical direction.

In the case of an inkjet head having a two-dimensional nozzle array, the projection nozzle array in which the nozzles in the two-dimensional nozzle array are projected (orthographically projected) along the nozzle array direction achieves the maximum recording resolution in the nozzle array direction. It can be considered that the nozzle density is equivalent to a single nozzle row in which each nozzle is arranged at approximately equal intervals. The “substantially equidistant” means that the droplet ejection points that can be recorded by the ink jet printing apparatus are substantially equidistant. For example, the concept of “equally spaced” also includes cases where the intervals are slightly different in consideration of manufacturing errors and movement of droplets on the medium due to landing interference. In consideration of the projection nozzle row (also referred to as “substantial nozzle row”), a nozzle number representing a nozzle position can be associated with each nozzle in the arrangement order of the projection nozzles arranged along the nozzle row direction.

The nozzle arrangement form in each of the inkjet heads 246C, 246M, 246Y, and 246K is not limited, and various nozzle arrangement forms can be adopted. For example, instead of a matrix-like two-dimensional array, a linear array of lines, a V-shaped nozzle array, a polygonal nozzle array such as a W-shape with a V-shaped array as a repeating unit, and the like are also possible. It is.

Ink droplets are ejected from the inkjet heads 246C, 246M, 246Y, and 246K toward the paper P conveyed by the drawing drum 242, and the ejected liquid droplets adhere to the paper P, whereby an image is formed on the paper P. To be recorded.

The drawing drum 242 functions as a means for relatively moving the inkjet heads 246C, 246M, 246Y, 246K and the paper P. The drawing drum 242 moves the paper P relative to the inkjet heads 246C, 246M, 246Y, and 246K, and corresponds to one form of relative moving means. The ejection timings of the ink jet heads 246C, 246M, 246Y, and 246K are synchronized with a rotary encoder signal obtained from a rotary encoder installed on the drawing drum 242. 29, the rotary encoder is not shown, and is illustrated as a rotary encoder 382 in FIG. The ejection timing is the timing at which ink droplets are ejected, and is synonymous with the droplet ejection timing.

In this example, the configuration of CMYK standard colors (four colors) is illustrated. However, the combination of ink colors and the number of colors is not limited to this embodiment, and light ink, dark ink, and special color ink are used as necessary. Etc. may be added. For example, it is possible to add an inkjet head that ejects light-colored inks such as light cyan and light magenta, and an inkjet head that ejects special color inks such as green and orange. The arrangement order of the heads is not particularly limited.

The inkjet heads 246C, 246M, 246Y, and 246K are examples of image forming units.

The in-line scanner 248 is an optical reading device that optically reads an image recorded on the paper P by the ink jet heads 246C, 246M, 246Y, and 246K and generates electronic image data indicating the read image. The inline scanner 248 includes an imaging device that captures an image recorded on the paper P and converts it into an electrical signal indicating image information. The inline scanner 248 may include an imaging optical device, an illumination optical system that illuminates a reading target, and a signal processing circuit that processes a signal obtained from the imaging device and generates digital image data.

The inline scanner 248 is composed of, for example, a line scanner using a CCD line sensor. The inline scanner 248 corresponds to the optical reading device 100 shown in FIG. The inline scanner 248 is preferably configured to be able to read a color image. In the inline scanner 248 of this example, for example, a color CCD linear image sensor is used as an imaging device. The color CCD linear image sensor is an image sensor in which light receiving elements having color filters of R (red), G (green), and B (blue) colors are arranged linearly. Instead of the color CCD linear image sensor, a color CMOS linear image sensor can be used. The inline scanner 248 reads an image on the paper P while the paper P is being conveyed by the drawing drum 242. The drawing drum 242 serves as a scanning device that relatively moves the inline scanner 248 and the chart.

CTF measurement is performed based on the data of the read image read by the inline scanner 248. Further, based on the data of the read image read by the in-line scanner 248, information such as image density and ejection defects of the inkjet heads 246C, 246M, 246Y, and 246K can be obtained.

The ink drying unit 250 performs a drying process on the paper P on which the image is recorded by the drawing unit 240. The ink drying unit 250 includes a chain delivery 310, a paper guide 320, and a hot air blowing unit 330.

The chain delivery 310 receives the paper P from the drawing drum 242 and transfers the received paper P to the stacking unit 260. The chain delivery 310 includes a pair of endless chains 312 that travel along a prescribed travel route, and grips the leading end of the paper P with a gripper 314 provided on the pair of chains 312 to convey the paper P in a prescribed manner. Transport along the route. A plurality of grippers 314 are provided in the chain 312 at regular intervals.

The paper guide 320 is a member that guides the conveyance of the paper P by the chain delivery 310. The paper guide 320 includes a first paper guide 322 and a second paper guide 324. The first paper guide 322 guides the paper P that is transported in the first transport section of the chain delivery 310. The second sheet guide 324 guides the sheet conveyed in the second conveyance section subsequent to the first conveyance section. The hot air blowing unit 330 blows hot air on the paper P conveyed by the chain delivery 310.

The stacking unit 260 includes a stacking device 262 that receives and stacks the paper P conveyed from the ink drying unit 250 by the chain delivery 310.

The chain delivery 310 releases the paper P at a predetermined stacking position. The stacking device 262 includes a stacking tray 262A, receives the paper P released from the chain delivery 310, and stacks the sheets P on the stacking tray 262A. The stacking unit 260 corresponds to a paper discharge unit.

<System configuration>
FIG. 30 is a block diagram illustrating a main configuration of a control system of the inkjet printing apparatus 201. The ink jet printing apparatus 201 is controlled by the control apparatus 202. The control device 202 includes a system controller 350, a communication unit 352, a display device 122, an input device 120, an image processing unit 358, a CTF measurement device 360, a conveyance control unit 362, an image recording control unit 364, Is provided. These elements of each part can be realized by one or a plurality of computers. That is, the control device 202 can be configured by a combination of computer hardware and software.

The system controller 350 functions as a control unit that performs overall control of each unit of the ink jet printing apparatus 201 and also functions as a calculation unit that performs various calculation processes. The system controller 350 includes a CPU (Central Processing Unit) 370, a ROM (read-only memory) 372, and a RAM (random access memory) 374, and operates according to a predetermined program. The ROM 372 stores programs executed by the system controller 350 and various data necessary for control.

The communication unit 352 includes a required communication interface. The inkjet printing apparatus 201 is connected to a host computer (not shown) via the communication unit 352, and can send and receive data to and from the host computer. The “connection” here includes a wired connection, a wireless connection, or a combination thereof. The communication unit 352 may be equipped with a buffer memory for speeding up communication.

The communication unit 352 serves as an image input interface unit for acquiring image data representing an image to be printed.

The image processing unit 358 performs various conversion processes, correction processes, and halftone processes on image data to be printed. The conversion process includes pixel number conversion, gradation conversion, color conversion, and the like. The correction processing includes density correction, non-ejection correction for suppressing the visibility of image defects due to non-ejection nozzles, and the like. The image processing unit 358 performs correction processing based on the read image obtained from the inline scanner 248. The halftone process is a digital halftoning process represented by a dither method or an error diffusion method. Further, the image processing unit 358 generates print image data for printing a chart used for CTF measurement.

The CTF measuring device 360 is equivalent to the device configuration of the data processing device 110 described in FIG. The function of the CTF measurement device 360 may be configured by a computer different from the control device including the system controller 350, or may be included as a functional block in the control device including the system controller 350. Good.

The transport control unit 362 controls the medium transport mechanism 380. The medium transport mechanism 380 includes the entire mechanism of the paper transport unit related to transport of the paper P from the paper feed unit 210 to the stacking unit 260 described in FIG. The medium transport mechanism 380 includes the paper feed drum 216, the processing liquid coating drum 222, the processing liquid drying drum 232, the drawing drum 242, the chain delivery 310, and the like described with reference to FIG. The medium transport mechanism 380 includes a drive unit such as a motor and a motor drive circuit (not shown) as a power source.

The conveyance control unit 362 controls the medium conveyance mechanism 380 in accordance with a command from the system controller 350 so as to convey the paper P from the paper feeding unit 210 to the stacking unit 260.

The inkjet printing apparatus 201 includes a rotary encoder 382 as means for detecting the rotation angle of the drawing drum 242 (see FIG. 29) in the medium transport mechanism 380. The ink jet heads 246C, 246M, 246Y, and 246K each have a discharge timing controlled according to a discharge timing signal generated from a rotary encoder signal output from the rotary encoder 382.

The image recording control unit 364 controls each drive of the ink jet heads 246C, 246M, 246Y, and 246K in accordance with a command from the system controller 350. The image recording control unit 364 records the predetermined image on the paper P conveyed by the drawing drum 242 based on the dot data of each ink color generated through the halftone process of the image processing unit 358. Each discharge operation of 246C, 246M, 246Y, and 246K is controlled. The image recording control unit 364 performs control for printing a chart used for CTF measurement.

The control device 202 includes a storage device such as a hard disk drive (not shown). The storage device can store a program executed by the CPU 370 and various types of data necessary for calculation. The storage device may be built in the control device 202 or may be configured to be connected to the control device 202 via a communication line.

<Hardware configuration of each processing unit and control unit>
The contrast calculation processing unit 116, the pixel interval determination unit 142, the inter-pixel CTF calculation unit 144, and the CTF calculation unit 146 of the data processing device 110 described in FIG. 28, and the transport control unit 362 of the control device 202 illustrated in FIG. The hardware structure of a processing unit (processing unit) that executes various processes such as the image recording control unit 364, the image processing unit 358, and the CTF measuring apparatus 360 includes, for example, various processors (processors) as shown below. ).

Various processors are processors that can change their circuit configuration after manufacturing, such as a CPU (Central Processing Unit) or FPGA (Field Programmable Gate Array) that is a general-purpose processor that functions as various processing units by executing programs. Examples include a dedicated electric circuit that is a processor having a circuit configuration specifically designed to execute a specific process such as a programmable logic device (PLD) and an ASIC (Application Specific Integrated Circuit).

One processing unit may be configured by one of these various processors, or may be configured by two or more processors of the same type or different types. For example, one processing unit may be configured by a plurality of FPGAs or a combination of a CPU and an FPGA. Further, the plurality of processing units may be configured by one processor. As an example of configuring a plurality of processing units with one processor, first, as represented by a computer such as a client or a server, one processor is configured with a combination of one or more CPUs and software. There is a form in which the processor functions as a plurality of processing units. Second, as represented by a system-on-chip (SoC), a form of using a processor that implements the functions of the entire system including a plurality of processing units with a single IC (integrated circuit) chip. is there. As described above, various processing units are configured using one or more of the various processors as a hardware structure.

Further, the hardware structure of these various processors is more specifically an electric circuit (circuitry) in which circuit elements such as semiconductor elements are combined.

<Discharge method of inkjet head>
An ejector of an inkjet head includes a nozzle that discharges liquid, a pressure chamber that communicates with the nozzle, and a discharge energy generating element that applies discharge energy to the liquid in the pressure chamber. With respect to the ejection method for ejecting liquid droplets from the nozzle of the ejector, the means for generating the ejection energy is not limited to the piezoelectric element, and various ejection energy generating elements such as a heating element and an electrostatic actuator can be applied. For example, it is possible to employ a method in which droplets are ejected using the pressure of film boiling caused by heating of a liquid by a heating element. Corresponding ejection energy generating elements are provided in the flow path structure according to the ejection method of the inkjet head.

<< Advantages of the CTF measurement method according to this embodiment >>
(1) According to the present embodiment, an accurate CTF can be obtained by a simple algorithm that combines contrast calculation of each pixel and maximum value search.

(2) In the conventional method, if the chart line is missing due to nozzle discharge or the like, the line search algorithm becomes complicated. In this respect, according to the present embodiment, if a line is missing due to non-ejection or the like, the image data of the read image is white, so CTF = 0, which can be calculated without special distinction. That is, according to the present embodiment, it is possible to perform processing that copes with line defects by a simple algorithm. Further, according to the present embodiment, it is not necessary to search for the maximum density of lines, the minimum density position between lines, and the like.

(3) Further, according to the present embodiment, the CTF is obtained even when the line width or drawing resolution of the chart is close to the resolution of the read pixel and the read data changes periodically depending on the phase of the read position. be able to.

<Other effects>
In a CTF measurement chart printed using an actual inkjet printing apparatus, the density and / or width of the CTF measurement chart line slightly changes due to variations in the ink discharge amount of the inkjet head that records each line. If the ink discharge amount is small, the darkness becomes light and the width becomes narrow.

When the CTF measurement chart printed using this actual inkjet printing apparatus is read and the CTF value is calculated using the CTF measurement method of the present disclosure, the above-described dispersion effect of the ink discharge amount affects the CTF calculation result. That is, if the ink discharge amount is small, the CTF calculation result is small.

FIG. 31 is a graph showing an example of the CTF calculated from the read image data of the chart printed using the ink jet printing apparatus 201. The horizontal axis represents the pixel number of the reading sensor (imaging sensor) of the optical reader, and the vertical axis represents the CTF value. In the graph shown in FIG. 31, there is a portion where CTF values appear discontinuous in the vicinity of pixel number 4000. The discontinuous portion is a boundary portion between two head modules in a line-type inkjet head configured by connecting a plurality of head modules. There is a difference in the ink discharge amount between the head modules, and the difference appears in the CTF calculation result at the boundary between the two head modules.

The CTF of the optical reading device is high or low within the reading range, but it does not change rapidly locally unless there is a lens striae (defect due to nonuniformity of glass components). Therefore, the local non-uniformity of the ink discharge amount of each head module can be captured from the CTF calculation result as shown in FIG. In addition, about the high peak-like numerical value seen in the place of the graph of FIG. 31, the process which excludes as an abnormal value and / or smoothing processes, such as a moving average, is processed into the data according to the purpose of use suitably May be.

The control device 202 functioning as the data processing device 110 includes a detection processing unit that performs processing for detecting local non-uniformity in the ink ejection amount of the head module in the line-type inkjet head using the CTF calculation result. Also good. The process for detecting the local non-uniformity of the ink discharge amount includes at least one of the detection process of the variation in the ink discharge amount and the non-discharge detection.

<< Modification 1 >>
The chart is not limited to black and white stripes, as long as at least a part of the image area to be read includes light and shade stripes. Depending on the application, the gray stripes can be applied as white red stripes, white blue stripes, white green stripes, white cyan stripes, white magenta stripes, or white yellow stripes. A chart may be printed for each ink color used in the printing apparatus.

In addition, even if a dedicated CTF measurement chart prepared in advance is used, the CTF can be calculated by applying the CTF measurement method of the present disclosure.

<< Modification 2 >>
One chart may include a plurality of gray stripes having different stripe intervals. That is, the first gray stripe with the stripe interval P1 and the second gray stripe with the stripe interval P2 may be included in one chart. Moreover, the direction of the light and shade stripes displayed on the chart is not limited to the vertical stripes, and may be horizontal stripes, or both vertical stripes and horizontal stripes. For example, the first shading stripe may be a vertical stripe, and the second shading stripe may be a horizontal stripe. The CTF can be calculated by the CTF measurement method of the present disclosure for each of the first and the second light and shade stripes. The line direction of the vertical stripe is an example of the first direction, and the line direction of the horizontal stripe is an example of the second direction.

<< Modification 3 >>
In the above-described embodiment, the single-pass inkjet printing apparatus has been described as an example of the printing apparatus. However, the present invention can be applied to various forms of printing apparatuses. For example, the present invention can be applied to a multi-scan ink jet printing apparatus that forms an image by reciprocating a short ink jet head. The means for forming an image on a recording medium in the practice of the present invention is not limited to an ink jet printing apparatus, but may be an electrophotographic apparatus or a plate type printing apparatus such as an offset printing apparatus.

<< Modification 4 >>
The configuration of the data processing apparatus 110 illustrated in FIG. 28 can be grasped as a resolution measuring apparatus that performs a calculation process of CTF measurement separately from the printing apparatus. In addition, the configuration of the control device 202 illustrated in FIG. 30 can be understood as one form of the resolution measuring device.

<About a program that allows a computer to function as a resolution measuring device>
As a resolution measuring apparatus that implements the CTF calculation method described in the above embodiment, a program for causing a computer to function is recorded on an optical disk, a magnetic disk, or other computer-readable medium (a non-transitory information storage medium that is a tangible object), It is possible to provide a program through this information storage medium.

Further, instead of a mode in which a program is stored in an information storage medium and provided, the program data can be provided as a download service using a communication network such as the Internet.

By incorporating this program into a computer, the function of the resolution measuring device can be realized by the computer. Also, a mode in which a part or all of a program for realizing control of the printing apparatus including the CTF calculation processing function described in the present embodiment is incorporated in a host control apparatus such as a host computer, or operation of the CPU of the printing apparatus It is also possible to apply as a program.

<About recording media>
“Paper” is a medium used for image recording, and is included in the concept of a recording medium. The term “recording medium” is a collective term for a variety of terms such as recording paper, printing paper, printing medium, printing medium, printing medium, image forming medium, image forming medium, image receiving medium, and ejection medium. The material, shape, and the like of the recording medium are not particularly limited, and various sheet bodies can be used regardless of the sealing paper, resin sheet, film, cloth, nonwoven fabric, and other materials and shapes. The recording medium is not limited to a single sheet medium, and may be a continuous medium such as continuous paper. Further, the sheet recording medium is not limited to a cut sheet that is preliminarily adjusted to a predetermined size, and may be obtained by cutting a continuous medium to a predetermined size at any time.

《Terminology》
The term “printing apparatus” is synonymous with terms such as a printing press, a printer, a printing apparatus, a printing apparatus, an image forming apparatus, an image recording apparatus, an image output apparatus, or a drawing apparatus.

“Image” is to be interpreted in a broad sense and includes color images, black and white images, single color images, gradation images, uniform density (solid) images, and the like. The “image” is not limited to a photographic image, but is used as a comprehensive term including a pattern, a character, a symbol, a line drawing, a mosaic pattern, a color painting pattern, other various patterns, or an appropriate combination thereof.

“Image recording” includes the concept of terms such as image formation, printing, printing, drawing, and printing. “Printing” includes the concept of digital printing based on digital data.

In the present specification, the term “orthogonal” or “perpendicular” refers to a case of intersecting at an angle of substantially 90 ° in an aspect of intersecting at an angle of less than 90 ° or greater than 90 °. The mode which produces the same operation effect as is included. In addition, the term “parallel” includes a permissible range that can be treated as substantially parallel.

《About image height》
The image height is a value representing the image position in terms of the distance from the optical axis on the evaluation surface of the optical system (usually, the in-focus position on the imaging surface). The image height includes an ideal image height and a real image height. The imaginary height is an ideal image height that can be obtained by paraxial magnification. On the other hand, the image height of a normal optical system deviates from the ideal image height due to lens aberration. The real image height is a position where an image is actually formed on the evaluation surface, and is an image height including aberration. The paraxial magnification is defined as follows.

Paraxial magnification γ: Lens magnification obtained by paraxial ray tracing.

γ = y ′ / y
y ': image height y: object height
<Distortion and distortion>
Distortion and distortion are optical aberrations, and are geometric distortion (position error) of an image. Distortion and distortion are caused by the fact that the magnification of an image changes depending on the location, resulting in an image height error. Simply, there are a pincushion type (positive distortion aberration) and a barrel type (negative distortion aberration).

<< Combination of Embodiments and Modifications >>
The matters described in the above-described embodiments and the modifications can be used in appropriate combinations, and some of the matters can be replaced.

In the embodiment of the present invention described above, the configuration requirements can be changed, added, or deleted as appropriate without departing from the spirit of the present invention. The present invention is not limited to the embodiments described above, and many modifications can be made by those having ordinary knowledge in the related fields within the technical idea of the present invention.

DESCRIPTION OF SYMBOLS 10 Chart 12 Line 14 White part 20 Read image 22 Square 24 Black line part position center 26 Black line part position periphery 30 Chart image 31 Black-and-white stripe 32 Black-and-white stripe 33 Black-and-white stripe 100 Optical reader 102 Chart 110 Data processor 112 Data acquisition part 114 Data storage unit 116 Contrast calculation processing unit 118 Data output unit 120 Input device 122 Display device 132 Read image storage unit 134 Inter-pixel CTF storage unit 136 CTF calculation result storage unit 142 Pixel interval determination unit 144 Inter-pixel CTF calculation unit 146 CTF calculation Unit 201 Inkjet printing device 202 Control device 210 Paper feed unit 212 Paper feed device 212A Paper feed stand 214 Feeder board 216 Paper feed drum 220 Treatment liquid application unit 222 Treatment liquid application drums 223, 233, 243 Gripper 224 Treatment liquid application device 230 Liquid drying unit 232 Treatment liquid drying drum 234 Hot air blower 240 Drawing unit 242 Drawing drum 244 Head unit 246C, 246M, 246Y, 246K Inkjet head 248 Inline scanner 250 Ink drying unit 260 Stacking unit 262 Stacking device 262A Stacking tray 310 Chain delivery 312 Chain 314 Gripper 320 Paper guide 322 First paper guide 324 Second paper guide 330 Hot air blowing unit 350 System controller 352 Communication unit 358 Image processing unit 360 CTF measuring device 362 Transport control unit 364 Image recording control unit 380 Medium transport mechanism 382 Rotary Encoder P Paper Ww White width,
Wb Black width Pb Interval Pw Interval r Specified distance S1 to S4 Steps S11 to S15 of CTF measurement processing Steps S21 to S25 of print image data generation processing Steps S31 to S36 of printing processing Steps S41 to S44 of reading processing Steps of CTF calculation processing S51 to S64 Contrast calculation processing steps

Claims

At least in part, a read image data acquisition step of acquiring image data of a read image obtained by reading a chart including a light and dark stripe whose density cross section changes into a rectangular shape at a specific interval;
For each pixel belonging to at least a part of the calculation target range of the image data of the read image, between pixels between the pixels separated by a number of pixels not less than 1 and not more than a specific number of pixels in the interval direction of the light and shade stripes A pixel-to-pixel contrast calculating step for calculating contrast;
A resolution calculation step of calculating a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel by the inter-pixel contrast calculation step,
The specific pixel number is defined as a pixel number that is equal to or less than a stripe interval pixel number that is the number of pixels corresponding to the specific interval of the light and shade stripes in the image data of the read image, and the specific pixel number is d, the pixel number indicating the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range of the image data of the read image is n, and the value of the pixel Xn represented by the pixel number n is X (n), the value of a pixel X n + m apart m pixels in the spacing direction of the streaks from the pixel X n and X (n + m), m is 1 ≦ m ≦ d, or a -d ≦ m ≦ -1 When taking an integer value that satisfies
The inter-pixel contrast calculation step has the following formula:
CTF (X n , X n + m ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
According to the process of calculating the contrast between the pixels,
Processing for obtaining a maximum value among CTFs (X n , X n + m ) obtained for different m with respect to the pixel X n ;
Resolving power measuring method.
In the inter-pixel contrast calculating step, CTF (X n , X n + m ) is calculated for all m satisfying 1 ≦ m ≦ d or −d ≦ m ≦ −1 for the pixel X n ,
And,
The resolving power measurement method according to claim 1, wherein a process of setting a maximum value of CTF (X n , X n + m ) among all the m values as a pixel contrast value in the pixel X n is performed.
The resolving power calculation step includes:
For pixels arranged in the interval direction of the light and shade stripes, set a group of pixels equal to or greater than the number of stripe interval pixels, and
The resolving power measurement method according to claim 2, further comprising a process of setting a maximum value among the contrast values of the pixels calculated for each pixel in each group as a contrast value at each pixel position belonging to the group.
The resolving power calculation step calculates, for the pixel Xn , the maximum value among the pixel contrast values calculated for each of a plurality of pixels arranged in a line in a line direction parallel to the light and shade stripes. The resolving power measurement method according to claim 2, further comprising a process of setting a contrast value at a position of a pixel column to which n belongs.
The chart includes a plurality of the shade stripes in which two or more shade stripes are arranged in a line direction parallel to the shade stripe, and the plurality of shade stripes have the same stripe interval, and the shade The position of the stripe is shifted by a specific distance in the direction of the stripe interval,
The resolving power calculation step includes:
The contrast value of the pixel calculated for each of a plurality of pixels arranged in a line in a direction parallel to the line direction with respect to the pixel Xn from the image data of the read image of the chart including the plurality of gray stripes 5. The resolution measurement method according to claim 2, further comprising: processing for setting a maximum value of the contrast value at the position of the pixel column to which the pixel Xn belongs.
The specific distance in the interval direction of the light and shade stripes is a positive integer of the stripe interval pixel number,
The resolution measuring method according to claim 5, wherein the number of the gray stripes arranged in the line direction is a positive integer multiple of (number of stripe interval pixels / specific distance).
The resolving power calculation step includes, for the pixel Xn , out of the contrast values of the pixels calculated for a plurality of pixels arranged in a line in a line direction parallel to the shading stripes, a contrast average of the pixels in the line direction The resolving power measurement method according to claim 3, further comprising a process of excluding a value larger than a specified ratio by a specified ratio or more as an abnormal value.
The resolving power calculation step includes, for the pixel Xn , out of the contrast values of the pixels calculated for a plurality of pixels arranged in a line in a line direction parallel to the shading stripes, a contrast average of the pixels in the line direction A value larger than a specified ratio by more than a specified ratio is set as an abnormal value, and the contrast value of a pixel indicating the abnormal value is replaced with an average value of contrast values of pixels that do not correspond to the abnormal value in a range of the surrounding specified number of pixels. The resolving power measuring method according to any one of claims 3 to 6 including processing.
In the case where the interval between the light and shade stripes is P millimeters and the interval between the reading pixels of the optical reading device on the chart is S millimeters, P ≧ 2 × S,
The resolution measuring method according to claim 1, wherein the specific number of pixels is a minimum integer d that satisfies d ≧ P / (2 × S).
When the interval between the light and shade stripes is P millimeters and the interval between the reading pixels of the optical reading device on the chart is S millimeters, P ≧ S,
The resolution measuring method according to claim 1, wherein the specific pixel number is a minimum integer d that satisfies d ≧ P / S.
In the case where the interval between the light and shade stripes is P millimeters and the interval between the reading pixels of the optical reading device on the chart is S millimeters, P ≧ 2 × S,
9. The specific pixel number is an integer d that is equal to or larger than a minimum integer satisfying d ≧ P / (2 × S) and equal to or smaller than a minimum integer satisfying d ≧ P / S. The resolving power measuring method described in 1.
The resolution measurement method according to any one of claims 1 to 11, wherein the chart includes a plurality of different patterns of gray stripes at the specific intervals.
The direction of the light and shade stripes included in the chart is the first direction, the second direction intersecting the first direction, or both the first direction and the second direction. The resolving power measuring method according to claim 1.
The resolution measuring method according to any one of claims 1 to 13, including a step of determining a focus state of the optical reading device from a calculation result of a contrast transfer function calculated for pixels in the entire reading range of the optical reading device.
The resolution measurement method according to any one of claims 1 to 14, wherein the chart is a chart in which the shading stripes are printed on a recording medium using a printing apparatus.
The resolving power measuring method according to any one of claims 1 to 15, further comprising a chart reading step of reading the chart using the optical reading device.
The resolution measuring method according to claim 16, wherein the optical system of the optical reading device includes an imaging lens having a single optical axis or a lens array having a plurality of optical axes.
The resolution measuring method according to any one of claims 1 to 17, further comprising a chart printing step of printing the chart including the shading stripes using a printing apparatus.
The printing apparatus includes the optical reader;
The resolution measuring method according to claim 18, wherein the chart is printed by the printing device, and the chart is read by the optical reading device in the printing device.
The resolution measuring method according to claim 18 or 19, wherein the printing device is an inkjet printing device.
The resolution measuring method according to claim 20, wherein each line of the light and shade stripes is recorded by droplet ejection from a single nozzle in an ink jet head.
The resolution measuring method according to claim 20 or 21, wherein the optical reading device is an in-line scanner mounted on the inkjet printing apparatus.
At least in part, a read image data acquisition unit that acquires image data of a read image obtained by reading a chart including a light and dark stripe whose density cross section changes to a rectangular shape at a specific interval;
For each pixel belonging to at least a part of the calculation target range of the image data of the read image, between pixels between the pixels separated by a number of pixels not less than 1 and not more than a specific number of pixels in the interval direction of the light and shade stripes An inter-pixel contrast calculation unit for calculating contrast;
A resolving power calculation unit that calculates a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel by the inter-pixel contrast calculation unit,
The specific pixel number is defined as a pixel number that is equal to or less than a stripe interval pixel number that is the number of pixels corresponding to the specific interval of the light and shade stripes in the image data of the read image, and the specific pixel number is d, the pixel number indicating the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range of the image data of the read image is n, and the value of the pixel Xn represented by the pixel number n is X (n), the value of a pixel X n + m apart m pixels in the spacing direction of the streaks from the pixel X n and X (n + m), m is 1 ≦ m ≦ d, or a -d ≦ m ≦ -1 When taking an integer value that satisfies
The inter-pixel contrast calculation unit has the following formula:
CTF (X n , X n + m ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
According to the process of calculating the contrast between the pixels,
Processing for obtaining a maximum value among CTFs (X n , X n + m ) obtained for different m with respect to the pixel X n ;
Resolving power measuring device.
Data processing for measuring the resolving power of the optical reading device from image data of a read image obtained by reading a chart including at least part of a density stripe whose density cross section changes into a rectangular shape at a specific interval by an optical reading device. A program to be executed,
On the computer,
A read image data acquisition step of acquiring read image data of the chart;
For each pixel belonging to at least a part of the calculation target range of the image data of the read image, between pixels between the pixels separated by a number of pixels not less than 1 and not more than a specific number of pixels in the interval direction of the light and shade stripes A pixel-to-pixel contrast calculating step for calculating contrast;
A resolving power calculation step of calculating a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel by the inter-pixel contrast calculation step,
The specific pixel number is defined as a pixel number that is equal to or less than a stripe interval pixel number that is the number of pixels corresponding to the specific interval of the light and shade stripes in the image data of the read image, and the specific pixel number is d, the pixel number indicating the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range of the image data of the read image is n, and the value of the pixel Xn represented by the pixel number n is X (n), the value of a pixel X n + m apart m pixels in the spacing direction of the streaks from the pixel X n and X (n + m), m is 1 ≦ m ≦ d, or a -d ≦ m ≦ -1 When taking an integer value that satisfies
The inter-pixel contrast calculation step has the following formula:
CTF (Xn, Xn + m ) = {X (n + m) -X (n)} / {X (n + m) + X (n)}
According to the process of calculating the contrast between the pixels,
Processing for obtaining a maximum value among CTFs (X n , X n + m ) obtained for different m with respect to the pixel X n ;
Including programs.
A computer-readable recording medium on which the program according to claim 24 is recorded.
An image forming unit for forming an image on a recording medium;
An optical reader that optically reads an image formed on the recording medium using the image forming unit;
A data processing device that performs arithmetic processing on image data of a read image read by the optical reading device,
The image forming unit prints a chart including light and shade stripes in which a density cross section changes to a rectangular shape at a specific interval, at least in part.
The optical reader reads the chart;
The data processing device includes a read image data acquisition unit that acquires image data of a read image of the chart read by the optical reading device;
For each pixel belonging to at least a part of the calculation target range of the image data of the read image, between pixels between the pixels separated by a number of pixels not less than 1 and not more than a specific number of pixels in the interval direction of the light and shade stripes An inter-pixel contrast calculation unit for calculating contrast;
A resolving power calculation unit that calculates a contrast transfer function at each stripe position of the light and shade stripes from the maximum value of the contrast value calculated for each pixel by the inter-pixel contrast calculation unit,
The specific pixel number is defined as a pixel number that is equal to or less than a stripe interval pixel number that is the number of pixels corresponding to the specific interval of the light and shade stripes in the image data of the read image, and the specific pixel number is d, the pixel number indicating the position in the interval direction of the gray stripes of each pixel belonging to at least a part of the calculation target range of the image data of the read image is n, and the value of the pixel Xn represented by the pixel number n is X (n), the value of a pixel X n + m apart m pixels in the spacing direction of the streaks from the pixel X n and X (n + m), m is 1 ≦ m ≦ d, or a -d ≦ m ≦ -1 When taking an integer value that satisfies
The inter-pixel contrast calculation unit has the following formula:
CTF (X n , X n + m ) = {X (n + m) −X (n)} / {X (n + m) + X (n)}
According to the process of calculating the contrast between the pixels,
Processing for obtaining a maximum value among CTFs (X n , X n + m ) obtained for different m with respect to the pixel X n ;
Printing device to do.
The image forming unit includes a line-type inkjet head configured by connecting a plurality of head modules,
27. The printing apparatus according to claim 26, wherein the data processing apparatus performs a process of detecting a local non-uniformity in the ink ejection amount of the head module using the contrast transfer function calculated by the resolving power calculation unit.