JP2008085564A - Device for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for forming an image capable of setting a gamma correction curve so as to reduce gradation jumpings in both a low gradation region and a high gradation region when the same density is detected by a plurality of test-patch images in both the low gradation region and the high gradation region. <P>SOLUTION: When the same density is detected by a plurality of the test-patch images in both the low gradation region R1 and the high gradation region R2, either a maximum-gradation data value or a minimum-gradation data value is selected discretely in each of the low gradation region R1 and the high gradation region R2. Concretely, the maximum-gradation data value is selected in the low gradation region R1 and the minimum-gradation data value in the high gradation region R2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention performs gamma correction processing based on preset gamma correction information (gamma correction curve, gamma correction table, etc.) on input image data, and then based on the image data after the gamma correction processing. In particular, the present invention relates to a method for setting the gamma correction information.

Image forming apparatuses such as printers, copiers, facsimile machines, and multifunction machines that form black-and-white and color images on recording paper by an electrophotographic process using an image carrier such as a photosensitive drum and a transfer belt are widely known. ing. Generally, in the image forming apparatus, after gamma correction processing is performed on input image data, image formation is executed based on the image data after the gamma correction processing.
In the gamma correction process, the image data is corrected based on a gamma correction curve (gamma correction information) set in advance according to the gamma characteristic (input / output characteristic) of the image forming apparatus. However, the gamma characteristic of the image forming apparatus changes due to deterioration of the photosensitive drum and the transfer belt. If the gamma characteristic changes, the image forming apparatus cannot obtain a desired image density and high color reproducibility unless the gamma correction curve is changed.
For this reason, for example, Patent Document 1 proposes that an optimal gamma correction curve is selected from a plurality of gamma correction curves when the gamma characteristic of the image forming apparatus changes.

Further, correcting the gamma correction curve is also known as a conventional method. Specifically, a plurality of test patch images corresponding to a plurality of gradation data values (for example, 32 points) are formed on a transfer belt or paper, and the density of each of the test patch images is detected, and then detected. The gamma correction curve is corrected based on the density.
At this time, if the dot reproducibility is reduced due to deterioration of the photosensitive drum or changes in development characteristics, when the density of each test patch image is detected, the same density is detected in different test patch images. May be.
FIG. 5 shows an example of the relationship between the gradation data value of the test patch image and the detected density, and FIG. 9 shows an example of a conventional gamma correction curve. The broken line shown in the figure indicates the ideal relationship between the gradation data value and the density.
When the dot reproducibility is degraded, as shown in FIG. 5, the gradation data value is small (the density is low), the low gradation area R1, and the gradation data value is large (the density is high) high gradation. In the region R2, the same density is detected in a plurality of test patch images corresponding to a plurality of gradation data values. In this case, either the highest highest gradation data value or the lowest lowest gradation data value is selected from a plurality of gradation data values (hereinafter referred to as “duplicate data”) from which the same density is detected. It is necessary to select a tone data value as a tone data value corresponding to the same density.

At this time, conventionally, the selection of the highest gradation data value or the lowest gradation data value is the same in the low gradation area R1 and the high gradation area R2. Specifically, the highest gradation data value is selected in both the low gradation region R1 and the high gradation region R2, or the lowest floor is selected in both the low gradation region R1 and the high gradation region R2. The key data value is selected.
Here, FIG. 9A shows a gamma correction curve C1 generated when the highest gradation data value is selected in both the low gradation area R1 and the high gradation area R2. FIG. 9B shows a gamma correction curve C2 generated when the lowest gradation data value is selected in both the low gradation region R1 and the high gradation region R2.
Here, when the gamma correction curve C1 shown in FIG. 9A is compared with the gamma correction curve C2 shown in FIG. 9B, the gamma correction curve C1 includes the gamma correction curve C1 in the low gradation region R1. A steep slope occurs in a region where the density is lower than the correction curve C2. In general, it is known that in human vision, the lower the density in the low gradation region R1, the harder it is to recognize the density difference, and the higher the density, the easier it is to recognize the density difference. That is, in the gamma correction curve C1, there are fewer gradation skips in the low gradation region R1 than in the gamma correction curve C2, which occurs in a region that can be easily recognized by human vision.
However, the gamma correction curve C1 has a steeper slope than the gamma correction curve C2 in the high gradation region R2. In general, it is known that it is difficult for human vision to recognize a density difference when a certain level of density is exceeded in the high gradation region R2. That is, in the gamma correction curve C1, the gradation skip occurring in the high gradation region R2 is larger than that in the gamma correction curve C2.
JP 09-172547 A

As described above, in the conventional method, when the highest gradation data value is selected in the low gradation region R1, the highest gradation data value is similarly selected in the high gradation region R2 (FIG. 9 ( a)), when the lowest gradation data value is selected in the low gradation region R1, the lowest gradation data value is similarly selected in the high gradation region R2 (see FIG. 9B). ). Therefore, there is a problem that gradation skip cannot be reduced in both the low gradation region R1 and the high gradation region R2.
Accordingly, the present invention has been made in view of the above circumstances, and the object of the present invention is to detect the same density when a plurality of test patch images are detected in both a low gradation region and a high gradation region. An object of the present invention is to provide an image forming apparatus capable of setting a gamma correction curve so as to reduce gradation skipping in both a low gradation area and a high gradation area.

In order to achieve the above object, the present invention provides an image forming unit for forming an image on an arbitrary image forming object based on input image data, and gamma correction information preset in the image data. Gamma correction processing means for performing gamma correction processing, test pattern image output means for outputting test pattern image data including a plurality of test patch images corresponding to a plurality of different gradation data values to the image forming unit, and the test pattern A density detector for detecting the density of each of the plurality of test patch images formed on the image forming object based on image data, and the same density detected in the plurality of test patch images by the density detector; The highest gradation data value or the lowest lowest gradation data among the gradation data values corresponding to each of the plurality of test patch images. The gradation data value selection means for selecting one of the values as the gradation data value corresponding to the same density, the detection result by the density detection means and the selection result by the gradation data value selection means based on the selection result. Gamma correction information setting means for setting gamma correction information, wherein the gradation data value selection means includes a low gradation region having a low gradation data value, When the same density is detected in the plurality of test patch images by the density detection means in both of the high gradation areas where the gradation data value is high, the highest gradation data value or the lowest gradation data value is determined. The image forming apparatus is characterized in that whether to select is determined individually for each of the low gradation region and the high gradation region.
According to the present invention configured as described above, either the highest gradation data value or the lowest gradation data value can be individually selected in each of the low gradation area and the high gradation area. Therefore, it is possible to reduce gradation skipping in both the low gradation area and the high gradation area.
Specifically, by selecting the highest gradation data value in the low gradation area and the lowest gradation data value in the high gradation area, the gradation in both the low gradation area and the high gradation area is selected. It becomes possible to reduce flying.

By the way, when the developing performance of the image forming unit is high, it is considered that the number of the test patch images in which the same density is detected by the density detecting unit in the high gradation region is small. If the number of the test patch images in which the same density is detected is small, the selection of either the highest gradation data value or the lowest gradation data value does not cause a large gradation skip. On the other hand, when the development performance by the image forming unit is high, when the highest gradation data value is selected in the high gradation area, the high gradation area is compared with the case where the lowest gradation data value is selected. It is well known that the number of tones that can be expressed can be increased.
Therefore, in the high gradation area, when the density of the test patch image corresponding to the maximum value of the gradation data value is equal to or higher than the preset first density, that is, when the development performance is sufficiently high. It is conceivable to select the highest gradation data and select the lowest gradation data when the density is less than the first density, that is, when the development performance is poor. As a result, when the development performance is sufficiently high, priority can be given to an increase in the number of gradations that can be expressed in the high gradation region.

In addition, when the dot reproducibility by the image forming unit is high, it is considered that the number of the test patch images in which the same density is detected by the density detecting unit in the low gradation region is small. If the number of the test patch images in which the same density is detected is small, the selection of either the highest gradation data value or the lowest gradation data value does not cause a large gradation skip. On the other hand, when the reproducibility of dots by the image forming unit is high, when the lowest gradation data value is selected in the low gradation region, the accuracy is higher than when the highest gradation data value is selected. It is well known that high gamma correction information can be set.
Therefore, in the low gradation area, when the density of the test patch image corresponding to 25% of the maximum value of the gradation data value is equal to or higher than the preset second density, that is, the dot reproducibility is high. If it is sufficiently high, the lowest gradation data is selected, and if it is less than the second density, that is, if the dot reproducibility is poor, the highest gradation data may be selected. Thereby, when the dot reproducibility is sufficiently high, priority can be given to the setting of the gamma correction information with high accuracy.
Here, in order to evaluate the reproducibility of dots in the image forming unit, the test patch image is formed so as to be isolated so that a plurality of dots constituting the test patch image do not contact each other. It is desirable. Thereby, the dot reproducibility can be determined with high accuracy.
By the way, the present invention can be applied to both monochrome image forming apparatuses and color image forming apparatuses. Specifically, when applied to a color image forming apparatus, that is, an image forming apparatus in which the image forming unit includes a plurality of image forming units corresponding to a plurality of colors, the gamma correction unit, the test pattern image output The means, the density detection means, the gradation data value selection means, and the gamma correction information setting means may be configured to execute each process for each of the plurality of colors.

  According to the present invention, since either the highest gradation data value or the lowest gradation data value can be individually selected in each of the low gradation area and the high gradation area, the low gradation area It is possible to reduce gradation skipping in both the area and the high gradation area. Specifically, by selecting the highest gradation data value in the low gradation area and the lowest gradation data value in the high gradation area, the gradation in both the low gradation area and the high gradation area is selected. It becomes possible to reduce flying.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
FIG. 1 is a block diagram showing a schematic configuration of the color printer X according to the embodiment of the present invention. 2 is a schematic diagram of the image forming unit 5 of the color printer X, FIG. 3 is a diagram showing the relationship between the G value and the magenta density, and FIG. 4 is a procedure of a gamma correction curve setting process executed in the color printer X. FIG. 5 is a diagram showing a relationship between gradation data values and detected densities, and FIG. 6 is a diagram showing an example of a gamma correction curve.
First, a schematic configuration of the color printer X according to the embodiment of the present invention will be described with reference to FIG. The color printer X is merely an example of an image forming apparatus according to the present invention, and the present invention can also be applied to a monochrome printer. In addition, a copying machine, a facsimile machine, a multifunction machine, and the like also correspond to the image forming apparatus according to the present invention.
As shown in FIG. 1, the color printer X includes a CPU and other peripheral devices (ROM, RAM, etc.), and executes a process according to a predetermined program stored in the ROM to set a gamma correction curve described later. The control unit 1 that performs processing (see the flowchart in FIG. 4) and the like, and controls the color printer X in an integrated manner, a liquid crystal display that performs various information display and input operations in the color printer X, a touch panel, and the like A data communication unit 3 for performing data communication via the LAN 10 between the operation display unit 2 and an information processing apparatus Y such as a personal computer, and an image for storing image data received by the data communication unit 3 An image to be printed on the paper supplied from a paper cassette (not shown) based on the memory 4 and the input image data. A forming unit 5, a density detection sensor 6 used to detect the concentration of toner image formed on the transfer belt 61 described later provided in the image forming unit 5, it is schematically configured with a.
In the color printer X, similarly to the conventional apparatus, gamma correction processing based on a preset gamma correction curve (an example of gamma correction information) is executed on the image data input from the information processing apparatus Y. Print output is executed based on the image data after the gamma correction processing is executed. The gamma correction processing is executed by the control unit 1, and the control unit 1 when executing the gamma correction processing corresponds to gamma correction processing means. A gamma correction curve used for the gamma correction processing is stored in an internal memory of the control unit 1 or a storage unit (not shown) such as an HD.

Next, the image forming unit 5 will be described with reference to the schematic diagram of FIG.
As shown in FIG. 2, the image forming unit 5 includes image forming units 50a to 50d provided corresponding to cyan (C), magenta (M), yellow (Y), and black (K) colors, An intermediate transfer device 60 and a transfer roller 70 are generally provided.
The image forming units 50a to 50d are configured to be uniformly charged to a predetermined potential by charging devices 51a to 51d, and to irradiate the photosensitive drums 52a to 52d with light. An exposure device (not shown) such as a laser scanning unit that forms electrostatic latent images on the surfaces of the drums 52a to 52d, and an electrostatic latent image formed on the surfaces of the photosensitive drums 52a to 52d are not shown. Developing devices 53a to 53d that visualize (develop) toner images as toner images of the respective colors supplied from the toner cartridge. In place of the exposure apparatus (not shown), for example, a plurality of LED head type exposure apparatuses provided corresponding to the photosensitive drums 52a to 52d may be used.

The intermediate transfer device 60 is disposed in contact with the transfer belt 61 and a transfer belt 61 on which the toner images formed on the photosensitive drums 52a to 52d are sequentially combined and transferred by the developing devices 53a to 53d. And a driving roller 62 that travels the transfer belt 61 by the rotational force transmitted from the DC motor 62a, and is also disposed in contact with the transfer belt 61 and supported in a state where the transfer belt 61 is stretched with a predetermined tension. A tension roller 63 and a driven roller 64. Each of the photosensitive drums 52a to 52d rotates while pressing the transfer belt 61 between the photosensitive drums 52a to 52d and a primary transfer roller (not shown) provided corresponding to each of the photosensitive drums 52a to 52d. A toner image is transferred to the transfer belt 61.
The transfer roller 70 sandwiches the transfer belt 61 together with the driven roller 64, and is formed on the transfer belt 61 on the sheet by rotating the transfer belt 61 and the sheet in pressure contact with each other. The transferred toner image is transferred.

The density detection sensor 6 detects the luminance of each of RGB of the toner image formed on the transfer belt 61 by a CCD element or the like, converts it into data, and inputs it to the control unit 1.
The controller 1 determines the density of the toner image based on the RGB values input from the density detection sensor 6. Specifically, FIG. 3 is a G value-density correspondence graph showing a known relationship between the G value of the RGB values detected by the density detection sensor 6 and the magenta density. The controller 1 obtains the magenta density (0 to 255) based on the G value (0 to 255) input from the density detection sensor 6 and the G value-density correspondence graph shown in FIG. Similarly, for other colors, it is possible to calculate the density based on the R value for cyan, the B value for yellow, and the G value for black. The method for detecting the density of the toner image is not limited to this, and other methods may be used.

By the way, as described above, the gamma characteristics in the image forming units 50a to 50d of the image forming unit 5 change due to deterioration of the photosensitive drums 52a to 52d and the transfer belt 61.
Therefore, conventionally, when the power of the color printer X is turned on, a gamma correction curve (gamma) used for gamma correction processing applied to the image data input to the color printer X by the control unit 1. A gamma correction curve setting process (see the flowchart of FIG. 4) for setting an example of correction information is executed. Here, the control unit 1 when executing such gamma correction curve setting processing corresponds to gamma correction information setting means. Thus, the color printer X obtains a desired image density and high color reproducibility by performing gamma correction processing based on the gamma correction curve set by the gamma correction curve setting processing on the input image data. be able to.

Hereinafter, an example of the procedure of the gamma correction curve setting process executed by the control unit 1 in the color printer X will be described with reference to the flowchart of FIG. Here, S1, S2,... Shown below represent processing procedure (step) numbers. In addition, here, the processing when setting a gamma correction curve corresponding to magenta will be described as an example, but the same processing is executed for each of the other colors, and a gamma correction curve corresponding to each color is set. .
The gamma correction curve setting process is executed by the control unit 1 when the main power of the color printer X is turned on, and the process is started from step S1.
First, in step S 1, the control unit 1 outputs test pattern image data including a plurality of test patch images corresponding to a plurality of different gradation data values to the image forming unit 5. Here, the control unit 1 when executing such output processing corresponds to test pattern image output means.
Specifically, test pattern image data including 32 test patch images corresponding to gradation data values in 32 steps corresponding to magenta is input to the image forming unit 50b. The test pattern image data is stored in the internal memory of the control unit 1, the image memory 4 or the like.
Thereby, in the image forming unit 50b, 32 test patch images are formed on the photosensitive drum 52b based on the test pattern image data. Thereafter, each of the test patch images formed on the photosensitive drum 52 b is transferred to the transfer belt 61.

In step S2, the control unit 1 detects the density of each of the plurality of test patch images transferred onto the transfer belt 61. Specifically, the density of each test patch image based on the G value of the RGB values of each of the test patch images detected by the density detection sensor 6 and the G value-density correspondence graph shown in FIG. Is required. Here, the control unit 1 and the density detection sensor 6 when executing such density detection processing correspond to density detection means.
Here, the description will be continued assuming that the detection result shown in the graph of FIG. 5 is obtained in step S2. As described above, in the detection result shown in FIG. 5, due to the deterioration of the dot reproducibility in the image forming unit 50b, a plurality of levels are obtained in both the low gradation data value and the high gradation area R2 having the high gradation data value. The same density is detected in a plurality of test patch images corresponding to the tone data value.

Next, in step S3, the control unit 1 determines whether or not the same density is detected in a plurality of test patch images in the low gradation region R1 and the high gradation region R2.
If it is determined that the same density is detected in a plurality of test patch images, the process proceeds to step S4. If it is determined that the same density is not detected, the process proceeds to step S5. The step S5 will be described later.
In step S4, the highest highest gradation data value or lowest lowest gradation among a plurality of gradation data values (hereinafter referred to as “duplicate data”) corresponding to each of the plurality of test patch images in which the same density is detected. Processing for selecting one of the data values as the gradation data value corresponding to the same density is executed by the control unit 1. Here, the control unit 1 when executing such selection processing corresponds to gradation data value selection means.
At this time, conventionally, when the highest gradation data value (or lowest gradation data value) is selected in the low gradation region R1, the highest gradation data value (or lowest value) is similarly applied to the high gradation region R2. Since the gradation data value is selected, there is a problem that gradation skip cannot be reduced in both the low gradation region R1 and the high gradation region R2.
However, in the gamma correction curve setting process, whether the highest gradation data value or the lowest gradation data value is selected by the control unit 1 depends on each of the low gradation region R1 and the high gradation region R2. Determined separately.
Specifically, the highest gradation data value of the overlapping data is selected in the low gradation area R1, and the lowest gradation data value of the overlapping data is selected in the high gradation area R2.

In the subsequent step S5, a gamma correction curve is set based on the density (see FIG. 5) of each of the plurality of test patch images detected in step S2 and the selection result in step S4.
FIG. 6 is a graph showing a gamma correction curve C3 set when the detection result shown in FIG. 5 is obtained.
As shown in FIG. 6, in the gamma correction curve C3, the low gradation region R1 has a steep slope in a region where the density is lower than the gamma correction curve C2 (see FIG. 9B). In the low gradation region R1, gradation skip occurring in a region that is easily recognized by human vision is less than that in the gamma correction curve C2 (see FIG. 9B).
Moreover, since the slope of the gamma correction curve C3 is smoother than that of the gamma correction curve C1 (see FIG. 9A) in the high gradation region R2, the gamma correction curve C1 (see FIG. 9A). ) Less gradation skipping than
That is, in the color printer X, the control unit 1 executes the gamma correction curve setting process, so that gradation skip can be reduced in both the low gradation region R1 and the high gradation region R2. is there.

By the way, when the developing performance of the image forming unit 50b is high, it is considered that the number of the test patch images in which the same density is detected in the high gradation region R2 is small. If the number of the test patch images in which the same density is detected is small, the selection of either the highest gradation data value or the lowest gradation data value does not cause a large gradation skip. On the other hand, when the highest gradation data value is selected in the high gradation region R2, the number of gradations that can be expressed in the high gradation region can be increased compared to the case where the lowest gradation data value is selected. It is well known that it can be done.
In addition, when the dot reproducibility by the image forming unit 50b is high, it is considered that the number of the test patch images in which the same density is detected in the low gradation region R1 is small. If the number of the test patch images in which the same density is detected is small, the selection of either the highest gradation data value or the lowest gradation data value does not cause a large gradation skip. On the other hand, when the lowest gradation data value is selected in the low gradation region R1, it is possible to set the gamma correction information with higher accuracy than when the highest gradation data value is selected. This is well known.
Therefore, in the first exemplary embodiment, when the overlapping data exists in the low gradation region R1 and the high gradation region R2, the dot reproducibility and the development performance of the image forming unit 50b are set according to the quality of the image. A configuration for switching between the highest gradation data value and the lowest gradation data value will be described.

FIG. 7 is a flowchart for explaining the gamma correction curve setting process according to the first embodiment. In addition, the same code | symbol is attached | subjected about the component and process sequence which are common in the said color printer X demonstrated in the said embodiment, and the description is abbreviate | omitted.
As shown in FIG. 7, in the gamma correction curve setting process according to the first embodiment, after step S3, in step S31, the control unit 1 determines whether the developing performance of the image forming unit 50b is good or bad. To do. Specifically, the development performance of the image forming unit 50b is that the density of the test patch image having the maximum gradation data value among the test patch images is equal to or higher than a preset first density. It is determined depending on whether or not. Here, the first density is for determining whether the developing performance of the image forming unit 50b is good or bad, and is stored in advance in the internal memory of the control unit 1.
Here, when it is determined that the density of the test patch image having the maximum gradation data value is less than the first density, that is, the developing performance is poor (No side of S31), the process proceeds to step S32. If it is determined that the density is equal to or higher than the first density, that is, the development performance is good (Yes in S31), the process proceeds to step S33.

Next, in step S32 and step S33, the control unit 1 determines whether the dot reproducibility of the image forming unit 50b is good or bad. Specifically, the dot reproducibility of the image forming unit 50b is that the density of the test patch image corresponding to 25% of the maximum gradation data value among the test patch images is set in advance. Judgment is made according to whether or not it is the second concentration or more. Here, the second density is for determining the dot reproducibility of the image forming unit 50b, and is stored in the internal memory of the control unit 1 in advance.
Here, if it is determined that the density of the test patch image corresponding to the gradation data value of 25% of the maximum value is less than the second density, that is, the dot reproducibility is poor (No in S32 or 33). , The process proceeds to step S41 or S43, and if it is determined that the density is equal to or higher than the second density, that is, the dot reproducibility is good (Yes in S32 or S33), the process proceeds to step S42 or S44. .

Steps S41 to S44 are processes executed by the control unit 1 instead of the step S4 (see FIG. 4). For each of the low gradation region R1 and the high gradation region R2, the middle of the duplicate data is processed. From the above, either the highest gradation data value or the lowest gradation data value is selected.
Specifically, in step S41, the highest gradation data value is selected in the low gradation region R1, and the highest gradation data value is selected in the high gradation region R2.
In step S42, the lowest gradation data value is selected in the low gradation region R1, and the highest gradation data value is selected in the high gradation region R2. In this case, the gamma correction information with high accuracy can be set.
On the other hand, in step S43, the highest gradation data value is selected in the low gradation region R1, and the highest gradation data value is selected in the high gradation region R2. In this case, the number of gradations that can be expressed in the high gradation region R2 can be increased.
In step S44, the lowest gradation data value is selected in the low gradation area R1, and the highest gradation data value is selected in the high gradation area R2. In this case, the gamma correction information with high accuracy can be set and the number of gradations that can be expressed in the high gradation region R2 can be increased.

In the first embodiment, the test patch image used in the gamma correction curve setting process is used to determine the dot reproducibility and development performance of the image forming unit 50b. It may be used.
For example, in general, in an image forming apparatus such as the color printer X, when the power is turned on, not only the gamma correction curve setting process but also the developing rollers (not shown) of the developing devices 53a to 53d provided in the image forming unit 5 are used. ) Is also executed.
In the bias voltage setting process, a test patch image corresponding to the maximum value of the gradation data value and a test patch image corresponding to 25% of the maximum value of the gradation data value are placed on the transfer belt 61. Then, the density is detected by the density detection sensor 6. The bias voltage is set based on the density. At this time, in the bias voltage setting process, since it is necessary to accurately determine the dot reproducibility and development performance of the image forming unit 50b, a test patch image different from the gamma correction curve setting process is output.

Here, FIG. 8A shows a test patch image output during the gamma correction curve setting process, and FIG. 8B shows a test patch image output during the bias voltage setting process. It is.
As shown in FIG. 8 (a), during the gamma correction curve setting process, as in the normal image forming process, a test that has been subjected to a screen process that connects a plurality of dots constituting the test patch image is performed. A patch image is formed. In this test patch image, since a plurality of dots are connected, the dot reproducibility cannot be determined with high accuracy.
However, as shown in FIG. 8B, at the time of the bias voltage setting process, a test patch image is formed on which screen processing is performed so that a plurality of dots constituting the test patch image are not in contact with each other. Is done. Specifically, in the test patch image, only one pixel is formed in four pixels. In this test patch image, since each of a plurality of dots is isolated, the dot reproducibility can be judged with high accuracy.

It is also conceivable that the test patch image output in the gamma correction curve setting process has a form in which each of a plurality of dots is isolated as shown in FIG. In this case, it is possible to accurately determine dot reproducibility using the test patch image output in the gamma correction curve setting process.
In the first embodiment and the second embodiment, the density of the test patch image in which the gradation data value corresponds to the maximum value or the test patch image in which the gradation data value corresponds to 25% of the maximum value is equal to or higher than a predetermined density. The example of determining the degree of deterioration of the development performance and dot reproducibility of the image forming unit 50b according to whether or not the image forming unit 50b is determined has been described. The degree of deterioration of the development performance and dot reproducibility of the unit 50b may be determined. That is, processing may be performed so as to switch between the highest gradation data value and the lowest gradation data value depending on the number of gradation data values included in the duplicate data.

1 is a block diagram showing a schematic configuration of a color printer according to an embodiment of the present invention. 1 is a schematic diagram of an image forming unit of a color printer according to an embodiment of the present invention. The figure which shows the relationship between G value and the density of magenta. 6 is a flowchart for explaining an example of a procedure of gamma correction curve setting processing executed in the color printer according to the embodiment of the present invention. The figure which shows the relationship between a gradation data value and detection density. The figure which shows an example of a gamma correction curve. 6 is a flowchart for explaining a gamma correction curve setting process according to the first embodiment of the invention. The figure for demonstrating an example of a test patch image. The figure which shows an example of the conventional gamma correction curve.

Explanation of symbols

X ... Color printer (an example of an image forming apparatus)
DESCRIPTION OF SYMBOLS 1 ... Control part 2 ... Operation display part 3 ... Data communication part 4 ... Image memory 5 ... Image formation part 6 ... Density detection sensor R1 ... Low gradation area | region R2 ... High gradation area C1-C3 ... Gamma correction curve (gamma correction information) )
S1, S2, ... Processing procedure (step) numbers

Claims (6)

An image forming unit that forms an image on an arbitrary image forming object based on the input image data;
Gamma correction processing means for performing gamma correction processing on the image data based on preset gamma correction information;
Test pattern image output means for outputting test pattern image data including a plurality of test patch images corresponding to a plurality of different gradation data values to the image forming unit;
Density detecting means for detecting the density of each of the plurality of test patch images formed on the image forming object based on the test pattern image data;
When the same density is detected in the plurality of test patch images by the density detection means, the highest highest gradation data value or lowest lowest gradation value among gradation data values corresponding to each of the plurality of test patch images. Gradation data value selection means for selecting any one of the gradation data values as gradation data values corresponding to the same density;
Gamma correction information setting means for setting the gamma correction information based on the detection result by the density detection means and the selection result by the gradation data value selection means;
An image forming apparatus comprising:
The gradation data value selection means detects the same density in the plurality of test patch images by the density detection means in both the low gradation area where the gradation data value is low and the high gradation area where the gradation data value is high. In this case, it is determined whether to select the highest gradation data value or the lowest gradation data value individually for each of the low gradation area and the high gradation area. Forming equipment.   2. The image forming apparatus according to claim 1, wherein the gradation data value selection unit selects the highest gradation data value in the low gradation region and the lowest gradation data value in the high gradation region.   When the density of the test patch image corresponding to the maximum value of the gradation data value is equal to or higher than a first density set in advance, the gradation data value selection unit selects the highest gradation in the high gradation region. The image forming apparatus according to claim 1, wherein data is selected and the lowest gradation data is selected in the high gradation region when the density is less than the first density.   When the gradation data value selection means has a density of the test patch image corresponding to 25% of the maximum value of the gradation data value equal to or higher than a preset second density, the low gradation area 4. The image forming apparatus according to claim 1, wherein the lowest gradation data is selected, and the highest gradation data is selected in the low gradation region when the density is less than the second density. 5.   The image forming apparatus according to claim 4, wherein the test patch image is formed so as to be isolated so that a plurality of dots constituting the test patch image do not contact each other. The image forming unit includes a plurality of image forming units corresponding to a plurality of colors;
The gamma correction means, the test pattern image output means, the density detection means, the gradation data value selection means, and the gamma correction information setting means execute each process for each of the plurality of colors. Item 6. The image forming apparatus according to any one of Items 1 to 5.

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