JP2016092751A - Image formation device and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reconcile the accuracy and followability of a gamma correction table, when performing calibration by estimating the density of other patch from one measurement patch.SOLUTION: After power is turned on, or during replacement of cartridge, on the first page at the time of environmental variation of temperature or humidity, a gamma correction table with an emphasis on follow-up is generated, by using a patch of intermediate density having a wide allowable range for variation. In the following page, accuracy of a gamma correction table is enhanced, by using a low density patch or a high density patch, having high sensitivity for variation of measurement patch.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electrophotographic image forming apparatus and a control method thereof, and more particularly, to a density correction technique in the image forming apparatus.

  2. Description of the Related Art Conventionally, in an image forming apparatus, calibration for updating a halftone gamma correction table is performed in order to keep a print density constant. However, an image forming apparatus equipped with a plurality of halftone screens has different degrees of change in output characteristics for each halftone screen. Therefore, since a calibration process is required for each halftone screen, it is necessary to perform a plurality of calibration processes, and it takes time to update the gamma correction table. From the viewpoint of shortening the update time, Patent Document 1 is proposed in which the gamma correction tables of a plurality of other halftone screens are updated from the calibration result for the gamma correction table of one halftone screen.

JP2013-13045A

  However, according to the method disclosed in Patent Document 1, although it is not necessary to perform calibration for each halftone screen, a plurality of halftone patches are imaged in order to update the gamma correction table of the model halftone screen. It is necessary to output.

  An image forming apparatus according to the present invention includes an output unit that outputs a patch image composed of a specific dot pattern, a measurement unit that measures the density of the patch image output by the output unit, and an output unit that outputs the patch image. Deriving means for deriving an estimated density value of a patch composed of other dot patterns based on the difference between the density read from the storage means for storing the density of the patch image and the density measured by the measuring means; Update means for updating information used in gamma correction using the estimated density value thus derived, and a patch image output by the output means at a predetermined timing, the first patch having a wide tolerance range of density fluctuation And control means for switching between the image and the second patch image having a narrow allowable range of density fluctuation.

  According to the present invention, when performing calibration, it is possible to estimate the density variation of each patch for generating a gamma correction table from one measurement patch and update the gamma correction table. The accuracy of the gamma correction table can be improved by repeating the update of the gamma correction table while changing the measurement patch at the time of subsequent calibration.

1 is a diagram illustrating an example of a block of an image forming apparatus according to a first exemplary embodiment. FIG. 3 is a diagram illustrating a plurality of measurement patches according to the first embodiment. 3 is a flowchart for explaining the operation of the first embodiment. 6 is a diagram illustrating a method for deriving a density difference according to the first embodiment. FIG. FIG. 6 is a diagram illustrating an example of a block of an image forming apparatus according to a second embodiment. 10 is a flowchart for explaining the operation of the second embodiment. FIG. 10 is a diagram illustrating a method for deriving a density difference according to the second embodiment. FIG. 10 is a diagram illustrating an example of a block of an image forming apparatus according to a third embodiment. 10 is a flowchart for explaining the operation of the third embodiment. FIG. 10 illustrates a block diagram of an image forming apparatus according to a fourth exemplary embodiment. 10 is a flowchart for explaining the operation of the fourth embodiment. 10 is a flowchart for explaining the operation of the fifth embodiment. 10 is a flowchart for explaining the operation of the sixth embodiment. It is a figure for demonstrating the characteristic model of a density fluctuation.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the accompanying drawings. In addition, the structure shown in the following Examples is only an example, and this invention is not limited to the structure shown in figure.

[Example 1]
In an electrophotographic image forming apparatus, it is known that the output density for the same input image changes due to environmental changes and changes with time. In order to correct this, a density correction process is performed. In general density correction processing, patches with different gradations from low density to high density are formed on an intermediate transfer belt, etc., the density of each gradation patch is measured with an optical sensor, etc., and output at the input image density This is done by obtaining a concentration curve. The patches of each gradation are formed on the intermediate transfer belt by outputting halftone image data of different gradations (that is, different dot shapes or different densities) used for printing.

  On the other hand, density fluctuations due to environmental changes and changes with time appear as fluctuations in the area ratio of dots on the recording medium. Thus, the inventors have proposed a technique that uses the correlation between the variation of the dot shape and the dot area by utilizing the fact that the variation rate of the dot area depends on the dot shape. According to this method, the density fluctuation rate of an arbitrary dot shape can be obtained from a certain dot shape and density fluctuation rate. That is, the density fluctuation value of each patch for generating a gamma correction table is estimated from the density fluctuation rate of one halftone patch for density fluctuation measurement. Further, it is possible to estimate the density of each patch from the estimated density fluctuation value of each patch and regenerate the gamma correction table using the estimated density of each patch. That is, the gamma correction table can be updated by measuring only one patch for density fluctuation measurement with respect to an arbitrary gamma correction table. Details of this processing will be described later.

  In this embodiment, the density of each patch is estimated from one measurement patch as described above, and the process of updating the gamma correction table using the estimated density of each patch is performed during the subsequent calibration process. Switch the measurement patch used for. As a result, the accuracy of calibration is improved.

  In this embodiment, a medium density patch is used as a measurement patch for calibration when printing is performed for the first time after power is turned on, when a cartridge is replaced, or when environmental changes such as temperature and humidity occur. Measure the density variation and update the gamma correction table. This medium-density patch is a patch in which dots are easily struck stably and has low sensitivity to density changes with respect to cartridge replacement and environmental changes (having a wide tolerance range). In the calibration when the subsequent printing is performed, the gamma correction table is updated by using a plurality of types of patches such as a low density patch or a high density patch with high density variation sensitivity as a measurement patch. That is, the gamma correction table is updated using the low density patch or the high density patch on the page after the first page to be printed. By such processing, the accuracy of the gamma correction table can be improved.

  FIG. 1 is a block diagram illustrating an example of an image forming apparatus according to the present exemplary embodiment. The image forming apparatus includes a gamma correction unit 100, a dot pattern generation unit 101, a halftone processing unit 102, an image forming unit 103, a density measurement unit 104, and a lookup table (LUT) storage unit 105. In addition, it includes a density variation rate deriving unit 106, a patch density storage unit 107, a density variation estimation unit 108, a gamma correction table update unit 109, and a control unit 110.

  The gamma correction unit 100 is used to correct the output density of the above-described image forming apparatus, and a method for correcting the output density using a look-up table (LUT) that makes the output characteristics for input data linear is known. Yes. The gamma correction unit 100 performs correction using the LUT stored in the LUT storage unit 105. The dot pattern generation unit 101 generates a dot pattern constituting a measurement patch necessary for generating a lookup table used by the gamma correction unit 100. The dot pattern generation unit 101 generates a measurement patch in which a predetermined dot pattern is arranged in a specific area. In this embodiment, a measurement patch is generated using a patch image composed of a 4 × 4 screen size low density, medium density, and high density dot pattern shown in FIGS. . As described above, the measurement patch of the present embodiment is image data indicating an image (patch image) in which a plurality of 4 × 4 screen size dot patterns shown in FIGS. Based on. In this embodiment, when performing gamma correction, one measurement patch is selected from the low density, medium density, and high density measurement patches generated in advance and printed on the intermediate transfer belt. A concentration measurement is performed. Alternatively, when performing gamma correction, one measurement patch may be generated each time from among low, medium, and high density measurement patches.

  Here, the patch image composed of the low density dot pattern of FIG. 2A and the high density dot pattern of FIG. 2C is composed of the medium density dot pattern of FIG. 2B. Sensitivity to density fluctuations is higher than patch images. For example, in the patch image with the low density dot pattern of FIG. 2A, there is a high possibility that the dots will disappear due to density fluctuation. Further, in the patch image by the high density dot pattern in FIG. 2C, there is a high possibility that the area (white dots) where the dots are not applied will be crushed due to density fluctuation. On the other hand, the patch image of the medium density dot pattern in FIG. 2B is less sensitive to density fluctuations than the patch image of the low density or high density dot pattern. In the present embodiment, processing is performed using different patch images having such characteristics.

  The halftone processing unit 102 multi-values the input data after the gamma correction. In this embodiment, binarization is performed, and binarization is performed using, for example, a dither method or an error diffusion method. The image forming unit 103 prints the binarized data by an electrophotographic method. The image forming unit 103 forms the measurement patch generated by the dot pattern generation unit 101 on an intermediate transfer belt (not shown). The density measuring unit 104 detects the density of the measurement patch on the intermediate transfer belt in the image forming unit 103 using an optical sensor or the like. The LUT storage unit 105 stores the LUT used in the gamma correction unit 100.

  The density fluctuation rate deriving unit 106 derives the patch density fluctuation rate between the previous density correction (calibration) and the current density correction. The patch density storage unit 107 is an initial set reference density value, a patch density measurement value measured by the density measurement unit 104 at the previous density correction, or a patch density estimation value estimated by the density variation estimation unit 108. Remember. The density variation rate deriving unit 106 is a reference density value stored in the patch density storage unit 107, or a patch density estimated value estimated at the previous density correction and a density measuring unit 104 at the current density correction. The density difference from the measured density value of the patch is calculated. Then, the density variation rate is derived by normalizing the density difference with the reference density value of the patch. The density fluctuation estimating unit 108 estimates the density of an arbitrary patch from the density fluctuation rate derived by the density fluctuation rate deriving unit 106. That is, the density fluctuation estimation unit 108 estimates the density of an arbitrary patch that has not been measured by the density measurement unit 104. The estimated density value thus estimated is stored in the patch density storage unit 107.

  The gamma correction table update unit 109 updates the LUT stored in the LUT storage unit 105 based on the density of the plurality of patches estimated by the density variation estimation unit 108. The control unit 110 controls a series of printing flows.

<Description of concentration fluctuation estimation unit>
Next, the processing of the density fluctuation estimation unit 108 will be described. The density fluctuation estimation unit 108 acquires the density fluctuation rate derived by the density fluctuation rate deriving unit 106. If the density fluctuation rate is V _m , the density measurement value of the patch measured by the density measuring unit 104 is D _m , and the reference density value is _Dr , the density fluctuation rate is V _m = (D _m −D _r ) / D _r. Is derived by Further, the density variation estimation unit 108 acquires information indicating the shape of the dot pattern used for the measurement patch from the halftone processing unit 102 and information indicating the shape of the dot pattern of an arbitrary patch to be estimated. Here, information indicating the shape of the dot pattern is referred to as a dot shape parameter. For example, the dot shape parameter is represented by a dot area and a dot circumference in a predetermined unit region. The example of FIG. 2B shows a pattern in which a predetermined unit region is 4 × 4 pixels, and a 2 × 2 pixel at the center is turned on to form one dot. FIG. 2B shows an example in which six such patterns are arranged. In the example of FIG. 2B, the dot area as the dot shape parameter indicates the number of pixels in which dots are turned on in a predetermined unit area, and is 4. Further, the dot circumference is 1 when the length of one side of the pixel is 1, and the circumference of the edge of the entire dot in a predetermined unit region is 8. Here, the dot shape parameter is the dot circumference / dot area, and the dot shape parameter F _m of the measurement patch is 8/4 = 2.

The density fluctuation estimation unit 108 obtains a density fluctuation characteristic model from the dot shape parameter F _m of the measurement patch and the density fluctuation rate V _m . Then, the density model of the arbitrary patch is estimated by applying the characteristic model of the density fluctuation to the shape parameter of the dot of another arbitrary patch.

  It has been found by the inventor's verification that there is a proportional relationship between the dot shape parameter (dot circumference / dot area) and the density fluctuation rate. Therefore, a characteristic model of density fluctuation is constructed using this proportional relationship. The density variation characteristic model is defined as V = G × F where the dot perimeter / area is the shape parameter F, the density variation rate of the image is V, and the density variation gradient is G. Here, the density fluctuation gradient G indicates the slope of a linear function established between the dot shape parameter F and the density fluctuation rate V. The density fluctuation gradient G is a parameter that changes in accordance with conditions when the image forming unit 103 outputs an image. In order to construct a characteristic model of density fluctuation when the image forming unit 103 outputs an image under a certain condition, it is necessary to obtain the density fluctuation gradient G.

The calculation of the density variation characteristic model V = G × F will be described. FIG. 14 is a graph for explaining a characteristic model of density variation. The graph shown in FIG. 14 shows that the image forming unit 103 outputs a patch image including a dot pattern represented by the halftone image data shown in FIG. This is a result of creating a characteristic model of fluctuation. With respect to the reference density _{D r} of the patch image, when the result obtained by measurement is the concentration _{D m,} as described above, the concentration change rate _{V m} is defined in _{(D m -D r) / D} r . In the dot pattern shown in FIG. 2B, as described above, the shape parameter F _m is derived as F _m = 2 (= 8/4). From these two values, the position of P _m points associating the dot shape parameters and the concentration of the variation rate is shown as in Figure 14. The slope of the straight line passing through the origin at this time from the point _{P m} (gradient) is obtained as _{G = (D m -D r)} / 2D r. From the above, the characteristic model of density fluctuation in FIG. 14 is defined as V = (D _m −D _r ) / 2D _r × F. When the density variation characteristic model is created, the density variation rate V can be uniquely derived by inputting the dot shape parameter F of an arbitrary patch image. That is, the density variation rate V is obtained for halftone image data (patch image) representing a dot pattern composed of dots having an arbitrary dot shape. Then, by using the obtained density fluctuation rate V, it is possible to estimate the image density when an arbitrary patch image is actually output as a toner image.

  Using the method described above, the density of one measurement patch whose dot shape is known can be measured, and the gamma correction table for the halftone whose dot shape is known can be corrected. One measurement patch is a patch having a single density, not a patch having a plurality of halftone densities. In this embodiment, a process of generating (updating) a more accurate gamma correction table when performing calibration based on the density of a plurality of patches estimated in this way will be described. In the present embodiment, an example in which a medium density measurement patch is used when the first page is printed and the measurement patch is switched to a different patch when the subsequent page is printed will be described.

  Next, FIG. 3 shows a processing flow performed in this embodiment. The processing shown in FIG. 3 is realized by the CPU reading a program stored in a ROM (not shown) to the RAM and functioning as each unit shown in FIG. Of course, it may be realized by hardware that performs equivalent processing.

  The processing in FIG. 3 is performed when a print job is first received, for example, after power is turned on, when a cartridge is replaced, or when an environmental change such as temperature or humidity occurs. Here, the update process of the gamma correction table is described, and the description of the process of printing the input image using the input image data included in the print job is omitted. The gamma correction itself of the input image is performed using the updated LUT stored in the LUT storage unit 105 by the gamma correction unit 100 after the gamma correction table shown in FIG. 3 is updated.

  In step S301, control unit 110 performs initial setting. For example, the control unit 110 initializes the LUT stored in the LUT storage unit 105 and sets the reference density value of the medium density patch stored in the patch density storage unit 107.

  The next steps S302 to S305 show the processing flow for updating the gamma correction table for the first page. More specifically, when the first page is printed, a measurement patch for density estimation is generated and its density is measured, and the gamma correction table is updated by the above-described method based on the measurement result. Hereinafter, the processing of each step will be described.

  In step S302, the control unit 110 selects a medium density patch as a measurement patch. The control unit 110 controls the dot pattern generation unit 101 to select the medium density patch shown in FIG. 2B and cause the dot pattern generation unit 101 to generate it. Although an example in which the selected patch is generated has been described here, processing may be performed in which a patch is generated in advance, and the generated patch is selected in step S302.

  In step S303, the image forming unit 103 outputs the patch image of the medium density patch selected in step S302 on the intermediate transfer belt as a measurement patch. In step S304, the density measurement unit 104 measures the patch density of the measurement patch output on the intermediate transfer belt in step S303, and acquires a patch density measurement value. In step S305, the gamma correction table is updated. The detailed procedure of step S305 will be described. First, the density fluctuation rate deriving unit 106 calculates a density difference using the reference density value stored in the patch density storage unit 107 and the patch density measurement value obtained in step S304, and calculates the density fluctuation rate. A derivation process is performed. Thereafter, the density fluctuation estimation unit 108 estimates the density fluctuation of each patch on the grid point of the gamma correction table based on the density fluctuation rate derived by the density fluctuation rate deriving unit 106, and from the density fluctuation estimated value of each patch. The density value of each patch not formed by the image forming unit 103 is estimated. Then, the gamma correction table update unit 109 derives gamma correction information based on the measured or estimated patch density value. The gamma correction information is information used when updating the gamma correction LUT stored in the LUT storage unit 105. For example, an input density value, an LUT indicating an output density value corresponding to the input density value, information indicating a correction amount of the LUT density value stored in the LUT storage unit 105, and the like. The gamma correction table update unit 109 updates the gamma correction table (LUT) stored in the LUT storage unit 105 based on the gamma correction information. Here, the update of the gamma correction table includes replacing the LUT already stored in the LUT storage unit 105 with a newly generated LUT or correcting the stored LUT. The patch density estimation value estimated by the density fluctuation estimation unit 108 is recorded in the patch density storage unit 107.

  The processing in the next steps S306 to S309 is processing at the time of printing the second page after the printing of the first page is finished. Here, for convenience of explanation, the description will be made as processing at the time of printing the second page, but the present invention is not limited to this. For example, the processes in steps S306 to S309 may be performed on the third page, the fifth page, and the like.

  In step S306, the control unit 110 controls the dot pattern generation unit 101 to select the high density patch shown in FIG. 2C and generate it as a measurement patch. In step S307, the image forming unit 103 outputs the generated measurement patch onto the intermediate transfer belt, similarly to step S303. The measurement patch output here is based on the patch image corresponding to the high density patch. In step S308, the density measuring unit 104 measures the patch density of the measurement patch output on the intermediate transfer belt in the same manner as in step S304. Here, the patch density of the patch image corresponding to the high density patch output as the measurement patch in step S307 is measured. In step S309, the gamma correction table is updated as in step S305. In the process of step S309, the estimated density value of the high density patch estimated in the process of step S305, that is, estimated in the first page is read from the patch density storage unit 107. The density variation rate is derived based on the estimated density value of the high density patch and the density measurement value measured in step S308.

  The next processing in steps S310 to S313 is processing at the time of printing the third page. Of course, for the sake of convenience of explanation, it is only described as the third page, and is not limited to the third page. In step S310, the control unit 110 controls the dot pattern generation unit 101 to select the low density patch shown in FIG. 2A and generate it as a measurement patch. In step S311, the image forming unit 103 outputs the generated measurement patch onto the intermediate transfer belt, similarly to step S303. In step S312, the density measuring unit 104 measures the patch density of the measurement patch output on the intermediate transfer belt in the same manner as in step S304. Here, the patch density of the patch image corresponding to the low density patch output as the measurement patch in step S311 is measured. In step S313, the gamma correction table is updated as in step S305. In the process of step S311, the density fluctuation rate is derived by reading out the estimated density value of the low density patch estimated in the process of step S309, that is, estimated in the second page from the patch density storage unit 107. Done.

  Step S314 shows the processing flow for updating the gamma correction table for the fourth and subsequent pages, and the same processing as in steps S306 to S313 is repeated. That is, the accuracy of the gamma correction table is improved by repeating the calibration with the high-density patch and the low-density patch, which have high sensitivity.

  Here, FIG. 4 shows a conceptual diagram of a method for deriving a density difference that is a source of the density fluctuation rate derived by the density fluctuation rate deriving unit 106 in each density patch. When using the medium density patch, the density difference 402 is derived from the medium density measurement value 401 and the medium density reference density value 400. On the other hand, when the high density patch is used, the density difference 405 is derived from the high density measured value 404 and the previous high density estimated value 403. When a low density patch is used, a density difference 408 is derived from the low density measurement value 407 and the previous low density estimation value 406.

  According to the present embodiment, on the first page such as when the power is turned on or when the cartridge is replaced, a gamma correction table is generated using a medium density patch that is easy to stably hit dots and has a wide tolerance range for fluctuation. The density correction can be surely performed. In addition, the subsequent update of the gamma correction table uses high-density patches and low-density patches that are highly sensitive to density fluctuations. Therefore, when multiple sheets are printed, gamma correction is performed using patches from low density to high density. The table will be updated. This makes it possible to update the gamma correction table with high accuracy over the entire area, as compared with the case where the gamma correction table is updated only with the medium density patch. That is, there is an effect that high-precision gamma correction can be performed.

  In this embodiment, the timing for switching the measurement patch is described for each page. However, the measurement patch may be switched for every predetermined number of pages other than one page. Further, the processing using the high-density patches in S306 to S309 and the processing using the low-density patches in S310 to S313 may be performed in reverse order.

[Example 2]
In the first embodiment, when a low density patch or a high density patch is output as a measurement patch, the density difference from the previous low density estimated value or the previous high density estimated value is obtained. In the second embodiment, even when a low density patch or a high density patch is output as a measurement patch, the density difference between the low density reference density value and the high density reference density value, which are the respective reference density values, is derived. The density difference between the high density reference density value and the low density reference density value is referred to herein as a density fluctuation value.

  In the second embodiment, when the density variation value (density difference from the reference density) is larger than a certain value, the gamma correction table is updated using the medium density patch on the next page. That is, when the density fluctuation value derived when a certain page (page A) is calibrated is larger than a predetermined value, the calibration of page A is performed using the derived density fluctuation value. On the other hand, in the calibration at the time of printing the subsequent page (page B), a medium density patch is output instead of outputting a high density or low density patch as a measurement patch. Then, the medium density patch measurement patch is measured, the above-described processing is performed, and the gamma correction table is updated. This prevents the density fluctuation value from exceeding the allowable range of the high density patch and the low density patch. As a result, even if there is a large fluctuation, it is possible to immediately suppress the density fluctuation. That is, when the density fluctuation value is large, followability is emphasized, and when the density fluctuation value is small, correction accuracy is emphasized.

  Details of this embodiment will be described with reference to FIG. The same functions as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted. The density fluctuation estimation unit 500 estimates the density of an arbitrary patch from the density fluctuation rate derived by the density fluctuation rate deriving unit 106. However, it differs from the density fluctuation estimation unit 108 of the first embodiment in that the estimated density of each patch is not stored in the patch density storage unit 107. Also, the control flow of the control unit 501 is different.

  Next, FIG. 6 shows a processing flow performed in the present embodiment. The same steps as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  First, in step S <b> 601, the control unit 501 initializes the LUT stored in the LUT storage unit 105 and sets the reference density values of the low density, medium density, and high density patches stored in the patch density storage unit 107. To do. In the first embodiment, the reference density value of the medium density patch is set. In the second embodiment, the reference density values of the low density and high density patches are also set.

  Steps S302 to S602 are a flow for generating a gamma correction table using a medium density patch. Note that steps S302 to S602 correspond to processing for printing the first page, as in the first embodiment. Further, in this embodiment, when it is determined that the density fluctuation is large in the process described later, the processes of steps S302 to S602 are performed for the process of the next page.

  In steps S302 to S304, as in the first embodiment, the output of the patch image of the medium density patch and the density measurement are performed. In step S602, the gamma correction table is updated as in step S305. In this process, the estimated density value of each patch estimated by the density fluctuation estimation unit 108 is not recorded in the patch density storage unit 107.

  Next, steps S306 to S603 show a flow of generating a gamma correction table using a high density patch. Similar to the first embodiment, the processing in steps S306 to S603 is processing for printing a page different from steps S302 to S602. In steps S306 to S308, as in the first embodiment, the output of the patch image of the high density patch and the density measurement are performed. In step S603, the gamma correction table is updated as in step S602. In step S603, the density variation rate is derived by reading the reference density value of the high density patch set in step S601 from the patch density storage unit 107. That is, the density fluctuation value that is the density difference is derived as the density fluctuation rate using the reference density value instead of the estimated density value. The density fluctuation estimation unit 108 estimates the density value of each patch from the density fluctuation rate based on the density difference, and the gamma correction table update unit 109 updates the gamma correction table using the estimated density value of each patch. To do. In step S604, the control unit 501 determines whether the density fluctuation value derived by the density fluctuation estimation unit 108 in the process of step S603 is greater than a predetermined threshold value. If larger, the process returns to step S302. In the subsequent calibration of the page, the follow-up property of the gamma correction table is guaranteed by updating the gamma correction table by using the patch for measurement of the medium density patch having low sensitivity. On the other hand, if the density variation value is smaller than the predetermined threshold value, the process proceeds to step S311. In the subsequent calibration of the page, the gamma correction table is updated using the low-sensitivity patch with high sensitivity, thereby density based on the gamma correction table. Ensures correction accuracy.

  Steps S310 to S605 indicate page calibration processing after step S306 to step S603, and more specifically, update processing of the gamma correction table using the low density patch. In steps S310 to S312, as in the first embodiment, the output of the low density patch measurement patch and the density measurement are performed.

  In step S605, the gamma correction table is updated as in step S602. However, in step S605, the density fluctuation rate is derived by the density fluctuation estimating unit 108 reading out the reference density value of the low density patch set in step S601 from the patch density storage unit 107. In step S606, it is determined whether the density fluctuation value is larger than a predetermined threshold value. If larger, the process returns to step S302. In the calibration of the next page, the gamma correction table is updated by using a medium density patch having a wide tolerance range for fluctuation as a measurement patch. As a result, the density fluctuation value is prevented from exceeding the allowable range of the measurement patch, and the followability of the gamma correction table is guaranteed. That is, even when the density fluctuation becomes large, the gamma correction table can be corrected without exceeding the allowable range, and the correction accuracy (followability) by the gamma correction table is improved. On the other hand, if the density fluctuation value is smaller than the predetermined threshold value, the process proceeds to step S607. Step S607 is a flow of the subsequent page calibration process, and the processes of S306 to S606 are repeated.

  Here, FIG. 7 shows a conceptual diagram of a density difference deriving method by the density variation rate deriving unit 106 in each density patch. When the medium density patch is used, the density difference 402 is derived from the medium density measured value 401 and the medium density reference density value 400 as in the first embodiment. When a high density patch is used, a density difference 705 is derived from the high density measurement value 704 and the high density reference density value 703. When the low density patch is used, the density difference 708 is derived from the low density measurement value 707 and the low density reference density value 706.

  In this embodiment, the process using the high density patch in steps S306 to S603 and the process using the low density patch in steps S310 to S605 may be switched. Further, the threshold value used for determining the density fluctuation value is not fixed and may be switched according to the measurement patch. Further, the threshold value may be set separately according to the sign of the density fluctuation value (density difference from the reference density). For example, in the high-density patch, the allowable fluctuation range on the negative side (thinning direction) is large, but the allowable fluctuation range on the positive side (darkening direction) is small. On the other hand, in the low-density patch, the fluctuation tolerance range on the negative side (lightening direction) is small, but the fluctuation tolerance range on the positive side (darkening direction) is large. In this embodiment, since it is important to control so as not to exceed the allowable fluctuation range, it is desirable to change the threshold value according to the measurement patch.

[Example 3]
In the first or second embodiment, the example in which one measurement patch is used has been described. In the third embodiment, an example in which two patch images are output as measurement patches will be described. Specifically, an example will be described in which one high density and one low density are used simultaneously as measurement patches. Thereby, the accuracy when updating the gamma correction table is improved as compared with the first and second embodiments. It should be noted that the penalty with respect to the printing speed due to the arrangement of the measurement patches on the left and right of the intermediate transfer belt in the main scanning direction is the same as in the case of one patch, and is superior to the case of using many halftone patches. is there.

  Details of this embodiment will be described with reference to FIG. The same functions as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted. The dot pattern generation unit 800 of the present embodiment selects two of the 4 × 4 screen pattern low density, medium density, and high density patch images shown in FIGS. 2A to 2C and selects them. A patch image is generated as a measurement patch. The image forming unit 801 according to the present exemplary embodiment outputs the selected and generated two measurement patches to the left and right ends of the intermediate transfer belt.

  The density measuring unit 802 of this embodiment detects the density of two patches on an intermediate transfer belt (not shown) in the image forming unit 801. In order to measure the measurement patches at the left and right ends, two optical sensors may be used. The density variation rate deriving unit 803 according to the present embodiment reads the density of two patches estimated by the density variation estimation unit 805 at the previous density correction from the patch density storage unit 804 and measures the two patches measured at the current density correction. The density difference from the patch density is calculated, and the density fluctuation rate is derived. The density fluctuation estimation unit 805 estimates the density value of an arbitrary patch from the density fluctuation rates for two patches derived by the density fluctuation rate derivation unit 803. Then, the estimated density value of each estimated patch is recorded in the patch density storage unit 804. A control unit 806 controls a series of printing flows.

  Next, FIG. 9 shows a processing flow performed in the present embodiment. First, in step S <b> 901, the control unit 806 initializes the LUT stored in the LUT storage unit 105 and sets the reference density value of the medium density patch stored in the patch density storage unit 804.

  Steps S902 to S905 indicate the calibration process when printing the first page. In step S902, the control unit 806 controls the dot pattern generation unit 800 to select the medium density patch shown in FIG. 2B and generate it as a measurement patch. In step S903, the image forming unit 801 outputs the medium density measurement patches selected in step S902 to the left and right ends of the intermediate transfer belt. In step S904, the density measurement unit 802 measures the patch density of the measurement patch output in step S902. In step S905, the gamma correction table is updated. As a detailed procedure in step S905, the density variation rate deriving unit 803 calculates the density difference of two measurement patches at the left and right ends using the reference density value of the medium density patch stored in the patch density storage unit 804. Then, the density fluctuation rate is derived. Thereafter, the density fluctuation estimation unit 805 derives an average value of the two density fluctuation rates, and estimates the density fluctuation of the arbitrary patch from the average value. Then, the density fluctuation estimation unit 805 estimates the density value of the arbitrary patch from the density fluctuation value of the arbitrary patch. Then, the gamma correction table update unit 109 derives gamma correction information from the estimated or measured density value of each patch and sets the gamma correction table. The estimated patch density is stored in the patch density storage unit 804.

  In the next steps S906 to S908, as a result of performing the processing of the first page, the control unit 806 determines which patches are to be generated on the left and right ends on the intermediate transfer belt in the printing of the second and subsequent pages. is there. In step S906, the control unit 806 compares the right end patch density and the left end patch density on the intermediate transfer belt measured in step S904. If the right end patch density is higher than the left end patch density, the process advances to step S907, and the control unit 806 places the low density patch shown in FIG. It is determined that the high density patch shown in FIG. On the other hand, if the right end patch density is lower than the left end patch density, the process advances to step S908, and the control unit 806 applies the high density patch shown in FIG. 2C to the right end on the intermediate transfer belt at the next calibration. It is determined that the low density patch shown in FIG. Note that the right end and the left end may be on the right side and the left side relative to the central portion on the intermediate transfer belt, and may not be generated on a complete end.

  The next steps S909 to S911 show a calibration process when printing the second and subsequent pages. In step S909, the image forming unit 801 outputs the measurement patches of the density determined in step S907 or S908 as measurement patches to the left and right ends of the intermediate transfer belt determined in step S907 or step S908. In step S910, the density measurement unit 802 measures the patch density of the measurement patches output to the left and right ends. Thereafter, in step S911, a gamma correction table for each of the low density side and the high side is generated from the estimated density value based on the density variation of the low density measurement patch and the estimated density value based on the high density measurement patch. . That is, the density fluctuation estimation unit 805 derives the estimated density value of each patch based on the low density measurement patch, and the gamma correction table update unit 109 calculates the derived estimated density value and the measured density value. A gamma correction table is generated based on the above. Similarly, the density variation estimation unit 805 derives the estimated density value of each patch based on the high density measurement patch, and the gamma correction table update unit 109 calculates the derived estimated density value and the measured density value. Based on the above, a gamma correction table is generated. In this way, a gamma correction table is generated from each of the low density measurement patch and the high density measurement patch. However, these gamma correction tables may not be the same due to density sticking to the high density side or the low density side due to density fluctuation. Therefore, an update gamma correction table is generated by complementing the gamma correction table based on the low density measurement patch and the gamma correction table based on the high density measurement patch with a known spline curve or the like.

  As described above, in the present embodiment, the measurement is performed by outputting the low-density patch as the darker one and the high-density patch as the measurement patch in the lighter left and right in the previous calibration process. As a result, even when density variation occurs in a highly sensitive patch, the density of the low density patch sticks to the low density side, or the density of the high density patch sticks to the high density side, making it impossible to measure. Can be suppressed. Therefore, it is possible to secure a dynamic range of high sensitivity patch measurement, and to obtain the advantages of estimating the density of each density patch from the measurement result of the high sensitivity patch. As a result, there is an effect that a highly accurate update of the gamma correction table can be realized.

  In this embodiment, the calibration is performed for each page from the second page onward, but may be performed for each predetermined number of pages. Further, as described in the second embodiment, the variation allowable range is different between the high density patch and the low density patch. Therefore, by using only the value of the fluctuation allowable range (that is, excluding the value that cannot be measured due to sticking due to density fluctuation), it is possible to achieve both patch measurement sensitivity and dynamic range.

  In the above embodiment, the gamma correction table is interpolated according to the density, but the present invention is not limited to this. When the measured values at the left and right ends are within the allowable range of patch fluctuations, the gamma correction table may be interpolated after multiplying the density value of each gamma correction table by a coefficient corresponding to the distance from the left and right ends. good.

[Example 4]
The fourth embodiment is basically the same processing as the third embodiment. However, in the fourth embodiment, when the high density patch and the low density patch are used, the density difference is derived using the reference density value as in the second embodiment, and the gamma correction table is updated. If the density fluctuation is larger than a certain value, the next page updates the gamma correction table using the medium density patch. As a result, even when the density fluctuation increases, the gamma correction table can be corrected without exceeding the allowable range, and the accuracy (followability) of updating the gamma correction table is improved.

  Details of this embodiment will be described with reference to FIG. The same functions as those in the third embodiment are denoted by the same reference numerals and description thereof is omitted. The density fluctuation estimation unit 1000 estimates the density of an arbitrary patch from the density fluctuation rate derived by the density fluctuation rate derivation unit 803. However, it differs from the density variation estimation unit 805 in that the estimated density is not stored in the patch density storage unit 804. Also, the control flow of the control unit 1001 is different.

  Next, FIG. 11 shows a processing flow performed in this embodiment. First, in step S1101, the control unit 1001 initializes the LUT stored in the LUT storage unit 105, and the reference density of each of the low density, medium density, and high density patches stored in the patch density storage unit 804. Set the value.

  Steps S902 to S1102 show the flow of updating the gamma correction table using the medium density patch. Similar to the second embodiment, this process includes a process for the first page and a process for the next page determined to have a large density fluctuation.

  In steps S902 to S904, the output of the medium density patch and the density measurement are performed as in the third embodiment. In step S1102, the gamma correction table is updated as in step S905. Unlike step S905, the estimated patch density is not stored in the patch density storage unit 804. In steps S <b> 906 to S <b> 908, as in the third embodiment, the control unit 1001 determines which patches are to be generated on the left and right edges on the intermediate transfer belt in the calibration for printing the next page.

  Steps S909 to S1104 show a processing flow of calibration at the time of printing the next page. In steps S909 to S910, as in the third embodiment, the patch images of the patches determined in step S907 or S908 are output as measurement patches to the left and right ends of the intermediate transfer belt. Then, the density measuring unit 802 measures the patch density. In step S1103, the gamma correction table is updated in the same procedure as in step S911. Unlike step S911, the estimated patch density is not stored in the patch density storage unit 804.

  In step S1104, the control unit 1001 determines whether the density fluctuation rate (value) of any of the left and right patches is greater than a predetermined threshold. If it is larger, the process returns to step S902, and on the next page, the gamma correction table is updated by using the patch for measurement for the medium density patch having low sensitivity, and the accuracy of the gamma correction table is guaranteed. As a result, even when the density fluctuation increases, the gamma correction table can be corrected without exceeding the allowable range, and the accuracy (followability) of generating the gamma correction table is improved. If it is smaller than the predetermined threshold value, the process proceeds to step S1105. Step S1105 is processing at the time of printing the next page of steps S909 to S1104, and the processing of steps S909 to S1104 is repeated.

  As described in the second embodiment, the variation allowable range is different between the high density patch and the low density patch. Therefore, by changing only patches that are likely to exceed the allowable fluctuation range to medium-density patches, it is possible to achieve both patch measurement sensitivity and dynamic range.

[Example 5]
In the fifth embodiment, an example in which a high-density patch or a low-density patch is switched according to the density difference will be described. Thereby, the accuracy of updating the gamma correction table is improved as compared with the first embodiment.

  Since the configuration diagram of the present embodiment is the same as that of the first embodiment, the description thereof is omitted. However, since the control of the control unit 110 is different, it will be described with reference to FIG.

  In step S1201, the printing flow for pages 1 to 3 shown in steps S301 to S313 in FIG. 3 is performed, and the density measurement unit 104 measures the density using the measurement patches determined on each page.

  Next, steps S1202 to S1207 show the printing flow of the fourth page. In step S1202, the control unit 110 evaluates whether the density derived by the density variation rate deriving unit 106 on the fourth page has increased. If it has increased, the process advances to step S1203 to cause the dot pattern generation unit 101 to select the low density patch shown in FIG. If it has decreased, the process advances to step S1204 to cause the dot pattern generation unit 101 to select the high density patch shown in FIG. In step S1205, the image forming unit 103 outputs the patch image having the selected density as a measurement patch on the intermediate transfer belt. In step S1206, the density measurement unit 104 measures the patch density. After that, the gamma correction table is updated in step S1307 when the high density patch is selected, and in step S1207 as in step S313 when the low density patch is selected. Step S1208 is the flow on and after the fifth page, and the processing of steps S1202 to S1207 is repeated.

  As described in the second embodiment, the variation allowable range is different between the high density patch and the low density patch. For example, in the high-density patch, the allowable fluctuation range on the negative side is large, but the allowable fluctuation range on the positive side is small. On the other hand, in the low density patch, the negative fluctuation tolerance is small, but the positive fluctuation tolerance is large. In this embodiment, this variation allowable range imbalance is used to select a patch having a wider variation range with respect to the variation direction (density increases / decreases), that is, according to increase / decrease in the density difference. The patch for measurement is switched between the high density patch and the low density patch. This improves the accuracy and followability of the gamma correction table. Note that the patch for measurement may be switched according to the measured value of the patch (difference value from the reference density) instead of increasing or decreasing the density difference. That is, when measuring with a low density patch, when the measured value of the patch falls below the first predetermined value, the patch is switched to the high density patch, and when measured with the high density patch, the measured value of the patch is the second predetermined value. If this is the case, it may be switched to a low density patch.

  In this embodiment, the calibration is performed for each page, but may be performed for each predetermined number of sheets.

[Example 6]
The sixth embodiment performs basically the same processing as the fifth embodiment, but when the density fluctuation rate is larger than a predetermined threshold, it is determined that the gamma correction table cannot be updated with high accuracy, and the medium density with low sensitivity is determined. The difference is that the gamma correction table is regenerated using the patch for measurement. This guarantees the accuracy of the gamma correction table. Since the configuration diagram of the present embodiment is the same as that of the first embodiment, the description thereof is omitted. However, since the control of the control unit 110 is different, it will be described with reference to FIG.

  In step S1301, the printing flow for pages 1 to 3 shown in steps S301 to S313 in FIG. 3 is performed, and the density measurement unit 104 measures the density using the measurement patch determined on each page.

  The next steps S1302 to S1309 show the printing flow of the fourth page. In step S1302, the control unit 110 determines whether the density fluctuation rate derived by the density fluctuation rate deriving unit 106 on the fourth page is greater than a predetermined threshold (for example, 30%). If larger, the process advances to step S1303, and the control unit 110 causes the dot pattern generation unit 101 to select and generate the medium density patch shown in FIG. On the other hand, if it is smaller, the process advances to step S1304 to determine whether or not the density derived by the density fluctuation rate deriving unit 106 on the fourth page has increased. If it has increased, the process advances to step S1305, and the control unit 110 causes the dot pattern generation unit 101 to select and generate the low density patch shown in FIG. On the other hand, if it has decreased, the process proceeds to step S1306, and the dot pattern generation unit 101 selects and generates the high density patch shown in FIG. Thereafter, the process advances to step S1307, and the image forming unit 103 outputs the patch image having the selected density as a measurement patch on the intermediate transfer belt. In step S1308, the density measuring unit 104 measures the patch density, and in step S1309, the gamma correction table is updated in the same procedure as in step S306 described above.

  Step S1310 is a flow for the fifth and subsequent pages, and the processing of steps S1302 to S1309 is repeated. That is, when the density fluctuation rate is larger than a predetermined threshold, a gamma correction table is generated by using a patch for measurement with a medium density patch having low sensitivity (a wide tolerance range for fluctuation), and the accuracy of the gamma correction table ( (Followability) is guaranteed.

  In this embodiment, the calibration is performed for each page, but may be performed for each predetermined number of sheets.

(Other examples)
The present invention supplies a program that realizes one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program This process can be realized. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

DESCRIPTION OF SYMBOLS 100 Gamma correction part 101 Dot pattern generation part 102 Halftone process part 103 Image formation part 104 Density measurement part 105 LUT memory | storage part 106 Density fluctuation rate derivation part 107 Measurement patch density memory | storage part 108 Density fluctuation estimation part 109 Gamma correction table update part 110 Control unit

Claims (16)

Output means for outputting a patch image composed of a specific dot pattern;
Measuring means for measuring the density of the patch image output by the output means;
Based on the difference between the density read from the storage means for storing the density of the patch image output by the output means and the density measured by the measurement means, the estimated density value of the patch constituted by another dot pattern Derivation means for deriving
Updating means for updating information used in gamma correction using the derived estimated density value;
Control means for switching the patch image output by the output means between a first patch image having a wide tolerance range of density fluctuation and a second patch image having a narrow tolerance range of density fluctuation at a predetermined timing. An image forming apparatus.   The control means causes the output means to output the second patch image after the information is updated by the update means in response to the output of the first patch image by the output means. The image forming apparatus according to claim 1, wherein:   3. The control unit according to claim 1, wherein when the difference is larger than a predetermined threshold, the control unit switches the patch image output from the output unit to the first patch image at the predetermined timing. 4. The image forming apparatus described. The second patch image includes a plurality of types of patch images,
The switching means switches between the plurality of types of second patch images at the predetermined timing after switching from the first patch image to the second patch image. The image forming apparatus according to any one of claims 1 to 3. The control means includes
Determine whether the concentration has increased based on the difference,
When the density increases, at the predetermined timing, switch to a low density patch image among the plurality of types of second patch images,
5. The image forming apparatus according to claim 4, wherein when the density is not increased, the image forming apparatus switches to a high-density patch image among the plurality of types of second patch images at the predetermined timing. 6. The output means outputs a plurality of the patch images arranged in the main scanning direction,
The deriving means derives the estimated density value based on a difference between the density read from the storage means and the average of the densities of a plurality of patch images measured by the measuring means,
The control means outputs a low-density patch image as the second patch image at a position where the density of the plurality of patch images measured by the measuring means is high, and the control means outputs the first density image at a position where the density is low. 5. The image forming apparatus according to claim 4, wherein control is performed so as to output a high-density patch image as the second patch image.   The update means includes first information obtained using the estimated density value derived from the low-density patch image and second information obtained using the estimated density value derived from the high-density patch image. The image forming apparatus according to claim 6, wherein the information used for the gamma correction is updated by complementing the information.   The image forming apparatus according to claim 7, wherein the update unit supplements the first information and the second information obtained by using a value within an allowable range of the density fluctuation.   The image forming apparatus according to claim 7, wherein the update unit performs the complementation using a coefficient corresponding to a position where the patch image is output.   The derivation means estimates a density fluctuation rate based on the difference, and derives the estimated density value of a patch composed of the other dot pattern from the estimated density fluctuation rate. The image forming apparatus according to claim 9. The derivation means includes
Storing the estimated density value of the patch composed of the derived other dot pattern in the storage means;
In the derivation process after the switching is performed by the control unit, the estimated density value of the patch corresponding to the patch image used after the switching is stored, which is stored in the storage unit, is read. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the derivation unit reads a predetermined reference density value from the storage unit.   The image forming apparatus according to claim 1, wherein the predetermined timing is a timing in units of a predetermined page to be printed.   The image forming apparatus according to claim 1, wherein the update unit updates a gamma correction table used in the gamma correction. An output step of outputting a patch image composed of a specific dot pattern;
A measurement step for measuring the density of the patch image output in the output step;
Based on the difference between the density read from the storage means for storing the density of the patch image output in the output step and the density measured in the measurement step, the estimated density value of the patch constituted by another dot pattern A derivation step for deriving
An update step of updating information used in gamma correction using the derived estimated density value;
A control step of switching the patch image output in the output step between a first patch image having a wide allowable density variation range and a second patch image having a narrow allowable density variation range at a predetermined timing. And a control method for the image forming apparatus.   The program for functioning a computer as each means of the image forming apparatus as described in any one of Claims 1-14.

Images