JP2006261811A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus suppressing the extreme correction of density. <P>SOLUTION: In the apparatus 10 for forming an image, the gradation of image data inputted on the basis of specified reference-gradation characteristics and measured density measured by a density measuring section is corrected and the image is formed on the basis of the corrected image data. Such a device for forming the image has a set-gradation characteristic acquiring section 110 obtaining new set-gradation characteristics on the basis of the measured density measured by the density measuring section and a gradation-characteristic selecting section 111 selecting either of the reference-gradation characteristics and the set-gradation characteristics. The device further has a gradation correcting section 107 correcting gradation on the basis of selected gradation characteristics and the measured density measured by the density measuring section. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus having a density correction function.

Recent image forming apparatuses such as printers and copiers can form highly accurate images. However, since the quality of an image to be formed varies depending on the aging of the apparatus and environmental changes such as temperature and humidity, as disclosed in Patent Document 1, gradation and density A device for maintaining the quality of the image by correcting the relationship is devised.
Japanese Patent Laid-Open No. 10-164305

  However, when there is a large variation from the reference image characteristics, there is a clear difference in the quality of the output image before and after correction. In particular, when density correction is performed for each predetermined printing unit, if density correction is performed during the printing process of a plurality of pages, the hue and density are extremely changed before and after the correction is output. This was a problem.

  In view of the foregoing problems, an object of the present invention is to provide an image forming apparatus capable of suppressing extreme density correction.

The present invention adopts the following configuration in order to solve the above points.
In an image forming apparatus that performs gradation correction of input image data based on a predetermined reference gradation characteristic and a measured density measured by a density measuring unit, and forms an image based on the corrected image data, density measurement A set tone characteristic acquisition unit that obtains a new set tone characteristic based on the measured density measured by the unit, a tone characteristic selection unit that selects either the reference tone characteristic or the set tone characteristic, and a selection And a tone correction unit that performs tone correction based on the measured tone characteristics and the measured density measured by the density measuring unit.

  The gradation characteristic selection unit selects the set gradation characteristic when the set gradation characteristic is acquired.

  In an image forming apparatus that performs gradation correction of input image data based on a predetermined reference gradation characteristic and a measured density measured by a density measuring unit, and forms an image based on the corrected image data, density measurement A setting gradation characteristic acquisition unit that obtains a new setting gradation characteristic based on the measured density measured by the unit, a weighting coefficient setting unit that sets a weighting coefficient indicating weighting for the reference gradation characteristic and the setting gradation characteristic, A gradation characteristic calculation unit that calculates a gradation characteristic for correction based on the reference gradation characteristic and the set gradation characteristic based on a weighting factor, and a measurement measured by the calculated gradation characteristic and density measurement unit And a tone correction unit that performs tone correction based on the density.

  According to the present invention, for example, by obtaining a new set tone characteristic based on the measured density measured by the density measuring unit for each predetermined printing unit, the tone correction is performed with the obtained new set tone characteristic as a target. It can be performed. Therefore, the result of the gradation correction has no change in hue or density difference compared to the correction using the reference gradation characteristics, and extreme density correction can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same constituent elements in the drawings used in each embodiment will be given the same reference numerals, and redundant description will be omitted as much as possible.

The image forming apparatus of the present invention is a color electrophotographic printer, a color electrophotographic copying machine or the like. In this embodiment, a color electrophotographic printer (hereinafter simply referred to as a printer) will be described as an example.
As shown in FIG. 1, the printer 10 of the present invention is based on an image processing unit 100 that corrects gradation based on a gradation correction table to correct the density of input image data and the corrected data. The print engine 101 that forms an image, the page counter 102 that counts the number of print media on which the image is formed, and the density of the gradation patch formed on the print engine 101 based on patch data indicating a predetermined patch pattern. When the count value in the density sensor 104 to be measured and the page counter 102 exceeds a predetermined value, an instruction is output to the image processing unit 100 to regenerate the gradation correction table, and when an instruction is received from the outside, it will be described later. In order to acquire custom target gradation characteristics, a gradation correction control unit 103 that outputs an instruction to the image processing unit 100 is provided.

  The image processing unit 100 includes a color conversion processing unit 105, a gradation correction table holding unit 106, a gradation correction processing unit 107, a gradation processing unit 108, a standard target gradation characteristic holding unit 109, and a custom target floor. A tone characteristic holding unit 110, a target tone characteristic selecting unit 111, and a tone correction table creating unit 112 are provided.

The color conversion processing unit 105 performs color conversion processing that matches the input image data with the color reproduction characteristics of the printer.
The gradation correction table holding unit 106 holds a gradation correction table for correcting the gradation of the color-converted image data.

The tone correction processing unit 107 corrects the tone of the color-converted image data based on the tone correction table held by the tone correction table holding unit 106.
The gradation processing unit 108 performs gradation processing for converting the data into dot pattern format data that can be processed by the print engine 101 based on the image data subjected to gradation correction.

The standard target gradation characteristic holding unit 109 holds a gradation characteristic predetermined as a standard of the printer as a standard gradation target characteristic (reference gradation characteristic).
The standard gradation target characteristic is used as an index (target) at the time of printing. In this embodiment, it is assumed that the standard gradation target characteristic has a linear characteristic, and the following description will be given.

The custom target gradation characteristic holding unit 110 holds the gradation characteristic obtained based on the density value measured by the density sensor 104 as a custom target gradation characteristic (set gradation characteristic).
The target tone characteristic selection unit 111 is held by the standard target tone characteristic holding unit 109 or the custom target tone characteristic holding unit 110 in accordance with an instruction from the tone correction control unit 103. One of the custom target gradation characteristics to be selected is selected.

  The tone correction table creation unit 112 generates a tone correction table based on the tone characteristics selected by the target tone characteristic selection unit 111 and holds the tone correction table in the tone correction table holding unit 106. .

Here, the operation of the printer 10 during normal printing will be described with reference to the flowchart of FIG.
When the image data is input to the image processing unit 100, the color conversion processing unit 105 performs color conversion processing that matches the color reproduction characteristics of the printer on the image data (step S1).

  In this embodiment, color conversion processing is performed by a known multidimensional LUT (Look Up Table) method. Specifically, in the signal based on the input image data, Cyan, Magenta, Yellow, Black (hereinafter referred to as “Red”, “Green”, “Blue” (hereinafter simply referred to as “R”, “G”, “B”)). Image data of a CMYK signal system is generated by performing signal interpolation processing based on a multidimensional LUT indicating a relationship with an output signal indicating simply C, M, Y, K).

After the color conversion processing, the tone correction processing unit 107 performs tone correction processing based on the tone correction table held by the tone correction table holding unit 106 (step S2).
In this embodiment, gradation correction processing is performed by a known one-dimensional LUT method. Specifically, correction processing is performed on the CMYK image data based on a gradation correction table (one-dimensional LUT) in which the relationship between input and output levels is indicated by, for example, 256 gradations, and C′M′Y ′ is obtained as a processing result. K ′ series image data is generated.

After the gradation correction processing, the gradation processing unit 108 performs gradation processing that converts the data into dot pattern format data that can be processed by the print engine 101 based on the corrected image data (step S3).
In this embodiment, gradation processing is performed by a known dither method, and image data of 256 gradation C′M′Y′K ′ series is converted into a two gradation dot pattern format. M ″ Y ″ K ″ series image data is generated.

  The print engine 101 performs a printing process on the print medium based on the C ″ M ″ Y ″ K ″ series image data, and forms an image on the print medium (step S4).

Next, the acquisition operation of the custom target gradation characteristic held by the custom target gradation characteristic holding unit 110 will be described with reference to the flowchart of FIG.
In this embodiment, the custom target tone characteristics are acquired in accordance with an instruction from the user. As an example, in the present embodiment, an example will be described in which the user instructs acquisition of custom target gradation characteristics when the total number of printed sheets reaches 100 from the initial state.

  The gradation correction control unit 103 is on standby until an acquisition request for a custom target gradation characteristic is received from the user via an operation panel (not shown) (step 11). The tone correction control unit 103 outputs image data indicating a predetermined patch pattern to the tone correction processing unit 107 as patch data.

  By the way, in this embodiment, the patch pattern has a gradation patch area ratio of 20 percent, that is, 0 percent, 20 percent, 40 percent, 60 percent, 80 percent, as the patch pattern, in each of C, M, Y, and K. Six patterns of percent and 100 percent are used. Here, when the 20 percent equal interval is expressed by 256 gradation levels from 0 to 255, 0 percent is 0 gradation level, 20 percent is 51 gradation level, 40 percent is 102 gradation level, 60 percent is 153 floor. The gradation level, 80 percent is the 204 gradation level, and 100 percent is the 255 gradation level.

The tone correction processing unit 107 performs tone correction processing on the patch data based on the tone correction table held by the tone correction table holding unit 106 (step S12).
In this embodiment, gradation correction processing is performed by a known one-dimensional LUT method, and C′M′Y′K ′ system patch data is generated as a processing result.

  After the gradation correction processing, the gradation processing unit 108 performs gradation processing for converting the data into a dot pattern format that can be processed by the print engine 101 based on the corrected patch data, and C ″ M ″ as a processing result. Y ″ K ″ patch data is generated (step S13).

The print engine 101 performs a printing process on the transfer belt based on the C ″ M ″ Y ″ K ″ system patch data, and forms a gradation patch on the transfer belt (step S14).
That is, gradation patches having a total of 24 types of gradation levels 0, 51, 102, 153, 204, and 255 are formed on the transfer belt in C, M, Y, and K, respectively.

  The density sensor 104 measures the density of the formed gradation patch (step S15). That is, the density sensor 104 has patches of gradation level 0, gradation level 51, gradation level 102, gradation level 153, gradation level 204, and gradation level 255 in each of C, M, Y, and K. Are measured, and a total of 24 kinds of density values are obtained.

  A part of the result measured by the density sensor 104 is shown in FIG. In FIG. 4, C (Cyan) in C, M, Y, and K is shown as an example, and gradation level 0, gradation level 51, gradation level 102, gradation level 153, gradation level 204, and The measured density value of each patch at the gradation level 255 is shown in a table and a graph.

  In the graph shown in FIG. 4A, the horizontal axis is the gradation level and the vertical axis is the measured density value, and the measurement result is indicated by a solid line. Note that the measurement results in FIG. 4 are the results when the total number of printed sheets has passed 100 from the initial state, and the measurement results are indicated by a solid line, which is the ideal target in the initial state (0th sheet). The gradation characteristics (linear characteristics) are indicated by broken lines.

FIG. 4 shows the measurement results of C (Cyan) in C, M, Y, and K as examples. However, since M, Y, and K other than C are similar measurement results, the description thereof is omitted. To do.
Further, in the following description, description will be made using the same graph and table as in FIG. 4 as appropriate, but in the description, C (Cyan) will be described as a representative, and other descriptions of M, Y, and K will be given. Is omitted as above.

  As is apparent from the graph of FIG. 4A, the measurement result is different from the gradation characteristics in the initial state even though the gradation correction processing is performed in step S12 described above prior to the density measurement. .

  By the way, the measurement result described above is held as a custom target tone characteristic by the custom target tone characteristic holding unit 110 (step S16), and the fact that the custom target tone characteristic has been acquired is not shown by the tone correction control unit 103. It is held in the storage unit (step S17).

Next, the update operation of the gradation correction table held by the gradation correction table holding unit 106 will be described with reference to the flowchart of FIG.
The page counter 102 counts the number of printed sheets. For example, every 200 sheets, a tone correction table update request held by the tone correction table holding unit 106 is output to the tone correction control unit 103.
The gradation correction control unit 103 is on standby until there is an update request from the page counter 102 (step S31), and the gradation correction control unit 103 that has received an instruction to update the gradation correction table from the page counter 102 Image data indicating a predetermined patch pattern is output to the gradation processing unit 108 as patch data.

  Here, patch data when the gradation correction table is updated will be described. The patch data is the same as the patch data at the time of obtaining the custom target gradation characteristics described above, but the output destination from the gradation correction control unit 103 is different. That is, when custom target tone characteristics are acquired, patch data is output to the tone correction processing unit 107, and tone correction processing is performed by the tone correction processing unit 107, but tone correction processing is performed when the tone correction table is updated. The tone correction processing is performed directly after the tone correction processing, that is, tone processing.

  Upon receiving the patch data from the gradation correction control unit 103, the gradation processing unit 108 performs gradation processing for converting the patch data into data in a dot pattern format that can be processed by the print engine 101, and C ″ M is obtained as a processing result. “Y” K ”patch data is generated (step S32).

  The print engine 101 performs a printing process on the transfer belt based on C ″ M ″ Y ″ K ″ patch data, and the gradation level is 0 on each of C, M, Y, and K on the transfer belt. , 51, 102, 153, 204, and 255 are formed as gradation patches (step S33).

  The density sensor 104 measures the density of the formed gradation patch (step S34), and for each of C, M, Y, and K, the gradation level 0, the gradation level 51, the gradation level 102, and the gradation level. 153, the density values of the patches of the gradation level 204 and the gradation level 255, that is, all 24 kinds of density values are acquired.

  A part of the result measured by the density sensor 104 is shown in FIG. In FIG. 6, C (Cyan) is shown as an example, and each patch at the gradation level 0, gradation level 51, gradation level 102, gradation level 153, gradation level 204, and gradation level 255 is shown. The measured concentration values are shown in tables and graphs.

  In the graph shown in FIG. 6A, the horizontal axis is the gradation level and the vertical axis is the measured density value, and the measurement result is indicated by a solid line. Note that the measurement results in FIG. 6 are the results when the total number of printed sheets has passed 200 from the initial state, and the measurement results are indicated by solid lines, which is the ideal target in the initial state (0th sheet). (Standard) gradation characteristics (linear characteristics) are indicated by broken lines.

  When the measured density value is acquired, the target tone characteristic selection unit 111 makes an inquiry to the tone correction control unit 103, and whether the custom target tone characteristic is acquired in the storage unit of the tone correction control unit 103. It is determined whether or not (step S35).

  The target gradation characteristic selection unit 111 acquires the standard target gradation characteristic to be applied to the gradation correction table update from the standard target gradation characteristic holding unit 109 when the custom target gradation characteristic is not acquired. When the tone characteristic is acquired, the custom target tone characteristic is acquired from the custom target tone characteristic holding unit 110.

  Note that the standard target gradation characteristics acquired from the standard target gradation characteristic holding unit 109 are shown in FIG. FIG. 7 shows standard target gradation characteristics taking C (Cyan) as an example, and the solid line in the graph of FIG. 7A shows the standard target gradation characteristics.

  The tone correction table creating unit 112 updates the tone correction table based on the target tone characteristics acquired by the selection by the target tone characteristic selecting unit 111 and the measurement result obtained by the density sensor 104 shown in FIG. (Step S36, Step S37).

Here, the tone correction table update process will be described in detail.
First, in the target gradation characteristic acquired by selection by the target gradation characteristic selection unit 111, a density value at each gradation level is obtained.
When the required density value is D, an arbitrary gradation level is L, and an inconstant corresponding to the gradation level is n, the relationship is shown in Equation 1.

_{D = (D n + 1 -D} n) · (L-L n) / (L n + 1 -L n) + D n ( Equation 1)
(However, n = 0, 1, 2, 3, 4, 5)
Note that n = 0 corresponds to gradation levels 0 to 50, n = 1 corresponds to gradation levels 51 to 101, n = 2 corresponds to gradation levels 102 to 152, and n = 3 corresponds to gradation levels. Corresponding to the levels 153 to 203, n = 4 corresponds to the gradation levels 204 to 254, and n = 5 corresponds to the gradation level 255.

For example, the calculation of density values when the custom target gradation characteristic shown in FIG. 4 is selected and acquired by the target gradation characteristic selection unit 111 and the arbitrary gradation level L is 102 will be described as an example.
When an arbitrary gradation level L is 102 and an unconstant corresponding to the gradation level is determined as n = 2, D _n = 0.64, D _{n + 1} = 0.95, L _n = 102, L _{n + 1} = 153. By substituting each of these values into Equation 1, a density value D = 0.64 is calculated.

Next, in the measurement result obtained by the density sensor 104 shown in FIG. 6, the gradation level corresponding to each density value is obtained.
When the required gradation level is L, an arbitrary density value is D, and an inconstant corresponding to the density is m, the relationship is shown in Equation 2.

_{L = (L m + 1 -L} m) · (D-D m) / (D m + 1 -D m) + L m ( Equation 2)
(However, m = 0, 1, 2, 3, 4, 5)
Note that m = 0 corresponds to a density of 0 to 0.35, m = 1 corresponds to a density of 0.36 to 0.69, m = 2 corresponds to a density of 0.70 to 1.00, and m = 3 corresponds to a density of 1.01 to 1.28, m = 4 corresponds to a density of 1.29 to 1.49, and m = 5 corresponds to a density of 1.50.

Here, assuming that D = 0.64 calculated by Expression 1 is an arbitrary density value and an indefinite constant m = 1 is obtained, D _m = 0.36, D _{m + 1} = 0.70, L _m = 51, L _{n + 1} = 102. By substituting these values into Equation 2, the gradation level L = 94 is calculated.

  Here, the relationship obtained by Equation 1 and Equation 2 described above is shown in the graph of FIG. 8, and specifically, on the curve indicating the gradation characteristic of the measurement result measured by the density sensor 104, the target floor is shown. The density corresponding to the gradation level of the tone characteristic is indicated by a black circle. The numerical value shown next to the black circle mark indicates the gradation level in the gradation characteristics detected by the density sensor 104 with respect to the measured density.

  As described above, the gradation correction table creating unit 112 changes the gradation level from the gradation level 0 so that the density value corresponding to the gradation level in the target gradation characteristics and the density value measured by the density sensor 104 are equal. These are obtained in all 255, and their relationship is summarized as a gradation correction table. The relationship between the gradation correction table generated by the gradation correction table creating unit 112 as the gradation level at the time of input and the gradation level at the time of output is shown in the graph and table of FIG.

  In addition, when the target gradation characteristic selection unit 111 selects the standard target gradation characteristic, the gradation correction table creation unit 112 generates a gradation correction table in the same manner as described above. Note that the gradation correction table in the standard target gradation characteristics is shown in the graph and table of FIG. 10 as the gradation level at the time of input and the gradation level at the time of output.

  Here, paying attention to the density value near 50 percent (about 127 gradation levels) in the 256 gradation levels, in which the variation in gradation characteristics due to changes with time is maximized, that is, the effect of the present invention is most effective. The explanation will focus on time-series changes.

When the change rate of the density value due to the change with time is ΔD, ΔD = T · ΔP (where D = density value, P = total number of printed sheets, T = 5.0 × 10 ⁻⁴ ), and initial state When the total number of printed sheets is 1 and the density value near the gradation level of 50% is 0.75, the change in density value due to the change with time is shown in FIG.

  In the graph shown in FIG. 11, “a” indicates the density value when the custom target gradation characteristic is acquired (total number of printed sheets is 100), and “b” is immediately before the gradation correction table is updated (total number of printed sheets is 200). The density value indicates the density value immediately after the gradation correction table is updated (the total number of printed sheets is 201), and d indicates the density value assumed when the total number of printed sheets is the 300th sheet.

  In the graph shown in FIG. 11, c ′ is a density value when the gradation correction table is updated with the conventional standard target gradation characteristic as a target (ideal), and d ′ is a total number of printed sheets of 200. After that, when the correction of the gradation correction table is not performed, the density value assumed when the total number of printed sheets is the 300th sheet is shown.

  According to the present invention, the amount of change in the discontinuous density value before and after the update of the gradation correction table (the total number of printed sheets is 200) is ΔD = (bc), and the conventional standard target gradation characteristics are targeted. As described above, the amount of change ΔD = (b−c ′) is smaller than the discontinuous density value change when the tone correction table is updated.

  That is, it is possible to reduce the amount of change in discontinuous density values and to prevent deterioration in print quality due to the update of the gradation correction table. When the update of the gradation correction table is cancelled, the density value assumed when the total number of printed sheets is 300 is d ', and the amount of change in the density value with respect to the initial state is ΔD = (d′-0. 75). On the other hand, in the present invention, the change amount of the density value with respect to the initial state is ΔD = (d−0.75), and the variation amount of the gradation characteristic due to the change with time can be suppressed.

  As described above, according to the image forming apparatus of the present invention, by obtaining the custom target tone characteristic based on the measurement result measured by the density sensor 104, the tone correction table targeting the custom target tone characteristic. Can be updated. Accordingly, the image data is corrected based on the updated gradation correction table, and the result of image formation on the print medium based on the corrected image data is the result of discontinuous density values before and after the gradation correction table is updated. The amount of change can be reduced.

Next, a printer (image forming apparatus) 20 in which a target gradation characteristic weighting coefficient setting unit 902 that sets weighting to target gradation characteristics is added to the configuration of the first embodiment will be described with reference to FIG.
As shown in FIG. 12, the printer 20 includes a print engine 101, a page counter 102, a density sensor 104, an image processing unit 900, and a tone correction control unit 901, and further, a target tone that is a feature of this embodiment. A characteristic weight coefficient setting unit 902 is provided.

Since the print engine 101, the page counter 102, and the density sensor 104 are the same as those in the first embodiment, description thereof is omitted.
A target gradation characteristic weighting coefficient setting unit 902 sets a weighting coefficient related to target gradation characteristics input via an operation panel (not shown). The weighting factor is a value from 0.0 to 1.0 in 0.1 unit, and the target gradation characteristic weighting factor setting unit 902 holds the value.

  Although the image processing unit 900 and the gradation correction control unit 901 have the same basic functions as the image processing unit 100 and the gradation correction control unit 103 of the first embodiment, the target floor added in this embodiment is used. This corresponds to the tone characteristic weighting coefficient setting unit 902.

  That is, the image processing unit 900 according to the second embodiment is based on the weighting factor from the target tone characteristic weighting factor setting unit 902 which is a feature of this embodiment, instead of the target tone characteristic selection unit 111 according to the first embodiment. A target gradation characteristic calculation unit 903 that calculates target gradation characteristics is provided. Further, since the target gradation characteristic selection unit 111 used in the first embodiment is not required, it is not necessary to respond to the inquiry about whether or not the custom target gradation characteristic is acquired from the target gradation characteristic selection unit 111. A corresponding gradation correction control unit 901 is provided instead of the gradation correction control unit 103.

Next, operations of the printer 20, particularly operations different from those of the first embodiment will be mainly described with reference to the drawings.
Since the normal printing operation is the same as that of the first embodiment, the operation of the custom target gradation characteristic will be described with reference to the flowchart of FIG.

  In this embodiment, the custom target tone characteristics are acquired in accordance with an instruction from the user. As an example, in this embodiment, when the total number of printed sheets reaches 100 from the initial state, the user instructs acquisition of custom target gradation characteristics.

In response to the acquisition instruction of the custom target gradation characteristic, the gradation correction control unit 901 starts the acquisition operation of the custom target gradation characteristic target gradation characteristic, and the custom target gradation characteristic is stored in the custom target gradation characteristic holding unit. The processing until it is held at 110 (steps S41 to S46) is the same as in the first embodiment.
However, in the first embodiment, the fact that the custom target gradation characteristic is acquired is stored in the storage unit (not shown) of the gradation correction control unit 103. However, in this embodiment, the custom target gradation characteristic is acquired. Do not record.

Next, the update operation of the gradation correction table held by the gradation correction table holding unit 106 will be described with reference to FIG.
Note that the weighting factor is the target tone characteristic weighting factor setting unit 902, the custom target tone characteristic shown in FIG. 4 is the custom target tone characteristic holding unit 110, and the standard target tone characteristic shown in FIG. The description will be made on the assumption that the characteristic holding unit 109 already holds the characteristic.

  The page counter 102 counts the number of printed sheets. For example, every 200 sheets, a tone correction table update request held by the tone correction table holding unit 106 is output to the tone correction control unit 901.

  The gradation correction control unit 901 that has received the instruction to update the gradation correction table starts the gradation correction table update operation, and the density sensor 104 measures the density of the gradation patch (step S34). In each of Y and K, the density values of each patch of gradation level 0, gradation level 51, gradation level 102, gradation level 153, gradation level 204, and gradation level 255, that is, all 24 kinds of density values. The processing until acquiring (steps S51 to S54) is the same as in the first embodiment, and the description thereof is omitted. In addition, the measurement result shown in FIG. 6 similar to Example 1 mentioned above is acquired.

  When the density value of the density patch is measured, the target tone characteristic calculation unit 903 has the custom target tone characteristic held by the custom target tone characteristic holding unit 110 and the standard target held by the standard target tone characteristic holding unit 109. Based on the weighting coefficient set in the gradation characteristic and the target gradation characteristic weighting coefficient setting unit 902, the target gradation characteristic to be applied to the gradation correction table update is calculated based on the following Expression 3 (step S55). .

The target gradation characteristic applied to the gradation correction table update is D ″, the density value with respect to the gradation level of the custom target gradation characteristic is Dn, the density value with respect to the gradation level of the standard target gradation characteristic is D′ n, and the target floor When the weighting coefficient held in the tone characteristic weighting coefficient setting unit 902 is W (W = 0.0 to 1.0) and the inconstant with respect to the gradation level is n,
D ″ _n = (1.0−W) · D ′ _n + W · D _n Formula 3
(However, n = 0, 1, 2, 3, 4, 5).
Note that n = 0 corresponds to gradation levels 0 to 50, n = 1 corresponds to gradation levels 51 to 101, n = 2 corresponds to gradation levels 102 to 152, and n = 3 corresponds to gradation levels. Corresponding to levels 153 to 203, n = 4 corresponds to gradation levels 204 to 254, and n = 5 corresponds to gradation level 255.

  By the way, when the weighting factor is 1.0, the target gradation characteristic D ″ applied to the gradation correction table update is the custom target gradation characteristic (gradation level and level) held in the custom target gradation characteristic holding unit 110. When the weighting factor is 0.0, the target tone characteristic D ″ applied to the tone correction table update is held in the standard target tone characteristic holding unit 109. It becomes the same as the standard target gradation characteristic.

When the weighting factor is 0.5, the target tone characteristic D ″ applied to the tone correction table update is an intermediate tone between the weighting factor of 1.0 and the weighting factor of 0.0. It becomes a characteristic.
The standard target gradation characteristics indicated by the relationship between the gradation level and the density value with respect to the weighting factor W = 1.0, 0.5 and 0.0 are shown in FIGS. 15, 16 and 17, respectively. .

  The gradation correction table creation unit 112 updates the gradation correction table based on the target gradation characteristic obtained by the target gradation characteristic calculation unit 903 and the measurement result of the gradation patch, as in the first embodiment. Is performed (step S56).

  Therefore, since the gradation correction table of this embodiment is updated using the target gradation characteristic obtained based on the weighting factor, when the weighting factor is close to 1, the correction table corresponds to the custom target gradation characteristic. When the weight coefficient is close to 0, a correction table corresponding to the standard target gradation characteristic is obtained.

Here, a time-series change in density value in the present embodiment will be described.
When the change rate of the density value due to the change with time is ΔD, ΔD = T · ΔP (where D = density value, P = total number of printed sheets, T = 5.0 × 10 ⁻⁴ ), and initial state When the total number of printed sheets is 1 and the density value near the gradation level of 50 percent is 0.75, the change in density value due to the change with time is shown in FIG.

  In the graph shown in FIG. 18, “a” indicates the density value when the custom target gradation characteristic is acquired (the total number of printed sheets is 100), and “b” is the state immediately before the gradation correction table is updated (the total number of printed sheets is 200). C, c ′ and c ″ are density values immediately after the gradation correction table is updated (the total number of printed sheets is 201), and the weighting factors W are 1.0, 0.5 and 0, respectively. Density values when set to 0 are shown, and d, d ', and d "are density values assumed when the total number of printed sheets is the 300th, and the weighting factors W are 1.0 and 0.5, respectively. And density values when 0.0 is set.

  According to the present embodiment, as apparent from the graph shown in FIG. 18, by setting the weighting factor to 1.0, it is possible to suppress the variation amount (change rate) of the gradation characteristics due to the change over time. By setting the weighting factor to 0.0, the absolute fluctuation amount of the gradation characteristic due to the change with time can be suppressed to the minimum.

  As described above, according to the image forming apparatus of the second embodiment, the user sets the weighting coefficient according to the use situation, so that the custom target gradation characteristic and the standard target floor are set based on the set weighting coefficient. Obtaining a target gradation characteristic indicating characteristics between the gradation characteristics, updating the gradation correction table based on the target gradation characteristic, and performing gradation correction based on the updated gradation correction table Can do. Therefore, according to the image forming apparatus of the second embodiment, it is possible to perform appropriate gradation correction according to the use situation, and to obtain an optimal printing result.

1 is a block diagram of an image forming apparatus according to Embodiment 1. FIG. It is a flowchart which shows the operation | movement at the time of printing. It is a flowchart which shows acquisition operation | movement of a custom target gradation characteristic. It is a figure which shows the measurement result of the gradation patch at the time of custom target gradation characteristic acquisition. 6 is a flowchart illustrating an operation for updating a gradation correction table according to the first exemplary embodiment. It is a figure which shows the measurement result of the gradation patch at the time of gradation correction table update. It is a figure which shows a standard target gradation characteristic. It is explanatory drawing for gradation correction. It is a figure which shows the relationship of the gradation level at the time of the input at the time of the gradation correction table at the time of custom target gradation characteristic selection. It is a figure which shows the relationship of the gradation level at the time of the input in the gradation correction table at the time of standard target gradation characteristic selection, and the time of output. FIG. 6 is a diagram illustrating a change amount of a density value due to a change with time in Example 1. 6 is a block diagram of an image forming apparatus according to Embodiment 2. FIG. 10 is a flowchart illustrating an operation of acquiring custom target gradation characteristics in the second embodiment. 10 is a flowchart illustrating an operation of updating a gradation correction table according to the second exemplary embodiment. It is a figure which shows the target gradation characteristic when a weighting coefficient is 1.0. It is a figure which shows the target gradation characteristic when a weighting coefficient is 0.5. It is a figure which shows the target gradation characteristic when a weighting coefficient is 0.0. FIG. 6 is a diagram showing changes in density values due to changes over time in Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printer 100, 900 Image processing part 101 Print engine 102 Page counter 103,901 Gradation correction control part 104 Density sensor 105 Color conversion processing part 106 Gradation correction table holding part 107 Gradation correction processing part 108 Gradation processing part 109 Standard Target tone characteristic holding unit 110 Custom target tone characteristic holding unit 111 Target tone characteristic selecting unit 112 Tone correction table creating unit 902 Target tone characteristic weighting factor setting unit 903 Target tone characteristic calculating unit

Claims (3)

In an image forming apparatus that performs gradation correction of input image data based on a predetermined reference gradation characteristic and a measured density measured by a density measuring unit, and forms an image based on the corrected image data.
A set tone characteristic acquisition unit for obtaining a new set tone characteristic based on the measured density measured by the density measuring unit;
A gradation characteristic selection unit that selects one of the reference gradation characteristic and the set gradation characteristic;
An image forming apparatus, comprising: a gradation correction unit that performs gradation correction based on the selected gradation characteristic and the measured density measured by the density measurement unit.   2. The image forming apparatus according to claim 1, wherein the gradation characteristic selection unit selects the set gradation characteristic when the set gradation characteristic is acquired. In an image forming apparatus that performs gradation correction of input image data based on a predetermined reference gradation characteristic and a measured density measured by a density measuring unit, and forms an image based on the corrected image data.
A set tone characteristic acquisition unit for obtaining a new set tone characteristic based on the measured density measured by the density measuring unit;
A weighting coefficient setting unit for setting a weighting coefficient indicating weighting for the reference gradation characteristic and the set gradation characteristic;
A gradation characteristic calculation unit that performs calculation of gradation characteristics for correction based on the reference gradation characteristics and the set gradation characteristics based on the weight coefficient;
An image forming apparatus comprising: a gradation correction unit that performs gradation correction based on the calculated gradation characteristic and the measured density measured by the density measurement unit.

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