JP2009302669A - Image forming apparatus, tone correction method, and tone correction program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform tone correction with respect to a plurality of tone expression methods with simple processing by using fewer data. <P>SOLUTION: An image forming apparatus measures density of many different dither patches by a first tone expression method. Then, the image forming apparatus acquires correlation between an amount of density fluctuation from tone characteristics in a reference environment by a first tone expression method and an amount of density fluctuation from tone characteristics in a reference environment by a second tone expression method different from the first tone expression method (S203). The image forming apparatus converts the amount of density fluctuation of measured density from the tone characteristics in the reference environment by the first tone expression method into the amount of density fluctuation from the tone characteristics in the reference environment by the second tone expression method according to the acquired correlation (S204) and calculates (estimates) density of a dither patch when an image is formed by the second tone expression method by using the converted amount of density fluctuation and the tone characteristic in the reference environment by the second tone expression method (S205). <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, a gradation correction method, and a gradation correction program.

  For example, in an electrophotographic image forming apparatus, it is difficult to express gradation in print pixel units. Therefore, such an image forming apparatus expresses a halftone in printing by performing binarization processing by a gradation expression method such as a dither method or an error diffusion method on an image including a halftone.

  In addition, in order to obtain good results for various image data such as photographic images where gradation is important and character images where resolution is important, there is a function that can implement multiple gradation expression methods. There is an image forming apparatus.

  However, when performing gradation correction for a plurality of gradation expression methods, an operation for measuring the density of a plurality of different halftone patches is required a plurality of times. For this reason, there is a problem that the time for gradation correction increases and the amount of printing color material (toner, ink) consumed for gradation correction also increases.

  In order to solve this problem, a plurality of combinations of “tone characteristics according to representative gradation expression methods” and “tone characteristics according to other gradation expression methods” corresponding thereto are stored in advance, An image forming apparatus that performs gradation correction for a plurality of gradation expression methods based on actually measured “gradation characteristics by a typical gradation expression method” has been proposed (see Patent Document 1).

In this case, a plurality of pre-stored “gradation characteristics according to representative gradation expression methods” that are close to the actually measured “gradation characteristics according to representative gradation expression methods” are first selected. The Subsequently, the “gradation characteristics by other gradation expression methods” corresponding to the selected “gradation characteristics by representative gradation expression methods” are employed. Then, using the adopted “tone characteristics by other gradation expression methods”, gradation correction for other gradation expression methods is performed.
JP-A-8-156330

  However, the image forming apparatus described in Patent Document 1 has a problem that it is necessary to store a lot of “combination of gradation characteristics”. In addition, the process of selecting the one close to the actually measured “gradation characteristic by the representative gradation expression method” from the plurality of “gradation characteristics by the representative gradation expression method” stored in advance. It is cumbersome and time consuming.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to perform tone correction for a plurality of tone expression methods with simple processing using less data. An image forming apparatus, a gradation correction method, and a gradation correction program are provided.

  The above object of the present invention is achieved by the following means.

  (1) A measurement unit that measures the density of a plurality of different halftone patches formed with an image by the first gradation expression method, and a density from gradation characteristics in a reference environment according to the first gradation expression method A storage unit that stores a correlation between a variation amount and a density variation amount from a gradation characteristic in a reference environment according to a second gradation expression method that is different from the first gradation expression method; The density fluctuation amount from the gradation characteristic in the reference environment according to the first gradation expression method, and the density fluctuation amount from the gradation characteristic in the reference environment according to the second gradation expression method according to the correlation. An image is formed by the second tone expression method using the conversion unit for converting to the image, the density fluctuation amount converted by the conversion unit, and the tone characteristics in the reference environment by the second tone expression method. Multiple when Image forming apparatus characterized by having a calculation unit for calculating the concentration of the different halftone patches.

  (2) a first generation unit that generates a first gradation correction table for obtaining predetermined gradation characteristics in image formation by the first gradation expression method from the density measured by the measurement unit; A second generation unit that generates a second gradation correction table for obtaining predetermined gradation characteristics in image formation by the second gradation expression method from the density calculated by the calculation unit; The image forming apparatus as described in (1) above.

  (3) The gradation characteristic in the reference environment is a gradation characteristic acquired by density measurement under a predetermined condition relating to image formation, as described in (1) or (2) above Image forming apparatus.

  (4) The image forming apparatus according to (1) or (2), wherein the gradation characteristic in the reference environment is a predetermined gradation characteristic.

  (5) The first gradation expression method and the second gradation expression method are dither methods having different screen line numbers or dither dot shapes, respectively. The image forming apparatus according to claim 1.

  (6) The first gradation expression method and the second gradation expression method are dither methods with different screen line numbers, respectively, and the correlation is based on the one with the lower screen line number. The image forming apparatus according to (5), wherein the ratio is set as a ratio.

  (7) In any one of the above (1) to (4), the first gradation expression method and the second gradation expression method are a dither method and an error diffusion method, respectively. The image forming apparatus described.

  (8) The image forming apparatus according to (7), wherein the correlation is set as a ratio based on a dither method.

  (9) Step (a) of measuring the density of a plurality of different halftone patches formed with an image by the first gradation expression method, and the first gradation expression method stored in the storage unit Obtaining a correlation between the density fluctuation amount from the gradation characteristics in the reference environment and the density fluctuation amount from the gradation characteristics in the reference environment by the second gradation expression method different from the first gradation expression method ( b) and the density variation amount of the density measured in the step (a) from the gradation characteristics in the reference environment by the first gradation expression method according to the correlation, the second gradation expression Step (c) for converting the density fluctuation amount from the gradation characteristic in the reference environment by the method, the density fluctuation amount converted in the step (c) and the gradation characteristic in the reference environment by the second gradation expression method When Used, gradation correction method characterized by having a step (d) of calculating a plurality of different halftone density patch when an image is formed by the second gradation representing method.

  (10) Generating a first gradation correction table for obtaining predetermined gradation characteristics in image formation by the first gradation expression method from the density measured in the step (a) (e) And (f) generating a second gradation correction table for obtaining predetermined gradation characteristics in the image formation by the second gradation expression method from the density calculated in the step (d). The gradation correction method according to (9), further comprising:

  (11) A procedure (a) for causing the measurement unit to measure the densities of a plurality of different halftone patches formed by the first gradation expression method, and the first gradation stored in the storage unit Obtaining a correlation between the density fluctuation amount from the gradation characteristic in the reference environment by the expression method and the density fluctuation amount from the gradation characteristic in the reference environment by the second gradation expression method different from the first gradation expression method. Step (b), and the density variation amount of the density measured in the procedure (a) from the gradation characteristics in the reference environment according to the first gradation expression method according to the correlation. The procedure (c) for converting to the density fluctuation amount from the gradation characteristics in the reference environment by the gradation expression method, the density fluctuation amount converted in the procedure (c) and the reference environment by the second gradation expression method With gradation characteristics Tone correction program for executing the processing including, a procedure (d) for calculating a plurality of the concentration of different halftone patch when it is the second image by the gradation representing method forming the computer.

  (12) The processing generates a first gradation correction table for obtaining predetermined gradation characteristics in image formation by the first gradation expression method from the density measured in the procedure (a). A procedure for generating a second gradation correction table for obtaining a predetermined gradation characteristic in image formation by the second gradation expression method from the procedure (e) and the density calculated in the procedure (d). The gradation correction program according to (11), further comprising (f).

  (13) A computer-readable recording medium on which the gradation correction program according to (11) or (12) is recorded.

  According to the present invention, the second gradation representation can be performed from the actually measured densities of a plurality of different halftone patches formed by the first gradation representation method with simple processing using less data. The density of a plurality of different halftone patches when an image is formed by the method is accurately calculated. As a result, it is possible to generate a gradation correction table for obtaining predetermined gradation characteristics in image formation by another different gradation expression method only by actually measuring gradation characteristics by one representative gradation expression method. It becomes possible. As a result, since less halftone patch printing and correction calculation are required, the time required for gradation correction processing can be shortened, and the consumption of printing color material can be reduced.

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

  FIG. 1 is a block diagram showing a configuration of an image forming apparatus 1 according to the first embodiment of the present invention. In the present embodiment, the image forming apparatus 1 is an electrophotographic laser printer.

  The image forming apparatus 1 includes a CPU 11 that controls each part in the image forming apparatus 1 and performs various arithmetic processes, a RAM 12 that temporarily stores programs and data as a work area, and a ROM 13 that stores various programs and data. An image memory 14 for storing image data obtained by analyzing print data, a memory access control unit 15 for sequentially sending image data from the image memory 14, and a gradation for image data from the image memory 14 Connected to the CPU 11 via a screening module 50 that generates image data that can be imaged by performing binarization processing by an expression method, a printing unit 30 that performs printing based on the sent image data, and a DA / AD converter 21 Halftone patch density It has a density sensor 22 used to measure

  The image memory 14 stores in advance image data having gradation developed in a bitmap format at a printing resolution. In this embodiment, the image data has 8 bits per pixel and 256 gradations.

  The memory access control unit 15 controls to sequentially send pixel data from the image memory 14 according to a command from the CPU 11 and outputs the coordinates of the pixel being sent in synchronization with the pixel data sending. Transmission of pixel data from the image memory 14 and output of pixel coordinates from the memory access control unit 15 are executed in synchronization with a clock signal (not shown).

  The pixel data transmitted from the image memory 14 and the pixel coordinates output from the memory access control unit 15 are input to the screening module 50.

  The screening module 50 includes a dither module 40 and an LUT (Look Up Table) 16.

  The LUT 16 replaces the sequentially input pixel density data with the dither pattern number Pn to be referred to in the processing in the subsequent dither module 40. The dither pattern number Pn indicates a dither pattern in which Pn pixels in the dither matrix are turned on. Prior to the start of processing, the CPU 11 sets data of a gradation correction table (print density correction table), which will be described later, in the LUT 16.

  The dither module 40 includes a dither matrix memory 19 that stores the dither matrix, address conversion circuits 17 and 18 that convert input pixel coordinates into addresses that refer to the dither matrix, and the dither pattern number Pn and the dither matrix replaced by the LUT 16. It comprises a comparator (CMP) 20 that compares array element data read from the memory 19.

  Prior to the start of processing, the dither module 40 stores dither matrix data and its size (m, n) by the CPU 11.

  FIG. 2 is a diagram illustrating an example of gradation characteristics of a binarized image by the dither method, and FIG. 3 is a diagram illustrating a dither matrix corresponding to the gradation characteristics of FIG.

  The dither matrix shown in FIG. 3 is given by an array of 24 × 24 pixels. Each pixel of the dither matrix is assigned a number from 0 to 575. By turning on the pixels in order from the pixel number 0 in the dither matrix, 577 gradations can be logically expressed.

  FIG. 4 is a diagram showing a part of the halftone pattern by the dither matrix of FIG. 4A shows a halftone pattern of 25% dot ratio (Dot Ratio) in which 25% of all 576 pixels of the dither matrix are ON, and similarly, FIG. 4B shows a dot ratio of 50%. % Shows a halftone pattern, and (c) shows a halftone pattern with a dot rate of 75%.

  In FIG. 2, the horizontal axis represents the above-described dot rate, and the vertical axis represents the normalized density of 0% when the dot rate is 0% and 100% when the dot rate is 100%. Yes. In general, in an electrophotographic image forming apparatus, in consideration of the stability of printing pixels, printing reproducibility of oblique lines, and the like, pixels that are actually printed are printed as indicated by black circles in FIG. The circular shape has a diameter larger than the pixel pitch, specifically, a diameter approximately 1.5 times the printing pixel pitch. For this reason, when the pixel pattern shown in FIG. 5A is printed, as shown in FIG. 5B, a dot (print pixel) covers an area larger than the area represented by the dot rate. As a result, the gradation characteristic of the printed image is not the gradation characteristic proportional to the dot rate indicated by the broken line in FIG. 2, but the characteristic indicated by the solid line (upwardly convex curve).

Returning to the explanation of FIG. 1, the address conversion circuits 17 and 18 convert the coordinates X and Y of the pixels sequentially read from the image memory 14 into
Mx = X mod m
My = Y mod n
(“Mod” is a function of remainder calculation, that is, Mx = X−m * Floor (X / m). Here, “Floor” indicates an integer part.)
Is converted into the coordinates (Mx, My) of the dither matrix element to be referred to.

  A dither matrix element value Dn indicated by array element coordinates (Mx, My) is output from the dither matrix memory 19.

The CMP 20 compares the two input values, the dither pattern number Pn and the dither matrix element value Dn,
When Pn> Dn, “1” (with dots)
When Pn ≦ Dn, “0” (no dot)
Is output.

  Note that the gradation characteristics as shown in FIG. 2 are not necessarily gradation characteristics suitable for halftone image printing. By setting appropriate gradation correction table data in the LUT 16 of the screening module 50, desired gradation characteristics can be obtained.

  For example, when the dither matrix shown in FIG. 3 is implemented and the gradation characteristic shown by the solid line in FIG. 2 is shown, an 8-bit density value that is an input value is converted into an output value that is a dither pattern number Pn. By setting the gradation correction table data shown in (2), it is possible to obtain a print output density proportional to the 8-bit density value that is the input value.

  In printing a halftone image, it is desirable to faithfully reproduce the halftone gradation of the input image. In a color image forming apparatus that reproduces colors by superimposing four color images of cyan (C), magenta (M), yellow (Y), and black (K), the tone characteristics of the density of each color are more important. It is. This is because even if one color has a density gradation characteristic that is out of order, it is printed out in a color different from the desired color.

  However, an electrophotographic image forming apparatus is not always exhibiting constant gradation characteristics due to the influence of environmental conditions in a broad sense such as temperature, humidity, and sensitivity of a photoconductor that forms an electrostatic latent image. For example, the gradation characteristics of the binarized image by the dither method using the first dither matrix shown in FIG. 3 illustrated in FIG. 3 are the environmental conditions as shown in FIGS. Indicates fluctuations due to influence.

  Therefore, in order to obtain a print output having a desired gradation characteristic, the gradation correction table data set in the LUT 16 of the screening module 50 is appropriately set according to the gradation characteristic of the binarized image (dithered image). Must be set to In the present embodiment, as a method for obtaining the gradation characteristics by one gradation expression method, images of different predetermined halftone patterns (halftone patches) are created by the one gradation expression method, A method of measuring (detecting) the concentration is employed.

  FIG. 8 is a diagram illustrating an example of a mechanism configuration of the image forming apparatus 1 of the present embodiment.

  The image forming apparatus 1 includes a photosensitive drum 33 as an image carrier, and the photosensitive drum 33 is uniformly charged by a charging device 34. The print data is input to the LUT 16 as multi-value image data, and is binarized by the dither module 40 and input to the laser drive unit 31 as image data that can be imaged. The laser beam sent from the laser drive unit 31 is reflected by the polygon mirror 32 to the photosensitive drum 33 to be exposed, and an electrostatic latent image is formed on the photosensitive drum 33. The electrostatic latent image is developed by the developing device 35 and a toner image is formed on the photosensitive drum 33. The toner image is transferred onto the paper by the transfer roller 36 and the residual toner is removed by the cleaner 37.

  In the present embodiment, a series of halftone pattern images (halftone patches) as illustrated in FIG. 9 are created on the photosensitive drum 33, and the density is measured using the density sensor 22.

  FIG. 10 is a diagram illustrating an example of the configuration of the density sensor 22.

  The density sensor 22 includes a light emitting element 22a that emits light, and a light receiving element 22b that receives light reflected from the surface of the photosensitive drum 33 of light emitted from the light emitting element 22a. On the toner image T created on the photosensitive drum 33, light is irregularly reflected and absorbed, thereby reducing the amount of reflected light entering the light receiving element 22b. As a result, the density sensor 22 can detect the ratio of the toner image covering the photosensitive drum 33. The light emitting element 22a is connected to the CPU 11 via a DA converter (DAC), and the light emission amount can be adjusted by a command from the CPU 11. The amount of light received by the light receiving element 22b is converted into digital data by an AD converter (ADC) and transmitted to the CPU 11.

  Prior to detecting the density of the halftone patch, the CPU 11 adjusts the light emission amount of the light emitting element 22a so that the amount of light reflected on the surface of the photosensitive drum becomes a predetermined amount during the period indicated by “A” in FIG. Next, during the period indicated by “B” in FIG. 9, the CPU 11 captures the edge of the first halftone patch by the variation in the light emission amount, and detects the reflected light amount of each halftone patch at a predetermined timing after the edge detection. .

  The dot ratio (Dot Ratio) is, for example, 100%, 87.5%, 75.0%, 62.5%, 50.0%, 37.5%, 25.0%, and 12.5%. A key patch is created. Then, the density of each halftone patch is measured, and an interpolation process such as a spline is applied to the measured density to obtain a gradation characteristic curve as shown in FIG. From the obtained tone characteristic curve, tone correction table data set in the LUT 16 of the screening module 50 for obtaining a print output having a desired tone characteristic is generated.

  The image forming apparatus 1 of the present embodiment has a function of performing a binarization processing method as a plurality of gradation expression methods.

  For example, the present embodiment includes a plurality of print modes such as “photo mode” and “character mode”, and the binarization processing method to be applied is switched depending on the print mode setting. In this case, in the “photo mode”, binarization processing is performed by a dither screen having a low number of lines suitable for smooth gradation expression with less influence of process noise of the electrophotographic mechanism. On the other hand, in the “character mode”, binarization processing is performed using a high-line-number dither screen that emphasizes the reproduction of sharp character edges. Here, the low line number means that the screen line number is low (small), and the high line number means that the screen line number is high (large). The number of screen lines indicates the fineness of halftone dots (how many halftone dots are included in one inch), and the unit is lpi (line per inch).

  In addition, a configuration may be employed in which the binarization processing method to be applied is switched as needed (dynamically) according to the attribute of the object to be printed. This method has attribute information for each pixel to be printed. In this case, a binarization process using a dither screen with a low number of lines suitable for smooth gradation expression with less influence of process noise of the electrophotographic mechanism is applied to the “image object”. On the other hand, a binarization process using a high-dither dither screen that emphasizes the reproduction of sharp edges is applied to “text objects” and “graphics objects”.

  Further, when the image forming apparatus 1 is an MFP (multifunctional peripheral device) having a copying function and a printer function, a configuration is adopted in which the binarization processing method applied between the printer mode and the copying mode can be switched. Good. In this case, the binarization processing by the dither method is applied in the printer mode, and the binarization processing by the error diffusion method is applied in the copy mode, which hardly causes interference (moire) of the original with the dither screen.

  Here, FIG. 11 illustrates a second dither matrix having a higher number of lines than the first dither matrix shown in FIG.

  FIG. 12 is a diagram showing a part of the halftone pattern by the dither matrix of FIG. 12A shows a halftone pattern with a dot rate of 25% in which 25% of all 576 pixels of the dither matrix are ON, and similarly, FIG. 12B shows a halftone pattern with a dot rate of 50%. (C) shows a halftone pattern with a dot rate of 75%.

  FIG. 13 shows the variation of the gradation characteristics of the binarized image by the dither method using the second dither matrix shown in FIG. 11 due to the influence of environmental conditions. Here, the gradation characteristics (a), (b), and (c) in FIG. 13 are respectively collected in the same environment as the gradation characteristics (a), (b), and (c) in FIG.

  FIG. 14 is a diagram showing the ratio of the gradation characteristic values shown in FIGS. 7 and 13 under each environment. As can be seen from FIG. 14, the correlation of the ratio of the gradation characteristic values between the different binarization processing methods cannot be determined uniquely. FIG. 15 shows the first dither matrix using the ratio of the gradation characteristic values of FIG. 7A based on the first dither matrix and the gradation characteristic values of FIG. 13A based on the second dither matrix. FIG. 6 is a diagram showing an error rate (error ratio) in a case where a tone characteristic based on a second dither matrix is estimated from a measured value obtained from the above. That is, FIG. 15 uses the ratio of the two gradation characteristic values to estimate the gradation characteristic by the second dither matrix from FIGS. 7B and 7C, which are actually measured values by the first dither matrix. The ratio of the calculated result to the gradation characteristic values of FIGS. 13B and 13C which are actually measured values is shown. As can be seen from FIG. 15, the error is large in the highlight portion where the dot rate is small, and in this case, in particular, a composite expressing gray with three colors of cyan (C), magenta (M), and yellow (Y) in a color printer. In the black method, the gray balance will be lost.

  The variation in the gradation characteristics illustrated in FIGS. 7 and 13 shows the result that the area covered by the actually printed dots varies. That is, as shown in FIG. 16, it can be considered that the size of the dots actually printed varies.

  FIG. 16 is a diagram illustrating a manner in which the area covered by a dot varies due to a variation in the size of a dot that is actually printed. In FIG. 16, (a) shows three types of dots having different sizes, and (b) to (d) show results of printing the same pixel pattern with dots of each size.

  It can be seen from FIG. 16 that the change in the area covered by the dots due to the change in the dot size occurs at the outer periphery of the pixel pattern. That is, density variation due to variation in dot size occurs at the outer periphery of the pixel pattern. In addition, in the image binarized by the dither method, the pixel pattern at each dot rate is uniquely determined by the dither matrix, and even in the image binarized by the error diffusion method, the dot pattern has a sufficiently large surface area. It is already known that the pixel pattern is almost fixed. Based on such knowledge, this embodiment provides a gradation correction method having the following steps.

  (1) The (reference) gradation characteristics in the reference environment by the first and second different image binarization processing methods (gradation expression methods) are acquired and stored in advance.

  (2) Correlation between the density fluctuation amount from the gradation characteristics in the reference environment by the first image binarization processing method and the density fluctuation amount from the gradation characteristics in the reference environment by the second image binarization processing method. Obtain and store in advance.

  (3) The density of a plurality of different halftone patches formed by the first image binarization processing method is actually measured, and the variation amount from the (reference) tone characteristics in the reference environment is calculated.

  (4) The density fluctuation amount of the halftone patch by the second image binarization processing method is calculated from the calculated density fluctuation amount of the halftone patch by the first image binarization processing method and the correlation of the density fluctuation amount. Calculate (estimate).

  (4) Second image binarization processing from the calculated density fluctuation amount of the halftone patch by the second image binarization processing method and the (reference) gradation characteristics in the second image binarization processing method Calculate (estimate) the density of the halftone patch by the method.

  (5) A gradation characteristic by the second image binarization processing method is generated from the calculated density of the halftone patch by the second image binarization processing method.

  (6) Screening module 50 for obtaining a print output having a desired gradation characteristic to be applied to the second image binarization process from the generated gradation characteristic by the second image binarization processing method. Tone correction table data to be set in the LUT 16 is generated.

  Next, an operation relating to gradation correction in the image forming apparatus of the present embodiment will be described.

  The ROM 13 of the image forming apparatus 1 stores in advance a laser printer control program executed by the CPU 11 and various data necessary for executing the program.

  Various data necessary for program execution stored in the ROM 13 is based on the first dither matrix shown in FIG. 3, the second dither matrix shown in FIG. 11, and the first dither matrix shown in FIG. The first reference tone characteristic in the reference environment of the halftone patch, the second reference tone characteristic in the reference environment of the halftone patch by the second dither matrix shown in FIG. 17B, the first reference shown in FIG. A correlation indicating a ratio of a variation amount from the second reference gradation characteristic of the halftone patch by the second dither matrix to a variation amount from the first reference gradation characteristic of the halftone patch by the dither matrix of FIG. 19, which is a target gradation characteristic at the time of print output. As the correlation between the first reference gradation characteristic, the second reference gradation characteristic, and the fluctuation amount, data obtained in advance by actual measurement is stored.

  FIG. 20 is a flowchart illustrating the procedure of the generation process of the gradation correction table (1) in the image forming apparatus 1. The algorithm shown in the flowchart of FIG. 20 is stored as a program in a storage unit such as the ROM 13 of the image forming apparatus 1, and the program is executed by the CPU 11.

  The gradation correction table (1) is a conversion table to be applied to the first image binarization processing method (gradation expression method). This gradation correction table (1) is set in the LUT 16 of the screening module 50 in order to obtain a print output having a desired gradation characteristic.

  First, eight first dithers having a dot rate of 100%, 87.5%, 75.0%, 62.5%, 50.0%, 37.5%, 25.0%, and 12.5% A halftone patch (toner image) binarized by the matrix is formed on the photosensitive drum 33 (S101).

  Subsequently, the density (1) of each halftone patch formed on the photosensitive drum 33 is measured using the density sensor 22 (S102) and stored in the RAM 12 (S103).

  Spline interpolation processing is performed on the measured density of each halftone patch, and the gradation characteristic (1) by the first image binarization processing method is calculated (S104). Here, the gradation characteristic (1) is given by a table.

  Subsequently, the target gradation characteristic stored in the ROM 13 is acquired (S105).

  Then, from the calculated gradation characteristic (1), data of the gradation correction table (1) for obtaining a print output having a desired target gradation characteristic when the image binarization process using the first dither matrix is applied. It is generated (S106) and stored in the RAM 12 (S107).

  FIG. 21 is a flowchart illustrating the procedure of the gradation correction table (2) generation process in the image forming apparatus 1. The algorithm shown in the flowchart of FIG. 21 is stored as a program in a storage unit such as the ROM 13 of the image forming apparatus 1, and the program is executed by the CPU 11.

  The gradation correction table (2) is a conversion table to be applied to the second image binarization processing method (gradation expression method). This gradation correction table (2) is set in the LUT 16 of the screening module 50 in order to obtain a print output having a desired gradation characteristic.

  First, the first reference gradation characteristic in the reference environment by the first image binarization processing method (gradation expression method) is acquired from the ROM 13 (S201).

  Subsequently, the variation (difference) (1) from the first reference tone characteristic of the density (1) of each halftone patch stored in step S103 is calculated (S202).

  Subsequently, the correlation of the fluctuation amount is acquired from the ROM 13 (S203).

  Then, the fluctuation amount (1) calculated in step S202 is converted into the fluctuation amount (2) from the second reference gradation characteristic according to the acquired fluctuation amount correlation (S204).

  The density of the halftone patch binarized by the second dither matrix is calculated (estimated) from the converted fluctuation amount (2) and the second reference tone characteristic (S205) and stored in the RAM 12. (S206).

  Spline interpolation processing is performed on the calculated density of each halftone patch, and the gradation characteristic (2) according to the second image binarization processing method is calculated (S207). Here, the gradation characteristic (2) is given by a table.

  Then, from the calculated gradation characteristic (2), there is data of the gradation correction table (2) for obtaining a print output having a desired target gradation characteristic when the image binarization process using the second dither matrix is applied. It is generated (S208) and stored in the RAM 12 (S209).

  The gradation correction tables (1) and (2) generated by the processing shown in the flowcharts of FIGS. 20 and 21 are stored in the RAM 12 as described above. Then, when performing printing that applies image binarization processing using the first dither matrix, the first dither matrix shown in FIG. 3 is set in the dither matrix memory 19 of the dither module 40, and at the same time, the LUT 16 of the screening module 50 is set. Is set with data of the gradation correction table (1). On the other hand, when performing printing that applies image binarization processing using the second dither matrix, the second dither matrix shown in FIG. 11 is set in the dither matrix memory 19 of the dither module 40, and at the same time the LUT 16 of the screening module 50. The data of the gradation correction table (2) is set in

  Next, the effect by 1st Embodiment is demonstrated.

  FIG. 22 shows an example of the density of a halftone patch binarized by the first dither matrix, measured in two different environments (Case 1 and Case 2). Here, there are 8 halftone patches with a dot rate of 100%, 87.5%, 75.0%, 62.5%, 50.0%, 37.5%, 25.0%, 12.5%. It consists of patches.

  FIG. 23 shows the (predicted) density of the halftone patch binarized by the second dither matrix calculated from the actually measured density of the halftone patch shown in FIG.

  On the other hand, FIG. 24 shows the density of the halftone patch binarized by the second dither matrix, measured in the two different environments (Case 1 and Case 2).

  FIG. 25 is a diagram illustrating the ratio (error ratio) of the calculated halftone patch density to the actually measured halftone patch density. From FIG. 25, it can be seen that the density fluctuation is within a sufficiently small error in the low density part where the density fluctuation is particularly noticeable.

  As shown in FIGS. 7 and 13, the second dither matrix having a higher number of lines tends to saturate near 100% earlier as the dot rate increases. Therefore, in the present embodiment, the correlation between the fluctuation amounts is set as a ratio based on the lower screen line number. In other words, the first dither matrix that is the reference for the correlation of the fluctuation amount is selected to have a lower screen line number. As a result, a preferable gradation correction table can be obtained even in the high density portion. In addition, when combining the dither method and the error diffusion method, the error diffusion method can be regarded as a dither screen with a very high number of lines, so the correlation of variation is set as a ratio based on the dither method. The

  As described above, in this embodiment, the image forming apparatus 1 measures the densities of a plurality of different halftone patches on which images are formed by the first gradation expression method. Subsequently, the image forming apparatus 1 stores the second gradation that is different from the first gradation expression method and the density variation amount from the gradation characteristics in the reference environment stored in the ROM 13 in the reference environment by the first gradation expression method. The correlation with the amount of density fluctuation from the gradation characteristics in the reference environment by the expression method is acquired. Then, the image forming apparatus 1 uses the second gradation expression method to calculate the density variation amount of the measured density from the gradation characteristics in the reference environment according to the first gradation expression method according to the acquired correlation. An image is converted by the second gradation expression method using the converted density fluctuation amount and the gradation characteristic in the reference environment according to the second gradation expression method. The density of a plurality of different halftone patches when formed is calculated (estimated).

  Therefore, according to the present embodiment, the second density can be calculated from the actually measured densities of a plurality of different halftone patches formed by the first gradation expression method by simple processing using less data. The density of a plurality of different halftone patches when an image is formed by the gradation expression method is accurately calculated. As a result, it is possible to generate a gradation correction table for obtaining predetermined gradation characteristics in image formation by another different gradation expression method only by actually measuring gradation characteristics by one representative gradation expression method. It becomes possible. As a result, since less halftone patch printing and correction calculation are required, the time required for gradation correction processing can be shortened, and the consumption of printing color material can be reduced.

  Next, a second embodiment will be described.

  The image forming apparatus according to the second embodiment has the same configuration as that of the first embodiment, but among the various data necessary for program execution stored in the ROM 13, the following is different from the first embodiment. .

  That is, the ROM 13 of the image forming apparatus according to the second embodiment stores the first reference gradation characteristic in the reference environment of the halftone patch by the first dither matrix shown in FIG. ) In the reference environment of the halftone patch based on the second dither matrix shown in FIG. 27 and the variation amount from the first reference tone characteristic of the halftone patch based on the first dither matrix shown in FIG. Is stored, the correlation indicating the ratio of the amount of variation from the second reference tone characteristic of the halftone patch by the second dither matrix is stored.

  In the second embodiment, the first reference gradation characteristic and the second reference gradation characteristic represent a virtual ideal state in which the dot rate matches the density of the halftone patch. On the other hand, data obtained by measuring the densities of the halftone patches by the first and second dither matrices in advance in a reference environment are stored as correlations of fluctuation amounts.

  Next, the effect by 2nd Embodiment is demonstrated.

  FIG. 28 shows an example of the density of a halftone patch binarized by the first dither matrix, measured in two different environments (Case 1 and Case 2). Here, there are 8 halftone patches with a dot rate of 100%, 87.5%, 75.0%, 62.5%, 50.0%, 37.5%, 25.0%, 12.5%. It consists of patches.

  FIG. 29 shows the (predicted) density of the halftone patch binarized by the second dither matrix calculated from the actually measured density of the halftone patch shown in FIG.

  On the other hand, FIG. 30 shows the density of the halftone patch binarized by the second dither matrix, measured in the two different environments (Case 1 and Case 2).

  FIG. 31 is a diagram illustrating the ratio (error ratio) of the calculated halftone patch density to the actually measured halftone patch density. From FIG. 31, it can be seen that in the low density portion where the density fluctuation is particularly conspicuous, it is within a sufficiently small error although it is slightly larger than that of the first embodiment.

  As described above, according to the second embodiment, the same effect as that of the first embodiment can be obtained.

  In the first embodiment, the density of the variation is measured by measuring the density of the halftone patch using the first and second dither matrices at least in two environments, ie, the reference environment and an environment different from the reference environment. It is necessary to find the correlation. On the other hand, in the second embodiment, the virtual ideal state is the reference gradation characteristic. Therefore, in the second embodiment, there is an advantage that it is only necessary to actually measure the densities of the halftone patches by the first and second dither matrices in one environment and to obtain the correlation between the fluctuation amounts.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims.

  For example, in the above-described embodiment, a dither method having a different number of screen lines is used as a plurality of gradation expression methods. Further, as a plurality of gradation expression methods, an example of a combination of a dither method and an error diffusion method is given. However, the present invention is also applicable when the plurality of gradation expression methods are dither methods having different dither dot shapes. FIG. 32 is a diagram illustrating a part of a halftone pattern having a dither dot shape different from that in FIG. The effects of the present invention can also be obtained when a plurality of gradation expression methods are performed in which halftone patterns having different dither dot shapes shown in FIGS. 4 and 32 are obtained.

  Further, the present invention is applicable not only to an electrophotographic image forming method but also to other image forming methods such as an ink jet method.

  In the above-described embodiment, the case where the image binarization circuit for realizing the image binarization processing method (gradation expression method) is included in the image forming apparatus has been described. The image processing apparatus including the image forming circuit and the image forming apparatus may be separated and connected to each other.

  In the above embodiment, a printer is exemplified as the image forming apparatus, but the present invention is not limited to this. The present invention can also be applied to other image forming apparatuses such as an MFP (Multi-Function Peripheral) and a copying machine.

  The means and method for performing various processes in the image forming apparatus of the present embodiment can be realized by either a dedicated hardware circuit or a programmed computer. The program may be provided by a computer-readable recording medium such as a flexible disk or a CD-ROM, or may be provided online via a network such as the Internet. In this case, the program recorded on the computer-readable recording medium is usually transferred to and stored in a storage unit such as a hard disk. The program may be provided as a single application software, or may be incorporated into the software of the device as one function of the device.

1 is a block diagram illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. It is a figure which shows an example of the gradation characteristic of the binarization process image by a dither method. It is a figure which shows the dither matrix corresponding to the gradation characteristic of FIG. FIG. 4 is a diagram showing a part of a halftone pattern by the dither matrix of FIG. 3. It is a figure which shows a mode that it prints with the circular dot which has a diameter of about 1.5 times the printing pixel pitch. It is a figure which shows an example of gradation correction table data. It is a figure which shows the fluctuation | variation by the influence of the environmental condition of the gradation characteristic of the binarization process image by the dither method using the 1st dither matrix shown in FIG. 2 is a diagram illustrating an example of a mechanism configuration of an image forming apparatus. FIG. FIG. 3 is a diagram illustrating an example of a series of halftone patches formed on a photosensitive drum. It is a figure which shows an example of a structure of a density sensor. It is a figure which shows an example of the 2nd dither matrix used as a line number higher than the 1st dither matrix shown in FIG. It is a figure which shows a part of halftone pattern by the dither matrix of FIG. It is a figure which shows the fluctuation | variation by the influence of the environmental condition of the gradation characteristic of the binarization process image by the dither method using the 2nd dither matrix shown in FIG. It is a figure which shows the ratio of the gradation characteristic value shown in FIG. 7 and FIG. 13 in each environment. Using the ratio of the gradation characteristic value of FIG. 7A based on the first dither matrix and the gradation characteristic value of FIG. 13A based on the second dither matrix, from the actually measured value based on the first dither matrix. It is a figure which shows the error rate (error ratio) at the time of estimating and calculating the gradation characteristic by a 2nd dither matrix. It is a figure which shows a mode that the area which a dot covers is fluctuate | varied by the fluctuation | variation of the size of the dot currently printed. It is a figure which shows the 1st reference gradation characteristic in the reference environment of the halftone patch by the 1st dither matrix, and the 2nd reference gradation characteristic in the reference environment of the halftone patch by the 2nd dither matrix. A correlation which is a ratio of a variation amount from the second reference gradation characteristic of the halftone patch by the second dither matrix to a variation amount from the first reference gradation characteristic of the halftone patch by the first dither matrix. FIG. It is a figure which shows the gradation characteristic made into the target at the time of print output. 6 is a flowchart illustrating a procedure of processing for generating a gradation correction table (1) in the image forming apparatus. It is a flowchart which shows the procedure of the production | generation process of the gradation correction table (2) in an image forming apparatus. It is a figure which shows an example of the density | concentration of the halftone patch binarized by the 1st dither matrix measured in two different environments. FIG. 23 is a diagram illustrating a (predicted) density of a halftone patch binarized by a second dither matrix calculated from the actually measured density of the halftone patch shown in FIG. 22. It is a figure which shows the density | concentration of the halftone patch binarized by the 2nd dither matrix measured in two different environments. It is a figure which shows the ratio (error ratio) of the density of the calculated halftone patch with respect to the density of the measured halftone patch. The first reference tone characteristic in the reference environment of the halftone patch by the first dither matrix and the second reference tone characteristic in the reference environment of the halftone patch by the second dither matrix in the second embodiment. FIG. In the second embodiment, the variation from the second reference tone characteristic of the halftone patch by the second dither matrix with respect to the variation amount from the first reference tone characteristic of the halftone patch by the first dither matrix in the second embodiment It is a figure which shows the correlation which is a ratio of quantity. It is a figure which shows an example of the density | concentration of the halftone patch binarized by the 1st dither matrix measured in two different environments in 2nd Embodiment. It is a figure which shows the (predicted) density | concentration of the halftone patch binarized by the 2nd dither matrix calculated from the actually measured density of the halftone patch shown in FIG. It is a figure which shows the density | concentration of the halftone patch binarized by the 2nd dither matrix measured in two different environments in 2nd Embodiment. It is a figure which shows the ratio (error ratio) of the density of the calculated halftone patch with respect to the density of the measured halftone patch in 2nd Embodiment. FIG. 5 is a diagram illustrating a part of a halftone pattern having a dither dot shape different from that in FIG.

Explanation of symbols

1 image forming apparatus,
11 CPU
12 RAM,
13 ROM,
14 Image memory,
15 memory access controller,
16 LUT,
17, 18 address conversion circuit,
19 dither matrix memory,
20 CMP,
21 DA / AD converter,
22 concentration sensor,
22a light emitting element,
22b light receiving element,
30 printing department,
33 Photosensitive drum,
40 dither modules,
50 screening modules,

Claims (13)

A measurement unit that measures the density of a plurality of different halftone patches formed by an image according to the first gradation expression method;
The amount of density fluctuation from the gradation characteristics in the reference environment by the first gradation expression method and the density fluctuation from the gradation characteristics in the reference environment by the second gradation expression method different from the first gradation expression method. A storage unit for storing the correlation with the quantity;
The density variation amount of the density measured by the measurement unit from the gradation characteristics in the reference environment according to the first gradation expression method is calculated according to the correlation. A conversion unit for converting the concentration fluctuation amount from the adjustment characteristics;
A plurality of different cases in which an image is formed by the second gradation expression method using the density fluctuation amount converted by the conversion unit and the gradation characteristics in the reference environment by the second gradation expression method. A calculation unit for calculating the density of the halftone patch;
An image forming apparatus comprising: A first generation unit for generating a first gradation correction table for obtaining predetermined gradation characteristics in image formation by the first gradation expression method from the density measured by the measurement unit;
A second generator for generating a second gradation correction table for obtaining predetermined gradation characteristics in image formation by the second gradation expression method from the density calculated by the calculator;
The image forming apparatus according to claim 1, further comprising:   The image forming apparatus according to claim 1, wherein the gradation characteristics in the reference environment are gradation characteristics acquired by density measurement under a predetermined condition relating to image formation.   The image forming apparatus according to claim 1, wherein the gradation characteristic in the reference environment is a predetermined gradation characteristic.   5. The dither method according to claim 1, wherein each of the first gradation expression method and the second gradation expression method is a dither method having a different screen line number or dither dot shape. Image forming apparatus.   The first gradation expression method and the second gradation expression method are dither methods having different screen line numbers, respectively, and the correlation is a ratio based on the lower screen line number. The image forming apparatus according to claim 5, wherein the image forming apparatus is set.   5. The image forming apparatus according to claim 1, wherein the first gradation expression method and the second gradation expression method are a dither method and an error diffusion method, respectively.   The image forming apparatus according to claim 7, wherein the correlation is set as a ratio based on a dither method. Measuring the density of a plurality of different halftone patches formed by the first gradation expression method;
In the reference environment by the second gradation expression method different from the first gradation expression method, the density variation amount from the gradation characteristics in the reference environment by the first gradation expression method stored in the storage unit A step (b) of acquiring a correlation with the density fluctuation amount from the gradation characteristics;
Based on the correlation, the reference environment according to the second gradation expression method is used to calculate the density variation amount from the gradation characteristics in the reference environment according to the first gradation expression method of the density measured in the step (a). A step (c) of converting into a density fluctuation amount from the gradation characteristic in FIG.
A plurality of cases in which an image is formed by the second gradation expression method using the density fluctuation amount converted in the step (c) and the gradation characteristics in the reference environment by the second gradation expression method. Calculating the density of halftone patches of different (d),
A gradation correction method characterized by comprising: Generating a first gradation correction table for obtaining predetermined gradation characteristics in image formation by the first gradation expression method from the density measured in the step (a);
Generating a second gradation correction table for obtaining predetermined gradation characteristics in image formation by the second gradation expression method from the density calculated in the step (d);
The gradation correction method according to claim 9, further comprising: A procedure (a) for causing the measurement unit to measure the densities of a plurality of different halftone patches formed by the first gradation expression method;
In the reference environment by the second gradation expression method different from the first gradation expression method, the density variation amount from the gradation characteristics in the reference environment by the first gradation expression method stored in the storage unit A procedure (b) for obtaining a correlation with the density fluctuation amount from the gradation characteristics;
Based on the correlation, the reference environment according to the second gradation expression method uses the density fluctuation amount from the gradation characteristics in the reference environment according to the first gradation expression method of the density measured in the procedure (a). A procedure (c) for converting into a density fluctuation amount from the gradation characteristics in FIG.
A plurality of cases in which an image is formed by the second gradation expression method using the density fluctuation amount converted in the procedure (c) and the gradation characteristics in the reference environment by the second gradation expression method. (D) calculating the density of halftone patches of different
A gradation correction program for causing a computer to execute processing including the above. In the processing, a procedure (e) for generating a first tone correction table for obtaining predetermined tone characteristics in image formation by the first tone expression method from the density measured in the procedure (a). )When,
A step (f) of generating a second gradation correction table for obtaining predetermined gradation characteristics in image formation by the second gradation expression method from the density calculated in the step (d). The gradation correction program according to claim 11, further comprising:   A computer-readable recording medium on which the gradation correction program according to claim 11 is recorded.