JP2008254341A - Image formation device and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform a natural tone correction suited to a human being's visual sense with high precision in an image formation device. <P>SOLUTION: A test pattern is formed when turning the power on, or printing sheets of the number predetermined, or being demanded by users. Test patterns with different densities are formed according to an image formation processing. In the case of a printing processing, the test pattern is formed using a dither pattern corresponding to the printing processing. In the case of a copy processing, the test pattern is formed using the dither pattern corresponding to the copy processing. The density of the test pattern is detected, and then the density value is converted into a luminance or a dot area ratio. Based on the luminance or the dot area ratio, a γ correction table is corrected. The density correction for image data input is made according to the corrected γ correction table, then the image is formed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that performs a copying process and a printing process by correcting a density correction table using a test pattern.

  In an electrophotographic image forming apparatus, density fluctuations occur when power is turned on or after mass printing. Therefore, a density sensor is provided on the main body side, and the measured density value is transmitted to the controller side. Based on the density value received from the density sensor, correction processing is performed on the controller side, thereby realizing highly accurate gradation correction. For example, a dither pattern corresponding to copy processing or printing processing is formed, and the gamma correction table is corrected from the read density value. Examples of related art related to this will be given below.

  The “image forming method” disclosed in Patent Document 1 is a method of selecting a gradation correction table so that a human eye does not feel a density shift. The density gradation pattern detection data is converted to density or brightness. The difference between the conversion result and the ideal characteristic is corrected with a weight function according to the sensitivity characteristic of the human eye. This weighting function places importance on the degree of matching at a relatively low density portion so as to match the sensitivity of the human eye. A plurality of correction tables are stored in the ROM, and the correction table having the highest degree of match is selected based on the difference corrected by the weight function. The correction table is used in the γ correction circuit to correct the input data so that the desired characteristics are obtained, and to control the laser output. It is easier to convert the ideal characteristic into the output of the optical sensor in advance and select the gamma correction table according to the difference between the ideal output of the optical sensor and the detected value.

  The “image forming apparatus” disclosed in Patent Document 2 has a good output image by the dither method. A toner pattern is created on the photoreceptor by an electrophotographic process. The toner pattern on the photoconductor is read. A γ correction table is created based on the output of the reading means. The dot concentration type dither method is used, and the dither pattern used for output and the same matrix type pattern are used as tone characteristic correction dot patterns.

  The “image processing apparatus” disclosed in Patent Document 3 always sets a halftone density value for forming a dither pattern in the multi-value dither method to an optimal value following the change in the process conditions of the apparatus. This enables high-quality gradation conversion. When the apparatus is turned on, an optimum halftone level is set in the current environment by actually forming a corrected image in the initial processing unit and detecting its density. Thereafter, the input image signal is subjected to multi-value dither processing in a dither processing unit and converted to a predetermined gradation. The gradation is converted into a predetermined density level value based on the halftone level in the level conversion unit, and is modulated and output from the PWM unit.

The “composite copying apparatus” disclosed in Patent Document 4 corrects the γ correction table separately in the copying process and the printing process. When the power is turned on, a dither pattern corresponding to the copying process is formed, and the γ correction table for copying is corrected based on the result of reading the dither pattern. When a printer job is first received, a dither pattern corresponding to the printing process is formed, and the printer gamma correction table is rewritten based on the result of reading the dither pattern. At the time of image formation, γ correction is performed on the image forming characteristics with reference to the γ correction table, and an image based on the γ corrected characteristics is formed.
Japanese Patent Laid-Open No. 05-244418 Japanese Unexamined Patent Publication No. 07-123264 JP 09-326926 A JP 2004-181670 A

  However, the conventional gradation correction method has the following problems. Even if the density value is read from the test pattern corresponding to each of the copying process and the printing process and the correction is performed, the density value is not compatible with human vision, so that a natural tone correction cannot be performed. In addition, the method of selecting a correction table based on the result of correcting the difference between the result of converting density gradation pattern detection data to lightness and the ideal characteristic with a weight function according to the sensitivity characteristic of the human eye is approximately Only density correction is possible. Even if these methods are combined, it is impossible to realize optimum density correction that matches the sensitivity of the human eye according to the image forming processing mode.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described conventional problems and to perform gradation correction corresponding to copying and printing processing with high accuracy in accordance with human vision in an image forming apparatus.

  In order to solve the above problems, in the present invention, a pattern forming unit that forms a plurality of test patterns having different densities in order to check a print density, a density detecting unit that detects the density of the formed test pattern, and a detection The pattern forming unit of the image forming apparatus includes a correction control unit that corrects the density correction table based on the density value and a unit that forms an image by performing density correction on the input image data according to the density correction table. And a means for forming a test pattern using a dither pattern corresponding to the copying process, and the correction control unit supplies the detected density value with brightness or halftone dots. A configuration comprising means for converting to an area ratio and means for correcting the density correction table based on the brightness or halftone dot area ratio of the detected density value It was. The correction control unit includes means for correcting the density correction table so that the brightness becomes linear. The pattern forming unit includes means for forming a test pattern when the power is turned on, when a predetermined number of sheets are printed, or when requested by the user.

  With the above-described configuration, natural tone correction suitable for human vision can be performed with high accuracy in the image forming apparatus.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to FIGS.

  In an embodiment of the present invention, a test pattern is formed using a dither pattern corresponding to a printing process and a dither pattern corresponding to a copying process, and a density value obtained from the test pattern is converted into a brightness or a dot area ratio. Thus, the image forming apparatus corrects the γ correction table.

  FIG. 1 is a functional block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. The basic configuration of the image forming apparatus in the embodiment of the present invention is the same as the conventional one. A correction method such as a γ correction table is different from the conventional one. In FIG. 1, a print medium 1 is a photosensitive member, an intermediate transfer member, paper, or the like on which a test pattern is printed. The detector 2 is means for detecting the density of the test pattern. The control unit 3 is means for correcting from the detected density. The A / D converter 4 is means for A / D converting the signal from the detector. The CPU 5 is an arithmetic control device that controls the whole. The correction control unit 6 is means for storing a dither pattern to be generated.

  The ROM 7 is a fixed memory that stores programs and fixed data. The NVRAM 8 is a non-volatile memory that stores table data and the like. The gradation correction pattern 9 is a pattern printed with a dither pattern. The image forming unit 10 is a means for forming an image such as a test pattern. The dither pattern generation unit 11 is means for generating an image for density detection. The panel control unit 12 is an interface with the operation panel 13. The operation panel 13 is a display panel for operation.

  FIG. 2 shows a color path configuration from input to output in the image forming apparatus according to the embodiment of the present invention. This color path configuration is also the same as a general one. In FIG. 2, an application 16 is an application program that requests printing. The color matching module 17 is means for converting an RGB signal into a CMYK signal. The printer γ conversion module 18 is means for performing γ conversion of the CMYK signal. The total amount regulation module 19 is a means for determining the total amount of CMYK signals that have been γ-converted. The gradation conversion module 20 is a means for expressing gradation with respect to the CMYK signal.

  The color profile 21 is means for holding a profile for performing color matching. The LUT 22 is a color profile lookup table. The IN / OUT table 23 is a color profile table for input / output. The γ correction table 24 is a γ correction table used for γ conversion. The total amount restriction value 25 is a means for holding the total amount restriction value. The dither pattern 26 is a means for holding a pattern for expressing gradation.

  FIG. 3 is a flowchart showing a processing procedure for executing automatic gradation correction when the power is turned on in the image forming apparatus according to the embodiment of the present invention. FIG. 4 is a diagram in which brightness and density are set linearly from 0 to 255 (white to black). FIG. 5 is a diagram showing the relationship between the dot area ratio and the density when the Yule-Nielsen coefficient is changed.

  The function and operation of the image forming apparatus in the embodiment of the present invention configured as described above will be described. First, an overview of the functions of the image forming apparatus will be described with reference to FIG. A test pattern is formed when the power is turned on, when a predetermined number of sheets are printed, or when requested by the user. In the case of a printing process, a test pattern is formed using a dither pattern corresponding to the printing process, and in the case of a copying process, a test pattern is formed using a dither pattern corresponding to the copying process. The density of the formed test pattern is detected, and the detected density value is converted into lightness or halftone dot area ratio to correct the γ correction table. An image is formed by applying density correction to the input image data.

  That is, using the dither pattern stored in the correction control unit 6, the dither pattern generation unit 11 generates a test pattern for density detection. A gradation correction pattern 9 is printed by the image forming unit 10 on a printing medium 1 such as a photosensitive member, an intermediate transfer member, or paper. The detection unit 2 detects the density of the test pattern. The signal from the detection unit 2 is A / D converted by the A / D conversion unit 4. The control unit 3 corrects the γ correction table based on the detected density value.

  Next, the color path will be described with reference to FIG. The color matching module 17 converts the RGB signal received from the application 16 into a CMYK signal. When performing color matching, the LUT 21 holding the color profile with the color profile 21 and the IN / OUT table 23 are used. The printer γ conversion module 18 performs γ conversion of the color-matched CMYK signal. A γ correction table 24 is used when performing γ conversion. The total amount regulation module 19 determines the total amount of the CMYK signal that has been γ-converted. At that time, the total amount regulation value 25 is used. The gradation conversion module 20 expresses gradation for the CMYK signal whose total amount is restricted. At that time, the dither pattern 26 is used. Then, the image forming unit 10 forms an image.

  Next, a processing procedure for automatic gradation correction at power-on will be described with reference to FIG. The processing flow is the same not only when the power is turned on, but also when a specified number of sheets is printed or when a user specifies. When an automatic gradation correction trigger comes, in step 1, the controller calls a CMYK dither pattern corresponding to the printing process, and forms a test pattern on the photoconductor using the dither pattern. Usually, since screen processing differs depending on the printer, a dither pattern used for actual printing is used.

Thereafter, in step 2, the density of the test pattern for gradation correction using the dither pattern is detected by a sensor. At this time, the detected density is a density on an intermediate transfer member such as a photosensitive drum or a transfer belt. Then, in step 3, the read density value is converted into lightness (L ^* ) or halftone dot area ratio that is visually highly correlated by the controller. In step 4, the difference from the current γ correction table is obtained from the data, and the γ correction table is corrected. This table is in the CMYK color space. In the case of copy processing, automatic gradation correction can be performed in the same manner as in the case of a printer by correcting a gamma correction table for copy using a dither pattern for copy processing.

Next, the difference between brightness linearity and density linearity will be described with reference to FIG. In the field of colorimetry, as an evaluation scale of brightness, there is lightness obtained from luminous reflectance. In the field of color hard copy, it is often used in L ^* in the L ^* a ^* b ^* color space. On the other hand, in the field of monochrome hard copy, a density which is a logarithmic value of the reciprocal of reflectance has been used to represent the density of an image. Generally, it is said that the lightness L ^* has better linearity with human vision. This meaning will be described with reference to FIG. FIG. 4 shows 0 to 255 (white to black) in the case of lightness linearity and density linearity. It can be seen that the brightness linearity is visually equidistant.

The digital electrophotographic output method is based on area modulation as in offset printing. In printing, density management is performed based on the dot area ratio. What is actually measured is the concentration. The concentration can be converted into a halftone dot area ratio by the following Yule-Nielsen equation.
Da = −n × log [(1-a) + a × 10 ^{−Ds / n} ] + Dp
Here, Da is density, and n is a coefficient determined by the number of screen lines and paper quality. a is the dot area ratio, Ds is the solid density, and Dp is the density of the paper.

  Next, the density linear characteristic and the brightness linear characteristic will be described with reference to FIG. FIG. 5 shows a graph showing the relationship between the dot area ratio and the concentration by changing the Yule-Nielsen coefficient. When n = ∞, the density is linear, and when n≈3.0, the brightness is linear.

  As described above, in the embodiment of the present invention, the image forming apparatus forms a test pattern using the dither pattern corresponding to the printing process and the dither pattern corresponding to the copy process, and obtains the density value obtained from the test pattern. Is converted into lightness or halftone dot area ratio and the γ correction table is corrected, so that natural gradation correction suitable for human vision can be performed easily and with high accuracy.

  The image forming apparatus of the present invention is optimal as a copying machine or printer that corrects a γ correction table with a test pattern and prints by a dither method.

1 is a functional block diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a color path configuration from input to output of the image forming apparatus according to the exemplary embodiment of the present invention. 5 is a flowchart showing a processing procedure of automatic gradation correction at power-on in the image forming apparatus in the embodiment of the present invention. It is a figure explaining the difference between brightness linearity and density linearity. It is the figure which changed the Yule-Nielsen's coefficient and showed the relationship between a dot area ratio and a density | concentration.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Print medium, 2 ... Detection part, 3 ... Control part, 4 ... A / D conversion part, 5 ... CPU, 6 ... Correction control part, 7 ... ROM, 8 ... NVRAM, 9 ... Tone correction pattern, 10 Image forming unit, 11 Dither pattern generation unit, 12 Panel control unit, 13 Operation panel, 16 Application, 17 Color matching module, 18 Printer gamma conversion module, 19 Total control module, 20 Gradation Conversion module, 21 ... color profile, 22 ... LUT, 23 ... IN / OUT table, 24 ... γ correction table, 25 ... total amount regulation value, 26 ... dither pattern.

Claims (6)

  A pattern forming unit that forms a plurality of test patterns having different densities to check the print density, a density detecting unit that detects the density of the formed test pattern, and a correction that corrects the density correction table based on the detected density value In the image forming apparatus comprising: a control unit; and means for forming an image by performing density correction on input image data according to the density correction table, the pattern forming unit uses a dither pattern corresponding to a printing process to test a pattern And a means for forming a test pattern using a dither pattern corresponding to a copying process, wherein the correction control unit converts the detected density value into lightness or halftone dot area rate; And a means for correcting the density correction table based on the brightness or halftone dot area ratio. .   The image forming apparatus according to claim 1, wherein the correction control unit includes a unit that corrects the density correction table so that the brightness becomes linear.   The image forming apparatus according to claim 1, wherein the pattern forming unit includes means for forming the test pattern when power is turned on, when a predetermined number of sheets are printed, or when a user requests.   In order to check the print density, in the case of print processing, a test pattern is formed using a dither pattern corresponding to the print processing. In the case of copy processing, a test pattern is formed using a dither pattern corresponding to the copy processing. The density of the formed test pattern is detected, the detected density value is converted into lightness or halftone dot area ratio, the density correction table is corrected, and the input image data is subjected to density correction according to the density correction table to form an image. An image forming method.   The image forming method according to claim 4, wherein the density correction table is corrected so that the brightness is linear.   5. The image forming method according to claim 4, wherein the test pattern is formed when power is turned on, when a predetermined number of sheets are printed, or when a user request is made.

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