JP2002374415A - Printer and density correcting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To precisely makes density corrections of a plurality of half-tone processing without making the time needed for a density measuring processing long. SOLUTION: Patch patterns generated by dithers 1 and 2 are printed on a form (S401) and the density values of respective density levels of the printed patch patterns are measured (S402) by using a scanner 119 and density characteristics to the input density level of the dither 1 are computed (S403) and a density correction table for correcting the input density level is generated (S404). Further, density characteristics to the input density level of the dither 2 are computed (S405) and a density correction table for correcting the input density level is generated (S406). The density correction tables of the dithers 1 and 2 are sent to a printer and stores in a density correction table 113c in a RAM 113 (S407). The kinds of half-tones after density corrections and the number of the kinds are set in a half-tone specification area 113b (S408).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printing apparatus and a density correction method, and more particularly, to a printing apparatus and a density correction method having density correction means for performing density correction of a plurality of halftone processes.

[0002]

2. Description of the Related Art In a conventional printing apparatus, density correction for a density level of a formed image in order to eliminate color fluctuation due to density fluctuation due to aging changes is performed at a plurality of density levels obtained by performing a specific type of halftone processing. The patch pattern is printed on paper, the density is measured by an external reading device such as a scanner, and the density characteristic of the halftone processing is measured based on the measured density value. Thus, a density correction table is created so that the density correction is performed, and the density correction is performed using the density correction table. Further, the density correction of the halftone processing in which the measurement is not performed is performed using the density correction table of the measured halftone processing.

[0003]

However, in such a conventional printing apparatus, since the density fluctuation varies depending on the type of the halftone processing, the accuracy of the density correction in the halftone processing without measurement is insufficient. There was a problem. Further, performing the density measurement processing a plurality of times for each type of halftone processing has a problem in that the printing speed is reduced.

The present invention has been made in view of such a problem, and an object of the present invention is to accurately perform density correction of a plurality of halftone processes without increasing the time required for the density measurement process. It is an object of the present invention to provide a printing apparatus and a density correction method capable of simultaneously measuring density fluctuations of a plurality of halftone processes and improving the durability of a plurality of density correction tables.

[0005]

In order to achieve the above object, the present invention is directed to a printing apparatus having a density correcting means for correcting a plurality of halftone processes. A patch pattern printing means for printing a sufficient number of halftone processed density level patch patterns on the same paper to obtain density characteristics with respect to an input density level; and Patch pattern measurement means for measuring the density of the patch pattern printed on the paper by the printing means; and density characteristic calculation for obtaining the density characteristics of each of a plurality of halftone processes based on the density values measured by the patch pattern measurement means. Means, and an input density level correction table for each of the plurality of halftone processes so that the density characteristics calculated by the density characteristic calculation means become specified density characteristics. A density correction table creation means for creating, characterized in that a density correction table setting means for setting a plurality of density correction table of the halftone processing created by the concentration correction table creation means.

[0006] The invention described in claim 2 is the invention according to claim 1.
Wherein the density correction halftone setting means sets the type and number of halftones set by the density correction means, and an area in which the patch pattern can be formed is set by the density correction halftone setting means. Patch pattern forming area dividing means for dividing the number of halftones into halftone areas and assigning them as respective halftone areas, and densities at several points where each halftone processing is performed on the area allocated by the patch pattern forming area dividing means. It is characterized by comprising a patch pattern forming means for forming a level patch pattern, and a density measuring means for measuring the density of the patch pattern formed by the patch pattern forming means using a sensor.

[0007] Further, the invention according to claim 3 is based on claim 2.
In the invention described in the above, at the time when the density correction table of each halftone is created by the density correction means, the density level measured by the density measurement means is set to which density level by the density correction table of each halftone. A sensor reference value determining unit that determines whether the correction has been performed, and uses the sensor measurement value of each halftone measured by the density measurement unit as a reference density level and a sensor reference value of each halftone, respectively. The density measuring means is performed continuously with the density correcting means.

[0008] The invention described in claim 4 is the invention according to claim 3.
In the invention described in the above, after the sensor reference value determination means is performed, the density measurement means is performed at a specified timing, and compares the measured sensor measurement value of each halftone with the sensor reference value. Density fluctuation amount calculating means for calculating the density fluctuation amount of each halftone by the increase / decrease thereof, and the density fluctuation amount calculated by the density fluctuation amount calculation means, the density characteristic of each halftone is defined as a specified density characteristic. Density correction table correction means for correcting the density correction table for each halftone.

According to a fifth aspect of the present invention, there is provided a density correction method for a printing apparatus having a density correction means for performing density correction of a plurality of halftone processes, wherein a density characteristic with respect to an input density level is obtained on the same sheet. A patch pattern printing step of printing a sufficient number of halftone processes of a plurality of halftone processes, and a patch pattern measuring step of measuring the density of the patch pattern printed on the paper by the patch pattern printing step And the density value measured in the patch pattern measurement step,
A density characteristic calculating step of obtaining density characteristics of each of the plurality of halftone processes; and an input density level correction of each of the plurality of halftone processes so that the density characteristics calculated by the density characteristic calculation steps become prescribed density characteristics. A density correction table creating step for creating a table and a density correction table setting step for setting a plurality of density correction tables for halftone processing created in the density correction table creating step are provided.

The invention described in claim 6 is the same as the invention in claim 5
Wherein the density correction halftone setting step of setting the type and number of halftones set in the density correction step, and an area where the patch pattern can be formed are set by the density correction halftone setting step. A patch pattern forming area dividing step of dividing the number of halftones into halftone areas and assigning each of them as a halftone area; and densities of several points obtained by performing each halftone processing on the area allocated by the patch pattern forming area dividing step. A patch pattern forming step of forming a level patch pattern;
A density measuring step of measuring the density of the patch pattern formed in the patch pattern forming step using a sensor.

The invention described in claim 7 is the same as the claim 6.
In the invention described in the above, at the time when the density correction table of each halftone is created by the density correction step, the density level measured by the density measurement step is:
The density correction table of each halftone is used to determine which density level has been corrected, and the sensor measurement value of each halftone measured in the density measurement step is used as the reference density level of each halftone and the sensor. There is a sensor reference value determining step as a reference value, and the density measuring step is performed continuously with the density correcting step.

[0012] The invention described in claim 8 is the same as in claim 7.
In the invention described in the above, after the sensor reference value determination step is performed, the density measurement step is performed at a specified timing, and compares the measured sensor measurement value of each halftone with the sensor reference value. The density fluctuation amount calculation step of calculating the density fluctuation amount of each halftone by the increase / decrease, and the density fluctuation amount calculated in the density fluctuation amount calculation step, the density characteristics of each halftone are defined as the specified density characteristics. A density correction table correction step for correcting the density correction table for each halftone.

According to a ninth aspect of the present invention, there is provided a program for causing a computer to execute the steps according to any one of the fifth to eighth aspects.

According to the present invention, it is possible to accurately perform the density correction of a plurality of halftone processes without increasing the time required for the density measurement process. Also,
In the printing apparatus, the density fluctuations of a plurality of halftone processes can be measured at the same time, and the durability of the plurality of density correction tables can be improved.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram for explaining a control configuration in an embodiment of a printing apparatus (hereinafter, referred to as a printer) according to the present invention. In the figure, reference numeral 101 denotes a host computer, and color information, characters, graphics, image images,
The print information such as the number of copies to be printed is sent to the printer 102. A scanner 119 measures the density of a density correction scanner patch pattern described later.

The printer 102 includes an image processing unit 103,
A printer engine 114 that forms an actual image based on an image signal sent from the image processing unit 103 and a density measuring unit 115 that measures a density value of a density level are roughly classified.

The main configuration and operation of the image processing section will be described below. In the image processing unit 103,
Reference numeral 104 denotes an interface for controlling transmission / reception of print information to / from the host computer 101, and reference numeral 105 denotes a reception buffer for holding input print information. In the object generation unit 106, information such as colors, characters, graphics, and image images, which are print information input from the host computer 101, is converted into intermediate information (hereinafter, referred to as an object) and stored in the object buffer 107. At this time, if the print information is color-related data such as a gray level setting, a color level setting, and a multi-valued image image, the data correction unit 116 described later corrects the density level using a density correction table.

Next, based on the object stored in the object buffer 107, the rendering unit 108
Generates a bit image to be rendered. At this time, the dither processing unit 110 performs halftone processing to reduce the output gradation. The generated bit image is stored in the band buffer 109. In this way, the bit image stored in the band buffer 109 is transmitted from the printer engine 114 and is formed on a recording medium.

Reference numeral 112 denotes a central processing unit (CP)
In U), various processes are determined and controlled according to the program stored in the ROM 111. 111 is ROM
(Read only memory), which stores various control programs 111a including the programs shown in the flowcharts of FIGS. 2, 4, 6, 7, 12, and 14.

Reference numeral 113 indicates that the CPU 112
The RAM (random access memory) used as a work area stores data and status information for performing determination control of each process according to the program stored in the storage unit 1, and stores halftone designation data 113b and a density correction table 113c to be described later. is there.

The density value of the scanner patch pattern measured by the scanner 119 is converted into a density correction table in a
The type and number of the measured halftones stored in the density correction table area 113c of 113 are set in the halftone designation area 113b of the RAM 113. A density measurement pattern forming unit 118, which will be described later, forms a plurality of density level patch patterns of a specified halftone, and the density measurement unit 115 uses a sensor to measure the density value of the patch pattern formed by the density measurement pattern formation unit. The density correction table correction unit 117 described later calculates the density fluctuation amount from the measured value, corrects the density correction table, and performs data correction.
In step 6, data density correction is performed.

Hereinafter, the printing process according to the present embodiment will be described with reference to the flowchart shown in FIG.
Note that the control program for realizing the processing shown in the flowchart shown in FIG. 2 is stored in the ROM 111 and executed by the CPU 112 as described above.

In FIG. 2, first, print data is received from the host computer 101 (S201), and held in the reception buffer 105 (S202). Then, data for one processing unit is extracted from the reception buffer 105 (S
203), it is determined whether or not all data has been extracted (S204). If it is determined that the data processing has not been completed, it is determined whether the data processing for one page has been completed (S205).

If it is determined that the printing has not been completed, it is determined whether the print data is color-related data such as color information or a color image (S206). If it is determined that the data is color-related data, the data correction unit 116 corrects the density level using a density correction table (S207). Then, an object is created (S20).
8) and store it in the object buffer 119 (S20)
9) Return to step S323 to retrieve the next data.

On the other hand, if it is determined in step S206 that the data is not color-related data, it is determined whether or not the data is mask data such as a character or a figure (S210). If it is determined that the object is the mask data, an object of the mask data is created (S208), and the process returns to step S209. On the other hand, if it is determined that the data is not mask data, print data processing is performed according to the type of data (S21).
1) Return to step S203.

If it is determined in step S205 that one page of data has been completed, rendering processing is performed based on the object held in the object buffer 107 (S212), and a bit image is transmitted to the printer engine 114. Then, a printing process for printing on paper is performed (S213). Step S2
If it is determined in 04 that the data is completed, the processing is terminated.

Next, a scanner density correction process according to the present embodiment will be described with reference to FIGS. In the present embodiment, two types of halftone processing for performing density correction, dither 1 and dither 2, are used.

FIG. 3 is a diagram showing an example of a patch pattern for scanner density correction. On the same paper, Y, M, C,
A patch pattern having an input density level of 4% and having two types of halftone processing of dither 1 and dither 2 is formed on the same sheet in a K color printing area in each color area. This patch pattern is printed on paper by a command from the host computer or a test print function of the printing apparatus.

FIG. 4 is a flowchart showing the scanner density correction processing. The patch patterns formed by dither 1 and dither 2 shown in FIG. 3 are printed on paper (S40).
1) The density value of each density level of the printed patch pattern is measured using the scanner 119 (S402), and the input density of the dither 1 is determined from the measured density value of each density level of the dither 1 patch pattern. A density characteristic for the level is calculated (S403), and a density correction table for correcting the input density level so that the density characteristic of the dither 1 becomes a specified density characteristic is created (S404).

Next, a density characteristic for the input density level of the dither 2 is calculated from the measured density value of each density level of the patch pattern of the dither 2 (S405), so that the density characteristic of the dither 2 becomes a specified density characteristic. A density correction table for correcting the input density level is created (S406). The density correction tables of dither 1 and dither 2 are sent to the printing device, and R
It is stored in the density correction table 113c of the AM 113 (S
407). Then, the type (dither 1, dither 2) and the number (two types) of the halftones for which the density correction has been performed are set in the halftone designation area 113b (S408).

FIG. 5 is a diagram showing an example of the density characteristic.
The solid line in (a) indicates that the patch pattern of FIG. 3 shown in step 404 of FIG.
Is a density characteristic with respect to the input density level obtained from the measured density value. The dashed line in FIG. 5A is a predetermined density characteristic which is determined in advance and is a linear characteristic. In order to change the density characteristic of the dither 1 shown by the solid line in FIG. 5A to the linear characteristic shown by the broken line in FIG. 5A, as shown by the solid line in FIG.
A density correction table for replacing the input density level with the corrected density level is created (a density correction table for dither 1). With respect to the density measurement value of dither 2, the density characteristics are similarly calculated, and a density correction table for dither 2 is created.

Next, a density measuring process in the printing apparatus according to the present embodiment will be described with reference to FIGS. FIG. 6 is a flowchart illustrating an example of the density measurement process. First, a patch pattern for performing density measurement is formed (S601). The density of the formed patch pattern is measured by a sensor in the printer (S602).

FIG. 7 is a flowchart showing the patch pattern forming process (S601) of FIG. As shown in step 408 in FIG. 4, two types of dither, dither 1 and dither 2, for which scanner density correction has been performed are designated in the halftone designation data 113b (S701). When the patch pattern formation area for the density measurement of the printer is an area in which a patch pattern of eight points for each color as shown in FIG. 8 can be formed, as shown in FIG. , And the upper portion is assigned as a dither 1 region and the lower portion is assigned as a dither 2 region (S702). Then, as shown in FIG. 10, a patch pattern having a density level of 20%, 40%, 60%, and 80% by dither 1 is formed in the upper region, and a density pattern of 20% by dither 2 is formed in the lower region. 40%, 60% and 80% patch patterns are formed (S703).

FIG. 11 shows the density measurement process (S60) of FIG.
It is a figure showing an example of 2). In the printer 102, the density of the patch pattern (FIG. 10) formed in the above-described patch pattern forming area on the intermediate transfer body 1101 is measured by the sensor 1102, and the measured density value is sent to the image processing unit 103. Note that the density measurement processing is performed continuously with the scanner density correction processing shown in FIG. 4 and the power supply is turned on and the specified number of prints is set. After printing the number of
This is performed at a predetermined time, for example, a predetermined elapsed time such as every 30 minutes, or at a timing such as an environmental change such as an in-machine temperature and humidity.

FIG. 12 is a flowchart showing an example of the sensor reference value setting process. The sensor reference value setting process
The density level value and the sensor measurement density value measured in the density measurement processing shown in FIG. 6 which is performed continuously with the sensor density correction processing shown in FIG. 4 are received from the density measurement unit 115 (S1).
201) First, the density level value of dither 1 is shown in FIG.
The input density level value of the input density level axis of FIG. 5B corresponding to the corrected density level value of the corrected density level axis of FIG. 5B is obtained and set as the reference density level of dither 1 (S1202). At the same time, the sensor measurement value is set as the sensor reference value of dither 1 (S1203).

The sensor reference value is indicated by ● (black dot) shown in FIG.
Becomes In this sensor reference value setting process, a sensor reference value is similarly set for the density level value measured by dither 2 and the sensor measurement density value.

Next, referring to FIGS. 14 and 15, FIG.
The operation of the density correction table correction unit shown in FIG. FIG. 14 is a flowchart showing the density correction table correction processing, in which a sensor measurement value is received from the density measurement unit 115 (S1401).
Is compared with the sensor reference value of the same density level set in the sensor reference value setting processing shown in (1), the amount of density fluctuation from the sensor reference value is determined (S1402), and the density characteristic corrected by the density correction table is obtained from the density fluctuation amount. Then, the density correction table of the dither 1 is corrected so that the density characteristics become the specified density characteristics (linear characteristics) (S1403). Similarly, for the dither 2, the density correction table of the dither 2 is corrected.

FIG. 15 shows an example of the density correction table correction processing. The horizontal axis in FIG. 15A indicates the input density level, and the vertical axis indicates the density value corrected by the density correction table. A dashed line is a density characteristic at the time of creating the density correction table, which is a specified density characteristic. ● on the wavy line indicates the sensor reference value, and 値 (white point) indicates the sensor measurement value. The density difference between the sensor reference value and the sensor measurement value is the density variation. The density characteristic corrected by the current density correction table from the density fluctuation amount becomes the density characteristic indicated by the solid line in FIG.

When correction is performed so that the density characteristic indicated by the solid line in FIG. 15A becomes a specified density characteristic (linear characteristic),
A density correction table indicated by a solid line in FIG. The density correction table shown by the solid line in FIG.
15B is a correction table of the input density level corrected by the density correction table indicated by the solid line in FIG. 15B, and includes a density correction table indicated by a solid line in FIG. 15B and a density correction table indicated by a solid line in FIG. When the density correction table is corrected by synthesizing the density correction table, the density correction table shown by the solid line in FIG.

The color data input from the host computer is converted into YMCK data, and the density level is always corrected by using the specified density correction table for halftone processing (solid line in FIG. 15B). Printing can be performed with colors having standard density characteristics.

As described above, according to the present invention, a sufficient number of halftone-processed density-level patch patterns are printed on the same sheet to obtain density characteristics with respect to the input density level, and are printed on the sheet. The density of the patch pattern is measured using a scanner, and the density characteristics of each of the plurality of halftone processes are obtained from the measured density values, and the input densities of each of the plurality of halftone processes are determined so that the density characteristics become the specified density characteristics. Creates a level correction table, sets a density correction table for multiple halftone processing in the printing device, sets the type and number of halftones that have been subjected to density correction by the scanner, and can form a patch pattern in the printing device The area is divided into the set number of halftones and assigned as each halftone area, and patch patterns of several density levels that have been subjected to each halftone processing are assigned to the assigned area. The density of the formed patch pattern is measured using a sensor, and when the density correction table for each halftone is created by the scanner, the density level measured in the printing apparatus is used for each halftone. It is also determined which density level has been corrected by the density correction table, and the measured sensor values of each halftone are used as the reference density level and sensor reference value of each halftone, respectively, and the sensor reference value is set. The sensor measurement value of each halftone measured in the subsequent density measurement is compared with the sensor reference value, and the density fluctuation amount of each halftone is calculated based on the increase / decrease of the sensor reference value. The time required for the density measurement process is not increased by correcting the density correction table of each halftone so that the density characteristic of the halftone becomes the specified density characteristic. , It is possible to accurately perform multiple density correction halftone processing. Further, in the printing apparatus, it is possible to simultaneously measure the density fluctuations of the plurality of halftone processes, and it is possible to improve the durability of the plurality of density correction tables.

In this embodiment, the density correction 2 is performed.
Although there are different types, an arbitrary number of types can be set, and an arbitrary type of density correction table can be created by dividing the patch pattern forming area according to the number of types.
Although the color correction is performed by a color printer, the present invention is also effective in devices such as a monochrome printer, a copying machine, and a facsimile. Further, although the density levels for measuring the density in the printing apparatus are eight density levels, the present invention is also effective in a printing apparatus that measures an arbitrary density level.

[0043]

As described above, according to the present invention, the density correction means performs a plurality of halftone processing steps on a single sheet to obtain a density characteristic corresponding to the input density level. A patch pattern printing unit that prints a patch pattern, a patch pattern measurement unit that measures the density of the patch pattern printed on the paper by the patch pattern printing unit, and a plurality of density values measured by the patch pattern measurement unit. A density characteristic calculating means for calculating each density characteristic of the halftone processing, and an input density level correction table for each of the plurality of halftone processing are created so that the density characteristic calculated by the density characteristic calculating means becomes a specified density characteristic. And a plurality of halftone processing density correction tables created by the density correction table creation means. The density correction table setting means makes it possible to measure multiple patch patterns for halftone processing in a single density measurement process, and to improve the durability of multiple halftone processes without increasing the time required for density measurement processing. Can be accurately corrected.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a control configuration of a printing apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a print information processing method according to the embodiment.

FIG. 3 is a diagram illustrating an example of a correction patch pattern according to the embodiment.

FIG. 4 is a flowchart illustrating a correction processing method according to the embodiment.

FIG. 5 is a diagram illustrating an example of a density correction table creation process according to the embodiment.

FIG. 6 is a flowchart illustrating an example of a density measurement process according to the embodiment.

FIG. 7 is a flowchart illustrating a pattern forming process according to the embodiment.

FIG. 8 is a diagram illustrating an example of a ぱ patch pattern formation region in the present embodiment.

FIG. 9 is a diagram illustrating an example of division of a patch pattern formation area according to the present embodiment.

FIG. 10 is a diagram illustrating an example of two types of dither patch pattern formation in the present embodiment.

FIG. 11 is a diagram illustrating an example of a density measurement process according to the present embodiment.

FIG. 12 is a flowchart illustrating an example of a sensor reference value setting process according to the embodiment.

FIG. 13 is a diagram illustrating an example of a sensor reference value setting process according to the present embodiment.

FIG. 14 is a flowchart illustrating an example of a density correction table correction process according to the embodiment.

FIG. 15 is a diagram illustrating an example of a density correction table correction process according to the present embodiment.

[Explanation of symbols]

 101 host computer 102 printer 103 image processing unit 104 interface 105 reception buffer 106 object generation unit 107 object buffer 108 rendering unit 109 band buffer 110 dither processing unit 111 ROM 111a control program 112 CPU 113 RAM 113a work area 113b halftone designation area 113c density Correction table area 114 Printer engine 115 Density measurement unit 116 Data correction unit 117 Density correction table creation unit 118 Pattern formation unit 119 Scanner 120 Scanner density correction unit 1101 Intermediate transfer body 1102 Sensor

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C061 AP01 AP04 KK18 KK25 KK28 KK32 2C262 AA24 AB12 AC07 BB01 BB36 BC01 BC10 EA04 FA13 GA02 2H027 DA09 DB01 DE07 DE10 EB03 EC03 EC06 EC07 EC09 EE08 EF09 CA28 A0812 CB01 CB07 CB12 CB16 CC01 CE13 DB02 DB06 DB09 DC22 DC33 5C077 LL04 LL19 MP01 MP08 NN08 PP15 PP27 PP28 PP33 PP54 PQ23 TT02

Claims (9)

[Claims] In a printing apparatus having a density correcting means for performing a density correction of a plurality of halftone processes, the density correcting means has a sufficient number of a plurality of density characteristics for obtaining an input density level on the same sheet. Patch pattern printing means for printing a patch pattern of a density level having undergone halftone processing, patch pattern measurement means for measuring the density of a patch pattern printed on paper by the patch pattern printing means, and patch pattern measurement means Density characteristic calculating means for obtaining the density characteristics of each of the plurality of halftone processes based on the density values measured in the step (a), and a plurality of intermediate characteristics such that the density characteristics calculated by the density characteristic calculating means become specified density characteristics. Density correction table creating means for creating each input density level correction table for the tone processing, and the density correction table creating means A printing apparatus comprising: a density correction table setting unit configured to set a plurality of density correction tables for halftone processing. 2. A density correction halftone setting means for setting the type and number of halftones set by the density correction means;
Dividing the area where the patch pattern can be formed into the number of halftones set by the density correction halftone setting means,
Patch pattern forming area dividing means to be assigned as each halftone area, and a patch pattern for forming a patch pattern of several density levels subjected to each halftone processing in the area assigned by the patch pattern forming area dividing means The printing apparatus according to claim 1, further comprising: a forming unit; and a density measuring unit configured to measure a density of the patch pattern formed by the patch pattern forming unit using a sensor. 3. When the density correction table of each halftone is created by the density correction means, the density level measured by the density measurement means is corrected to any density level by the density correction table of each halftone. It has a sensor reference value determination means as a reference density level and a sensor reference value of each halftone, respectively, the sensor measurement value of each halftone measured by the density measurement means, The printing apparatus according to claim 2, wherein the density measurement unit is performed continuously with the density correction unit. 4. After the sensor reference value determining means is performed, the density measuring means is performed at a prescribed timing, and compares the measured sensor measurement value of each halftone with the sensor reference value; The density fluctuation amount calculated by the density fluctuation amount calculating means for calculating the density fluctuation amount of each halftone by the increase and decrease, and the density characteristic of each halftone becomes a prescribed density characteristic by the density fluctuation amount calculated by the density fluctuation amount calculating means. 4. The printing apparatus according to claim 3, further comprising a density correction table correction unit for correcting the density correction table of each halftone. 5. A density correction method for a printing apparatus having density correction means for performing density correction of a plurality of halftone processes, wherein a sufficient number of a plurality of halftones are obtained on the same sheet to obtain density characteristics with respect to an input density level. A patch pattern printing step of printing a patch pattern of the processed density level, a patch pattern measuring step of measuring the density of the patch pattern printed on the paper by the patch pattern printing step, and a measurement at the patch pattern measuring step A density characteristic calculating step of obtaining the density characteristics of each of the plurality of halftone processes based on the obtained density values; and a plurality of halftone processes such that the density characteristics calculated in the density characteristic calculation step become specified density characteristics. Density correction table creating step of creating each input density level correction table of Density correcting method in the printing apparatus is characterized in that a density correction table setting step of setting a plurality of density correction table of the halftone processing created by Buru creation step. 6. A density correction halftone setting step for setting the type and number of halftones set in the density correction step, and an area where the patch pattern can be formed is set in the density correction halftone setting step. A patch pattern forming region dividing step of dividing the number of halftones and assigning them as respective halftone regions; and density levels of several points where each halftone process is performed on the region allocated by the patch pattern forming region dividing step. 6. The printing apparatus according to claim 5, further comprising: a patch pattern forming step of forming the patch pattern, and a density measuring step of measuring the density of the patch pattern formed by the patch pattern forming step using a sensor. Density correction method in. 7. When a density correction table for each halftone is created in the density correction step, the density level measured in the density measurement step is set to any density level according to the density correction table for each halftone. Finding whether or not the correction has been performed, and having a sensor reference value determination step of setting each halftone sensor measurement value measured in the density measurement step as a reference density level and a sensor reference value of each halftone, The method according to claim 6, wherein the density measuring step is performed continuously with the density correcting step. 8. After the sensor reference value determination step is performed, the density measurement step is performed at a specified timing, and compares the measured sensor measurement value of each halftone with the sensor reference value; The density fluctuation amount calculation step of calculating the density fluctuation amount of each halftone by the increase and decrease, and the density fluctuation amount calculated by the density fluctuation amount calculation step makes the density characteristic of each halftone a prescribed density characteristic. 8. A density correction method for a printing apparatus according to claim 7, further comprising a density correction table correction step of correcting said density correction table of each halftone. 9. A program for executing the steps according to claim 5 on a computer.