JP2007020111A - Image forming apparatus and image density correcting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To appropriately correct the concentration of a developer, without having a dedicated pattern image or patch image for concentration detection output. <P>SOLUTION: A partial image, that can be substituted as a patch for concentration detection, is extracted for gradation correction or developer concentration adjustment from a general image formed by an image forming means, the extracted partial image is read and according to the read partial image, the concentration of a developer is corrected. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for suitably correcting image density in an image forming apparatus.

  In an image forming apparatus that forms an image using a developer such as toner or ink, an image with stable quality can be formed by suitably controlling the concentration of the developer.

  According to Patent Document 1, a dedicated test pattern for density detection is formed on a sheet, the density is read by a sensor, a density correction table is created from the read result, and based on the created density correction table. A method for correcting the output density has been proposed.

  According to Patent Document 2, a patch image formed on a photoconductor is read by a sensor, a gradation correction table is adjusted based on the read result, and an output image is converted based on the adjusted gradation correction table. A correction method has been proposed.

Furthermore, according to Patent Document 3, when a large number of the same printed matter is printed using ink, the designated position is read in each printed matter, and if the reading result (color density) changes, the ink density A method of adjusting is proposed.
JP-A-4-193576 Japanese Patent Laid-Open No. 3-87768 JP-A-4-226762

  However, according to the invention described in Patent Document 1 or Patent Document 2, it is necessary to output a test pattern dedicated for density detection. For this reason, the image forming apparatus must stop forming a general image (an image other than a dedicated test pattern for the purpose of density adjustment) while executing density correction. That is, there is a time during which the general image desired by the operator cannot be output from the image forming apparatus.

  Further, according to the invention described in Patent Document 3, it is necessary for an operator to specify a reading position in an actual printed matter in advance, which is troublesome. Moreover, the operator had to grasp advanced knowledge about density correction.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

  According to the present invention, a partial image that can be used as a density detection patch is extracted from a general image formed by the image forming unit, the extracted partial image is read, and gradation is determined according to the read partial image. Correct the correction table.

  According to the present invention, by substituting a suitable partial image included in a general image as a patch, it is possible to suitably correct the image density without outputting a dedicated pattern image or patch image for density detection. it can.

[First Embodiment]
FIG. 1 is a diagram illustrating a schematic configuration of a color image forming apparatus according to an embodiment. A color image forming apparatus 100 shown in FIG. 1 includes a document reading unit 101 for reading a document and an image forming unit 150 for forming an image on a recording medium.

  In general, the document reading unit 101 is often arranged at the top of the color image forming apparatus 100. The document 102 is placed in the platen cover 103. The lamp 104 is a light source that irradiates light on the document surface. The mirror 105 guides the reflected light from the lamp 104 reflected by the document 102 to the mirror 106. The mirror 106 guides the light reflected by the mirror 105 to the lens 108.

  The lamp 104 and the mirror 105 are installed on the optical bench 112. The mirror 106 and the mirror 107 are installed on the optical bench 113. The optical benches 112 and 113 move by rotating the motor 111 forward or backward, respectively. By moving the optical benches 112 and 113, the document 102 can be scanned along the sub-scanning direction.

  The lens 108 condenses the reflected light from the document surface guided by the optical benches 112 and 113 onto the CCD 109. The CCD 109 receives light from the original surface collected by the lens 108 and performs photoelectric conversion. It is assumed that the CCD 109 is installed on the substrate 110.

  The reference white plate 114 is used when measuring the light quantity variation of the lamp 104 and the sensitivity variation of the CCD 109. The CCD 109 has three line sensors. Each line sensor corresponds to RED, GREEN, and BLUE (hereinafter referred to as RGB) in order to read a color image. For example, if the number of main scanning pixels of each sensor is 7500 pixels, an A3 document can be read at 600 dpi.

<Scanning original>
FIG. 2 is an exemplary functional block diagram of the document reading unit according to the embodiment. The document reading controller 201 transfers the image data read by the CCD 109 to the image forming unit 150.

  More specifically, when reading of a document is instructed from the operation unit 308 shown in FIG. 3 or the like, the document reading controller 115 controls the motor 111 to a position for reading the reference white plate 114 to the optical bench 112. And 113 are moved. Next, the controller 115 turns on the lamp 104 and causes the CCD 109 to read the reference white plate 114. From the reading result of the reference white plate 114, the controller 115 corrects the light amount variation of the lamp 104 and the sensitivity variation of the CCD 109 (so-called shading correction). The controller 150 further controls the motor 111 to sub-scan the document 102 and transfers the acquired RGB image data to the image forming unit 150.

<Image formation>
Referring again to FIG. 1, it can be seen that an image forming unit 150 is disposed below the color image forming apparatus 100. The recording paper 153 placed on the paper feeding unit 152 is fed one by one by the pickup roller 154. The recording paper 153 is conveyed to the transfer roller 159 portion via the paper supply roller 155, the guide plate 156 and the registration roller 157. In addition, RGB format image data read by the document reading unit 101 is converted into CYAN, MAGENTA, YELLOW, and BLACK (hereinafter, CMYK) image data.

  The developing devices 160y to 160k store different developers (hereinafter, toners). The photosensitive drums 163y to 163k uniformly charged by the chargers 162y to 162k are exposed by the exposure units 161y to k according to the image data. The electrostatic latent images formed on the photosensitive drums 163y to 163k are developed by developing units 162y to 162k. The toner images on the photosensitive drums 163y to 163k are multiple-transferred (primary transfer) to the intermediate transfer belt 158. Note that primary transfer chargers 166y to 166y are provided at positions facing the photosensitive drums 163y to 163k.

  The transfer roller 159 is pressed against the intermediate transfer belt 158 with an appropriate pressure. The toner image carried on the intermediate transfer belt 158 is secondarily transferred onto the recording paper 153 by the transfer roller 159. The fixing unit 164 includes a roller having a heat source such as a halogen heater inside. The fixing unit 164 melts and fixes the toner image on the recording paper 153.

  The CIS (Contact Image Sensor) unit 170 is a sensor that reads an image formed on the intermediate transfer belt 168, and is provided on the downstream side of the yellow photosensitive drum 163y that is the final development station. The CIS unit 170 is generally composed of a CCD or the like that receives LEDs of RGB colors as a light source and receives reflected light from a document.

<Density correction>
FIG. 3 is an exemplary functional block diagram of the image forming unit according to the embodiment. In general, the charge amount of the developer varies depending on various conditions such as environmental changes. If the charge amount changes, the density of the formed image becomes unstable. Therefore, in this embodiment, a partial image that can be used as a patch image (test pattern) included in a general image (original copy image, print image transmitted from a personal computer) is automatically extracted, and the extracted partial image Density correction is performed by reading.

  The image forming controller 300 mainly includes the following components. The image input I / F unit 301 receives image data transferred from the document reading unit 101 or image data transferred from the PC terminal 350 or the like. Note that the image input I / F unit 301 follows, for example, a selection instruction input from the operation unit 308 as to which image data is stored in the image memory 302. The CPU 320 is a control circuit that comprehensively controls each unit of the image forming controller 300.

  The LOG conversion circuit 303 converts RGB image data (luminance data for each RGB) stored in the image memory 302 into density data, and then converts the density data into CMYK image data using a predetermined look-up table. To do.

  The output γ correction unit 304 functions as a density correction unit. That is, the output γ correction unit 304 selects one of a plurality of γ correction tables prepared in advance according to the reading result of the CIS unit 170, and corrects the CMYK image data according to the selected γ correction table. Note that the output γ correction unit 304 may create a γ correction table that is suitable for the reading result.

  FIG. 4 is a diagram illustrating an example of the CIS unit according to the embodiment. The CIS unit 170 is only an example of an image reading unit, and other reading sensors may be used. The CIS unit 170 includes an LED light source 601 and a Selfoc lens array 602. Note that a CCD sensor is disposed below the SELFOC lens array 602. This CCD sensor has, for example, three lines corresponding to RGB. Each line sensor has 7500 pixels in the main scanning direction. With such a CIS unit 170, the toner image on the intermediate transfer belt 158 can be read.

  The CIS unit 170 may include a shading correction circuit for correcting (normalizing) CCD sensitivity unevenness, LED light source 601 variation, and the like. The CIS unit 170 is configured to be able to read the entire main scanning area. Therefore, even if a patch image (or partial image) is formed at any main scanning position, the patch image can be suitably read.

  The output γ correction unit 304 determines from the image data read by the CIS unit 170 whether the density of the patch image (or a substitute image thereof) has reached a predetermined value (reference value). For example, if the density value is brighter than the predetermined density, the output γ correction unit 304 corrects the data in the output γ correction table for approaching the predetermined density.

  FIG. 5 is a diagram illustrating an example of the output γ correction table according to the embodiment. The output γ correction unit 304 corrects the γ correction table so as to obtain an appropriate gradation correction characteristic, whereby the read result (actual gradation) is corrected to an ideal value.

  The input selector 305 selects and outputs either CMYK image data output from the output γ correction unit 304 or patch image data output from the patch image generation unit 312. Note that the input selector 305 and the patch image generation unit 312 are optional. For example, the general image may not include a partial image equivalent to the patch image, but in this case, there is a possibility that the density correction cannot be performed at all. Therefore, when a partial image cannot be extracted from the general image for a predetermined period or a predetermined number, the patch image is output from the patch image generation unit 312 so that the density correction can be suitably executed.

  FIG. 6 is a diagram illustrating an example of a patch image according to the embodiment. This patch image includes a plurality of patches that have a density of 5%, 50%, and 100% respectively for YMCK.

  A binarization processing unit 306 binarizes the image data output from the input selector 305. The image output control unit 307 controls each unit of the image forming unit 150 in accordance with the binarized image data.

  The extraction unit 309 determines whether the general image formed by the image forming unit 150 includes a partial image that can be used as a patch image. Thereby, the area of the partial image is specified.

  FIG. 7 is an exemplary flowchart of density correction according to the present embodiment. In step S <b> 701, the extraction unit 309 reads general image data from the image memory 302 and extracts a partial image that can be used as a patch.

  The partial image is desirably an image of an area where there is no change in density, for example. This is because density correction cannot be performed properly unless a partial image in an area where there is no change in density is used. In practice, it is needless to say that an image of a region having no change in density may include a region having a change in density (eg, several pixels) to such an extent that no problem occurs in density correction. Absent.

  Further, the extraction unit 309 may specify a region of a partial image included in the general image by analyzing image data in advance before forming the general image. This is because if it is possible to grasp in advance in which area of the general image a suitable partial image exists, the partial image can be easily read from the intermediate transfer belt 158.

  For example, the extraction unit 309 may specify an area in which a predetermined number of pixels having a predetermined density are continuous as a partial image area. This is because a partial image having a certain area must be used in order to substitute for the patch image. That is, if the partial image is not a predetermined area or larger, there is a possibility that sufficient accuracy of density correction cannot be ensured.

  In step S702, the extraction unit 309 determines whether the partial image has been extracted. If it cannot be extracted, the formation of the general image is interrupted, and the process proceeds to step S706. In step S706, the patch image generation unit 312 generates a patch image. The input selector 305 selects a patch image output from the patch image generation unit 312 and supplies it to the binarization processing unit 306. Thereafter, the image output control unit 307 controls each unit of the image forming unit 150 to form a patch image on the intermediate transfer belt 158.

  If a suitable partial image cannot be extracted, the density correction may be omitted. In this case, the extraction unit 309 counts the period or the number of sheets for which extraction has failed continuously. Then, when this count value reaches a predetermined period or a predetermined number, the extraction unit 309 causes the patch image generation unit 312 to output a patch image, and executes density correction based on the patch image. Thereby, it is possible to suitably suppress the deterioration of the image quality due to the fact that the density correction is not performed over a long period of time. Note that the CPU 320 may receive a notification indicating that the extraction has not been performed from the extraction unit 309 and issue a command for causing the patch image generation unit 312 to generate a patch.

  On the other hand, if the partial image has been successfully extracted from the general image, the process advances to step S703, and the image output control unit 307 forms the general image on the intermediate transfer belt 158 as usual.

  In step S704, the CIS unit 170 reads a patch image formed on the intermediate transfer belt 158 or a substitute image (partial image) thereof.

  In step S705, the output gamma correction unit 304 rewrites some correction data included in the created gamma correction table according to the density value of the read patch image or its substitute image. The output γ correction unit 304 may newly create a suitable γ correction table or select a suitable correction table from a plurality of correction tables.

  As described above, according to the present embodiment, by substituting a suitable partial image included in a general image as a patch, the image density is suitably corrected without outputting a dedicated test pattern for density detection. can do.

<Extraction of substitute image>
FIG. 8 is an exemplary flowchart of a substitute image extraction process according to the embodiment. Hereinafter, a process of extracting a partial image (substitute image) that can be substituted as a patch from the data of the general image will be described in detail with reference to FIG. This process corresponds to step S701 in FIG.

  In step S <b> 801, the extraction unit 309 reads out RGB image data to be imaged from the image memory 302.

  In step S802, the extraction unit 309 determines whether the target pixel has a predetermined density (first condition). If the first condition is satisfied, the process proceeds to step S803, and if not, the process proceeds to step S806.

  Note that the image data at this point is RGB data. Therefore, an area that can be used as a patch when it is converted into CMYK data by the LOG conversion circuit 303 is extracted. For example, the following conditions are part of the extraction conditions.

RED Xr to Yr,
GREEN Xg to Yg and
BLUE Xb or more Yb ... 1st condition Here, the value of Xr-b and Yr-b changes with each patch. For example, partial images for substituting a YELLOW 100% patch are RED 250 to 255, GREEN 250 to 255, and BLUE 0 to 5 inclusive. Here, the RGB image data is 8-bit luminance data. These conditions are only examples, and other conditions may be used.

  Furthermore, it is desirable that these conditions take into account the γ correction table selected by the output γ correction unit 304. This is because a suitable image quality cannot be achieved without considering the γ correction table.

  In step S803, the extraction unit 309 holds pixels that satisfy the first condition as candidates. For example, the extraction unit 309 increments the stored value corresponding to the main scanning position of the pixel in the area determination line memory (for example, incorporated in the extraction unit 309).

  This area determination line memory includes, for example, a storage area corresponding to the number of pixels in the main scanning direction (eg, 7500 pixels). The value of the area determination line memory is cleared to 0 at the first sub-scanning line of the image data.

  In step S804, the extraction unit 309 determines whether a predetermined number or more of pixels (candidate pixels) satisfying the first condition are continuous. For example, when a range of 0.5 mm × 0.5 mm is used as a patch image, it is determined that the candidate pixels are continuous for 12 or more in the main scanning direction (the second condition).

  This second condition means that the storage value of the above-described area determination line memory exceeds 12. Then, an area that matches the second condition is specified as an area of a partial image that can be used as a patch image. In step S805, the extraction unit 309 stores information representing the region of the partial image (main scanning position and sub-scanning position) in a memory or the like and holds the information.

  In step S802, the extraction unit 309 excludes pixels that do not satisfy the predetermined density from the candidates. For example, the storage value corresponding to the pixel in the area determination memory is cleared to zero.

  FIGS. 9A to 9D are diagrams illustrating detection examples of partial images that can be substituted as patches according to the embodiment. In particular, FIG. 9A shows an example of a general image 900. In FIG. 9B, a partial image 901 that can be substituted as a patch is highlighted. FIG. 9C shows a part of the target general image. FIG. 9D shows the transition of the storage value of the area determination line memory for the image shown in FIG. 9C.

  As can be seen from FIG. 9, it can be understood that a region where a predetermined number or more of pixels having a predetermined density are continuous is extracted as a partial image. The extracted partial image is read by the CIS unit 170 in step S704.

  It should be noted that the above-described density correction processing is similarly executed not only for the YELLOW 100% patch, but also for other density patches related to YELLOW, and patches based on combinations of other colors and other densities. Absent.

  As described above, according to the present embodiment, a suitable partial image included in a general image is used as a patch. Thereby, the process of outputting a test pattern dedicated for density detection can be omitted. Conventionally, the formation of a general image has been stopped in order to output a test pattern at the time of density correction. On the other hand, according to the present embodiment, since the formation of the general image can be continued, the throughput of the image formation is improved. Furthermore, according to the present embodiment, it is not necessary for the operator to have advanced knowledge about density correction, which will be very convenient.

  Further, according to the present embodiment, it is possible to improve the accuracy of density correction by automatically extracting an image with a stable density included in a general image as a partial image. This is because if an image with unstable density is used, the accuracy of density correction decreases. Since it is not necessary for the operator to search for and specify an area where the density is stable, the burden on the density correction is reduced for the operator.

  Furthermore, the extraction unit 309 can appropriately specify the region of the partial image included in the general image by analyzing the image data in advance before forming the general image. This is because if it is possible to grasp in advance in which area of the general image a suitable partial image exists, the partial image can be easily read from the intermediate transfer belt 158. There is also an advantage that the process of searching for the partial image from the read general image can be simplified.

  Further, according to the specifying process of the extraction unit 309, it is desirable to specify an area where a predetermined number of pixels having a predetermined density are continuous as a partial image area. This is because the accuracy of density correction cannot be suitably maintained unless it is a partial image having a certain area in order to be used as a patch image.

  When a partial image cannot be extracted, a patch image may be formed by the image forming unit 150 and the formed patch image may be read by the CIS unit 170. It is not desirable that the state in which the density correction cannot be performed continues for a long time. For this reason, when a partial image cannot be extracted for a predetermined period (or a predetermined number) or more, the gradation correction table can be properly maintained by forming a dedicated patch image. Even in this case, the frequency of forming the patch image is greatly reduced as compared with the prior art. Therefore, it goes without saying that the various merits described above can be enjoyed.

  By the way, in the above-described embodiment, the example in which the substitute image is extracted and used as the patch image for correcting the data of the γ correction table has been described. However, the present invention may be realized as a method of extracting and using a substitute image as a patch image for adjusting the toner density in the developing device 160.

  In general, toner density correction in the developing device 160 is performed by forming a patch image of a predetermined density on the photoconductor 163 at an appropriate timing, measuring the density, and controlling the replenishment of the developer according to the difference from the reference value. It is realized by doing. Therefore, if the above-described embodiment is applied to the patch image at this time, a substitute image of the patch image can be extracted from the general image. If extraction from the general image has failed, a patch image may be created based on data from the patch image generation unit 312 as in the above-described embodiment.

[Embodiment 2]
The CIS unit 170 according to the first embodiment reads the toner image primarily transferred onto the intermediate transfer belt 158. This toner image is formed by multiplexing all CMYK toners. However, the partial image that can be used as a patch image is preferably a single toner image formed at a predetermined density. Therefore, it is very difficult to find a suitable partial image from a toner image formed by multiplexing a plurality of developers.

  Therefore, in the second embodiment, in order to solve the problem of the first embodiment, improvement is made so that a single color toner image formed on an image carrier (for example, a photosensitive drum) for each color is read. . This also improves the probability that a partial image that can be substituted as a patch can be extracted from a general image, so that the throughput of the image forming apparatus can be further improved.

  FIG. 10 is a diagram illustrating a schematic configuration of an image forming apparatus according to the second embodiment. The description will be simplified by giving the same reference numerals to the parts already described.

  A difference from the configuration of the first embodiment is that, instead of the CIS unit 170, CIS units 1001y-k are arranged for each of the photosensitive drums 163y-k. As described above, by adopting a configuration in which the toner image on each photosensitive drum can be individually read, it becomes easy to individually detect the input / output characteristics of each of CMYK.

  FIG. 11 is an exemplary functional block diagram of the image forming controller according to the second embodiment. The description will be simplified by giving the same reference numerals to the parts already described.

  An extraction unit 309 that extracts a partial image that can be used as a patch reads out CMYK image data output from the output γ correction unit 304. Here, the extraction unit 309 individually extracts an area that can be used as a patch for each developer of CMYK. For example, when extracting a region that is a substitute for the YELLOW 100% patch, the extraction unit 309 uses the following conditions.

YELLOW 100% patch 250 or more and 255 or less... First condition Here, CMYK data is assumed to be 8-bit data. In addition, about the above-mentioned 2nd condition, you may employ | adopt the conditions similar to 1st Embodiment. After that, the extraction unit 309 specifies a partial image area individually for each CMYK developer. Of course, as described above, it goes without saying that an area determination line memory may be adopted. However, a line memory for area determination will be required for each developer.

  As described above, according to the second embodiment, for each photosensitive drum (or each single color toner image), a partial image that can be used as a patch is extracted, and the area is specified. In step S704 described above, the toner images formed on the photosensitive drums 163y to 163k are read by the corresponding CIS units 1001y to 1001k. The read partial image is used for density correction by the output γ correction unit 304.

  As described above, according to this embodiment, a partial image is extracted with respect to an image by a single developer, and is read from the photosensitive drum. Thereby, compared with the first embodiment, the extraction process is facilitated, and the extraction probability of the partial image is further improved. If the extraction probability of the partial image is improved, the number of times of stopping the formation of the general image and forming the dedicated patch image can be reduced, so that the throughput of the image formation is also improved.

  As described in the first embodiment, the second embodiment can also be applied to extraction of a substitute image used as a patch image for toner density adjustment.

[Third Embodiment]
In the above-described embodiment, basically, a partial image that can be used as a patch is extracted from one general image. For this reason, there is a risk of being vulnerable to noise. In addition, there is a possibility that an image in a region having a density change cannot be used as a partial image.

  Therefore, in the third embodiment, the reading results obtained from a plurality of general images are averaged. As a result, noise tolerance is improved, and an image in a region with a slight change in density can be used as a partial image.

  FIG. 12 is an exemplary functional block diagram illustrating a part of the image forming controller according to the third embodiment. The description will be simplified by giving the same reference numerals to the parts already described. It should be noted that the same configuration as that of the first or second embodiment may be adopted for the portion not shown.

  Compared with the first or second embodiment, the extraction unit 309 extracts an image of a region having a slight density change as a partial image. It should be noted that the extent to which the change in density can be allowed depends on the degree to which the uniform density can be regarded by the averaging process.

  The CIS units 170 or 1001y to k read the extracted partial images and output the read results to the averaging unit 1201. The averaging unit 1201 calculates an average value for each pixel from the reading results obtained from a plurality of general images, and outputs the average value to the output γ correction unit 304. The output γ correction unit 304 selects a γ correction table using the averaged reading result.

  As described above, according to the third embodiment, the noise component can be reduced by averaging the reading results of a plurality of general images. Accordingly, the extraction conditions of partial images that can be used as substitutes for patch images can be relaxed, so that the extraction probability is further improved and the throughput of image formation is also improved.

[Other Embodiments]
In the above-described embodiment, the color image forming apparatus 100 has been described as an example, but the present invention is not limited to this. For example, the present invention can be applied to a monocolor image forming apparatus.

  The color image forming apparatus shown in FIGS. 1 and 10 employs a copying machine or a multifunction machine as an example, but the present invention is not limited to this. For example, the present invention can be applied to other image forming apparatuses such as a printer and a facsimile.

  In the above-described embodiment, the electrophotographic image forming apparatus has been described. However, the present invention is not limited to this. For example, the present invention can also be applied to an inkjet image forming apparatus that uses ink as a developer.

1 is a diagram illustrating a schematic configuration of a color image forming apparatus according to an embodiment. 3 is an exemplary functional block diagram of a document reading unit according to the embodiment. FIG. 3 is an exemplary functional block diagram of an image forming unit according to the embodiment. FIG. It is a figure which shows an example of the CIS unit which concerns on embodiment. It is a figure which shows an example of the output (gamma) correction table which concerns on embodiment. It is a figure which shows an example of the patch image which concerns on embodiment. 5 is an exemplary flowchart of density correction according to the present embodiment. 6 is an exemplary flowchart of a substitute image extraction process according to the embodiment. It is a figure which shows the example of a detection of the partial image which can be substituted as a patch which concerns on embodiment. It is a figure which shows schematic structure of the image forming apparatus which concerns on 2nd Embodiment. FIG. 6 is an exemplary functional block diagram of an image forming controller according to a second embodiment. FIG. 10 is an exemplary functional block diagram illustrating a part of an image forming controller according to a third embodiment.

Claims (9)

Image forming means for forming an image according to image data using a developer;
An extraction unit that extracts a partial image that can be used as a patch used for density detection from the general image formed by the image forming unit;
Reading means for reading the extracted partial image;
An image forming apparatus comprising: density correction means for correcting density correction characteristics in accordance with the read partial image.   The image forming apparatus according to claim 1, wherein the partial image is an image of an area where there is no change in density. The extraction means includes
The image forming apparatus according to claim 2, wherein an area of the partial image included in the general image is specified by analyzing the image data in advance before forming the general image. The extraction means includes
The image forming apparatus according to claim 3, wherein an area in which a predetermined number of pixels having a predetermined density are continuous is specified as the area of the partial image. The reading means includes
5. The image forming apparatus according to claim 1, wherein each of the partial images formed on a plurality of image carriers carrying different developers is read. The density correction means includes
6. The image forming apparatus according to claim 1, wherein an averaging process is applied to the partial images read for each of the plurality of general images. The density correction means includes
When the partial image cannot be extracted, a patch image is formed by the image forming unit, and the density correction characteristic is corrected by causing the reading unit to read the formed patch image. The image forming apparatus according to claim 1. A density correction method in an image forming apparatus,
A step of reading a partial image that can be used as a patch used for density detection among general images formed using a developer by the image forming apparatus;
And a step of correcting a density correction characteristic in accordance with the read partial image. Image forming means for forming an image according to image data using a developer;
Extracting means for extracting a partial image that can be used as a patch used for correcting the developer density, out of the general image formed by the image forming means,
Reading means for reading the extracted partial image;
An image forming apparatus comprising: a correcting unit that corrects the density of the developer according to the read partial image.

Images