JP2011253068A - Image forming apparatus and control method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide gradation in image formation capable of executing high-precision CAL even when foreign matter such as paper dust is present at the time of CAL printing on paper (at the time of automatic gradation correction, multi-order color CAL, ICC profile creation). A correction method, an image forming apparatus, and a control method thereof are provided.
An image forming process of printing a density correction patch chart composed of a plurality of density patches on a recording medium, an image reading process of reading the printed patch chart, and the density of the read patch chart A gradation correction method in image formation having a density correction process for generating density correction data from data, and a read value in a patch of a read patch chart is changed from a predetermined value to a high brightness side or a low brightness side An area where the size of the shifted area exceeds a predetermined size is detected as an abnormal area, and in the density correction step, read data within a specific distance from the detected abnormal area is not used for density correction.
[Selection] Figure 9

Description

  The present invention relates to an image forming apparatus and a control method thereof. In particular, the present invention relates to automatic gradation correction executed to maintain the stability of the image forming apparatus or correction of multi-order colors called multi-order color CAL or ICC profile in an image forming apparatus that prints color materials on paper.

  There are various types of image forming apparatuses such as an electrophotographic system, an ink jet system, and an offset type printing apparatus, and various image stabilization techniques have been proposed for the electrophotographic system. In the electrophotographic system, an image carrier called a photoconductor is charged by a primary charger, and a latent image is formed by an optical system such as a laser. The toner is developed on the image carrier by a developing device having a color material called toner for the latent image. The toner developed and held on the image carrier is transferred onto the paper by one or two transfers, and is fixed to the paper by a thermal fixing device. Since the electrophotographic basic printing process uses the electrostatic phenomenon, it is easily affected by temperature and humidity. Therefore, many stabilization methods have been proposed to solve the above instability.

  Patent Document 1 is called automatic gradation correction (hereinafter referred to as CAL), which prints a test pattern on paper and detects the current engine gamma characteristic (relationship between input signal and density). By generating a LUT (lookup table: a table that performs primary conversion of the input signal into the same or different signal) so as to achieve a predetermined target gradation, and through the LUT during subsequent image formation It is corrected to a desired gradation. In the CAL, the output test pattern is read by a reader. A reader is a device that reads an image of a document used during copying, and can read the document image as RGB or gray (luminance information) data.

  When dust such as paper dust or dust is present in the image reading unit of the reader, the dust adhering to the image reading unit during the flow reading becomes a vertical streak image in the document transport direction (sub-scanning direction). When it is not a non-scanning reading, it is read as a white dot only in a portion where paper dust exists. As a method for detecting and correcting vertical stripes at the time of scanning, there is an image processing method for deleting image data of a portion where dust is detected as in Patent Document 2. In addition, as a method for detecting a foreign object on the glass surface, there is a detection method for detecting a foreign object by providing an imaging unit that reads an image on the surface of an image base different from the imaging unit for reading a document as in Patent Document 3. It was. Moreover, in patent document 1, the area | region where the paper powder etc. adhere and the density | concentration gradient of an adjacent patch is high at the time of CAL was determined, and the problem was avoided on the calculation. Further, as a noise avoidance method at the time of stabilization control of the image forming apparatus, there is a noise removal method in which a large value and a small value in a sample data string are replaced with an average value of other values as in Patent Document 4. .

JP-A-8-251413 JP 2002-247310 A JP 2005-167933 A Japanese Patent Application Laid-Open No. 2004-070184

  If foreign matter such as paper dust exists on the paper before the color material is transferred to the paper, the color material to be transferred around the foreign matter scatters. As shown in FIG. 19A, the color material scatters due to the transfer pressure distribution around the paper dust and the resistance value of the paper in the paper dust portion being changed by the paper dust. In the case of an image forming apparatus using a halftoning method called pseudo halftone processing, the ratio of the area where the color material covers the paper (hereinafter referred to as halftone dot%) increases when scattering occurs. Since the density is expressed by the white of the paper and the color material, the density is increased because the white of the paper is covered by scattering. Also, since the paper powder is peeled off by a fixing device or a roller after the color material is transferred, the color material does not exist and becomes a white spot. When CAL is executed in such a state, the original density is not known, and the accuracy is lowered. In Patent Documents 2 and 3 for the above problem, foreign matters such as paper dust and dust do not exist on the paper, and thus the problem cannot be solved.

  In Patent Document 1, random noise is superimposed when the gradient of the patch density is equal to or higher than a predetermined gradient. Therefore, when the engine has a large gradient, a desired gradation correction is performed for random noise. Not done. Alternatively, when the specified inclination amount is set high, tone correction is performed using the detected density although it is not an engine element characteristic due to paper dust or scattering. Furthermore, since paper dust and the like have a two-dimensional shape, lack of density information necessary for correction occurs unless noise can be removed two-dimensionally, and correction cannot be performed with high accuracy. In addition, Patent Document 4 is effective in the case of a small amount of noise because it eliminates a noise occurrence location or applies an average value, but the influence of paper dust in a patch as in the problem of the present invention is wide-ranging. If you can not respond.

  The present invention provides a gradation correction method, an image forming apparatus, and a control method thereof in image formation that can execute high-precision CAL even when foreign matter such as paper dust exists in an image reading unit of a reader.

  In order to solve this problem, an image forming apparatus of the present invention reads an image forming unit that prints a density correction patch chart composed of a plurality of density patches on a recording medium, and the printed patch chart. An image forming apparatus having image reading means and density correction means for generating density correction data from the read patch chart density data, wherein a read value in a patch of the read patch chart is determined from a predetermined value Detecting means for detecting, as an abnormal area, an area in which the size of the area shifted to the high lightness side or the low lightness side exceeds a predetermined size, and a calculating means for calculating the position of the detected abnormal area And the density correction means does not use read data within a specific distance from the calculated position of the abnormal area for density correction. That.

  It is possible to provide a gradation correction method, an image forming apparatus, and a control method therefor in image formation that can execute high-precision CAL even when foreign matter such as paper dust exists in the image reading unit of the reader.

  That is, when the CAL is executed, the foreign matter in the image reading unit is specified using the test pattern reading information. The inside of the specified peripheral pixel of the specified foreign object is not used for the CAL calculation, and the CAL calculation is performed using the remaining luminance information. Alternatively, the brightness gradient from a foreign object is detected two-dimensionally, a pixel that falls below a certain gradient is specified, and the brightness information up to the specified pixel and the foreign object is not used, and the remaining brightness information is used. CAL calculation is performed. In addition, the prescribed peripheral pixels have different influences on the ease of scattering and the density depending on the type of pseudo halftone processing. Therefore, the finer the screen frequency (also referred to as LPI (Line / Inch) or the number of lines), the smaller the prescribed peripheral pixels. You may make it the structure which enlarges a pixel. Furthermore, due to the characteristics of pseudo halftone processing, even if paper dust is present in the highlight portion, there is little influence because there is not much color material around it. In addition, even if the shadow portion is scattered, the color material is immediately present, so the density influence is small. Therefore, the specified peripheral pixels may have a relationship of highlight <halftone and shadow portion <halftone.

1 is a diagram illustrating a schematic configuration example of an image forming apparatus according to an embodiment. It is a figure which shows the schematic structural example of the scanner concerning this embodiment. It is a figure which shows the example of schematic structure of the operation part concerning this embodiment. FIG. 3 is a diagram illustrating a control configuration example of an image forming apparatus according to the present embodiment. (A) is a diagram illustrating a schematic configuration example of a RIP unit according to the present embodiment, (b) is a diagram illustrating a schematic configuration example of a scanner unit according to the present embodiment, and (c) is an output image processing according to the present embodiment. It is a figure which shows the schematic structural example of a part. It is a flowchart which shows the basic procedure of the automatic gradation correction concerning this embodiment. It is a figure which shows the example of the test pattern of the automatic gradation correction | amendment concerning this embodiment. It is a schematic diagram which shows the relationship between (gamma) characteristic concerning this embodiment, and LUT. 7 is a flowchart illustrating a procedure example of calculation steps in FIG. 6 according to the first embodiment. It is a figure which shows the example of the labeling matrix concerning Embodiment 1. FIG. (A) is a figure which shows the example of the labeling result concerning Embodiment 1, (b) is a figure which shows the example of the gravity center result concerning Embodiment 1. FIG. FIG. 10 is a schematic diagram for explaining edge extraction according to the second embodiment. FIG. 10 is a diagram illustrating an outline of dot gain according to a third embodiment. It is a figure explaining the outline of the foreign material concerning Example 3, and a jump. It is a figure explaining the relationship between the foreign material concerning Embodiment 4, and a density range. It is a figure explaining the deletion area | region from the foreign material edge concerning Embodiment 4. FIG. 10 is a flowchart illustrating an example of a procedure of calculation steps in FIG. 6 according to the fifth embodiment. 10 is a diagram illustrating an example of a smoothing filter according to Embodiment 5. FIG. It is a figure which shows the change of the brightness | luminance gradient calculation concerning Embodiment 5. FIG. FIG. 10 is a diagram for explaining an outline of luminance gradient calculation according to the fifth embodiment. FIG. 10 is a diagram for explaining the outline of a dither matrix according to a modification of the fifth embodiment.

<Example of Configuration of Image Forming Apparatus of Present Embodiment>
(Description of Printer Unit 11) FIG. 1 is a schematic configuration diagram of an electrophotographic printer unit 11, and FIG. 1 is a longitudinal sectional view showing a schematic configuration of a laser beam printer as a printer unit. A printer unit 11 shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 100 as an image carrier inside a printer unit main body A. Around the photosensitive drum 100, a charging unit 101, an exposure unit 102, a developing unit 103, an intermediate transfer unit 104, a cleaning unit 105, and a pre-charger 106 are disposed almost in order along the rotation direction. A primary transfer unit 107 is built in the intermediate transfer unit 104 opposite to the photosensitive drum 100. Further, a feeding / conveying unit 109, a secondary transfer unit 108, a conveying belt unit 110, a fixing unit 111, and a paper discharge tray 112 are arranged in order from the upstream side along the conveying direction of the recording medium (for example, paper) P. ing. A scanner input unit 200 is disposed on the upper part of the printer unit main body A. The photosensitive drum 100 described above has an a-Si photosensitive member provided in a layered manner on the outer peripheral surface of an aluminum cylinder, and is rotationally driven in a direction of an arrow at a predetermined process speed by a driving means (not shown). The surface of the photosensitive drum 100 is uniformly charged to a predetermined polarity and a predetermined potential by the charging unit 101. As the charging unit 101, for example, a corona charger that is not in contact with the photosensitive drum 100 can be used. An electrostatic latent image is formed on the photosensitive drum 100 after charging by the exposure unit 102.

  An original D is placed on the scanner input unit 200 and a reading operation is performed. Details of the operation and the like will be described later. The image information read by the scanner input unit 200 is processed by an input image processing unit (not shown) and an output image processing unit (not shown) in FIG. The exposure unit 102 includes a laser oscillator 102a, a polygon mirror 102b, a lens 102c, a reflection mirror 102d, and the like. The exposure unit 102 exposes the surface of the photosensitive drum 1 according to the image information input from the scanner input unit 200 and statically exposes it. An electrostatic latent image is formed. The electrostatic latent image formed on the surface of the photosensitive drum 100 is developed as a toner image by being attached with toner by the developing unit 103. On the other hand, the recording material P stored in the paper feeding cassette 109a of the paper feeding / conveying unit 109 is fed by the paper feeding roller 109b and enters the secondary transfer unit 108 by the conveying roller 8c. The toner image formed on the photosensitive drum 100 by the developing unit 103 described above is held on the intermediate transfer unit 104 by applying a transfer bias at the primary transfer unit 107 in the intermediate transfer unit 104, and further, the secondary image is further transferred. A transfer bias is applied at the transfer unit 108 and transferred onto the surface of the recording material P. The recording material P onto which the toner image has been transferred is conveyed to the fixing unit 111 by the conveying belt unit 110, where the toner image is fixed on the surface by being heated and pressed by the fixing roller 111a and the pressure roller 111b. Thereafter, the paper is discharged onto the paper discharge tray 112.

  (Description of Scanner Input Unit 200) FIG. 2 is a schematic configuration diagram of the scanner input unit 200 attached to the upper part of the printer unit 11. In FIG. 2, the document D to be read is placed on the document table glass 201, and the start key of the operation unit 300 (see FIG. 3) is pressed, or the OK key of the scanner driver is clicked. The scan operation is started as a trigger. When the scanning operation is started, the first mirror unit 202 and the second mirror unit 203 once return to the home position where the home position sensor 204 is located, and the document illumination lamp 205 is turned on to irradiate the document. The reflected light is imaged on the CCD sensor 20 through the lens 209 via the first mirror 206 in the first mirror unit 202, the second mirror 207 and the third mirror 208 in the second mirror unit 203, and the light. The signal is input to the CCD sensor 20 as a signal. The movement of the first mirror unit 202 and the second mirror unit is driven by the scanner motor 210. At the time of CAL, which will be described later, since the gradation is adjusted by relative density using the paper background information, the accuracy of the highlight portion is degraded under the condition of light quantity and gain that saturate the white portion of the paper. . For this reason, the light receiving condition of the scanner is changed so that the gain is 85% as compared with the normal image reading.

  (Description of Operation Unit 300) FIG. 3 is a schematic configuration diagram of the operation unit 300, and is a screen when the user selects user mode adjustment / CLN (cleaning). An operation unit 300 of the MFP includes a key input unit 301 and a touch panel unit 302. The key input unit 301 is a key input part that can perform routine operation settings. A user mode key 303 is a key for shifting to a system setting screen for each user. For the show-through reduction CAL related to this case, when the user mode key is pressed and the adjustment / CLN button is touched on the touch panel, a selection screen is displayed. When executing the show-through reduction CAL on new thin paper, etc., enter from the adjustment / CLN button on the operation screen and select the show-through reduction CAL.

  <Control Configuration Example of Image Forming Apparatus> FIG. 4 is a block diagram illustrating a control configuration example of the image forming apparatus according to the present embodiment. The image forming apparatus has a system configuration in which an interface that enables connection to a network and an image processing unit that executes various types of image processing are incorporated. Therefore, the image forming apparatus is referred to as an MFP (Multi Function Peripheral). The MFP includes a memory such as a hard disk capable of storing data of a plurality of jobs in its own apparatus. The multiple functions of the MFP include a copy function that allows the printer unit 11 to print job data output from the scanner input unit 200 via the memory. Also included is a print function that allows the print unit 11 to print job data output from an external device such as a computer via the memory.

  (Configuration Example Centering on MFP Control Unit 1) As shown in FIG. 4, the MFP control unit 1 has the following configuration units as interfaces for various information. An input image processing unit 2 that reads an image such as a paper document and performs image processing on the read image data. Further, it has a FAX unit 3 for transmitting and receiving images using a telephone line typified by a facsimile. In addition, a network interface card (NIC) unit 4 that exchanges image data and device information using a network is provided. Further, it has a dedicated interface unit (dedicated I / F unit) 5 for exchanging information such as image data with an external device. In addition, a USB interface (USB I / F) unit 6 that transmits and receives image data and the like with a USB device typified by a USB (Universal Serial Bus) memory (a type of removable media) is provided. The MFP control unit 1 (CPU) plays a role of traffic control such as temporarily storing image data or determining a route according to the use of the MFP.

  Next, the document management unit 8 includes a memory such as a hard disk capable of storing a plurality of image data. For example, the MFP controller 1 mainly controls the image data input from each interface so that a plurality of image data can be stored in the hard disk. Then, the image data stored in the hard disk is appropriately read out and transferred to the output unit of the printer unit 11 so that output processing such as print processing by the printer unit 11 can be executed. Further, in accordance with an instruction from the operator, the image data read from the hard disk is controlled so as to be transferred to an external device such as a computer or another image forming apparatus. When storing image data in the document management unit 8, the image data is compressed and stored as necessary, or conversely, when the stored image data is read out, it is decompressed to the original image data. Processing such as returning is performed via the compression / decompression unit 7. In addition, it is generally known that compressed data such as JPEG, JBIG, and ZIP is used when data passes through a network. After the data enters the MFP, the data is decompressed (expanded) by the compression / expansion unit 7. Is done. In addition, the resource management unit 9 stores various parameter tables that are handled in common, such as fonts and gamma tables, and can be called up as necessary, and a new parameter table can be stored or modified and updated. You can do it. The image forming condition information generated at the time of performing the calculation calculation of the optimum image forming condition (show-through reduction CAL) for reducing the show-through for each thin paper type of the present embodiment is stored in the resource management unit 9 and necessary. In response, the MFP control unit 1 refers to and sends information to the printer unit 11. Next, in the MFP control unit 1, when PDL data is input, the RIP unit 13 performs RIP (Raster Image Processor) processing. Further, the output image processing unit 12 performs image processing for printing on the image to be printed as necessary. Further, intermediate data or print ready data (bitmap data for printing or data obtained by compressing it) can be stored again by the document management unit 8 as necessary. Then, it is sent to the printer unit 11 for image formation. The sheet printed out by the printer unit 11 is sent to the post-processing unit 10 where sheet sorting processing and sheet finishing processing are performed.

Here, the MFP control unit 1 plays a role of smoothly flowing jobs, and path switching is performed as follows according to how the MFP is used. However, although it is generally known that image data is stored as intermediate data as needed, access other than the document management unit 8 serving as a start point and an end point is not shown here. In addition, processing such as the compression / decompression unit 7 and post-processing unit 10 used as necessary, or the MFP control unit 1 serving as the entire core is omitted, and is described so that an approximate flow can be understood.
(A) Copying function: input image processing unit 2 → output image processing unit 12 → printer unit 11
(B) FAX transmission function: input image processing unit 2 → FAX unit 3
(C) FAX reception function: FAX section 3 → output image processing section 12 → printer section 3
(D) Network scan: input image processing unit 2 → NIC unit 4
(E) Network printing: NIC unit 4 → RIP unit 13 → output image processing unit 12 → printer unit 11
(F) Scan to external device: input image processing unit 2 → dedicated I / F unit 5
(G) Printing from an external device: dedicated I / F unit 5 → output image processing unit 12 → printer unit 11
(H) Scan to external memory: input image processing unit 2 → USB I / F unit 6
(I) Printing from external memory: USB I / F unit 6 → RIP unit 13 → output image processing unit 12 → printer unit 11
(J) Scan box function: input image processing unit 2 → output image processing unit 12 → document management unit 8
(K) Box print function: document management unit 8 → printer unit 11
(L) Box reception function: NIC unit 4 → RIP unit 13 → output image processing unit 12 → document management unit 8
(M) Box transmission function: Document management unit 8 → NIC unit 4
(N) Preview function: document management unit 8 → operation unit 14
(P) Automatic gradation correction: output image processing unit 12 → printer unit 11 → input image processing unit 2 → output image processing unit 12 → resource management unit 9
(Configuration Example of Input Image Processing Unit 2) FIG. 5A is a block diagram showing the flow and configuration of image data in the scanner input unit 200. The read image is converted into an electric signal of 600 dpi using the single-color one-line CCD sensor 20, and is input to the A / D converter 21 as single-color image data. The A / D conversion unit 021 performs gain adjustment and offset adjustment, and is converted into 8-bit image data (density data). The shading correction unit 22 is an image processing block arranged in the input image processing unit 2. The shading correction unit 22 corrects variations in the sensitivity of each pixel of the CCD sensor 20 and variations in the amount of light of the document illumination lamp using the reference white plate reading signal. The filter processing unit 23 performs convolution integration including the target pixel and a plurality of peripheral pixels, and performs processing to make the image captured from the CCD sensor 20 sharper. The gradation correction unit 24 that performs density correction is a one-dimensional lookup table in consideration of the output characteristics of the printer unit 11 so as to obtain gradations according to three image modes such as a character mode, a photo mode, and a mixed mode thereof. Density correction data is held as (LUT). The character mode is a curve that reproduces the input image in extremely white or black from low to medium density in order to reproduce the character clearly. The photo mode is an almost linear curve to faithfully reproduce the original, and the mixed mode is an intermediate position. The halftone processing unit 25 expresses gradations in area ratios using processing of an error diffusion system (generally, the target pixel and its surrounding pixels are weighted with an error filter) in which moiré is unlikely to occur. A halftone is expressed in a pseudo manner by converting from a multi-value image to a binary image. Further, the notch processing unit 26 corrects unnecessary notches (jagged edges) using a window with respect to the target pixel and its peripheral pixels, and changes the target pixel when it matches a pattern prepared in advance. . At the time of automatic gradation correction related to the present embodiment, the processing after the shading correction unit 22 is bypassed, and the luminance information is sent to the MTF control unit.

  (Configuration Example of RIP Unit 13) The configuration of the RIP unit 13 will be described with reference to FIG. RIP reproduces vector information such as characters, line drawings, and figures described in PDL or image scanning line information such as colors, patterns, and photographs on a page at the same time. For this purpose, the processor develops each object information into a bitmap (raster image) on a memory. Originally, it was mounted on the output device side as hardware, but now it is realized by software by increasing the CPU speed. The RIP unit 13 generally includes two parts, an interpreter unit 30 and a rendering unit 31. The interpreter unit 30 includes a PDL interpretation unit 32 that translates PDL, and a DL (Display List) generation unit 33 that generates an intermediate file called a display list from the interpreted PDL data. On the other hand, the rendering unit 31 includes a DL expansion unit 34 that expands the display list into a bitmap (raster image). In addition, a gradation correction unit 35 for performing brightness adjustment and printer gradation correction instructed by the user is provided. In addition, the image processing apparatus includes a screen processing unit 36 that performs pseudo halftone processing for converting a multi-value grayscale image into a binary image. The PDL interpretation unit 32 is a part that analyzes various types of input PDL data with respect to the display list. As the input format, the PostScript (registered trademark) language of Adobe, the PCL (Printer Control Language) language of HP (Hewlett-Packard), and the like are well known. These are described as printer control codes for creating an image in units of pages, and include not only simple character codes but also graphic drawing codes and photographic image codes. A document display file format developed by Adobe called PDF (Portable Document Format) is also widely used in various industries, and this format directly thrown into the MFP without using a driver is also targeted. In addition, a format for VDP (Variable Data Print) called PPML (Personalized Print Markup Language) is also supported. It also supports a color image compression format called JPEG (Joint Photographic Experts Group) or TIFF (Tagged Image File Format). Note that automatic gradation correction, which will be described later, is also effective for PDL, and the LUT created and stored in the resource management unit 009 is set in the gradation correction unit 035.

  (Configuration Example of Output Image Processing Unit 12) The configuration of the output image processing unit 12 will be described with reference to FIG. The output image processing unit 12 has different processing blocks for normal image formation and automatic gradation correction. The output image processing unit 12 that has input binary image data for normal image formation from the MFP control unit 1 converts the line width from 600 dpi to 1200 dpi of the printer if necessary. The resolution converter 42 is provided. Then, the processed binary data is sent to the printer unit 11 via the MFP control unit 1. Note that image data input during normal image formation is binary data of 600 dpi or 1200 dpi, and the resolution conversion unit 042 is bypassed during 1200 dpi. When image data for automatic gradation correction is input and generation of gradation correction information is instructed from the MFP control unit, patch extraction 43 is performed to extract a patch from the input test pattern (600 dpi multivalue). . Next, a foreign matter detection unit 44 that detects foreign matter, a foreign matter surrounding removal unit 45 that removes the detected foreign matter and peripheral luminance information within a specific distance from the foreign matter, and remaining luminance information after the foreign matter is removed The process up to the averaging unit 46 to be averaged is performed on each cut out patch. Then, the data is input to the LUT generation unit 47. The foreign object detection unit 44 sends the position of the center of gravity in the abnormal area, which is position information where the foreign object exists, to the foreign object surrounding removal unit 45, and refers to the position information of the foreign object from the extracted patch image data. Thus, the foreign matter portion and its periphery are removed. The LUT generation unit 47 grasps engine gradation from each input patch luminance information, generates gradation correction information (LUT) so as to be a target gradation, and manages resources via the MFP control unit 001. Input to section 9.

<Operation Example of Image Forming Apparatus of this Embodiment>
(Basic Flow of Automatic Gradation Correction) FIG. 6 shows a basic flow of automatic gradation correction according to this embodiment. The automatic gradation correction is the content described in the program in the image processing apparatus, and the MFP control unit 1 (CPU) executes necessary operations and calculations. The MFP instructed to execute automatic gradation correction from the operation screen as shown in FIG. 3 (S1) prints a test pattern for density correction as shown in FIG. 7 (S2). The test pattern is composed of a plurality of gradations (a patch chart of 64 patches per color). The printed test pattern is placed on the document table of the scanner input unit 200 by the user's hand (S3), and reading is started by pressing a read button on the operation screen (S4) (S5). The read test pattern is converted into luminance information averaged for each patch. The averaged luminance information is converted into density information by a one-dimensional luminance density conversion table having a relationship between the luminance and density of the reading unit, and grasps the γ characteristic of the MFP. As shown in FIG. 8, an LUT is created so that the γ characteristic of the MFP becomes a desired γ characteristic (in this embodiment, brightness linear) (S6), and the LUT is stored in the resource management unit 9 (S7). In the calculation step (S6), the output image processing unit 12 cuts out patches, detects and removes foreign matters, and averages the luminance information. The process from cutting out the patch to averaging the luminance information is a characteristic part of the present invention, and will be described later using a detailed calculation flow (FIG. 9). The above is the basic automatic gradation correction flow.

[Embodiment 1]
(Details of Calculation Flow S6) Next, Embodiment 1 of the calculation step (S6 in FIG. 6) which detects the foreign matter on the test pattern and suppresses the influence of the automatic gradation correction by the foreign matter, which is the subject of the present invention. This will be described with reference to FIG. The calculation step means the patch cutout unit 43, the foreign matter detection unit 44, the foreign matter surrounding removal unit 45, the averaging unit 46, and the LUT generation unit 47 provided in the output image processing unit 12. Since the averaging and LUT generation flow has been described in the basic flow of automatic gradation correction, description thereof is omitted. The MFP controller 1 that has output the test chart reads the test chart with a scanner (S5 in FIG. 6). Since it is necessary to calculate the read test pattern for each gradation, patch extraction is executed (S16). In the patch cutout (S16), in accordance with an instruction from the MFP control unit, the patch cutout unit 43 is used to read the A4 size test pattern shown in FIG. Cut out. The test patterns in FIG. 7 are four types of automatic gradation correction patterns with 4 × 16 64 gradations as one set, but the four types of sets have different screen processing.

  Even if the test pattern is obliquely placed on the reading unit, it is necessary to cut out the test pattern. In FIG. 7, dark patches of the upper left patch A and the lower right patch B of the test pattern are detected, and then the luminances of the upper right patch C and patch D are compared. When the difference between the brightness of patch D and the brightness of patch C (DC) is positive, alignment adjustment (determining the inclination of the chart placed from the coordinates of the three points) is performed using patches A to C. Cut out the patch. When the difference (D−C) between the luminance of the patch D and the luminance of the patch C is 0 or negative, a display for instructing repositioning of the chart is performed on the touch panel unit 302. Since the test pattern is a patch of 16 mm for main scanning and 12 mm for sub-scanning, a margin of 2 mm is seen vertically and horizontally. This margin takes into account the reflection of light in the reading device and the instability of the edge portion of the image forming device, and is not related to the present invention.

  Next, the foreign matter detection unit 44 is an image processing block for detecting whether foreign matter such as paper dust is present from the cut out 12 mm × 8 mm patch. The foreign matter detection unit 44 includes a binarization step (S26), a labeling step (S36), an area ratio calculation step (S46), a center of gravity calculation step (S56), a foreign matter determination step (S66), and a foreign matter removal region determination step (S86). Consists of. In the binarization step (S26), the intermediate brightness value between the minimum brightness and the maximum brightness inside the patch is set as a threshold value, black (1) is printed on the portion where the low-lightness toner is printed, and white (0) on the high-lightness background. ) To be binarized. In the labeling step (S36), all black points and white points in the 12 mm × 8 mm patch are detected by an image search method called labeling from the main scanning sub-scanning start point (upper left) to the main scanning sub-scanning end point (lower right). Then, numbers are assigned in the order of detection. Then, the areas are detected in the order of the assigned numbers. It should be noted that no labels are given to black spots and white spots in contact with edges. Labeling is a method of assigning the same label to connected pixels in an input binarized image. For example, in a 3 × 3 pixel area as shown in FIG. 10, the binarized information of each pixel and its surrounding pixels is inspected from the upper left of the image data. “a” is a pixel of interest, and a label is added as necessary. If the same black as the target pixel exists in the eight pixels near the target pixel, a label is added to the target pixel. The same number is assigned to the connected pixels. If the connected pixels are interrupted at the inspection stage, the label information increased by 1 is added to the next independent black spot. The above inspection and label addition are inspected in two types: black point and white point. FIG. 11A is a diagram illustrating a labeling result according to the first embodiment. It will be understood that the same labels “1” and “2” are assigned to the connected pixels.

  In the area ratio calculation step (S46), the number of pixels having the labeled number is counted. In the case of the labeling result in FIG. 11A, the number of pixels in Table 1 is obtained. After confirming the number of pixels up to all the labels (Nth), the average value of the 1st to Nth pixels is obtained, and the area ratio of each label is calculated. In the case of FIG. 11A, the total number of pixels from labels 1 to 11 is “59”, the average value is “5” (≈59 / 11), and the area ratio of each label is the area ratio (label). = Number of pixels / average value × 100.

  The center-of-gravity calculation is performed using Equation 1 below.

Here, (xi, yi) indicates the coordinates of each pixel to which the same label is assigned. i is a natural number from 0 to n-1. n is the number of previous pixels. In the case of the image in FIG. 11A, the center of gravity is the dark hatched pixel portion in FIG. In the foreign matter determination step (S66), it is determined whether there is a black spot or a white spot with the area ratio exceeding 300%. If there are black spots and white spots that exceed the number, it is determined that the object is a foreign object. If it is 300% or less, it is determined that there is no foreign matter. If it is Table 1, it will be judged that the label 5 is a foreign material (S76).

  In the foreign substance removal region determination step (S86), an area of 4 mm around the center of gravity is grasped only for the label determined to be a foreign matter (S76). If it is determined that there are both white spots and black spots, both are instructed to the foreign object removal section 45 to remove overlapping portions. The calculation flow executed by the foreign matter detection unit 43 has been described above. The foreign object periphery removal step (S96) is a step processed by the foreign object periphery removal unit 45, and the area information of the 4 mm periphery around the center of gravity determined to be a foreign object by the foreign object detection unit 44 is replaced with zero and the number of replaced pixels is stored. Keep it.

  As described in the problem, if foreign matters such as paper dust exist before transferring the color material, the color material scatters at the secondary transfer portion. It was confirmed from experiments that the range in which the scattering and foreign matters such as paper dust affect the concentration is approximately 4 mm or less from the center of gravity. Therefore, in this embodiment, the area around the center of gravity of 4 mm from the center of gravity is not used as density information for automatic gradation correction.

  In the averaging step (S106), the averaging unit 46 is used to average the luminance information in the 12 mm × 8 mm patch. When the luminance information is replaced with zero by the foreign object periphery removing unit 45, the luminance information of the foreign object and the peripheral part of the foreign object is included by subtracting the number of replaced pixels from the total pixel number information when calculating the average value. There is no averaging. Since the density or color reading range of the image reading apparatus is wider than the density reproduction or color reproduction range of the image forming apparatus, no problem is caused by replacing with zero as described above. However, if it is a combination of an image forming apparatus and an image reading apparatus that also generates a brightness value of zero, processing that does not include the foreign substance removal area may be performed when the brightness is averaged.

  The calculated patch brightness is converted into density information as shown in Table 2 through a brightness density conversion table prepared in advance.

  Table 2 shows the patch input signal values in the vertical direction during automatic tone correction. The patch average is simply obtained by calculating the average luminance of the patch and converting it to the density. This is an example when foreign object detection and foreign object gravity center periphery (distance from the foreign material gravity center within 4 mm) of the present embodiment is deleted.

  In the conventional example 1 in Table 2, the n + 1 patch (input signal 32) whose slope has reached a certain level is not used, and the density value of the n + 2 patch (input signal 36) and the current n patch (input signal 28). The density information of the n + 1 patch (input signal 32) is determined by linear interpolation. Conventional example 2 does not use the n + 1 patch (input signal 36) having a negative slope, and linearly complements the density value of the n + 2 patch (input signal 40) and the current n patch (input signal 32). Thus, the density information of the n + 1 patch (input signal 36) is determined. Conventional example 3 is a simple replacement example in which the density information of the current n patch (input signal 32) is used without using the n + 1 patch (input signal 36) having a negative slope. Thus, in the conventional example, it is not specified in which patch the foreign matter is generated. Regardless of the original patch density, the patch density has been corrected in relation to the density before and after the input signal. Therefore, when a foreign object or the like is included, the gradation is not corrected with high accuracy.

  As a result of calculating the foreign matter detected by the method of the present embodiment and not including information on the circumference of the foreign matter center of gravity of 4 mm with respect to the conventional examples 1 to 3, it is first specified that the patch that is not the original density is the patch of the input signal 32 Is done. Moreover, it turned out that the average density | concentration after deleting foreign material gravity center 4mm is 22 levels. That is, it succeeded in obtaining the original density to be reflected in the automatic patch correction including the simple patch average and the foreign matter that cannot be corrected in the conventional examples 1 to 3 with high accuracy. In addition, although it converted into density | concentration information using the brightness | luminance density | concentration conversion table, 1.5 is normalized by 255 by 255 gradations by relative reflection density (paper reference | standard reflection density). For the patch density information converted into high-precision density information, a γLUT is created according to the flow of FIG. 6, and the automatic gradation correction is terminated.

  <Effect of First Embodiment> Thus, the foreign object is determined based on the deviation amount from the average black point or the average white point, and 4 mm around the center of gravity is deleted from the luminance average information of the patch. As a result, it is possible to accurately grasp the original gradation characteristics of the engine that do not include the density increase due to the scattering caused by the foreign matter. By performing automatic gradation correction using the above method, high-precision correction is possible.

[Embodiment 2]
In the first embodiment, from the viewpoint of calculation speed, the problem of the present invention has been solved by a simple calculation of 4 mm uniformly from the center of gravity of the foreign matter. However, in the second embodiment, the deletion area at the time of averaging taking into account the shape of the foreign matter Change the configuration to optimize. The function of the foreign substance removal region determination step (S96) described in the first embodiment is expanded to acquire the edge information of the foreign substance instead of the surrounding 4 mm from the center of gravity, and the luminance of the surrounding 2 mm from the edge is deleted. Edge information of the black point or white point determined as a foreign object in the foreign object determination step (S66, S76) is acquired. Edge information of a black point or white point determined to be a foreign object is determined by information similar to that at the time of labeling and based on information on the target pixel and its surrounding eight pixels shown in FIG. “A” to “e” in FIG. 12 are pixels of interest. 8 pixels around the target pixel are inspected. The number of different pixels (hereinafter referred to as background pixels) around the pixel of interest is inspected. In accordance with the inspection result, whether or not the pixel of interest is an edge or an edge is inspected using all the pixels of the label determined as foreign matter as the pixel of interest.

The relationship between the number of background pixels and whether they are edges or not can be determined as follows.
The number of background pixels is 0 pixels: labeled but internal pixels of foreign matter, not edges,
The number of background pixels is 1 to 7 pixels: edge,
The number of background pixels is 8 pixels: an independent pixel that is labeled but has only one pixel.
It should be noted that pixels that are 300% or more of the average number of pixels have already been extracted in the foreign substance determination step (S66), and in the present embodiment, there is no independent pixel with 8 background pixels. From the edge of the foreign object, the luminance information of the outer 2 mm is replaced with zero and at the same time the number of replaced pixels is stored. Since the subsequent description is the same as that of the first embodiment, the description is omitted.

  <Effects of Embodiment 2> Foreign matter may be not only circular but also paper fibers such as 1 mm x 2 mm may be attached, and from the viewpoint of calculation speed, if it is uniformly removed from the center of gravity, a certain range is removed. Had to do. On the other hand, if the edge information of a foreign object is acquired as in the present embodiment, the periphery of the foreign object can be removed and averaged according to the shape of the foreign object, so that it is not necessary to delete the luminance information that should originally be included in the averaging. Therefore, automatic gradation correction with higher accuracy can be executed.

[Embodiment 3]
The third embodiment shows a configuration in which the foreign object periphery removal region is optimized for each screen from pixels that are uniformly 2 mm away from the edge, and the number of pixels included in the averaging is increased while suppressing the deletion amount as much as possible. The test pattern of FIG. 7 is four types of automatic gradation correction patterns with 4 × 16 64 gradations as one set, but the four types of sets have different screen processing. Each has a dot growth of 1200 dpi, a 45 degree screen, 106, 141, 169, 212 lpi (also referred to as line / inch (number of lines)). The relationship between the scattering due to the foreign matter and the density increase is that the density increases because the color material that has been scattered to the part where the white of the background is originally visible covers the background. In the case of an image forming apparatus that expresses area gradation, the density must also consider the optical dot gain. FIG. 13 shows a conceptual diagram when printing on paper with the same dot area ratio even if the dot reproduction is digitally complete. In FIG. 13, the black part is toner, the upper left figure is 212 lpi, and the upper right figure is 106 lpi. What is indicated by an arrow at the end of the dot is a diagram showing incident transmission / scattering of light, and light wraps around the white background adjacent to the dot and darkens the white background. Comparing 212 lpi and 106 lpi, since 212 lpi has a narrower dot interval, the area of the white background is small and it is easily affected by optical dot gain. Even if dots can be reproduced with the same area ratio based on the above principle, 212 lpi appears darker due to the optical dot gain. In FIG. 13, small black spots 130a and 130b showing scattering are provided. The lower profile of FIG. 13 shows the profile of the darkness including the skip. In this way, when scattering occurs at 212 lpi, the optical dot gain at that scattering further increases, so the density increase rate with respect to the amount of scattering is high.

Furthermore, when the number of lines is high and the dot cycle is fine, the dots (coloring material) collide with the foreign matter and scatter at the secondary transfer unit as shown in FIG. 14A, but the number of lines as shown in FIG. 14B. When the dot period is low and the dot cycle is coarse, the collision amount of the foreign matter is reduced (the collision rate between the dot and the foreign matter is small). Therefore, the amount of splattering in the first place is smaller for the low number of lines. For two reasons, the optical dot gain and the dot / foreign matter collision rate, the amount of scattering on the foreign matter and the degree of influence on the density differ for each number of lines. For this reason, if the foreign matter removal area is enlarged in consideration of the high line number side, the portion not affected by the scattering by the foreign matter is deleted, and further improvement in accuracy cannot be expected. Therefore, in this embodiment, after labeling, the deletion area after detecting a foreign object is defined to be equal to or less than the following distance from the center of gravity.
106lpi: 1.4mm
141lpi: 1.5mm
169lpi: 1.7mm
212lpi: 2.0mm
Since the subsequent description is the same as that of the second embodiment, the description is omitted.

  <Effect of Embodiment 3> Even with the same foreign substance, the amount of scattering and the degree of influence on the concentration differ depending on the number of lines. By setting the foreign substance removal area according to the density area taking the influence degree into account, more accurate gradation correction can be realized.

[Embodiment 4]
The fourth embodiment is characterized in that the foreign matter removal area is changed according to the input signal value. As described with reference to FIG. 14 of the third embodiment, even if foreign matter is present, scattering does not occur unless a color material is present around the foreign matter. Further, even in the presence of a color material, in a shadow portion with few white background portions, the density starts to be expressed by overlapping toner (black portions) as shown in FIG. 15, and even if scattering occurs, the influence on the density is not affected. rare. That is, the halftone is more susceptible to foreign matter than the highlight and the halftone than the shadow portion, and the input signal should be considered. In the fourth embodiment, based on the above phenomenon, the input signal is set on the horizontal axis and the removal area (mm) from the foreign substance edge is set on the vertical axis as shown in FIG. Since the subsequent description is the same as that of the second embodiment, the description is omitted.

  <Effect of Embodiment 4> Even with the same foreign substance, the amount of scattering and the degree of influence on the concentration differ depending on the input signal (halftone dot%). By setting the foreign substance removal area in consideration of the degree of influence, more accurate gradation correction can be realized.

[Embodiment 5]
The fifth embodiment is characterized in that the foreign substance deletion area is determined after grasping the brightness gradient due to scattering, not the predetermined foreign substance removal area. As shown in FIG. 17, the difference from FIG. 9 is that a halftone blur filtering step (S126) for processing a multi-valued image is added in parallel with the processing after the binarization step (S26). As shown in FIG. 18, the blurring filter smoothes the screen image by replacing the average value of the surrounding 25 pixels with the pixel of interest with an averaging filter of 5 × 5 pixels. In the foreign matter removal area determination step (S86), the center of gravity information of the label determined to be a foreign matter is obtained from the foreign matter determination step (S66). In the foreign substance removal region determination step (S86), the luminance gradient profile of the multi-valued image subjected to halftone dot filtering from the foreign substance barycentric coordinates to the patch edge cut out is calculated for all patch edges. The flow of the above image processing is shown in FIGS. FIG. 19D shows the luminance gradient profile of the portion indicated as “A” in FIG. The horizontal axis is brightness and goes to the right (larger value). The foreign material at the center of gravity now has paper dust attached to it, and it has been peeled off by the fixing device. The inclination is calculated based on the brightness profile with respect to the patch edge from the foreign material centroid. In calculating the inclination, the difference in luminance when the position of the read luminance profile is shifted from n by n + 500 μm is calculated as the inclination. The patch image is read at 600 dpi, and it is possible to produce a fine tilt, but because the influence of paper dust is a large area, as shown in FIG. 20, the luminance is 500 μm apart from each pixel from a position close to the center of gravity. Calculate the slope with the information.

In the fifth embodiment, luminance information of the following areas is not used for automatic gradation correction.
Range of 0.5mm from the center of gravity: Not used for calculation even if the slope is gentle From the center of gravity to more than 0.5mm to the patch edge: The area from the n + 500 position to the center of gravity where the slope is 0.25 or more and -0.25 or less is not used <Embodiment Effect of 5> As in the present embodiment, it is possible to perform automatic gradation correction with high accuracy by optimizing the foreign substance removal region in consideration of the luminance gradient.

<Modification of Embodiment 5>
In the fifth embodiment, the removal area is calculated by calculating a luminance gradient profile. With this method, there is a possibility that density information cannot be obtained for very large foreign matters. Even if very little information can be extracted, it is difficult to determine whether the little area is worthy of the representative value of the corresponding patch. In order to deal with the above problems, in this modification, the minimum necessary area may be defined from the matrix structure of the screen. FIG. 21 is a dither matrix of 16 × 16 pixels = 256 pixels of 1200 dpi and 106 lpi. In the case of 141 lpi, a dither matrix can be assembled with 12 × 12 pixels, but the above 256 gradations cannot be expressed. Therefore, in many cases, 12 × 24 pixels or 24 × 24 pixels that are larger than an area where 256 gradations can be expressed. Generally, if the number of lines is high at the same resolution, the dither matrix becomes large. With the above background, in order to know the density of the patch reproduced by the engine, luminance information larger than the dither matrix size is required. Therefore, when a dither matrix having a high number of lines is adopted, the minimum necessary area must be increased. That is, the number of pixels necessary for the automatic gradation correction calculation may be larger than the dither matrix size.

  As described above, it is possible to provide an image forming apparatus capable of performing CAL with high accuracy even when foreign matter such as paper dust is present. In the present embodiment, detection of foreign matter such as paper dust has been described. However, the same effect can be obtained by the same configuration even when the detection is extended to detection of an abnormal area including a scratch on a recording medium.

[Other Embodiments]
The present invention may be applied to a system composed of a plurality of devices, or may be applied to an apparatus composed of one device. For example, a scanner, a printer, a PC, a copier, a multifunction machine, and a facsimile machine. The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (12)

Image forming means for printing a patch chart for density correction composed of a plurality of density patches on a recording medium, image reading means for reading the printed patch chart, and density correction from density data of the read patch chart An image forming apparatus having density correction means for generating data,
Detection means for detecting, as an abnormal area, an area in which the read value in the patch of the read patch chart is shifted from a predetermined value to a high lightness side or a low lightness side where the size of the area exceeds a predetermined size When,
Calculating means for calculating the position of the detected abnormal area;
The image forming apparatus, wherein the density correction unit does not use read data within a specific distance from the calculated position of the abnormal area for density correction.   2. The image according to claim 1, wherein, when the patch chart is formed by a plurality of pseudo halftone processes, the detection unit detects the detection under different detection conditions for each type of the pseudo halftone process. Forming equipment.   The image forming apparatus according to claim 1, wherein the detection unit detects a detection condition according to a density range.   The image forming apparatus according to claim 1, wherein the position of the abnormal area is a center of gravity in the abnormal area.   The image forming apparatus according to claim 1, wherein the specific distance from the position of the abnormal area varies depending on the density area.   The image forming apparatus according to claim 1, wherein the specific distance from the position of the abnormal area is determined using a density gradient of read information.   2. The specific distance from the position of the abnormal area is equal to or less than a distance from an edge of a patch used for calculation to the abnormal area and is equal to or greater than an area capable of expressing 256 gradations by pseudo halftone processing. 4. The image forming apparatus according to any one of items 1 to 3.   The image forming apparatus according to claim 1, wherein the specific distance from the position of the abnormal area differs for each type of pseudo halftone process. An image forming apparatus control method comprising:
An image forming step in which an image forming unit prints a density correction patch chart composed of a plurality of density patches on a recording medium;
An image reading unit that reads the printed patch chart; and
A density correction step in which density correction means generates density correction data from the read density data of the patch chart;
The area where the reading value in the patch of the read patch chart is shifted from the predetermined value to the high lightness side or the low lightness side exceeds the predetermined size is defined as an abnormal area. A detection process to detect;
A calculating means for calculating a position of the detected abnormal area;
In the density correction step, read data within a specific distance from the calculated position of the abnormal area is not used for density correction.   A program for causing a computer to execute the steps of the control method for an image forming apparatus according to claim 9.   A computer-readable storage medium storing the program according to claim 10. A gradation correction method in image formation,
The image forming unit prints a density correction patch chart composed of a plurality of density patches on the recording medium,
Image reading means reads the printed patch chart,
The density correction means generates density correction data from the read density data of the patch chart,
The area where the reading value in the patch of the read patch chart is shifted from the predetermined value to the high lightness side or the low lightness side exceeds the predetermined size is defined as an abnormal area. Detect
In the step of generating the density correction data, read data within a specific distance from the detected abnormal area is not used for density correction.