JP5146238B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  2. Description of the Related Art Conventionally, there has been known an image forming apparatus such as an electrophotographic method or an ink jet method that includes an image reading unit that reads an image on a sheet on which an image has been formed. In such an image forming apparatus, for example, it is determined whether or not the color of the image read by the image reading unit is an intended color, and correction is performed as necessary.

  Here, the image obtained by scanning the printing surface of the reference printed matter with a sensor camera is divided into a plurality of regions, and a reference density level is obtained for each region. Thus, a technique has been proposed in which an inspection density level is obtained for each area, and a color density difference is detected by comparing the reference density level and the inspection density level for each area (see, for example, Patent Document 1). ).

  In addition, a technique has been proposed in which the print density is detected based on the information on the ink application areas on the front and back surfaces of a newspaper printed on a rotary press (see, for example, Patent Document 2).

  Furthermore, a technique has been proposed in which an image is divided into regions based on timing pulses of a rotary encoder and density monitoring is performed for each region (see, for example, Patent Document 3).

For later reference, Patent Document 4 is cited.
Japanese Patent Laid-Open No. 05-077409 JP 06-1105050 A Japanese Patent Laid-Open No. 2002-310799 JP 2004-199548 A

  An object of the present invention is to provide an image forming apparatus capable of suppressing color fluctuations between the same images formed on a plurality of sheets based on the same image signal.

The image forming apparatus according to claim 1 comprises:
Paper transport means for transporting paper,
An image forming unit that is provided on a paper transport path through which the paper is transported by the paper transport unit and forms an image on the transported paper;
A color variation suppression processing unit that executes a color variation suppressing operation for suppressing color variation of an image formed by the image forming unit;
An image reading unit that is disposed on the downstream side of the image forming unit on the sheet conveyance path and that reads an image on a sheet on which an image is formed in the image forming unit;
An image processing unit that detects color variation by processing image data obtained by reading by the image reading unit and instructs the color variation suppression processing unit to suppress color variation;
The image reading unit includes a plurality of unit sensors arranged in a width direction crossing the conveyance direction of the paper, and has a line sensor that repeatedly reads an image on the paper while the paper passes through the image reading unit,
The image processing unit
Based on the image data obtained by the line sensor, a plurality of feature points are extracted in the transport direction, the positions of the feature points on the image in the transport direction are obtained, and the positions of the feature points in the transport direction A first measurement unit that measures the expansion / contraction magnification of each part in the conveyance direction of the image by obtaining the amount of deviation from each reference position;
By correcting the expansion / contraction of each part in the conveyance direction of the image based on the magnification of the expansion / contraction of each part in the conveyance direction of the image represented by the image data measured by the first measurement unit, the image A correction unit that generates correction image data representing a correction image in which the positions of the extracted plurality of feature points in the transport direction are matched with the respective reference positions;
A detection unit that detects a color variation from a reference color of a representative color representing the corrected image;
A color variation suppression operation instructing unit that instructs the color variation suppression processing unit to execute the color variation suppression operation based on the color variation detected by the detection unit.

An image forming apparatus according to a second aspect is the image forming apparatus according to the first aspect, wherein the color variation suppression operation instruction unit is
A necessity determination unit that determines whether or not to perform the color variation suppression operation based on the color variation detected by the detection unit;
In response to the determination that the necessity determination unit needs to execute the color variation suppression operation, the type of color variation suppression operation is determined based on the color variation, and the color variation suppression processing unit is determined. And a type determining unit for instructing the execution of the type color variation suppressing operation.

  An image forming apparatus according to a third aspect is the image forming apparatus according to the first or second aspect, wherein the first measurement unit is based on the image data obtained by the line sensor and is represented by the image data. A projection waveform is obtained by obtaining a plurality of cumulative values of the pixel values of the pixels arranged in the width direction in the transport direction, and the plurality of feature points are extracted based on the projection waveform.

  An image forming apparatus according to a fourth aspect is the image forming apparatus according to any one of the first to third aspects, wherein the image processing unit sequentially forms the same image based on the same image signal on a plurality of sheets. Based on the image data obtained by reading the image formed on the first sheet, a plurality of feature points are extracted in the transport direction, and the positions of the feature points on the image in the transport direction are obtained. The position of the feature point in the transport direction is the reference position.

  The image forming apparatus according to claim 5 is the image forming apparatus according to any one of claims 1 to 3, wherein the image processing unit sequentially forms the same image based on the same image signal on a plurality of sheets. The weighted average position of the feature points corresponding to each other obtained from each of the image data previously read by the line sensor is obtained as the reference position corresponding to the feature point. It is characterized by being.

According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the traveling speed of the paper when the image reading unit reads an image on the paper Comprising a speed measuring means for measuring at least a part,
A second measurement for measuring the expansion / contraction magnification of each portion in the transport direction of the image represented by the image data obtained by reading the image on the sheet based on the result of the sheet traveling speed measurement by the speed measuring unit. Part
The first measurement unit and the second measurement unit cooperate or share within the image the expansion / contraction magnification of each part in the transport direction of the image represented by the image data obtained by reading the image on the paper. Calculated,
The correction unit calculates the image based on the expansion / contraction magnification of each part in the transport direction of the image represented by the image data, which is calculated by the first measurement unit and the second measurement unit in cooperation or sharing. By correcting the expansion and contraction of each part in the conveyance direction, corrected image data representing a corrected image in which the positions in the conveyance direction of a plurality of feature points extracted for the image are matched with the respective reference positions is generated. The image forming apparatus according to claim 7 is the image forming apparatus according to claim 6, wherein the partial magnification measured by the first measuring unit and the partial magnification measured by the second measuring unit are calculated. A warning output unit is further provided for outputting a warning when there is a difference equal to or greater than a threshold value compared.

  In the image forming apparatus according to the first aspect, it is possible to suppress color variation between the same images formed on a plurality of sheets based on the same image signal.

  In the image forming apparatus according to claim 2, in response to the determination that the execution of the color variation suppression operation for suppressing the color variation of the image is necessary, the execution instruction according to the type of the color variation suppression operation is performed. Can do.

  In the image forming apparatus according to the third aspect, it is possible to extract the feature point representing the feature of the image.

  In the image forming apparatus according to the fourth aspect, the reference position can be easily obtained.

  In the image forming apparatus according to the fifth aspect, the reference position can be obtained with higher accuracy than in the case where the invention according to the fifth aspect is not adopted.

  The image forming apparatus according to claim 6 can measure with high accuracy compared with a case where only a means for extracting feature points on an image and measuring the expansion / contraction magnification of each part in the image transport direction from the feature points is provided. it can.

  In the image forming apparatus according to the seventh aspect, the user can be informed that the color variation of the image exceeds a predetermined range.

  Embodiments of the present invention will be described below.

  First, an image forming system including an image forming apparatus according to an embodiment of the present invention will be described.

  FIG. 1 is a block diagram illustrating a configuration example of an image forming system including an image forming apparatus according to an embodiment of the present invention.

  As shown in FIG. 1, the image forming system includes an image forming apparatus 1, an information processing apparatus 2, and an image reading apparatus 3.

  First, the information processing apparatus 2 will be described. The information processing apparatus 2 transmits image data to the image forming apparatus 1 to be described later, thereby instructing the image forming apparatus 1 to print based on the image data, or outputting information notified from the image forming apparatus 1. Device. The information processing apparatus 2 is realized by a personal computer (PC), for example.

  The image reading device 3 optically reads an image recorded on a recording medium such as paper, and outputs the image data to the image forming device 1 to instruct the image forming device 1 to perform printing based on the image data. It is. The image reading device 3 is realized by a scanner, for example.

  The image forming apparatus 1 is, for example, a printer that employs an electrophotographic system, and the image forming apparatus 1 includes a paper transport unit 10 that transports paper.

  In addition, the image forming apparatus 1 includes an image forming unit 20 that is provided on a paper transport path through which the paper is transported by the paper transport unit 10 and forms an image on the transported paper. The configuration of the image forming unit 20 will be described later.

  Further, the image forming apparatus 1 executes a calibration corresponding to an example of the color variation suppressing operation according to the present invention for suppressing the color variation of the image formed by the image forming unit 20. 30 is provided. In suppressing the color variation of the image, for example, hardware adjustment such as the laser light amount and the toner charge amount may be performed, or the gradation value of the image signal may be changed, or Both may be used in combination.

  Further, the image forming apparatus 1 includes an image reading unit 40 that is disposed on the downstream side of the image forming unit 20 on the paper conveyance path and reads an image on a sheet on which an image is formed in the image forming unit 20. It has been.

  Further, the image forming apparatus 1 detects the color variation by processing the image data read by the image reading unit 40 and instructs the color variation suppression processing unit 30 to suppress the color variation. 50 is provided. The image processing unit 50 determines the conveyance direction based on the image data obtained by reading the image formed on the first sheet when sequentially forming the same image based on the same image signal on a plurality of sheets. A plurality of feature points are extracted to obtain positions on the image in the transport direction of the feature points, and the positions of the feature points in the transport direction are used as reference positions.

  In addition, the image processing unit 50 includes a first measurement unit 51_1. The first measurement unit 51_1 extracts a plurality of feature points in the transport direction based on the image data obtained by the line sensor, and obtains a position on the image in the transport direction of each feature point. Further, the magnification of expansion / contraction of each part in the transport direction of the image is measured by obtaining the amount of deviation of the position in the transport direction of each feature point from each reference position. Further, the first measurement unit 51_1 obtains a plurality of cumulative values of the pixel values of the pixels arranged in the width direction of the image represented by the image data based on the image data obtained by the line sensor in the transport direction. Thus, a projection waveform is obtained, and a plurality of feature points are extracted based on the projection waveform.

  The image processing unit 50 includes a reading position / partial magnification correction unit 52 which is an example of a correction unit according to the present invention. The reading position / partial magnification correction unit 52 corrects the expansion / contraction of each part in the conveyance direction of the image based on the expansion / contraction magnification of each part in the conveyance direction of the image represented by the image data measured by the first measurement unit 51_1. Thus, corrected image data representing a corrected image in which the positions in the transport direction of the plurality of feature points extracted for the image are matched with the respective reference positions is generated.

Further, the image processing unit 50 includes a color variation detection unit 53 corresponding to an example of the detection unit of the present invention. The color variation detector 53 detects the color variation of the representative color representing the corrected image from the reference color. Here, an example of average RGB will be described as the representative color and reference color, but average CMYK, average L ^* a ^* b ^{*, and the} like can be adopted as other examples.

  The image processing unit 50 instructs the color variation suppression processing unit 30 to execute calibration, which is a color variation suppressing operation, based on the color variation detected by the color variation detecting unit 53. A calibration instruction unit 54 corresponding to an example of the operation instruction unit is provided. The calibration instruction unit 54 includes a calibration necessity determination unit 54_1 corresponding to an example of the necessity determination unit of the present invention, and a calibration type determination unit 54_2 corresponding to an example of the type determination unit of the present invention. ing.

  The calibration necessity determination unit 54_1 determines whether it is necessary to execute calibration based on the color variation detected by the color variation detection unit 53. The calibration type determination unit 54_2 receives the determination that the calibration needs to be executed by the calibration necessity determination unit 54_1, determines the calibration type based on the color variation, and performs the color variation. The suppression processing unit 30 is instructed to execute the determined type of calibration. Note that the types of calibration include a change in the amount of laser light, a change in the toner charge amount, a change in the tone value of the image signal, or a combination thereof.

  Further, the image forming apparatus 1 includes a paper conveyance speed measuring unit 60 that is an example of a speed measuring unit of the present invention. The paper transport speed measuring unit 60 is realized by a rotary encoder, for example. The paper conveyance speed measuring unit 60 measures the traveling speed of the paper when the image reading unit 40 reads an image on the paper, except for the rear end portion of the paper.

  In addition, the image reading unit 40 includes a second measurement unit 51_2. The second measuring unit 51_2 is an expansion / contraction magnification of each part in the conveyance direction of the image represented by the image data obtained by reading the image on the sheet based on the measurement result of the sheet traveling speed by the sheet conveyance speed measuring unit 60. Measure. The second measuring unit 51_2 and the first measuring unit 51_1 described above cooperate in the image with the expansion / contraction magnification of each part in the conveyance direction of the image represented by the image data obtained by reading the image on the sheet. To calculate. Further, the reading position / partial magnification correction unit 52 is based on the expansion / contraction magnification of each part in the conveyance direction of the image represented by the image data, which is calculated in cooperation with the first measurement unit 51_1 and the second measurement unit 51_2. Then, by correcting the expansion and contraction of each part in the transport direction of the image, corrected image data representing a corrected image in which the positions in the transport direction of a plurality of feature points extracted for the image are matched with the respective reference positions is generated.

  Further, the image forming apparatus 1 includes a warning output unit 70 and a storage unit 80.

  The warning output unit 70 compares the partial magnification measured by the first measurement unit 51_1 and the partial magnification measured by the second measurement unit 51_2, and outputs a warning when there is a difference equal to or greater than a threshold value. The storage unit 80 is used as a work area or the like for temporarily storing image data processed by the image processing unit 50.

  Next, a hardware configuration of a part of the image forming apparatus 1 that implements the color variation suppression processing unit 30, the image processing unit 50, the warning output unit 70, and the storage unit 80 will be described.

  FIG. 2 is a block diagram illustrating a hardware configuration example of a portion that realizes the color variation suppression processing unit, the image processing unit, the warning output unit, and the storage unit that constitute the image forming apparatus illustrated in FIG. 1.

  As shown in FIG. 2, the image forming apparatus 1 includes a CPU (Central Processing Unit) 101, a RAM (Random Access Memory) 102, a ROM (Read Only Memory) 103, an HDD (Hard Disk Drive) 104, A UI (User Interface) unit 105 and a communication unit 106 are provided.

  The CPU 101 is arithmetic means for controlling the above parts according to the procedure described in the control program.

  The RAM 102 is realized by a semiconductor memory, for example, and provides a work area for the CPU 101 to execute arithmetic processing.

  The ROM 103 is realized by a semiconductor memory, for example, and stores various control programs executed by the CPU 101.

  The HDD 104 is realized by a magnetic disk device, for example, and stores various data.

  The UI unit 105 includes, for example, a touch panel and various buttons in addition to the warning output unit 70 described above, and accepts an operation from the user, and notifies the user of information by an image or sound.

  The communication unit 106 is an interface device for performing communication via a network. For example, image data is received from an external device such as the information processing device 2 or the image reading device 3 via a network such as a LAN (Local Area Network).

  Here, the color variation suppression processing unit 30 and the image processing unit 50 shown in FIG. 1 are realized by the CPU 101 shown in FIG. 2 reading the program stored in the ROM 103 into the RAM 102 and executing it. The warning output unit 70 is realized by the UI unit 105, and the storage unit 80 is realized by the RAM 102 and the HDD 104.

  Next, in the image forming apparatus 1, the configurations of the paper transport unit 10, the image forming unit 20, the image reading unit 40, and the paper transport speed measuring unit 60 will be described.

  FIG. 3 is a diagram illustrating a configuration of a portion including a paper transport unit, an image forming unit, an image reading unit, and a paper transport speed measurement unit in the image forming apparatus illustrated in FIG.

  In FIG. 3, the paper transport unit 10, the image forming unit 20, the image reading unit 40, and the paper transport speed measuring unit 60 are shown. 3 also shows a color variation suppression processing unit 30 and an image processing unit 50. Further, a paper storage unit 90 that stores the paper P is also shown.

  The image forming unit 20 includes image forming units 21Y, 21M, 21C, and 21K corresponding to colors of Y (yellow), M (magenta), C (cyan), and K (black), an intermediate transfer member 22, and the like. A secondary transfer unit 24 and a fixing device 25 are provided. Here, in order to avoid complication of the drawings, only the components constituting the image forming unit 21Y are described with reference numerals, but the configurations of the remaining image forming units 21M, 21C, and 21K are also described. The configuration is the same as that of the image forming unit 21Y. Further, the paper conveyance speed measurement unit 60 is provided on the downstream side of the fixing device 25, and the image reading unit 40 is provided on the downstream side of the paper conveyance speed measurement unit 60.

  Hereinafter, the configuration of each unit of the image forming unit 20 will be described in detail.

  The image forming unit 21Y forms a toner image by electrophotography. The image forming unit 21Y includes a photoreceptor 211, a charging unit 212, an exposure unit 213, a developing unit 214, a primary transfer unit 215, and the like. The image forming unit 21Y and the image forming units 21M, 21C, and 21K transfer toner images of respective colors Y (yellow), M (magenta), C (cyan), and K (black) onto the intermediate transfer body 22 (primary transfer). )

  The photoconductor 211 is a drum-like member that rotates at a predetermined speed in the direction of arrow A about the axis. The charging unit 212 charges the surface of the photoconductor 211 to a predetermined potential. The exposure unit 213 irradiates the charged photoconductor 211 with a beam (symbol Bm in the drawing) to form an electrostatic latent image. The developing unit 214 forms a toner image by attaching toner to the electrostatic latent image formed on the photoconductor 211. The primary transfer unit 215 transfers the toner image formed on the photoconductor 211 to the intermediate transfer body 22. A voltage having a polarity opposite to the charging polarity of the toner is applied to the primary transfer unit 215, whereby the toner image on the photosensitive member 211 is sequentially electrostatically attracted to the intermediate transfer member 22, and the intermediate transfer is performed. A color toner image in which the toner images of the respective colors are superimposed on the body 22 is formed.

  The intermediate transfer member 22 is a belt-like member configured to be rotatable in the direction of arrow B shown in the drawing by various rolls. Here, as various types of rolls, a drive roll 221 that is driven by a motor (not shown) to rotate the intermediate transfer body 22, and has a function of giving a constant tension to the intermediate transfer body 22 and preventing meandering of the intermediate transfer body 22. A tension roll 222, an idle roll 223 that supports the intermediate transfer body 22, and a backup roll 242 are provided.

  The paper transport unit 10 includes a pickup roll 11 that picks up and transports the paper P stored in the paper storage unit 90 at a predetermined timing, a transport roll 12 that transports the paper P fed by the pickup roll 11, and a transport roll 12. A transport chute 234 that feeds the paper P transported by the secondary transfer unit 24 to a secondary transfer position, and a transport belt 235 that transports the paper P after the secondary transfer to the fixing unit 25.

  In the secondary transfer unit 24, a secondary transfer roll 241 disposed in pressure contact with the toner image holding surface side of the intermediate transfer body 22 and a counter electrode of the secondary transfer roll 241 disposed on the back surface side of the intermediate transfer body 22 are provided. A backup roll 242 is provided. A brush roll 244 that removes dirt adhering to the secondary transfer roll 241 is disposed in contact with the secondary transfer roll 241. In addition, a metal power supply roll 243 to which a secondary transfer bias is applied is disposed in contact with the backup roll 242.

  Further, a belt cleaner 224 for cleaning the surface of the intermediate transfer body 22 after the secondary transfer is provided on the downstream side of the secondary transfer roll 241. On the other hand, an image density sensor 225 for adjusting image quality is disposed upstream of the secondary transfer roll 241.

  Further, a reference sensor (home position sensor) 226 that generates a reference signal serving as a reference for taking image forming timings in the image forming units 21Y, 21M, 21C, and 21K is disposed upstream of the image forming unit 21Y. ing. The reference sensor 226 recognizes a predetermined mark provided on the back side of the intermediate transfer member 22 and generates a reference signal. Each reference sensor 226 generates a reference signal based on an instruction from the color variation suppression processing unit 30 based on the recognition of the reference signal. The image forming units 21Y, 21M, 21C, and 21K are configured to start image formation.

  The fixing device 25 includes a roll member for heating and pressurizing the paper P, and fixes the toner image transferred onto the paper P.

  Next, the configuration of the image reading unit 40 will be described. As shown in FIG. 3, the image reading unit 40 includes a light source 41, an imaging lens 42, and an image sensor 43.

  The light source 41 irradiates light at a predetermined position of the conveyed paper P.

  The imaging lens 42 forms an image of the reflected light from the paper P at the position of the image sensor 43.

  The image sensor 43 is a line sensor that includes a plurality of unit sensors arranged in the width direction intersecting the conveyance direction of the paper P, and repeatedly reads an image on the paper P while the paper P passes through the image reading unit 40. The image sensor 43 receives the light imaged by the imaging lens 42 and generates and outputs image data corresponding to the amount of received light. The output image data is stored in the storage unit 80 (see FIG. 1).

  FIG. 4 is a flowchart showing an operation example of the image forming apparatus shown in FIG.

  This operation example is an example in which a predetermined number of document images based on one page of document image data are printed. Here, it is assumed that feedback processing for automatically correcting the image forming conditions in the image forming unit 20 is performed as the calibration processing.

When document image data is input from the information processing apparatus 2 or the image reading apparatus 3 to the image forming apparatus 1, the image forming apparatus 1 starts its operation. First, in step S <b> 1, the image processing unit 50 and the color variation suppression processing unit 30 convert the color space of the document image data into the color space of the image forming unit 20. Here, the color space of the document image data input from the information processing apparatus 2 or the image reading apparatus 3 is one of color expression methods according to the convenience of the user (C ₁ , M ₁ , Y _1). , K ₁ ) or the color space of (R ₁ , G ₁ , B ₁ ), which is another one of the color expression methods. The image processing unit 50 changes the color space of the document image data from the color space of (C ₁ , M ₁ , Y ₁ , K ₁ ) or the color space of (R ₁ , G ₁ , B ₁ ). The color is converted to the color space of (C ₂ , M ₂ , Y ₂ , K ₂ ) of the color material used in 20. This color conversion is performed using, for example, a color conversion matrix, a DLUT (Direct Look Up Table), or the like. In this case, the color conversion into the color space of (C ₂ , M ₂ , Y ₂ , K ₂ ) is performed despite being input in the color space of (C ₁ , M ₁ , Y ₁ , K ₁ ). The reason for this is that the color space (C ₁ , M ₁ , Y ₁ , K ₁ ) of the color material of the document image data assumed by the user and the (C ₂ , M ₂ , This is because the case where the color space of Y ₂ , K ₂ ) is different is considered. For _{example, (C 1, M 1,} Y 1, K 1) when the color space is assumed offset printing ink, the toner and the color reproduction characteristics of the image forming apparatus 1 are _different, (C _{2, M} 2, Y ₂ , K ₂ ) color space needs to be converted.

  Next, in step S2, the color variation suppression processing unit 30 instructs the image forming unit 20 to print a document image based on the document image data subjected to color conversion in step S1, and the image forming unit 20 thereby The original image is printed on the paper P.

  Thereafter, the paper P on which the original image is printed is discharged to the outside of the image forming apparatus 1. However, before being discharged, the image reading unit 40 reads the image printed on the paper P in step S3.

  Further, in step S4, the sheet conveyance speed measuring unit 60 measures the traveling speed of the sheet P to obtain sheet conveyance speed data.

  Next, the process proceeds to step S5. In step S5, the image processing unit 50 creates a projection waveform of the read image read in step S3. Here, an example of creating a projection waveform will be described with reference to FIG.

  FIG. 5 is an explanatory diagram of an example of creating a projection waveform.

  The projection waveform in the conveyance direction of the paper P represents the distribution state of black pixels with respect to the coordinates in the conveyance direction of the image by the number of pixels. In other words, the count values of black pixels when pixels having coordinate values in the transport direction are scanned in the width direction are obtained and arranged for each coordinate in the transport direction. In the case of a multi-value image, instead of the number of pixels, the projection waveform may be created by accumulating the gradation value of each pixel. The description will be continued with reference to FIG.

  In step S6, it is determined whether or not printing of the first copy (image formed on the first sheet P) is performed. If it is determined that the first copy is printed, the process proceeds to step S7. In step S7, the sheet conveyance speed data and the projection waveform are stored in the storage unit 80 as the reference sheet conveyance speed data and the reference projection waveform.

In step S8, an image representing the reference sheet conveyance speed data and the reference projection waveform is converted into an arbitrary color space. Specifically, the image is converted into an RGB color space, which is one of the color expression methods. Here, an example of an RGB color space will be described, but an L ^* a ^* b ^* color space, which is another color expression method, may be used. In step S9, an average value in the RGB color space is obtained and stored in the storage unit 80, and the process returns to step S2.

  On the other hand, if it is determined in step S6 that the first copy is not printed, the process proceeds to step S10. In step S10, the expansion / contraction magnification is measured from the comparison of the first and n-th projection waveforms and the comparison of the first and n-th sheet conveyance speed data. Details will be described below.

  First, the first measurement unit 51_1 extracts a plurality of feature points in the transport direction based on the first and n-th projection waveforms, and the plurality of feature points in the plurality of feature points in the transport direction of each feature point. The data is created sequentially.

  FIG. 6 is an explanatory diagram of an example of feature point extraction processing.

  The feature points to be extracted are arbitrary, and may be peaks (peaks, valleys, peaks and valleys) of the projection waveform, edges extracted from the projection waveform using a differential filter, etc., or arbitrary frequency components, Any gradient may be used. In the example shown in FIG. 6, the peak of the projection waveform is shown as an example of the feature point, and peaks (illustrated by white circles) and valleys (illustrated by black circles) are illustrated. Here, the peak of the projection waveform (mountain: illustrated by a white circle) is used as a feature point, and the position on the projection waveform where the peak exists is referred to as data at the feature point (hereinafter referred to as feature data). Thereby, feature amount data in the transport direction of the first image (referred to as a reference image) and feature amount data in the transport direction of the second and subsequent images (referred to as inspection images) are created.

  Next, the feature amount data of the reference image is associated with the feature amount data of the inspection image. As a matching method, for example, DP matching is used.

  FIG. 7 is an explanatory diagram of an example of association of feature amount data by DP matching.

  The horizontal axis of FIG. 7 shows the feature amount data of the reference image. The vertical axis indicates the feature amount data of the inspection image.

  In FIG. 7, as a specific example, the feature amount data of the reference image is {10, 20, 30, 40, 50, 60, 70, 80, 90}, and the feature amount data of the inspection image is {3, 13, 21, 34, 44, 46, 49, 58, 74, 81, 95, 100}. Here, the feature amount data {3, 46, 100} of the inspection image is a false peak generated due to smearing of the printed material, that is, image noise, and the element to be associated with the peak data of the reference image is {13, 21 , 34, 44, 49, 58, 74, 81, 95}. If DP matching is used, it is correctly matched without being affected by the false peak as shown in FIG. DP matching is common and will be briefly described below.

  FIG. 8 is a lattice graph for explaining general DP matching.

Here, the feature amount data of the reference image is P = p ₁ , p ₂ ,..., P _{j ,.}
The feature amount data of the test image _{Q = q 1, q 2,} ..., q k, ..., q K
Then, the similarity (or distance) D (P, Q) is D (P, Q) = min _{k (j)} [Σ _{j = 1} ^J h (j, k)]
It is expressed as Further, the condition of k (j) is as follows.

1. k (j) is a continuous function of j 2. k (j) is a monotonically increasing function of j k (1) = 1, k (J) = K
The partial similarity (or distance) h (j, k) at the position (j, k) on the PQ plane is as follows.

h (j, k) = min [h (j-1, k-1) + 2d (j, k), h (j-1, k) + d (j, k), h (j, k-1) + d (J, k)]
However, d (j, k) = || p _j −q _k ||
By obtaining a path that gives this D (P, Q), the feature amount data P = p ₁ , p ₂ ,..., P _j , p _J of the reference image and the feature amount data Q = q _{1 of the} inspection image _{, q 2, ..., q k} , ..., can be associated with _{q K.} An example of the route thus obtained is shown in FIG. The example shown in FIG. 8 is a general example, and the paths obtained from the specific examples of the feature amount data of the reference image and the feature amount data of the inspection image are shown in FIG.

  FIG. 9 is an explanatory diagram of a specific example of the result of associating feature amount data.

  By associating the feature amount data of the reference image and the feature amount data of the inspection image by DP matching as described above, a result of association as shown in FIG. 9 can be obtained. As described above, with respect to the feature amount data {10, 20, 30, 40, 50, 60, 70, 80, 90} of the reference image, the feature amount data {13, 21, 34, 44, 49, 58, 74, 81, 95} are associated with each other. Returning to FIG. 4 again, the description will be continued.

  In step S10, in this way, the first measurement unit 51_1 extracts a plurality of feature points in the transport direction based on the first and n-th projection waveforms, and the feature points in the transport direction. Find the position on the image. Further, the amount of deviation of each feature point in the transport direction from each reference position is obtained, and the expansion / contraction magnification of each part in the transport direction of the image is measured.

  In step S10, the second measurement unit 51_2 obtains the images on the paper P by reading the traveling speed measurement results of the first and n-th paper P by the paper transport speed measurement unit 60. The expansion / contraction magnification of each part in the conveyance direction of the first and n-th images represented by the image data is measured. Further, the first measurement unit 51_1 and the second measurement unit 51_2 cooperate to calculate the expansion / contraction magnification of each part in the conveyance direction of the first and n-th images. In this way, step S10 shown in FIG. 4 is executed.

  Next, in step S11, the expansion / contraction of the read image is corrected. That is, the reading position / partial magnification correction unit 52 corrects the expansion / contraction of each part in the conveyance direction of the nth image based on the expansion / contraction magnification of each part in the conveyance direction calculated in step S10. Corrected image data representing a corrected image in which the positions in the transport direction of the plurality of feature points extracted for the image are matched with the respective reference positions is generated.

  In step S12, the corrected image is converted into an RGB color space, which is an arbitrary color space, and an average value in the color space is obtained in step S13. That is, the color variation detection unit 53 detects the average color variation (color difference) of the representative color representing the corrected image from the reference color.

  Next, in step S14, it is determined whether or not the average value color variation (color difference) is equal to or less than a predetermined value. That is, the calibration necessity determination unit 54_1 determines whether or not the calibration needs to be performed based on the color variation detected by the color variation detection unit 53. If the average value color variation (color difference) exceeds a predetermined value, the process proceeds to step S15.

  In step S15, a calibration method is selected according to the magnitude of the color difference, and calibration is executed in step S16. That is, the calibration type determination unit 54_2 determines the type of calibration (for example, the change of the laser light amount, the change of the toner charge amount, the change of the gradation value of the image signal, or the like depending on the magnitude of the color difference. The image forming conditions are corrected by instructing the color variation suppression processing unit 30 to execute the determined type of calibration. Further, in step S17, the image forming conditions are corrected and the process returns to step S2. In this way, color variation is suppressed based on the image obtained by printing the first copy and the image obtained by printing the nth copy.

  The image processing unit 50 described above is based on the image data obtained by reading the image formed on the first sheet when sequentially forming the same image based on the same image signal on a plurality of sheets. A plurality of feature points are extracted for the image, the position on the image in the transport direction of each feature point is obtained, and the position of each feature point in the transport direction is set as each reference position. However, the image processing unit may be the following. That is, the image processing unit conveys feature points corresponding to each other obtained from each of the image data read in advance by the line sensor when sequentially forming the same image based on the same image signal on a plurality of sheets. The weighted average position of the direction may be obtained, and the weighted average position may be used as a reference position corresponding to the feature point. For example, the weighting in the conveyance direction of the feature points in the projection waveform of the first copy image is maximized, and the weights of the feature points in the projection waveforms of the images of the second and subsequent copies are sequentially reduced to correspond to each other. The weighted average position in the transport direction of the feature points to be obtained is obtained. The weighted average position obtained in this way may be used as a reference position corresponding to the feature point.

  Note that the image forming apparatus referred to in the present invention may be a copier or a fuchsimily adopting an electrophotographic method as well as a printer adopting an electrophotographic method, and further, for example, exposing a photographic paper with a laser beam. It may be a photographic printer that records an image on a photographic paper by developing the photographic paper, a printer that records an image on a paper by an inkjet method, a printing machine that rotates a rotary press to create a large amount of printed matter, etc. .

  In the present embodiment, the first measurement unit 51_1, the second measurement unit 51_2, and the sheet conveyance speed measurement unit 60 are provided, and an example of calculating the expansion / contraction magnification of each part in the image conveyance direction has been described. The present invention is not limited to this, as long as the first measurement unit measures the expansion / contraction magnification of each part in the image conveyance direction based on the image data obtained by the line sensor.

  Further, in the above embodiment, the feature point is obtained from the projection waveform (see FIGS. 5 and 6) projected in the width direction of the paper, and the image distortion due to the expansion and contraction in the transport direction is corrected. In the case of the transport system in question, in addition to the projection waveform projected in the paper width direction, the projection waveform projected in the paper transport direction is also obtained, the feature points are obtained from both projection waveforms, and each of the paper transport direction and the width direction is determined. A two-dimensional distortion of the image may be corrected by applying a DP matching technique. This distortion correction method is described in Patent Document 4.

1 is a block diagram illustrating a configuration example of an image forming system including an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram illustrating a hardware configuration example of a part that realizes a color variation suppression processing unit, an image processing unit, a warning output unit, and a storage unit that constitute the image forming apparatus illustrated in FIG. 1. FIG. 2 is a diagram illustrating a configuration of a portion including a paper transport unit, an image forming unit, an image reading unit, and a paper transport speed measurement unit in the image forming apparatus illustrated in FIG. 1. 3 is a flowchart illustrating an operation example of the image forming apparatus illustrated in FIG. 1. It is explanatory drawing of an example of preparation of a projection waveform. It is explanatory drawing of an example of a feature point extraction process. It is explanatory drawing of an example of matching of feature-value data by DP matching. It is a lattice graph for demonstrating general DP matching. It is explanatory drawing of the specific example of the matching result of feature-value data.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Information processing apparatus 3 Image reading apparatus 10 Paper conveyance part 11 Pickup roll 12 Conveyance roll 20 Image formation part 21Y, 21M, 21C, 21K Image formation unit 22 Intermediate transfer body 24 Secondary transfer part 25 Fixing device 30 Color Fluctuation suppression processing unit 40 Image reading unit 41 Light source 42 Imaging lens 43 Image sensor 50 Image processing unit 51_1 First measurement unit 51_2 Second measurement unit 52 Reading position / partial magnification correction unit 53 Color variation detection unit 54 Calibration instruction Unit 54_1 calibration necessity determination unit 54_2 calibration type determination unit 60 sheet conveyance speed measurement unit 70 warning output unit 80 storage unit 90 sheet storage unit 101 CPU
102 RAM
103 ROM
104 HDD
105 UI part 106 Communication part 211 Photoconductor 212 Charging part 213 Exposure part 214 Development part 215 Primary transfer part 221 Driving roll 222 Tension roll 223 Idle roll 224 Belt cleaner 225 Image density sensor 226 Reference sensor (home position sensor)
234 Conveying chute 235 Conveying belt 241 Secondary transfer roll 242 Backup roll 243 Power feeding roll 244 Brush roll

Claims (7)

Paper transport means for transporting paper,
An image forming unit that is provided on a paper transport path through which the paper is transported by the paper transport unit and forms an image on the transported paper;
A color variation suppression processing unit that executes a color variation suppression operation for suppressing color variation of an image formed by the image forming unit;
An image reading unit that is disposed on the downstream side of the image forming unit on the paper transport path and reads an image on a sheet on which an image is formed in the image forming unit;
An image processing unit that detects color variation by processing image data obtained by reading by the image reading unit and instructs the color variation suppression processing unit to suppress color variation;
The image reading unit includes a plurality of unit sensors arranged in a width direction intersecting the conveyance direction of the paper, and has a line sensor that repeatedly reads an image on the paper while the paper passes through the image reading unit,
The image processing unit
Based on the image data obtained by the line sensor, a plurality of feature points are extracted in the transport direction to obtain positions on the image of the feature points in the transport direction, and the positions of the feature points in the transport direction A first measuring unit that measures the expansion / contraction magnification of each part in the conveyance direction of the image by obtaining a deviation amount from each reference position;
By correcting the expansion / contraction of each part in the conveyance direction of the image based on the magnification of the expansion / contraction of each part in the conveyance direction of the image represented by the image data measured by the first measurement unit, the image A correction unit that generates corrected image data representing a corrected image in which the positions of the extracted plurality of feature points in the transport direction are matched with the respective reference positions;
A detection unit that detects a color variation from a reference color of a representative color representing the corrected image;
An image forming apparatus comprising: a color variation suppression operation instruction unit that instructs the color variation suppression processing unit to execute the color variation suppression operation based on the color variation detected by the detection unit. The color variation suppression operation instruction unit is
A necessity determination unit that determines whether or not to perform the color variation suppression operation based on the color variation detected by the detection unit;
In response to the determination that the necessity determination unit needs to perform the color variation suppression operation, based on the color variation, the type of color variation suppression operation is determined and the color variation suppression processing unit is determined. The image forming apparatus according to claim 1, further comprising: a type determining unit that instructs execution of a type color variation suppressing operation.   Based on the image data obtained by the line sensor, the first measurement unit obtains a plurality of cumulative values of the pixel values of the pixels arranged in the width direction in the image represented by the image data in the transport direction. The image forming apparatus according to claim 1, wherein a projection waveform is obtained thereby, and the plurality of feature points are extracted based on the projection waveform.   The image processing unit is configured to perform the transport direction based on image data obtained by reading an image formed on a first sheet when sequentially forming the same image based on the same image signal on a plurality of sheets. The feature point is obtained by extracting a plurality of feature points to obtain positions of the feature points on the image in the transport direction, and using the positions of the feature points in the transport direction as the reference positions. The image forming apparatus according to any one of 1 to 3.   The image processing unit conveys the feature points corresponding to each other obtained from each of the image data read in advance by the line sensor when sequentially forming the same image based on the same image signal on a plurality of sheets. 4. The image forming apparatus according to claim 1, wherein a weighted average position in a direction is obtained and the weighted average position is set as the reference position corresponding to the feature point. A speed measuring unit that measures the traveling speed of the paper when the image on the paper is read by the image reading unit for at least a part of the paper;
Second measurement for measuring the expansion / contraction magnification of each part in the transport direction of the image represented by the image data obtained by reading the image on the sheet based on the result of the sheet traveling speed measurement by the speed measuring unit. Part
The first measurement unit and the second measurement unit cooperate or share the expansion / contraction magnification of each part in the transport direction of the image represented by the image data obtained by reading the image on the paper in the image. Calculated,
The correction unit is configured to calculate the image based on the expansion / contraction magnification of each part in the transport direction of the image represented by the image data, which is calculated by the first measurement unit and the second measurement unit in cooperation or sharing. By correcting the expansion and contraction of each part in the transport direction, corrected image data representing a corrected image in which the positions in the transport direction of a plurality of feature points extracted for the image are matched with the respective reference positions is generated. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.   The apparatus further comprises a warning output unit that compares the partial magnification measured by the first measurement unit and the partial magnification measured by the second measurement unit and outputs a warning when there is a difference equal to or greater than a threshold value. The image forming apparatus according to claim 6.

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