Image inspection apparatus, image forming apparatus, image inspection method, and program

Abstract

To improve the accuracy of detecting positional misalignment. [Solution] The image forming apparatus (image inspection apparatus) reads a recording medium on which an image has been formed and compares the resulting read image with a reference image to detect a misalignment of the image formation position on the recording medium. The control unit of the image forming apparatus aligns the image contained in the read image with the image contained in the reference image, and, in the aligned state, detects the misalignment of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image. Based on the position of the region on the recording medium in which the image is formed, the control unit selects a reference position to be used to detect the misalignment from among the reference positions contained in the read image and the reference image.

Description

Technical Field

[0001] The present invention relates to an image inspection apparatus, an image forming apparatus, an image inspection method, and a program.

Background Art

[0002] In an image forming apparatus (printer), adjusting so as to be able to form an image at a predetermined position on a recording medium is called registration. Conventionally, since a cross mark (a cross) is used as a registration mark for registration, registration is also called cross alignment. In registration, it is required to form a cross at a predetermined position on the recording medium and measure the amount of deviation of the position where the cross is actually formed. As registration marks, in addition to crosses, quadrilaterals, circles, etc. are used.

[0003] Patent Document 1 discloses a technique for detecting the positional deviation between a cut-in pattern and a printed pattern by reading the positions of the registration marks for the cut-in pattern and the registration marks for the printed pattern. According to this technique, it is not necessary for a person to visually confirm whether the position of the printed pattern is correct. Furthermore, by performing automatic control on the position of the printed pattern using the detected deviation amount, manual adjustment work becomes unnecessary.

[0004] By the way, the method for detecting positional deviation using a registration mark (image for position detection) cannot be used for jobs where a registration mark cannot be formed. Also, it takes time to add a registration mark to an image. Further, the registration mark used for detecting positional deviation needs to be finally cut off and removed, resulting in waste of the recording medium.

[0005] To address this problem, a method can be conceived to detect positional misalignment without using registration marks by comparing the image to be inspected, formed on a recording medium, with a pre-registered reference image. In this method, for example, the read image generated by reading the recording medium on which the image to be inspected is formed, and the corresponding areas in the region where the image actually exists, are found between the read image and the reference image, and the images are aligned. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2001-270086 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, there was a problem in that the accuracy of alignment decreased when there was a large blank area in the recording medium where no image existed. For example, consider the case where the printing area 111 on which the image is actually formed exists only at the top of the recording medium 110, as shown in Figure 15. In this case, when the read image and the reference image are aligned using the upper printing area 111 where the image exists as a reference, the variation in the lower corners T13 and T14 where the image does not exist becomes larger compared to the upper corners T11 and T12. As a result, the amount of misalignment at the lower corners T13 and T14 may exceed the threshold used to determine the misalignment, which could lead to a decrease in the accuracy of detecting the misalignment.

[0008] The present invention has been made in view of the problems in the prior art described above, and aims to improve the accuracy of detecting positional misalignment. [Means for solving the problem]

[0009] To solve the above problems, the invention described in claim 1 is an image inspection device that detects a misalignment of the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, the device comprising a control unit that aligns an image contained in the read image with an image contained in the reference image, and in the aligned state, detects the misalignment of the image formation position based on the misalignment between the reference position in the read image and the reference position in the reference image, the control unit selects a reference position to be used to detect the misalignment from among the reference positions contained in the read image and the reference image, based on the position of the region on the recording medium in which an image is formed.

[0010] The invention described in claim 2 is the image inspection apparatus described in claim 1, wherein the reference position is the four corners of the recording medium.

[0011] The invention described in claim 3 is an image inspection apparatus according to claim 2, wherein the control unit calculates the positions of the four corners of the recording medium based on the edge information of the recording medium.

[0012] The invention described in claim 4 is an image inspection apparatus according to claim 2, wherein the control unit excludes the reference position on the edge side of the recording medium from the reference position used to detect the misalignment when the distance from the region where the image is formed to the edge of the recording medium in the predetermined direction is greater than a preset threshold.

[0013] The invention described in claim 5 is an image inspection apparatus according to claim 2, wherein the control unit excludes reference positions whose distance from the region where the image is formed is greater than or equal to a preset threshold from the reference positions used to detect the positional misalignment.

[0014] The invention described in claim 6 is an image inspection apparatus according to claim 5, wherein the distance from the region where the image is formed to the reference position is the distance between the position in the region where the image is formed that is closest to the reference position and the reference position.

[0015] The invention described in claim 7 is an image inspection apparatus according to claim 2, wherein the control unit selects at least two of the four corners of the recording medium as reference positions to be used for detecting the positional misalignment.

[0016] The invention described in claim 8 is an image forming apparatus comprising: an image inspection apparatus according to any one of claims 1 to 7; an image forming unit for forming an image on the recording medium; and a reading unit for reading the recording medium on which the image has been formed and generating a read image.

[0017] The invention described in claim 9 is an image inspection method for detecting a misalignment of the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, the method comprising: aligning an image included in the read image with an image included in the reference image; detecting the misalignment of the image formation position based on the misalignment between a reference position in the read image and a reference position in the reference image in the aligned state, and further comprising selecting a reference position to be used to detect the misalignment from among the reference positions included in the read image and the reference image, based on the position of the region on which an image is formed on the recording medium.

[0018] The invention described in claim 10 is a program for causing a computer to function as a control unit that detects a positional shift of the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, aligns an image contained in the read image with an image contained in the reference image, and, in the aligned state, detects the positional shift of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image, wherein the control unit selects a reference position to be used to detect the positional shift from among the reference positions contained in the read image and the reference image, based on the position of the region on the recording medium in which an image is formed. [Effects of the Invention]

[0019] According to the present invention, the accuracy in detecting misalignment can be improved.

Brief Description of Drawings

[0020] [Figure 1] It is a schematic configuration diagram of an image forming apparatus in an embodiment of the present invention. [Figure 2] It is a block diagram showing a functional configuration of the image forming apparatus. [Figure 3] It is a flowchart showing misalignment detection processing executed in the image forming apparatus. [Figure 4] It is a diagram for explaining image positions serving as criteria for image matching in a reference image and a read image. [Figure 5] It is a diagram for explaining a method of detecting four corners of a recording medium from a reference image and a read image after alignment. [Figure 6] It is a diagram showing an image for comparing points at four corners of a recording medium in a reference image after alignment with points at four corners of the recording medium in a read image. [Figure 7] It is a flowchart showing reference position selection processing executed in the image forming apparatus. [Figure 8] It is a flowchart showing reference position selection processing executed in the image forming apparatus. [Figure 9] It is a flowchart showing reference position selection processing executed in the image forming apparatus. [Figure 10] It is an example of an image for explaining the position of a printing area with respect to a recording medium. [Figure 11] It is an example of an image for explaining the position of a printing area with respect to a recording medium. [Figure 12] It is an example of an image for explaining the position of a printing area with respect to a recording medium. [Figure 13] It is an example of an image for explaining the position of a printing area with respect to a recording medium. [Figure 14]This is an example image illustrating the position of the printing area on the recording medium. [Figure 15] This diagram illustrates the problems that arise when the printing area exists only at the top of the recording medium. [Modes for carrying out the invention]

[0021] Embodiments of the present invention will be described below with reference to the drawings. The advantages and features provided by the embodiments will be understood from the following detailed description and drawings. However, the scope of the present invention is not limited to the embodiments or illustrations disclosed below.

[0022] Figure 1 is a schematic diagram of the image forming apparatus 100 as an image inspection apparatus and image forming apparatus in this embodiment. The image forming apparatus 100 forms a color image on a recording medium such as paper using an electrophotographic method. The image forming apparatus 100 forms an image based on image data obtained by reading an image from a document, or image data received from an external device. The image forming apparatus 100 includes an operation unit 10, a display unit 20, a document reading unit 30, an image forming unit 40, a supply unit 50, a post-processing unit 60, an image reading unit 70, and the like.

[0023] The operation unit 10 outputs operation signals based on user operations to the control unit 81 (see Figure 2). The operation unit 10 is equipped with various operation keys and a touchscreen. The various operation keys receive various instruction operations from the user. The touchscreen is formed to cover the display screen of the display unit 20. The touchscreen receives touch operations on the display screen and detects the touch position.

[0024] The display unit 20 is composed of an LCD (Liquid Crystal Display). The display unit 20 displays various screens according to the instructions of the display signals input from the control unit 81.

[0025] The document scanning unit 30 includes an ADF (Automatic Document Feeder), a scanner, etc. The document scanning unit 30 reads the image of the document and outputs the resulting image data to the control unit 81.

[0026] The image forming unit 40 forms an image on the recording medium supplied from the supply unit 50. The image forming unit 40 is equipped with photosensitive drums 41Y, 41M, 41C, and 41K corresponding to yellow, magenta, cyan, and black, respectively. The image forming unit 40 further includes an intermediate transfer belt 42, a secondary transfer roller 43, a fixing unit 44, an inversion mechanism 45, and the like.

[0027] The image forming unit 40 uniformly charges the photoreceptor drum 41Y, then scans and exposes it with a laser beam based on yellow image data to form an electrostatic latent image. Next, the image forming unit 40 deposits yellow toner onto the electrostatic latent image on the photoreceptor drum 41Y, thereby developing the electrostatic latent image. The photoconductor drums 41M, 41C, and 41K are the same as the photoconductor drum 41Y, except for the different colors they handle, so we will omit their explanation.

[0028] The image forming unit 40 sequentially transfers the toner images of each color formed on the photoreceptor drums 41Y, 41M, 41C, and 41K onto the rotating intermediate transfer belt 42 (primary transfer). That is, a color toner image is formed on the intermediate transfer belt 42 by superimposing the toner images of the four colors. The image forming unit 40 then transfers the color toner image on the intermediate transfer belt 42 onto the recording medium all at once using the secondary transfer roller 43 (secondary transfer). The fixing unit 44 fixes the color toner image onto the recording medium by heating and pressurizing. The inversion mechanism 45 inverts the surface of the recording medium when forming images on both sides of the recording medium.

[0029] The supply unit 50 is equipped with supply trays 51 to 53 and supplies recording media to the image forming unit 40. Each supply tray 51 to 53 contains a predetermined type and size of recording media.

[0030] The post-processing unit 60 performs post-processing on the recording medium on which the image has been formed by the image forming unit 40, as needed. For example, post-processing may include cutting, sorting, stapling, punching, folding, and binding. The post-processing unit 60 discharges the recording medium on which the image has been formed into the discharge trays 61 and 62. Alternatively, the post-processing unit 60 stores the recording medium on which the image has been formed in the large-capacity stacker 63.

[0031] The image reading unit 70 (reading unit) is located downstream of the image forming unit 40 in the transport direction of the recording medium (Y direction shown in Figure 1). The image reading unit 70 reads the transported recording medium and generates a read image (image data). The image reading unit 70 outputs the generated read image to the control unit 81. The image reading unit 70 is a color sensor that receives light emitted from a light source and reflected from the surface of the recording medium with a photodetector and outputs a signal according to the intensity of the light. The image reading unit 70 is composed of a line sensor in which multiple photodetectors are arranged at predetermined intervals in a direction perpendicular to the transport direction of the recording medium (Y direction) (X direction shown in Figure 1). For example, the image reading unit 70 is used to read an image formed on the recording medium by the image forming unit 40 and to inspect the position of the image, the color of the image, dirt, etc.

[0032] Figure 2 is a block diagram showing the functional configuration of the image forming apparatus 100. As shown in Figure 2, the image forming apparatus 100 includes an operation unit 10, a display unit 20, a document reading unit 30, an image forming unit 40, a supply unit 50, a post-processing unit 60, an image reading unit 70, a control unit 81, a storage unit 82, a communication unit 83, a transport unit 84, and the like. Note that the functional units already described will not be explained further.

[0033] The control unit 81 includes a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc. The CPU reads various processing programs stored in the ROM in response to operation signals input from the operation unit 10 or instruction signals received by the communication unit 83, and loads the read programs into the RAM. The CPU centrally controls the operation of each part of the image forming apparatus 100 according to the loaded programs.

[0034] The storage unit 82 is composed of non-volatile storage devices such as hard disks and flash memory, and stores various types of data. For example, a reference value V is pre-stored in the storage unit 82.

[0035] The communication unit 83 transmits and receives data to and from external devices connected to a communication network such as a LAN (Local Area Network).

[0036] The transport unit 84 is equipped with transport rollers for transporting recording media. The transport unit 84 transports recording media within the image forming apparatus 100. For example, the transport unit 84 transports recording media stored in the supply trays 51-53 of the supply unit 50 to the image forming unit 40. The transport unit 84 transports recording media on which images are formed by the image forming unit 40. The transport unit 84 transports recording media on which images are read by the image reading unit 70. The transport unit 84 transports the recording media after image formation to the discharge trays 61, 62 or the large-capacity stacker 63.

[0037] The control unit 81 acquires a read image generated by reading the recording medium on which an image has been formed by the image forming unit 40 using the image reading unit 70.

[0038] The control unit 81 compares the read image generated by reading the recording medium on which the image has been formed with a reference image to detect a positional shift in the image formation position on the recording medium. The reference image is pre-registered in the storage unit 82. The reference image may be a RIP image (RIP data), or it may be an image generated by reading an image formed on a recording medium (an image without positional shifts or abnormalities). Positional misalignment includes deviations from the position where the image should be formed on the recording medium, front-to-back positional misalignment during double-sided printing, color misalignment, etc.

[0039] The control unit 81 aligns the image contained in the read image with the image contained in the reference image. With the images aligned, the control unit 81 detects the positional shift of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image.

[0040] The reference position is a point (position) within the read image and the reference image that is used as a reference to detect positional shifts in the image formation position by comparing the read image with the reference image. In this embodiment, the four corners of the recording medium are used as the reference position.

[0041] The control unit 81 selects a reference position to be used for detecting positional misalignment from among the reference positions included in the read image and the reference image, based on the position of the area in which an image is formed on the recording medium.

[0042] The control unit 81 calculates the positions of the four corners of the recording medium based on the edge information of the recording medium. The edge information includes the position of the edge, the direction of the edge, etc. For example, the control unit 81 detects the boundary line (edge) between the recording medium area and the background area from the read image. The recording medium area is the area in the image corresponding to the recording medium. The background area is the area outside the recording medium in the image. Specifically, the control unit 81 determines a threshold that separates the luminance values ​​corresponding to the recording medium area from the luminance values ​​corresponding to the background area from the distribution of luminance values ​​(histogram, etc.) in the read image. The control unit 81 uses this threshold to detect the edges of the recording medium in the read image. Since the recording medium is usually rectangular, the control unit 81 detects four straight lines from the read image as the edges of the recording medium. The control unit 81 obtains the positions of four points as the intersections of the four straight lines, and these four points are used as the positions of the four corners of the recording medium.

[0043] The control unit 81 determines whether the distance from the area where an image is formed to the edge of the recording medium in a predetermined direction is greater than a preset threshold. If the distance from the area where an image is formed to the edge of the recording medium in a predetermined direction is greater than the threshold, the control unit 81 excludes the reference position on the edge side of the recording medium from the reference position used to detect misalignment. The predetermined direction is, for example, the transport direction (sub-scanning direction) of the recording medium. Alternatively, the predetermined direction may be the width direction (main scanning direction) perpendicular to the transport direction of the recording medium. When using either the transport direction or the width direction of the recording medium as the predetermined direction, it is desirable to use the direction along the long side of the recording medium as the predetermined direction.

[0044] The control unit 81 selects a reference position that is relatively close to the area where the image is formed as the reference position used to detect positional misalignment. The control unit 81 excludes reference positions that are relatively close to the area where the image is formed from the reference positions used to detect positional misalignment. Here, the distance from the "area where the image is formed" to a certain reference position is the distance between the position in the "area where the image is formed" that is closest to the reference position and the reference position itself.

[0045] The control unit 81 selects at least two of the four corners of the recording medium as reference positions to be used for detecting misalignment.

[0046] The control unit 81 determines whether or not there is a positional misalignment of the image formed by the image forming unit 40 by determining the amount of deviation from the target position of the image formed on the recording medium. For example, the control unit 81 detects the front-to-back positional misalignment during double-sided printing based on the amount of deviation.

[0047] Next, the operation of the image forming apparatus 100 will be described. Figure 3 is a flowchart showing the positional misalignment detection process performed in the image forming apparatus 100. In the positional misalignment detection process, positional misalignment is detected by comparing it with a reference image without using registration marks (position detection images) such as registration marks. This process is realized by software processing through the cooperation of the CPU of the control unit 81 and the program stored in ROM.

[0048] The left panel of Figure 4 shows an example of a reference image A1. Reference image A1 includes a recording medium area 121 and a background area 122. Even when reference image A1 is a RIP image, a background area 122 corresponding to the area outside the recording medium area 121 is added to reference image A1. Note that the X direction shown in Figure 4 is the direction perpendicular to the transport direction of the recording medium (main scanning direction). The Y direction shown in Figure 4 is the transport direction of the recording medium (sub-scanning direction). The same applies to the X and Y directions shown in Figure 5.

[0049] First, the control unit 81 searches for image formation locations within the reference image A1 and obtains image positions O1 to O4, which will serve as reference points (model points) for image matching (step S1). At the same time, the control unit 81 obtains an image of the area surrounding image positions O1 to O4 as a template image. Image formation locations are the regions where images are actually formed. It is desirable that image positions O1 to O4 be located close to each of the four corners of the recording medium within the region where images are formed. The closer image positions O1 to O4 are to the four corners of the recording medium, the higher the accuracy of detecting positional misalignment. It is also desirable that the template image be an image suitable for template matching.

[0050] Next, the control unit 81 controls the image forming unit 40 to form the image to be inspected on the recording medium (step S2). The image to be inspected is an image corresponding to the reference image A1, and is an image that is inspected with reference image A1 as the reference.

[0051] Next, the control unit 81 controls the image reading unit 70 to read the recording medium on which the image to be inspected has been formed by the image forming unit 40, and generates a read image B1. The control unit 81 controls the transport unit 84 to transport the recording medium and cause the image reading unit 70 to read the recording medium. The control unit 81 obtains the read image B1 generated by reading the recording medium on which the image to be inspected has been formed from the image reading unit 70 (step S3). An example of the read image B1 is shown in the right-hand figure of Figure 4. The read image B1 includes a recording medium area 131 and a background area 132.

[0052] Next, the control unit 81 performs template matching on the read image B1 based on image positions O1 to O4 to obtain image positions P1 to P4 of the read image B1 (step S4). Specifically, the control unit 81 uses the template image obtained from the reference image A1 and image positions O1 to O4 to find the part of the read image B1 that has the highest similarity to the template image. The control unit 81 sets the points in the read image B1 that correspond to image positions O1 to O4 as image positions P1 to P4.

[0053] Next, the control unit 81 aligns the reference image A1 with the read image B1 based on the information of image positions O1 to O4 and image positions P1 to P4 (step S5). Specifically, the control unit 81 rotates, translates, etc., the reference image A1, including the edges of the recording medium, so that each of the image positions O1 to O4 in the reference image A1 overlaps with each of the image positions P1 to P4 in the read image B1. In this way, the control unit 81 aligns the image contained in the reference image A1 with the image contained in the read image B1. The left figure of Figure 5 shows the reference image A2 after alignment.

[0054] Next, as shown in the left diagram of Figure 5, the control unit 81 determines the four corner points Q1 to Q4 (coordinates) of the recording medium (recording medium area 121) from the reference image A2 after alignment (step S6). Specifically, the control unit 81 detects four straight lines as edges of the recording medium area 121 from the reference image A2 after alignment. The control unit 81 obtains the positions of four points as the intersections of the four straight lines, and these four points are designated as the positions of the four corners of the recording medium area 121 (points Q1 to Q4). Points Q1 to Q4 are the reference positions in the reference image A2 after alignment.

[0055] Next, as shown in the right-hand diagram of Figure 5, the control unit 81 determines the four corner points R1 to R4 (coordinates) of the recording medium (recording medium area 131) from the read image B1 (step S7). Specifically, the control unit 81 detects four straight lines as edges of the recording medium area 131 from the read image B1. The control unit 81 obtains the positions of four points as the intersections of the four straight lines, and these four points are designated as the positions of the four corners (points R1 to R4) of the recording medium area 131. Points R1 to R4 are the reference positions in the read image B1.

[0056] Next, the control unit 81 calculates the distance between points Q1 to Q4 in the reference image A2 after alignment and points R1 to R4 in the read image B1 as the displacement amounts D1 to D4 (step S8). The control unit 81 calculates the displacement amounts D1 to D4 from the difference between points Q1 to Q4 and points R1 to R4 in the X and Y directions, respectively. Figure 6 shows an image comparing point Q1 and point R1. The distance between point Q1 and point R1 is the displacement amount D1.

[0057] Next, the control unit 81 determines whether at least one of the displacement amounts D1 to D4 is greater than or equal to a preset reference value V (step S9). If all of the displacement amounts D1 to D4 are less than the reference value V (step S9; NO), the control unit 81 determines that there is no positional displacement (step S10).

[0058] In step S9, if at least one of the displacement amounts D1 to D4 is greater than or equal to the reference value V (step S9; YES), the control unit 81 determines that there is a positional displacement (step S11). After step S10 or step S11, the positional misalignment detection process is completed.

[0059] The above explanation described the case where all four corners of the recording medium are used as reference points for detecting misalignment. Next, we will explain how to determine which of the four corners of the recording medium to select as reference points for detecting misalignment, and which points to exclude from being used as reference points for detecting misalignment.

[0060] Figures 7 to 9 are flowcharts showing the reference position selection process performed in the image forming apparatus 100. The reference position selection process is performed prior to the positional misalignment detection process (see Figure 3). The reference position selection process is realized by software processing through the cooperation of the CPU of the control unit 81 and the program stored in ROM.

[0061] First, the control unit 81 acquires the area on the recording medium where an image is formed (printing area) from the target image (step S21). Specifically, the control unit 81 performs a binarization process on the recording medium area of ​​the reference image using a predetermined threshold and extracts pixels on which an image is formed. The control unit 81 acquires the smallest rectangle surrounding the pixels on which an image is formed as the printing area. Here, the acquired printing area is a rectangular area enclosed by straight lines along the transport direction and width direction of the recording medium.

[0062] Figures 10 to 14 show examples of images (reference images) with different positions of the print area 91 relative to the recording medium 90. Let L1 be the distance between the print area 91 and the upper edge of the recording medium 90 in the transport direction of the recording medium 90 (vertical direction in Figures 10 to 14), and L2 be the distance between the print area 91 and the lower edge of the recording medium 90 in the transport direction of the recording medium 90. Also, let W1 be the distance between the print area 91 and the left edge of the recording medium 90 in the width direction perpendicular to the transport direction of the recording medium 90 (horizontal direction in Figures 10 to 14), and W2 be the distance between the print area 91 and the right edge of the recording medium 90 in the width direction of the recording medium 90. Furthermore, let H be the length of the recording medium 90 in the transport direction, and W be the length of the recording medium 90 in the width direction.

[0063] Furthermore, let a be the distance between the top-left vertex of the printing area 91 (rectangle) and the top-left corner T1 of the recording medium 90. Let b be the distance between the top-right vertex of the printing area 91 and the top-right corner T2 of the recording medium 90. Let c be the distance between the bottom-left vertex of the printing area 91 and the bottom-left corner T3 of the recording medium 90. Let d be the distance between the bottom-right vertex of the printing area 91 and the bottom-right corner T4 of the recording medium 90.

[0064] The control unit 81 calculates the distance L1 between the printing area 91 and the upper edge of the recording medium 90 in the transport direction of the recording medium 90, and the distance L2 between the printing area 91 and the lower edge of the recording medium 90 (step S22). The control unit 81 also calculates the distance W1 between the printing area 91 and the left edge of the recording medium 90 in the width direction of the recording medium 90, and the distance W2 between the printing area 91 and the right edge of the recording medium 90 (step S22).

[0065] Next, the control unit 81 calculates the distances a, b, c, and d from the printing area 91 to the four corners T1 to T4 of the recording medium 90 (step S23).

[0066] Next, the control unit 81 determines whether the distance L1 between the printing area 91 and the upper end of the recording medium 90 in the transport direction of the recording medium 90 is greater than half the length H of the recording medium 90 in the transport direction (step S24). If L1 is greater than H / 2, the control unit 81 determines that there is no image in the upper half of the recording medium 90, and if L1 is less than H / 2, it determines that there is an image in the upper half of the recording medium 90.

[0067] If L1 is greater than H / 2 (step S24; YES), the process proceeds to step S25. In step S25, the control unit 81 determines whether the distance W1 between the print area 91 and the left edge of the recording medium 90 in the width direction is greater than half the length W of the recording medium 90 in the width direction. If W1 is greater than W / 2, the control unit 81 determines that there is no image in the left half of the recording medium 90, and if W1 is less than W / 2, the control unit 81 determines that there is an image in the left half of the recording medium 90.

[0068] If W1 is greater than W / 2 (step S25; YES), the control unit 81 uses the lower right corner T4 and one or more points from the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 for positional misalignment determination (step S26). The control unit 81 selects the lower right corner T4 and one or more points as reference positions to be used for detecting positional misalignment.

[0069] In step S25, if W1 is less than or equal to W / 2 (step S25; NO), the control unit 81 determines whether the distance W2 between the printing area 91 in the width direction of the recording medium 90 and the right edge of the recording medium 90 is greater than W / 2 (step S27). If W2 is greater than W / 2, the control unit 81 determines that there is no image in the right half of the recording medium 90, and if W2 is less than W / 2, the control unit 81 determines that there is an image in the right half of the recording medium 90.

[0070] If W2 is greater than W / 2 (step S27; YES), the control unit 81 uses the lower left corner T3 and one or more points from the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 for positional misalignment determination (step S28). The control unit 81 selects the lower left corner T3 and one or more points as reference positions to be used for detecting positional misalignment.

[0071] In step S27, if W2 is less than or equal to W / 2 (step S27; NO), the control unit 81 uses the lower left corner T3 and the lower right corner T4 as points closest to the printing area 91 among the four corners T1 to T4 of the recording medium 90 for positional misalignment determination (step S29). The control unit 81 selects the lower left corner T3 and the lower right corner T4 as reference positions to be used for detecting positional misalignment.

[0072] In step S24, if L1 is less than or equal to H / 2 (step S24; NO), the process proceeds to step S30 in Figure 8. In step S30, the control unit 81 determines whether the distance L2 between the printing area 91 and the lower end of the recording medium 90 in the transport direction of the recording medium 90 is greater than H / 2. If L2 is greater than H / 2, the control unit 81 determines that there is no image in the lower half of the recording medium 90, and if L2 is less than H / 2, it determines that there is an image in the lower half of the recording medium 90.

[0073] If L2 is greater than H / 2 (step S30; YES), the control unit 81 determines whether the distance W1 between the printing area 91 and the left edge of the recording medium 90 in the width direction of the recording medium 90 is greater than W / 2 (step S31).

[0074] If W1 is greater than W / 2 (step S31; YES), the control unit 81 uses the upper right corner T2 and one or more points from the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 for positional misalignment determination (step S32). The control unit 81 selects the upper right corner T2 and one or more points as reference positions to be used for detecting positional misalignment.

[0075] Figure 11 is an example of an image corresponding to step S32. In the example shown in Figure 11, L2 is greater than H / 2 and W1 is greater than W / 2. In other words, there is no image in the lower half of the recording medium 90 and no image in the left half of the recording medium 90. The printing area 91 is located slightly to the right of the recording medium 90. First, the control unit 81 selects the upper right corner T2 as the reference position. Next, the control unit 81 compares the distance a from the printing area 91 to the upper left corner T1 and the distance d from the printing area 91 to the lower right corner T4, and selects the lower right corner T4, which has a shorter distance, as the reference position. In this way, the control unit 81 uses the upper right corner T2 and the lower right corner T4 as reference positions for detecting positional misalignment.

[0076] Figure 12 is an example of an image corresponding to step S32. In the example shown in Figure 12, L2 is greater than H / 2 and W1 is greater than W / 2. In other words, there is no image in the lower half of the recording medium 90 and no image in the left half of the recording medium 90. The printing area 91 is located slightly to the right of the recording medium 90. First, the control unit 81 selects the upper right corner T2 as the reference position. Next, the control unit 81 compares the distance a from the printing area 91 to the upper left corner T1 with the distance d from the printing area 91 to the lower right corner T4, and since distance a and distance d are the same, it selects both the upper left corner T1 and the lower right corner T4. In this way, the control unit 81 uses the upper left corner T1, the upper right corner T2 and the lower right corner T4 as reference positions for detecting misalignment.

[0077] In step S31, if W1 is less than or equal to W / 2 (step S31; NO), the control unit 81 determines whether the distance W2 between the printing area 91 and the right edge of the recording medium 90 in the width direction of the recording medium 90 is greater than W / 2 (step S33).

[0078] If W2 is greater than W / 2 (step S33; YES), the control unit 81 uses the upper left corner T1 and one or more points from the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 for positional misalignment determination (step S34). The control unit 81 selects the upper left corner T1 and one or more points as reference positions to be used for detecting positional misalignment.

[0079] In step S33, if W2 is less than or equal to W / 2 (step S33; NO), the control unit 81 uses the upper left corner T1 and the upper right corner T2 as points closest to the printing area 91 among the four corners T1 to T4 of the recording medium 90 for positional misalignment determination (step S35). The control unit 81 selects the upper left corner T1 and the upper right corner T2 as reference positions to be used for detecting positional misalignment.

[0080] Figure 10 is an example of an image corresponding to step S35. In the example shown in Figure 10, L2 is greater than H / 2, and there is no image in the lower half of the recording medium 90. Also, W1 is less than or equal to W / 2, and W2 is less than or equal to W / 2. The control unit 81 excludes the two corners T3 and T4 on the lower end, where it is determined that there is no image, from the reference positions used for detecting misalignment. The control unit 81 uses the two corners T1 and T2 on the upper end of the four corners of the recording medium 90 as reference positions used for detecting misalignment. The control unit 81 may also select the two upper corners T1 and T2 from among the distances a to d between the printing area 91 and the four corners T1 to T4 of the recording medium 90, on the grounds that distance a and b are the shortest.

[0081] In step S30, if L2 is less than or equal to H / 2 (step S30; NO), the process proceeds to step S36 in Figure 9. In step S36, the control unit 81 determines whether the distance W1 between the printing area 91 and the left edge of the recording medium 90 in the width direction of the recording medium 90 is greater than W / 2.

[0082] If W1 is greater than W / 2 (step S36; YES), the control unit 81 uses the upper right corner T2 or the lower right corner T4 and one or more points from among the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 to determine the misalignment (step S37). The control unit 81 selects the upper right corner T2 or the lower right corner T4 and one or more points as reference positions to be used for detecting the misalignment.

[0083] Figure 13 shows an example of an image corresponding to step S37. In the example shown in Figure 13, L1 is less than or equal to H / 2, L2 is less than or equal to H / 2, and W1 is greater than W / 2. In other words, there is no image in the left half of the recording medium 90. The printing area 91 is located slightly to the right of the recording medium 90. First, the control unit 81 compares the distance b from the printing area 91 to the upper right corner T2 and the distance d from the printing area 91 to the lower right corner T4, and selects the corner with the shorter distance as the reference position. Next, the control unit 81 selects the corner with the shortest distance from the printing area 91 among the remaining three corners as the reference position. In the example shown in Figure 13, since distance b and distance d are the same, the control unit 81 selects the upper right corner T2 and the lower right corner T4. Thus, the control unit 81 uses the upper right corner T2 and the lower right corner T4 as reference positions for detecting positional misalignment. The control unit 81 may also select the two corners T2 and T4 on the right edge, on the grounds that distances b and d are the shortest among the distances a to d between the printing area 91 and the four corners T1 to T4 of the recording medium.

[0084] In step S36, if W1 is less than or equal to W / 2 (step S36; NO), the control unit 81 determines whether the distance W2 between the printing area 91 and the right edge of the recording medium 90 in the width direction of the recording medium 90 is greater than W / 2 (step S38).

[0085] If W2 is greater than W / 2 (step S38; YES), the control unit 81 uses the upper left corner T1 or the lower left corner T3 and one or more points from among the four corners T1 to T4 of the recording medium 90 as points close to the printing area 91 to determine the misalignment (step S39). The control unit 81 selects the upper left corner T1 or the lower left corner T3 and one or more points as reference positions to be used for detecting the misalignment.

[0086] In step S38, if W2 is less than or equal to W / 2 (step S38; NO), the control unit 81 uses the four corners T1 to T4 of the recording medium 90 for positional displacement determination (step S40). The control unit 81 selects all four corners T1 to T4 as reference positions to be used for detecting positional displacement.

[0087] Figure 14 is an example of an image corresponding to step S40. In the example shown in Figure 14, L1 is less than or equal to H / 2, L2 is less than or equal to H / 2, W1 is less than or equal to W / 2, and W2 is less than or equal to W / 2. Since the printing area 91 is located near the center of the recording medium 90, the control unit 81 uses four corners T1 to T4 as reference positions for detecting misalignment.

[0088] After step S26, step S28, step S29, step S32, step S34, step S35, step S37, step S39, or step S40, the reference position selection process ends.

[0089] The control unit 81 uses the reference positions selected in the reference position selection process (two or more of the upper left corner, upper right corner, lower left corner, and lower right corner) to perform a position misalignment detection process (Figure 3). Specifically, in step S6, the control unit 81 determines only the selected reference position from the four corner points Q1 to Q4 of the recording medium using the reference image A2 after alignment. In step S7, the control unit 81 determines only the selected reference position from the four corner points R1 to R4 of the recording medium from the read image B1. In step S8, the control unit 81 calculates the distance between two corresponding reference positions, based on the reference position obtained from the reference image A2 after alignment and the reference position obtained from the read image B1, as the amount of displacement. In this way, the control unit 81 calculates the amount of displacement for each reference position selected in the reference position selection process.

[0090] In step S9, the control unit 81 determines whether at least one of the calculated displacement amounts is greater than or equal to a preset reference value V. If all of the displacement amounts are less than the reference value V (step S9; NO), the control unit 81 determines that there is no positional displacement (step S10). If at least one of the displacement amounts is greater than or equal to the reference value V (step S9; YES), the control unit 81 determines that there is a positional displacement (step S11).

[0091] Furthermore, the control unit 81 may control the image formation position in the image forming unit 40 based on the amount of displacement calculated in step S8. Specifically, the control unit 81 corrects the image formation position in the image forming unit 40 so as to eliminate the positional displacement in the image forming unit 40. The control unit 81 feeds back the amount of displacement into an adjustment value for correcting the positional displacement.

[0092] Furthermore, the control unit 81 may also inspect for abnormalities in the read image B1 based on the reference image A2 after alignment and the read image B1. In this case, the control unit 81 inspects for abnormalities other than misalignment. Abnormalities in the read image B1 include image color, dirt, etc. For example, the control unit 81 generates a difference image between the reference image A2 after alignment and the read image B1. The control unit 81 generates a difference image by calculating the difference in pixel values ​​(luminance value, RGB value, etc.) of each pixel in both images. The control unit 81 inspects for abnormalities in the read image B1 by checking whether each pixel in the difference image exceeds a predetermined threshold. This allows the control unit 81 to perform normal inspection operations along with detecting misalignment.

[0093] Furthermore, if there is an abnormality in the read image B1, the control unit 81 may control the transport unit 84 to eject the abnormal recording medium to a different ejection tray than the ejection tray where normal recording media are ejected. The abnormality in the read image B1 may be a misalignment or an abnormality other than a misalignment.

[0094] As described above, according to this embodiment, the control unit 81 of the image forming apparatus 100 selects a reference position to be used to detect misalignment from among the reference positions included in the read image and the reference image, based on the position of the area (printing area) in which an image is formed on the recording medium. The control unit 81 can perform stable misalignment determination by selecting a reference position to be used to detect misalignment according to the position of the printing area on the recording medium and then determining the misalignment. Therefore, the control unit 81 can improve the accuracy of misalignment detection.

[0095] Since the control unit 81 uses the four corners of the recording medium as reference positions, it can detect positional misalignment based on information from the recording medium area, which is the largest available area. The control unit 81 calculates the positions of the four corners of the recording medium based on the edge information of the recording medium. This allows the control unit 81 to accurately obtain the positions of the four corners of the recording medium even when the four corners of the recording medium are not included in the read image due to corner bending or the like.

[0096] The control unit 81 excludes the reference position on the edge side of the recording medium from the reference position used to detect misalignment if the distance from the area where an image is formed (printing area) to the edge of the recording medium in a predetermined direction is greater than a threshold. Positions far from the printing area exhibit greater variation after alignment. The control unit 81 can improve the accuracy of detecting misalignment by excluding reference positions far from the printing area. Furthermore, the control unit 81 can determine whether the printing area and the reference position are far apart with relatively simple processing by using the distance from the printing area to the edge of the recording medium in a predetermined direction.

[0097] In the example shown in Figure 10, the distance L2 from the printing area 91 to the lower end of the recording medium 90 is greater than the threshold H / 2 in the transport direction of the recording medium 90. Therefore, the control unit 81 excludes the corners T3 and T4 on the lower end side of the recording medium 90 from the reference position used to detect misalignment.

[0098] The control unit 81 excludes reference positions that are relatively far from the area where the image is formed (printing area) from the reference positions used to detect misalignment. By excluding reference positions that are far from the printing area, the control unit 81 can improve the accuracy of detecting misalignment.

[0099] In the example shown in Figure 11, comparing the distances a to d from the printing area 91 to the four corners T1 to T4 of the recording medium 90, the distances from largest to smallest are c, a, d, and b. Therefore, the control unit 81 excludes the lower left corner T3 and the upper left corner T1, which are relatively far from the printing area 91, from the reference positions used to detect misalignment.

[0100] The control unit 81 selects at least two of the four corners of the recording medium as reference positions to be used for detecting misalignment. This allows the control unit 81 to improve the accuracy of misalignment detection.

[0101] The above-described embodiments are examples of the image inspection apparatus, image forming apparatus, image inspection method, and program according to the present invention, and are not limited thereto. The detailed configuration and operation of each part constituting the apparatus can also be modified as appropriate without departing from the spirit of the present invention.

[0102] For example, the above embodiment described a case in which the image forming apparatus 100 (image inspection apparatus) comprises an image forming unit 40 and an image reading unit 70. Alternatively, the image inspection apparatus may acquire a read image generated by reading a recording medium on which an image has been formed by an external image forming unit (image forming apparatus) using an external image reading unit (image reading apparatus), and use that as the object of inspection. Furthermore, in the above embodiment, each process performed by the image forming apparatus 100 may be performed in cooperation with multiple devices.

[0103] Furthermore, in the positional misalignment detection process (see Figure 3), even when the reference image is a RIP image, a background region corresponding to the outside of the recording medium area is assumed to be added to the reference image. Alternatively, when the reference image is a RIP image, a background region does not need to be added to the reference image. In this case, the control unit 81 can treat the edge of the image data of the reference image as a position corresponding to the edge of the recording medium.

[0104] Furthermore, the control unit 81 may use a simpler method when selecting reference positions from the four corners of the recording medium to detect misalignment. For example, the control unit 81 may exclude reference positions from which the distance from the area where the image is formed (printing area) is greater than a preset threshold from the reference positions used to detect misalignment. Here, the distance from the printing area to a certain reference position is the distance between the position in the printing area closest to the reference position and the reference position itself. By excluding reference positions far from the printing area, the control unit 81 can improve the accuracy of misalignment detection.

[0105] Furthermore, in the above embodiment, the control unit 81 acquired the area where the image is formed (printing area) as a rectangular area, but is not limited to this. The control unit 81 only needs to be able to recognize the approximate range in which the image will be formed as the area where the image will be formed.

[0106] Furthermore, in the reference position selection process (see Figures 7 to 9), the case where the threshold in the transport direction of the recording medium is H / 2 and the threshold in the width direction of the recording medium is W / 2 was described, but the thresholds in each direction can be changed as appropriate. In addition, the thresholds in each direction may be set according to the size of the recording medium.

[0107] Furthermore, in the reference position selection process (see Figures 7 to 9), the case where the reference image is used as the target image to determine the area where the image is formed (printing area) and select a reference position to be used for detecting positional misalignment was described. Alternatively, if there is sufficient processing time, the control unit 81 may use the read image as the target image to determine the area where the image is formed and select a reference position to be used for detecting positional misalignment.

[0108] Furthermore, if the control unit 81 is unable to select two or more reference positions from the read image or reference image for any reason, it may select a reference position using a threshold different from a preset threshold (H / 2, W / 2, etc.). Furthermore, if the control unit 81 cannot select two or more reference positions from the read image or reference image for any reason, it may choose not to perform an inspection to determine whether or not there is an abnormality in the read image (image to be inspected).

[0109] The computer-readable medium used to store the programs for executing each process is not limited to the examples above. Furthermore, a carrier wave may be used as the medium for providing program data via a communication line.

[0110] The embodiments disclosed herein are for illustrative purposes only and not intended to limit the scope of the invention. The scope of the invention should be interpreted as described in the claims. [Explanation of Symbols]

[0111] 40 Image forming unit 50 Supply section 70 Image reading unit 81 Control Unit 82 Memory section 84 Conveying section 90 Recording media 91 Print area 100 Image forming apparatus

Claims

1. An image inspection device that detects a misalignment of the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, The image included in the read image and the image included in the reference image are aligned. The system includes a control unit that detects the positional deviation of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image, in the state described above. The control unit is an image inspection device that selects a reference position to be used to detect the positional misalignment from among the reference positions included in the read image and the reference image, based on the position of the region in which an image is formed on the recording medium.

2. The aforementioned reference positions are the four corners of the recording medium. The image inspection apparatus according to claim 1.

3. The control unit calculates the positions of the four corners of the recording medium based on the edge information of the recording medium. The image inspection apparatus according to claim 2.

4. The control unit excludes the reference position on the edge side of the recording medium from the reference position used to detect the misalignment if the distance from the region where the image is formed to the edge of the recording medium in the predetermined direction is greater than a preset threshold. The image inspection apparatus according to claim 2.

5. The control unit excludes reference positions whose distance from the region where the image is formed is greater than or equal to a preset threshold from the reference positions used to detect the positional misalignment. The image inspection apparatus according to claim 2.

6. The distance from the region where the image is formed to the reference position is the distance between the position in the region where the image is formed that is closest to the reference position and the reference position. The image inspection apparatus according to claim 5.

7. The control unit selects at least two of the four corners of the recording medium as reference positions to be used for detecting the misalignment. The image inspection apparatus according to claim 2.

8. An image inspection apparatus according to any one of claims 1 to 7, An image forming unit that forms an image on the recording medium, A reading unit reads a recording medium on which the aforementioned image is formed and generates a read image, An image forming apparatus equipped with the following features.

9. An image inspection method for detecting a misalignment of the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, A step of aligning the image included in the read image with the image included in the reference image, In the aligned state, a step of detecting the positional misalignment of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image, Includes, An image inspection method further comprising the step of selecting a reference position to be used to detect the positional misalignment from among the reference positions included in the read image and the reference image, based on the position of the region in which an image is formed on the recording medium.

10. A computer that detects a positional shift in the image formation position on a recording medium by comparing a read image generated by reading a recording medium on which an image has been formed with a reference image, The image included in the read image and the image included in the reference image are aligned. A program for causing a control unit to function as a control unit that detects a positional deviation of the image formation position based on the difference between the reference position in the read image and the reference position in the reference image, in the state described above. The control unit is a program that selects a reference position to be used to detect the positional misalignment from among the reference positions included in the read image and the reference image, based on the position of the region in which an image is formed on the recording medium.

Images