Patent Publication Number: US-2023142237-A1

Title: Inspection system, method for controlling inspection system

Description:
BACKGROUND 
     Field of the Disclosure 
     The present disclosure relates to an inspection system, a method for controlling inspection system. 
     Description of the Related Art 
     A printing system including a printing apparatus and an inspection apparatus and configured to inspect a sheet printed by the printing apparatus by the inspection apparatus during sheet conveyance has recently been known. When inspecting a printed sheet, the inspection apparatus scans an image of the printed sheet that is conveyed thereto, and then performs image analysis on the scanned image to determine whether the printed sheet is OK or not. 
     To generate a reference image to be used for image analysis, in a method according to related art, an inspection apparatus scans a sheet having been printed in advance and being in sufficient print quality, and uses the scanning result as the reference image. 
     Japanese Patent Laid-open No. 2021-135197 discloses an inspection apparatus configured to, when comparing a print image obtained by scanning a conveyed printed sheet with a reference image for inspection, perform image-to-image alignment based on feature points extracted from each of these two images. 
     SUMMARY 
     In a case where a sufficient number of feature points for the image-to-image alignment fail to be extracted, an inspection apparatus disclosed in this publication uses information on sheet vertices of the print image to carry out the inspection. An inspection system according to a certain aspect of the present disclosure includes: one or more controllers having one or more processors and one or more memories, the one or more controllers being configured to: scan a printed material including a printed image to generate a scan image, extract one or more feature points from each of the scan image and a reference image, perform alignment of the scan image and the reference image based on at least one or more corresponding pair among the feature points extracted from the scan image and from the reference image having been registered in advance; carry out inspection of the printed material by using the reference image having been subjected to the alignment and the scan image having been subjected to the alignment, and perform processing in accordance with a method selected from among a plurality of methods including at least a first method and a second method in a case where a number of the feature points in the reference image is less than a predetermined number, wherein the processing is performed such that, in the case where the number of the feature points in the reference image is less than the predetermined number, the alignment using at least the extracted feature points is performed when the first method is selected, and the inspection is then carried out, whereas the inspection is not carried out when the second method is selected. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an example of an overall view of a printing system. 
         FIG.  2    is an example of a block diagram illustrating a system configuration of the printing system. 
         FIG.  3    is an example of a schematic mechanical sectional view of an image forming apparatus. 
         FIG.  4    is an example of a flowchart illustrating an overall flow of inspection processing. 
         FIG.  5 A  is a diagram illustrating an example of a captured image. 
         FIG.  5 B  is a diagram illustrating an example of an image after image shape transformation performed on the captured image. 
         FIG.  5 C  is an example of a diagram illustrating extracted feature points. 
         FIG.  5 D  is an example of an enlarged view of a certain feature point. 
         FIG.  6 A  is a diagram illustrating an example of an image having a few feature points. 
         FIG.  6 B  is a diagram illustrating an example of a feature point extracted from the image having a few feature points. 
         FIG.  7    is a diagram illustrating an example of a UI screen for alerting the user. 
         FIG.  8    is a diagram illustrating an example of a UI screen for making settings for an image having a few feature points. 
         FIG.  9    is an example of a flowchart of detailed settings of a method of inspection. 
         FIG.  10    is a diagram illustrating an example of a UI screen for making inspection settings. 
         FIG.  11    is an example of a processing flowchart for an inspection apparatus configured to perform inspection processing. 
         FIG.  12    is an example of a processing flowchart for the inspection apparatus configured to perform comparison with a reference image. 
         FIG.  13    is an example of a processing flowchart of inspection level coordinate information update processing. 
         FIG.  14 A  is a diagram illustrating an example of inspection level coordinate information. 
         FIG.  14 B  is a diagram illustrating an example of inspection level coordinate information. 
         FIG.  14 C  is a diagram illustrating an example of inspection level coordinate information. 
         FIG.  15    is an example of a flowchart of detailed settings of a method of inspection according to another embodiment. 
         FIG.  16    is a diagram illustrating an example of a UI screen for making inspection settings according to another embodiment. 
         FIG.  17    is an example of a flowchart illustrating an overall flow of inspection processing according to another embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     With reference to the accompanying drawings, the best mode for implementation of the disclosed technique will now be explained in detail. 
     In the description below, an external controller may be referred to as an image processing controller, a digital front end (DFE), a print server, or the like. An image forming apparatus may be hereinafter referred to as a multifunction printer, a multifunction peripheral (MFP), or the like. 
       FIG.  1    is an overall view of a hardware configuration of an image processing system according to the present embodiment. The image processing system includes an image forming apparatus  101  and an external controller  102 . The image forming apparatus  101  and the external controller  102  are connected to each other via an internal LAN  105  and a video cable  106  such that communication can be performed therebetween. The video cable  106 , instead of having its hardware configuration, may be configured such that its function is fulfilled by the internal LAN  105 . The external controller  102  is connected to a client PC  103  via an external LAN  104  such that communication can be performed therebetween. Print instructions are given from the PC  103  to the external controller  102 . 
     A printer driver having a function of converting print data into a print description language that can be processed by the external controller  102  is installed in the client PC  103 . A user who wants printing to be executed is able to give print instructions from various kinds of application via the printer driver. Based on the print instructions given by the user, the printer driver transmits the print data to the external controller  102 . Upon receiving the print instructions from the PC  103 , the external controller  102  performs data analysis processing and rasterization processing, and inputs the print data into the image forming apparatus  101  for execution of the print instructions. 
     Next, the image forming apparatus  101  will now be explained. In the image forming apparatus  101 , a plurality of apparatuses having different functions are connected. The image forming apparatus  101  is capable of performing complex print processing such as bookbinding. 
     A printing apparatus  107  forms an image using toners on a recording sheet (paper) conveyed from a sheet feeding unit provided near the bottom of the printing apparatus  107 . The configuration of the printing apparatus  107 , and its operation principle, is as follows. A ray of light such as a laser beam modulated according to image data is reflected by a rotating polygon mirror or the like. The reflected light impinges on a photosensitive drum as scanning light. An electrostatic latent image formed on the photosensitive drum by the laser beam is developed using a toner. The toner image is transferred onto a recording sheet clinging to a transfer drum. A series of image forming processes described above is executed sequentially for a yellow toner (Y), a magenta toner (M), a cyan toner (C), and a black toner (K), thereby forming a full-color image on the sheet. The sheet with the full-color image is conveyed from the transfer drum to a fixing device. The fixing device includes rollers, a belt, and the like. A heating source such as a halogen heater is built in a roller of the fixing device. Heat and pressure are applied to the sheet onto which the toner image has been transferred, thereby melting the toners and making the toners fixed to the sheet. 
     The reference numeral  108  denotes an inserter for inserting an insertion sheet. A sheet can be inserted from the inserter  108  at a desired position between printed sheets conveyed from the printing apparatus  107 . 
     An inspection apparatus  109  is an apparatus that scans an image of the conveyed sheet and compares the scanned image with a reference image that has been registered in advance, thereby determining whether the printed image is OK or not. 
     The reference numeral  110  denotes a large-capacity stacker having a large sheet accommodation capacity. The reference numeral  111  denotes a finisher that performs finish processing on sheets conveyed thereto. The finisher  111  is capable of performing stapling, punching, saddle stitching, or the like. The finish-processed sheets are ejected onto an ejection tray. 
     In the printing system described with reference to  FIG.  1   , the external controller  102  is connected to the image forming apparatus  101 . However, the scope of the present disclosure is not limited to a configuration with the external controller  102  connected thereto. 
     That is, the image forming apparatus  101  may be connected to the external LAN  104 , and print data that can be processed by the image forming apparatus  101  may be transmitted from the client PC  103 . In this case, data analysis processing and rasterization processing are performed in the image forming apparatus  101 . Then, print processing is performed. 
       FIG.  2    is a block diagram illustrating a system configuration of the image forming apparatus  101 , the external controller  102 , and the client PC  103 . 
     First, the configuration of the printing apparatus  107  of the image forming apparatus  101  will now be explained. The printing apparatus  107  of the image forming apparatus  101  includes a communication I/F  217 , a LAN I/F  218 , a video I/F  220 , an HDD  221 , a CPU  222 , a memory  223 , an operation unit  224 , and a display  225 . The printing apparatus  107  of the image forming apparatus  101  further includes a document exposure unit  226 , a laser exposure unit  227 , an imaging unit  228 , a fixing unit  229 , and a sheet feeding unit  230 . These components are interconnected via a system bus  231 . 
     The communication I/F  217  is connected via a communication cable  254  to the inserter  108 , the inspection apparatus  109 , the large-capacity stacker  110 , and the finisher  111 . Communication for control of each of these apparatuses is performed via the communication I/F  217 . 
     The LAN I/F  218  is connected via the internal LAN  105  to the external controller  102 . Communication of print data, etc., is performed via the LAN I/F  218 . 
     The video I/F  220  is connected via the video cable  106  to the external controller  102 . Communication of image data, etc., is performed via the video I/F  220 . 
     The HDD  221  is a storage device in which programs and data are stored. Based on the programs, etc. stored in the HDD  221 , the CPU  222  performs image processing control and print control comprehensively. Programs needed for the CPU  222  to perform various kinds of processing, and image data, are stored in the memory  223 . The memory  223  behaves as a work area. The operation unit  224  receives inputs for various kinds of setting and operation instructions from a user. Setting information of the image processing apparatus, print job processing status, and the like are displayed on the display  225 . The document exposure unit  226  performs document reading processing when a copy function or a scan function is used. While applying the light of its exposure lamp to the sheet placed by the user, the document exposure unit  226  captures an image by means of its CMOS image sensor to obtain document data. The laser exposure unit  227  is a device that performs primary charging for irradiating the photosensitive drum with a laser beam for toner-image transfer and performs laser exposure. In the laser exposure unit  227 , first, primary charging for charging the surface of the photosensitive drum to a uniform negative potential is performed. Next, a laser beam is applied to the photosensitive drum by means of a laser driver while adjusting a reflection angle by means of a polygon mirror. This neutralizes the negative electric charge at the irradiated portion, thereby forming an electrostatic latent image. The imaging unit  228  is a device for toner transfer to a sheet. The imaging unit  228  includes a development unit, a transfer unit, a toner replenishment unit, etc. The imaging unit  228  transfers the toner on the photosensitive drum onto the sheet. In the development unit, the toner charged to negative polarity is applied from its development cylinder to the electrostatic latent image on the surface of the photosensitive drum so as to visualize the image. In the transfer unit, primary transfer and secondary transfer are performed. The primary transfer is operation of transferring the toner on the surface of the photosensitive drum onto a transfer belt by applying a positive potential to a primary transfer roller. The secondary transfer is operation of transferring the toner on the transfer belt onto a sheet by applying a positive potential to a secondary transfer outer roller. The fixing unit  229  is a device for melting the toner on the sheet by applying heat and pressure thereto for fixing it. The fixing unit  229  includes a heater, a fixing belt, a pressing belt, etc. The sheet feeding unit  230  is a device for feeding a sheet. Its sheet-feeding/conveying operation is controlled using rollers and various sensors. 
     Next, the configuration of the inserter  108  of the image forming apparatus  101  will now be explained. The inserter  108  of the image forming apparatus  101  includes a communication I/F  232 , a CPU  233 , a memory  234 , and a sheet feeding control unit  235 . These components are interconnected via a system bus  236 . The communication I/F  232  is connected via the communication cable  254  to the printing apparatus  107 . Communication needed for control is performed via the communication I/F  232 . In accordance with control programs stored in the memory  234 , the CPU  233  performs various kinds of control needed for sheet feeding. The memory  234  is a storage device in which the control programs are stored. Based on instructions from the CPU  233 , the sheet feeding control unit  235  controls the feeding and conveyance of the sheet fed from the sheet feeding unit of the inserter and conveyed from the printing apparatus  107  while controlling rollers and sensors. 
     Next, the configuration of the inspection apparatus  109  of the image forming apparatus  101  will now be explained. The inspection apparatus  109  of the image forming apparatus  101  includes a communication I/F  237 , a CPU  238 , a memory  239 , an image capturing unit  240 , a display unit  241 , an operation unit  242 , and an HDD  255 . These components are interconnected via a system bus  243 . The communication I/F  237  is connected via the communication cable  254  to the printing apparatus  107 . Communication needed for control is performed via the communication I/F  237 . In accordance with control programs stored in the memory  239 , the CPU  238  performs various kinds of control needed for inspection. The memory  239  is a storage device in which the control programs are stored. Based on instructions from the CPU  238 , the image capturing unit  240  captures an image of a conveyed sheet. The CPU  238  stores, as a reference image, an image captured by the image capturing unit  240  into the memory  239 . Specifically, an average of a plurality of scan images is stored as the reference image. In addition, the CPU  238  compares the image captured by the image capturing unit  240  with the reference image stored in the memory  239  to determine whether the printed image is OK or not. The result of inspection, a setting screen, etc. are displayed on the display unit  241 . The operation unit  242  is operated by users and receives an instruction for changing the settings of the inspection apparatus  109 , an instruction for registering a reference image, and the like. The HDD  255  stores various kinds of setting information and images that are needed for inspection. The various kinds of setting information and images that are stored can be reused. 
     Next, the configuration of the large-capacity stacker  110  of the image forming apparatus  101  will now be explained. The large-capacity stacker  110  of the image forming apparatus  101  includes a communication I/F  244 , a CPU  245 , a memory  246 , and a sheet ejection control unit  247 . These components are interconnected via a system bus  248 . The communication I/F  244  is connected via the communication cable  254  to the printing apparatus  107 . Communication needed for control is performed via the communication I/F  244 . In accordance with control programs stored in the memory  246 , the CPU  245  performs various kinds of control needed for sheet ejection. The memory  246  is a storage device in which the control programs are stored. Based on instructions from the CPU  245 , the sheet ejection control unit  247  controls the conveyance of the sheet having been conveyed thereto to a stack tray, an escape tray, or the finisher  111  connected downstream of the large-capacity stacker  110 . 
     Next, the configuration of the finisher  111  of the image forming apparatus  101  will now be explained. The finisher  111  of the image forming apparatus  101  includes a communication I/F  249 , a CPU  250 , a memory  251 , a sheet ejection control unit  252 , and a finish processing unit  253 . These components are interconnected via a system bus  256 . The communication I/F  249  is connected via the communication cable  254  to the printing apparatus  107 . Communication needed for control is performed via the communication I/F  249 . In accordance with control programs stored in the memory  251 , the CPU  250  performs various kinds of control needed for finishing and sheet ejection. The memory  251  is a storage device in which the control programs are stored. Based on instructions from the CPU  251 , the sheet ejection control unit  252  controls sheet conveyance and sheet ejection. Based on instructions from the CPU  251 , the finish processing unit  253  controls finish processing such as stapling, punching, saddle stitching, etc. 
     Next, the configuration of the external controller  102  will now be explained. The external controller  102  includes a CPU  208 , a memory  209 , an HDD  210 , a keyboard  211 , a display  212 , a LAN I/F  213 , a LAN I/F  214 , and a video I/F  215 . These components are interconnected via a system bus  216 . 
     Based on programs and data stored in the HDD  210 , the CPU  208  performs reception of print data from the client PC  103  and raster image processor (RIP) processing comprehensively. In addition, the CPU  208  performs other processing such as transmission of the print data to the image forming apparatus  101 . The CPU  208  is also capable of performing RIP processing for reference image data. Specifically, in RIP processing for reference image data, for example, an image is generated while converting a resolution from 600 dpi to 300 dpi, whereas, in RIP processing for print data, an image is generated without such a resolution reduction. 
     Programs and data needed for the CPU  208  to perform various kinds of processing are stored in the memory  209 . The memory  209  behaves as a work area. Programs and data needed for operation such as print processing are stored in the HDD  210 . The keyboard  211  is a device for inputting operational instructions to the external controller  102 . Information on applications, etc. run on the external controller  102  is displayed on the display  212  by means of still-image and moving-image video signals. The LAN I/F  213  is connected via the external LAN  104  to the client PC  103 . Communication of print instructions etc., is performed via the LAN I/F  213 . The LAN I/F  214  is connected via the internal LAN  105  to the image forming apparatus  101 . Communication of print instructions etc., is performed via the LAN I/F  214 . The external controller  102  is able to interactively exchange various kinds of data with the printing apparatus  107 , the inserter  108 , the inspection apparatus  109 , the large-capacity stacker  110 , and the finisher  111  via the internal LAN  105  and the communication cable  254 . 
     The video I/F  215  is connected via the video cable  106  to the image forming apparatus  101 . Communication of print data, etc., is performed via the video I/F  215 . 
     Next, the configuration of the client PC  103  will now be explained. The client PC  103  includes a CPU  201 , a memory  202 , an HDD  203 , a keyboard  204 , a display  205 , and a LAN I/F  206 . These components are interconnected via a system bus  207 . Based on a document processing program, etc. stored in the HDD  203 , the CPU  201  generates print data and gives print instructions. In addition, the CPU  201  controls each device connected to the system bus comprehensively. Programs and data needed for the CPU  201  to perform various kinds of processing are stored in the memory  202 . The memory  202  behaves as a work area. Programs and data needed for operation such as print processing are stored in the HDD  203 . The keyboard  204  is a device for inputting operational instructions to the PC  103 . Information on applications, etc. run on the client PC  103  is displayed on the display  205  by means of still-image and moving-image video signals. The LAN I/F  206  is connected to the external LAN  104 . Communication of print instructions etc., is performed via the LAN I/F  206 . 
     In the foregoing description, the internal LAN  105  and the video cable  106  are connected to the image forming apparatus  101  and the external controller  102 . However, any alternative configuration may be adopted as long as data needed for printing can be transmitted and received. For example, a “video-cable-only” connection configuration may be adopted. Any storage device configured to store data and programs suffices for each of the memories  202 ,  209 ,  223 ,  234 ,  239 ,  246 , and  251 . For example, the disclosed memory may be replaced with a volatile RAM, a nonvolatile ROM, a built-in HDD, an external HDD, a USB memory, or the like. 
       FIG.  3    is a mechanical sectional view of the image forming apparatus  101 . The reference numeral  107  denotes a printing apparatus that forms an image to be printed on a sheet. The reference numerals  301  and  302  denote sheet feeding decks. Each of these sheet feeding decks is able to contain various kinds of sheet. Each of these sheet feeding decks is able to separate the uppermost one of a stack of sheets contained therein and to convey this top sheet to a sheet conveyance path  303 . The reference numerals  304  to  307  denote development stations. For color image production, they use respective color-component toners of C, M, Y, and K to form a toner image. The toner image formed here is primarily transferred to an intermediate transfer belt  308 . The intermediate transfer belt  308  turns clockwise in the figure. The toner image is transferred at a secondary transfer position  309  to a sheet coming from the sheet conveyance path  303 . The display  225  displays information on print status and information for the settings of the image forming apparatus  101 . The reference numeral  311  denotes a fixing unit for fixing a toner image to a sheet. The fixing unit  311  includes a pressing roller and a heating roller. The passing of the sheet through the nip of these rollers causes the melting of the toner and the press-fixing thereof, thereby fixing the toner image to the sheet. The sheet having been processed by the fixing unit  311  is conveyed to a sheet conveyance path  315  through a sheet conveyance path  312 . Depending on the type of the sheet, if further melting and press-fixing are needed for fixing the toner image, after passing through the fixing unit  311 , the sheet is conveyed to a second fixing unit  313  by using an upper sheet conveyance path, and after additional melting and press-fixing, the sheet is conveyed to the sheet conveyance path  315  through a sheet conveyance path  314 . If the image forming mode is a duplex mode, the sheet is conveyed to a sheet turnover path  316 . After being turned over thereat, the sheet is conveyed to a duplex conveyance path  317 , and image transfer to the second side of the sheet is performed at the secondary transfer position  309 . 
     The reference numeral  108  denotes an inserter for inserting an insertion sheet. The inserter  108  includes an inserter tray  321 , and causes a sheet fed through a sheet conveyance path  322  to enter a conveyance path. In this way, it is possible to insert a sheet at a desired position into a set of sheets conveyed from the printing apparatus  107 . 
     The sheet having passed through the inserter  108  is conveyed to the inspection apparatus  109 . Contact image sensors (CIS)  331  and  332  are disposed inside the inspection apparatus  109  such that they face each other. The CIS  331  is a sensor configured to scan the upper side of a sheet. The CIS  332  is a sensor configured to scan the lower side of the sheet. The image sensors for scanning may be line scan sensors instead of CISs. The inspection apparatus  109  is capable of scanning an image of the sheet by using the CISs  331  and  332  at the timing of arrival of the sheet conveyed along a sheet conveyance path  333  to a predetermined position, and determining whether the image in the apparatus is OK or not. The result of inspection conducted by the inspection apparatus  109 , etc. can be displayed on the display unit  241 . 
     The reference numeral  110  denotes a large-capacity stacker having a large sheet accommodation capacity. The large-capacity stacker  110  includes a stack tray  341  on which sheets can be stacked. The sheet having passed through the inspection apparatus  109  comes into the large-capacity stacker  110  through a sheet conveyance path  344 . The sheet conveyed from the sheet conveyance path  344  goes through a sheet conveyance path  345  to be outputted onto a stack of sheets on the stack tray  341 . The large-capacity stacker  110  includes an escape tray  346  as its ejection tray. The escape tray  346  is an ejection tray used for ejection of a sheet determined as being defective by the inspection apparatus  109 . The sheet to be outputted to the escape tray  346  is conveyed from the sheet conveyance path  344  via a sheet conveyance path  347  to the escape tray  346 . The sheet is conveyed via a sheet conveyance path  348  when outputted to the post-processing apparatus provided downstream of the large-capacity stacker  110 . The reference numeral  349  denotes a turnover portion for turning over the sheet. The turnover portion  349  is used when the sheet is to be outputted onto the stack on the stack tray  341 . When the sheet is to be outputted onto the stack on the stack tray  341  such that sheet orientation at the time of output will be the same as sheet orientation at the time of input, the sheet is turned over at the turnover portion  349  once. When the sheet is conveyed to the escape tray  346  or the downstream post-processing apparatus, turnover operation at the turnover portion  349  is not performed because the sheet is ejected directly without flipping when stacked. 
     The reference numeral  111  denotes a finisher that applies finish processing corresponding to the function designated by the user to sheets conveyed thereto. Specifically, the finisher  111  has a finishing function such as a stapling function (single stapling, double stapling), punching (two holes, three holes), saddle stitching, or the like. The finisher  111  has two ejection trays  351  and  352 . The sheet is outputted to the ejection tray  351  via a sheet conveyance path  353 . However, finish processing such as stapling cannot be performed on the sheet conveyance path  353 . The sheet is conveyed via a sheet conveyance path  354  when finish processing such as stapling is to be performed. The finishing function designated by the user is executed at a processing unit  355 . The finish-processed sheets are outputted to the ejection tray  352 . Each of the ejection trays  351  and  352  can move up and down. The sheets having been finish-processed at the processing unit  355  can also be ejected onto the ejection tray  351  after the ejection tray  351  is lowered. When saddle stitching is commanded, sheets are stapled together at the center at a saddle stitching processing unit  356 . After the stapling, the sheets are folded in half and are then outputted to a saddle stitching tray  358  via a sheet conveyance path  357 . The saddle stitching tray  358  has a belt-conveyor configuration. The bundle of the saddle-stitched sheets on the saddle stitching tray  358  is conveyed leftward. 
     In accordance with preset inspection items, the inspection apparatus  109  inspects an incoming sheet image. Sheet image inspection is carried out by comparing an incoming sheet image with a preset reference image. Some examples of a method for image comparison are: a method of comparing pixel values at each image position, a method of comparing object positions by edge detection, a method of extracting character data by optical character recognition (OCR), and the like. Some examples of inspection items are: precision error in print position, shades of color of an image, image density, streaking and fading, missing print dots, and the like. 
     Overall Flow of Inspection Processing 
     Next, with reference to the flowchart of  FIG.  4   , an overall flow from tasks performed before a start of inspection at the inspection apparatus  109  to execution of the inspection will now be explained. Steps S 401  to S 412  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
     Each processing in  FIG.  4    is performed by the inspection apparatus  109  in accordance with user operation via the client PC  103 . 
     First, in the step S 401 , the inspection apparatus  109  registers a reference image to be used as a reference for inspection OK determination. 
     There are two methods for generating a reference image. One of the two methods is to generate a reference image by executing a print job and scanning a print image by the image capturing unit  240 . 
     The inspection apparatus  109  waits in a reference image reading mode, and executes a print job for reference image registration commanded from the client PC  103 . Upon execution of printing, the inspection apparatus  109  detects the conveyance of a sheet, and scans the sheet by the image capturing unit  240 . The image obtained by scanning is stored as the reference image into the memory  239  of the inspection apparatus  109 . 
     The other method is to use, as a reference image, image data after RIP processing, which is generated through analysis of a print job, instead of using a scan image. In the description of the step S 402  and the subsequent steps below, a method of generating a reference image by executing a print job and scanning a print image by the image capturing unit  240  is assumed. The steps S 402  and S 403  are skipped if image data after RIP processing is used as a reference image. 
     In the step S 402 , the inspection apparatus  109  extracts the positions of sheet vertices from the image captured by the image capturing unit  240 . In the present embodiment, the term “sheet vertices” means the four corners of the sheet of paper. 
     In the step S 403 , based on the positions of the sheet vertices obtained in the step S 402 , the inspection apparatus  109  transforms the image into the shape of the sheet. This process may include processing of converting the resolution of the captured image into predetermined resolution. It is quite common that, in a captured image, an image portion corresponding to a sheet has a deformed shape due to the effect of a sheet skew and/or variations in the speed of conveyance. For example, in a case where the size of a sheet to be inspected is LTR size, where resolution in a main-scanning direction is 300 dpi, and where resolution in a sub-scanning direction is 300 dpi, the shape of the sheet described above will be as follows: a rectangle having a length WR in the main-scanning direction=11 inches×300=3,300 pixels and a length HR in the sub-scanning direction=8.5 inches×300=2,550 pixels. The shape of the sheet can be expressed in a coordinate system by four points of (0, 0), (3299, 0), (0, 2549), and (3299, 2549). The inspection apparatus  109  transforms the shape of the image data such that the four positions of the sheet vertices obtained from the image will coincide with the preset positions of the four points of the shape of the sheet to be inspected. Such image shape transformation is also called as geometric transform. Known methods such as affine transformation, etc. exist. Through the processing in the steps S 402  and S 403 , it is possible to convert the scanned reference image into the size of the sheet to be inspected. 
     In the step S 404 , the inspection apparatus  109  calculates feature points. Feature points represent, in an image, point positions suited for alignment of the image as a whole when compared with a reference image in inspection processing to be described later. As feature points suited for alignment of the image as a whole, points whose corner feature amount in the image is large are conceivable. A corner feature is a feature at a point whose local neighborhood stands in two dominant and different edge directions. A corner feature amount is an amount indicating the strength of this edge feature. Preferably, feature points to be used for alignment of the image as a whole should be scattered away from one another to some degree because there will be significant positional misalignment at positions away from the feature points if located in an unbalanced manner at only a certain part of the image. Therefore, from among points where the corner feature amount mentioned above is large, the inspection apparatus  109  extracts, as feature points, scattered points that are located at distributed positions within the entire area of the image. Extraction of feature points will be described later with reference to  FIGS.  5 A,  5 B, and  5 C . 
     In the step S 405 , the inspection apparatus  109  determines whether or not the number of the feature points extracted by executing the step S 404  is not less than a predetermined number. In the present embodiment, the predetermined number of the feature points means the minimum number of feature points that makes it possible to align the scan image with the reference image by using the feature points. In the present embodiment, the predetermined number of the feature points is assumed to be three, but is not limited thereto. The purpose of this determination is to, if the number of the extracted feature points is less than the predetermined number, determine to the effect that an image having a few feature points has been registered, display an alert screen, and prompt the user to select inspection operation to be performed in a case of a few feature points. In a case where the number of the extracted feature points is determined to be not less than the predetermined number and thus where a sufficient number of feature points for image-to-image alignment has been extracted, the process proceeds to the step S 407 . If the number of the extracted feature points is determined to be less than the predetermined number, the process proceeds to the step S 406 . 
     In the step S 406 , the inspection apparatus  109  stores the sheet vertices as alignment information into the memory  239 . 
     In the step S 407 , the inspection apparatus  109  stores the feature points extracted in the step S 404  as alignment information into the memory  239 . 
     In the step S 408 , for example, the inspection apparatus  109  displays a screen illustrated in  FIG.  7    on the display unit  241  of the inspection apparatus  109  to notify the user that the reference image having been registered is an image having a few feature points. The display on the display unit is controlled by the CPU  238  of the inspection apparatus  109 . 
     A user interface (UI)  700  illustrated in  FIG.  7    is a UI for letting the user know that the reference image having been registered is an image having a few feature points and confirming whether to go ahead to execute an inspection or to return to the step of registering a reference image without executing an inspection. 
     A button  701  is a button for going back to the step S 401  in order to redo reference image registration. 
     A button  702  is a button for performing inspection setting processing in order to make inspection settings. 
     A button  703  is a button for making the UI  700  disappear. 
     The inspection apparatus  109  keeps the UI  700  displayed until the button  701 , the button  702 , or the close button  703  is selected in accordance with user operation. 
     In the step S 409 , the inspection apparatus  109  determines whether either the button  701  or the button  702  is selected in accordance with user operation. If the button  701  is selected, the process returns to the step S 401  to read a reference image again. 
     If the button  702  is selected, the process proceeds to the step S 410  to set an inspection mode for a case where an image having a few feature points has been registered. 
     In the step S 410 , for example, the inspection apparatus  109  displays a screen illustrated in  FIG.  8    on the display unit  241  of the inspection apparatus  109  for the user to make settings about operation for a case where an image having a few feature points has been registered. 
     A UI  800  illustrated in  FIG.  8    is a UI screen for setting an inspection mode for a case where the image has a few feature points. 
     A button  801  is a button for executing an inspection in a normal inspection mode in a case where the image has a few feature points. In the normal inspection mode, alignment is performed based on information on the sheet vertices of the reference image and information on the sheet vertices of the scan image. As for the level of the inspection, in the normal inspection mode, the inspection is executed under the same settings as in a case where the number of the feature points is sufficient. 
     A button  802  is a button for executing an inspection in an inspection level change mode in a case where the image has a few feature points. The inspection level change mode is an inspection mode in which an inspection is executed with automatic changes in the level of the inspection made to inspection settings that have been configured. In the inspection level change mode, alignment is performed based on information on the sheet vertices of the reference image and information on the sheet vertices of the scan image. As for the level of the inspection, in the inspection level change mode, the inspection is executed at an automatically-changed predetermined level. 
     A button  803  is a button for executing an inspection in an exclusion mode in a case where the image has a few feature points. The exclusion mode is a mode to exclude a registered inspection image from inspection. 
     In the step S 411 , the inspection apparatus  109  sets detailed information such as the inspection level of a print image inspection, the type of the inspection, and the area of the inspection in accordance with user operation. A detailed explanation of this step will be given later. 
     In the step S 412 , in response to a print job for inspection commanded from the client PC  103 , the inspection apparatus  109  detects the conveyance of a sheet, scans the sheet by the image capturing unit  240 , and stores the scan image into the memory  239  of the inspection apparatus  109 . Then, the inspection apparatus  109  compares the scan image obtained by the scanning of the inspection job with the reference image registered in the step  401  to carry out the inspection thereof while using the inspection parameters set in the steps S 410  and  411 . A detailed explanation of this step will be given later. 
     Detailed Explanation of Method for Calculation of Feature Point Information 
     With reference to  FIGS.  5 A,  5 B, and  5 C , a detailed explanation of a method for calculation of feature point information will be given below. 
       FIGS.  5 A,  5 B, and  5 C  are diagrams for explaining the extraction of feature points in the step S 404 . 
       FIG.  5 A  illustrates the image captured by the image capturing unit  240 .  FIG.  5 B  illustrates an image after image shape transformation performed using sheet vertices. As explained earlier in the step S 404 , as feature points suited for alignment of the image as a whole, points whose corner feature amount in the image is large are conceivable. 
     Various methods have been devised for detecting a corner feature amount that is an amount indicating the strength of an edge feature; as one of methods for calculating a corner feature amount, there exists a known technique called as Harris corner detection. In the Harris corner detection method, a corner feature amount image is calculated from a derivative image in the main-scanning direction and a derivative image in the sub-scanning direction. The corner feature amount image is an image representing an edge amount of the weaker one of two edges constituting a corner feature. The amount of a corner feature is expressed based on whether the weaker one of two edges constituting the corner feature, both of which are supposed to be strong, has a large edge amount or not.  FIG.  5 C  illustrates that pixels whose feature amount is greater than a predetermined value are expressed in white by applying the Harris corner detection method to the image illustrated in  FIG.  5 B . There exist a plurality of points where the corner feature amount is comparatively large within the image; in the present embodiment, among them, six points whose corner feature amount ranks in a high level and whose positions are scattered within the entire area of the image are extracted as feature points to be used for alignment. In  FIG.  5 C , the positions of the extracted six points are indicated by white dotted-line circles. 
       FIG.  5 D  illustrates an image of an area of 33 pixels square corresponding to one of the six feature points extracted in  FIG.  5 C . In the flow of inspection processing to be described later, search processing is performed on the read image to be inspected to find a location of a match with the image illustrated in  FIG.  5 D  in the neighborhood of the coordinates corresponding to the feature point; by this means, it is possible to acquire the coordinates of the feature point in the read image to be inspected. 
     Examples of Case Where Image has a Few Feature Points 
     A case where the number of feature points is not less than a predetermined number and where extracted feature points are located in a well-distributed manner has been described with reference to  FIGS.  5 A,  5 B, and  5 C . Next, with reference to  FIGS.  6 A and  6 B , a case where an image has a few feature points and where feature points are located in an unbalanced manner will now be explained. This applies to a case where, for example, a reference image is registered using a blank sheet with no print image thereon or using a sheet having a print pattern including only points whose corner feature amount is small. From a read image obtained by scanning such a sheet, there is a possibility that a sufficient number of feature points for image-to-image alignment might not be able to be extracted. 
       FIG.  6 B  illustrates an example of an image with a feature point extracted in the step S 404  from a reference image illustrated in  FIG.  6 A . In the image illustrated in  FIG.  6 B , points whose feature amount is large are located in an unbalanced manner at one corner area (indicated by a dotted-line circle in the figure) of a sheet. When the CPU  238  extracts any one of these points whose feature amount is large as a feature point, the others of these points will not be extracted as feature points because they are very close to the extracted one. In this case, even with feature point extraction from the reference image, it could be difficult to align the target read image to be processed with the reference image by feature-point-based alignment. 
     Detailed Settings of Inspection Method 
     Next, with reference to the flowchart of  FIG.  9   , processing for setting detailed information such as the inspection level of a print image inspection, the type of the inspection, and the area of the inspection in the step S 411  will now be explained. Steps S 901  and S 902  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
     By performing processing in this flowchart, the inspection apparatus  109  sets various inspection parameters such as the inspection area of a print image inspection, the inspection level thereof, etc. With reference to  FIG.  10   , an example of UI regarding inspection settings will now be explained. 
     In the step S 901 , the inspection apparatus  109  sets print image inspection areas. The method of setting print image inspection areas according to the present embodiment is as follows. 
     In the present embodiment, four kinds of area setting are performed. Specifically, the four kinds of area setting are: setting an area of focus, setting a standard inspection area, setting a simple inspection area, and setting an out-of-the-scope-of-inspection area (exclusion area outside the scope). The area of focus is an area where a defect inspection should be executed more rigorously with greater importance in comparison with other areas. An example of the area of focus is a human face area. The standard inspection area is an area that should be inspected with a standard degree of rigorousness. The simple inspection area is an area for which a simpler inspection than at the standard inspection area suffices. The out-of-the-scope-of-inspection area is an area to be excluded from the scope of inspection. 
     First, to set an area of focus, the user presses a button  1021  labeled “Area-of-focus Setting”. Next, the user designates a regional range which the user wants to be inspected more rigorously in a page preview  1004 . The inspection apparatus  109  sets the designated regional range as an area of focus  1005 . 
     To set a standard inspection area, the user presses a button  1022  labeled “Standard Inspection Area Setting”. Next, the user designates a regional range which the user wants to be inspected with a standard degree of rigorousness in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as a standard inspection area  1007 . 
     To set a simple inspection area, the user presses a button  1023  labeled “Simple Inspection Area Setting”. Next, the user designates a regional range which the user wants to be inspected simply in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as a simple inspection area  1006 . 
     To set an out-of-the-scope-of-inspection area, the user presses a button  1024  labeled “Exclusion Area Setting”. Next, the user designates a regional range which the user wants to be excluded from the scope of inspection in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as an out-of-the-scope-of-inspection area  1008 . 
     In the step S 902 , the inspection apparatus  109  sets detection items for which defect detection should be performed in the print image inspection, and the inspection level thereof, on a UI screen  1009 . 
     The “detection item” in the print image inspection means an item regarding the characteristics of a defect which the user wants to be detected when a printed material is inspected. For example, the detection item is a defect having a round shape (a spot), a streaky defect (a streak), etc. In the present embodiment, a recording sheet with an image printed thereon will be referred to as “printed material”. The inspection level is a parameter set on a scale basis for specifying a threshold degree for determination as being defective for each characteristic of a detected defect. For example, the inspection level is set on a 7-point scale of “Level 1” to “Level 7”. Setting the inspection level to “Level 7” makes it possible to detect a fainter and finer defect than “Level 1. The level setting can be made for each inspection item, for example, “Level 7” for spot inspection and “Level 4” for streak inspection. The scale of the parameter, and the level thereof, are not limited to this example. The illustrated example of the UI screen  1009  shows that “Level 7” has been selected as the inspection level for a spot defect by the user, and “Level 7” for a streak defect.  FIG.  14 A  illustrates an example of inspection level coordinate information having been set in the step S 411 . The coordinates of the upper left corner of the area designated in the page preview  1004  are stored as a start coordinate. The coordinates of the lower right corner of the area designated in the page preview  1004  are stored as an end coordinate. 
     The foregoing is an explanation of processing regarding detailed settings made in the step S 411  for the inspection level of a print image inspection, the type of the inspection, and the area of the inspection. 
     Detailed Explanation of Inspection Processing 
     Next, with reference to the flowcharts of  FIGS.  11  and  12   , the flow of inspection processing performed by the inspection apparatus  109  in the step S 412  will now be explained. Steps S 1101  to S 1110  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
       FIG.  11    is a flowchart illustrating the flow of processing performed by the inspection apparatus  109  when carrying out inspection. The processing illustrated in  FIG.  11    is performed by the CPU  238  of the inspection apparatus  109 . 
     In the step S 1101 , the inspection apparatus  109  acquires print settings and alignment information. 
     In the step S 1102 , the inspection apparatus  109  determines whether an inspection end instruction is received or not. The inspection processing is ended if an inspection end instruction is received. The process proceeds to the step S 1103  if an inspection end instruction is not received. 
     In the step S 1103 , the inspection apparatus  109  determines whether a sheet is conveyed to the inspection apparatus  109  or not. The process returns to the step S 1102  if there is no sheet conveyed thereto. The process proceeds to the step S 1104  if a sheet is conveyed thereto. An image of the sheet is scanned using the CIS s  331  and  332 . The scanned image is stored into the memory  239  of the inspection apparatus  109 . 
     In the step S 1105 , the inspection apparatus  109  compares the image having been scanned in the step S 1104  with the reference image. The reference image used here is the image generated through the flow of  FIG.  4   . A flow as to how to compare the scan image with the reference image will be described later using  FIG.  12   . 
     Next, the process proceeds to the step S 1106 . Based on the result of comparison with the reference image in the step S 1105 , the inspection apparatus  109  determines whether the image that is being inspected is a non-defective image or a defective image. 
     If it is determined in the step S 1106  that the image that is being inspected is a non-defective image (inspection OK), the process proceeds to the step S 1107 , and the inspection result “OK” is displayed on the display unit  241  of the inspection apparatus  109 . 
     Next, the process proceeds to the step S 1108 . The inspection apparatus  109  instructs the printing apparatus  107  that the printed sheet should be ejected to the stack tray  341  of the large-capacity stacker  110 . Based on the instruction from the inspection apparatus  109 , the printing apparatus  107  causes the large-capacity stacker  110  to eject the printed sheet to the stack tray  341 . 
     Next, the process returns to the step S 1102  to continue the processing. 
     If it is determined in the step S 1106  that the image that is being inspected is a defective image (not OK), the process proceeds to the step S 1109 , and the inspection result “Failed” is displayed on the display unit  241  of the inspection apparatus  109 . 
     Next, the process proceeds to the step S 1110 . The inspection apparatus  109  instructs the printing apparatus  107  that the printed sheet should be ejected to the escape tray  346  of the large-capacity stacker  110 . Based on the instruction from the inspection apparatus  109 , the printing apparatus  107  causes the large-capacity stacker  110  to eject the printed sheet to the escape tray  346 . 
     Next, the process returns to the step S 1102  to continue the processing. If an inspection end instruction is received or if an inspection end instruction has been received before the next sheet is conveyed, the inspection processing is ended. 
     Detailed Explanation of Processing for Comparison with Reference Image 
       FIG.  12    is a flowchart illustrating the flow of processing performed by the inspection apparatus  109  when making a comparison with the reference image in the inspection processing. Steps S 1201  to S 1207  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
     In the step S 1201 , the inspection apparatus  109  performs processing for updating the inspection level and the coordinate information. A detailed explanation of this step will be given later. 
     In the step S 1202 , the inspection apparatus  109  extracts the positions of sheet vertices from the image captured by the image capturing unit  240 . 
     In the step S 1203 , the inspection apparatus  109  determines whether or not the number of the feature points in the reference image, which has been calculated in the step S 404 , is not less than the predetermined number. The process proceeds to the step S 1204  if it is determined that the number of the feature points is not less than the predetermined number. 
     If the number of the feature points is less than the predetermined number, the process proceeds to the step S 1206 . 
     In the step S 1204 , the inspection apparatus  109  performs search processing on the image having been read in the step S 1104  to find a location of a match, with the image at the position of each feature point in the reference image, in the neighborhood of the coordinates corresponding to the feature point from the sheet vertex position, thereby acquiring the positions of the feature points in the read image. 
     In the step S 1205 , the inspection apparatus  109  transforms the shape of the read image and performs alignment such that the positions of the feature points in the read image, which have been acquired in the step S 1204 , will coincide with the positions of the feature points in the reference image. This process may include processing of converting the resolution of the captured image into predetermined resolution. Such image shape transformation is also called as geometric transform. Known methods such as affine transformation, etc. exist. In affine transformation, a coefficient that is needed for affine transformation processing can be obtained based on the coordinates of locations where a match between a base image (herein, the reference image) and a scan image to be transformed in shape (herein, the read image) is wanted. 
     In the step S 1206 , the inspection apparatus  109  transforms the shape of the read image and performs alignment such that the positions of the sheet vertices having been acquired in the step S 1202  will coincide with the positions of the sheet vertices of the reference image. This process may include processing of converting the resolution of the captured image into predetermined resolution. 
     In the alignment in the step S 1206 , both of the positions of the feature points the number of which is less than the predetermined number and the positions of the sheet vertices may be used for alignment processing. 
     In the step S 1207 , the inspection apparatus  109  compares the image having been transformed in shape in the step S 1205  or S 1206  with the reference image, and then terminates the flow. 
     For example, if the difference between the pixel value (luminance value) of the inspection target pixel in the image having been transformed in shape in the step S 1205  or S 1206  and the pixel value (luminance value) of the comparison target pixel in the reference image is not greater than a threshold value, the inspection apparatus  109  determines the inspection target pixel as “Passed”. The threshold value differs from one inspection level to another, for example, as follows. The threshold level for “Level 1” inspection is 200. The threshold level for “Level 2” inspection is 180. The threshold level for “Level 3” inspection is 150. The threshold level for “Level 4” inspection is 130. The threshold level for “Level 5” inspection is 120. The threshold level for “Level 6” inspection is 100. The threshold level for “Level 7” inspection is 50. 
     Then, upon finishing the inspection of all of the pixels constituting the image having been transformed in shape in the step S 1205  or S 1206 , the inspection apparatus  109  determines whether or not the total number of pixels having been determined as “Failed” is not greater than a pass threshold value. If the total number of the pixels having been determined as “Failed” is not greater than the pass threshold value, the inspection apparatus  109  determines the image having been transformed in shape in the step S 1205  or S 1206  as “Passed”. If the total number of the pixels having been determined as “Failed” is greater than the pass threshold value, the inspection apparatus  109  determines the image having been transformed in shape in the step S 1205  or S 1206  as “Failed”. 
     In the present embodiment, the determination in the step S 1203  is performed based on the number of the feature points in the reference image. However, the determination in the step S 1203  may be performed based on the number of the feature points in the scan image. 
     Detailed Explanation of Processing for Updating Inspection Level and Coordinate Information 
     Next, with reference to the flowchart of  FIG.  13    in conjunction with  FIG.  14   , the flow of processing performed by the inspection apparatus  109  in the step S 1201  for updating the inspection level and the coordinate information will now be explained. Steps S 1301  to S 1305  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
       FIG.  13    is a flowchart illustrating the flow of processing performed by the inspection apparatus  109  when performing processing for updating the inspection level and the coordinate information.  FIGS.  14 A,  14 B, and  14 C  are diagrams for explaining the processing for updating the inspection level and the coordinate information. 
     In the step S 1301 , the inspection apparatus  109  acquires the inspection level coordinate information having been set in the step S 411 . 
     In the step S 1302 , in order to determine whether the inspection level and the coordinate information need to be updated or not, the inspection apparatus  109  determines whether or not the number of feature points in the reference image is not less than a predetermined number. It is assumed here that the processing in the step S 1302  is performed by storing the information having been acquired in the step S 404  and performing determination based on the stored information. However, the determination may be performed based on information obtained by performing feature point extraction from the reference image again in the step S 1302 . 
     If the number of the feature points is not less than the predetermined number, updating the inspection level coordinate information is determined to be unnecessary. In this case, the inspection level coordinate information is not updated, and the processing for updating the inspection level and the coordinate information is terminated. 
     If the number of the feature points is less than the predetermined number, the inspection level coordinate information is updated so that operation will be performed in the inspection mode having been set in the step S 410  for a case where the image has a few feature points. If the number of the feature points is less than the predetermined number, the process proceeds to the step S 1303 . 
     In the step S 1303 , in order to operate in the inspection mode having been selected in the step S 410 , the inspection apparatus  109  identifies the selected inspection mode. If it is determined that the normal inspection mode has been selected, the processing for updating the inspection level and the coordinate information is terminated without updating the inspection level coordinate information. 
     If it is determined that the inspection level change mode has been selected, the process proceeds to the step S 1304 , and the inspection level coordinate information is updated. If it is determined that the exclusion mode has been selected, the process proceeds to the step S 1305 , and the inspection level coordinate information is updated. 
     In the step S 1304 , the inspection apparatus  109  changes the inspection levels of all of the areas to be inspected (the area of focus, the standard inspection area, and the simple inspection area) to “Level 1”. Changing the inspection levels to “Level 1” makes it possible to reduce the occurrence of erroneous determination even in a case of low alignment precision due to the image having a few feature points. 
       FIG.  14 A  illustrates an example of the inspection level coordinate information before being updated. Since the inspection levels of all of the areas to be inspected (the area of focus, the standard inspection area, and the simple inspection area) are changed to “Level 1” in the step S 1304 , the “after-the-change” inspection level of all of the areas to be inspected (the area of focus, the standard inspection area, and the simple inspection area) is “Level 1” as illustrated in  FIG.  14 B . 
     In the step S 1305 , the inspection apparatus  109  changes information on the out-of-the-scope-of-inspection area in the inspection level coordinate information. Designating the entire image area as the out-of-the-scope-of-inspection area makes it possible to reduce the occurrence of erroneous determination even in a case of low alignment precision due to the image having a few feature points. 
       FIG.  14 A  illustrates an example of the inspection level coordinate information before being updated. When an instruction to designate the entire image area as the out-of-the-scope-of-inspection area is given in the step S 1305 , as illustrated in  FIG.  14 C , the information on the out-of-the-scope-of-inspection area is updated. Namely, the entire image area is designated as the out-of-the-scope-of-inspection area. In the table, Xmax denotes the length of the inspection image in the horizontal direction; this length changes depending on sheet size. In the table, Ymax denotes the length of the inspection image in the vertical direction; this length also changes depending on sheet size. 
     The method described above will be effective when applied to a case where a fine precision error in print position will be undesirably detected as a difference if the alignment is performed based on the sheet vertices, although a fine precision error in print position will be corrected to pass the inspection if the alignment is performed based on the feature points. It is possible to switch the inspection mode between a mode for an inspection target for which a fine precision error in print position should be detected as defective printing and a mode for an inspection target for which a fine precision error in print position should be tolerated. 
     Though a case where the number of the feature points is less than the predetermined number has been described above, it may be determined that the extracted feature points are not enough for image-to-image alignment also in cases where, though the number of the feature points is not less than the predetermined number, the feature points are arranged in a row or concentrated at one place, etc. 
     In the foregoing embodiment, a method of switching the inspection mode for a case where the image has a few feature points has been described. However, there is an issue that it is difficult to suppress erroneous determination if the normal inspection mode is selected. 
     To address this issue, described as another embodiment below is a method of imposing a limitation on a range of the levels of inspection settable in detailed settings of the method of inspection in a case where an image having a few feature points has been registered as the reference image. By limiting the range of the setting such that high inspection levels cannot be selected, it is possible to reduce the occurrence of erroneous determination even in a case of low alignment precision. 
     Only the differences from the foregoing embodiment will be described in detail below. 
     Detailed Settings of Inspection Method 
     With reference to the flowchart of  FIG.  15   , processing according to another embodiment for setting detailed information such as the inspection level of a print image inspection, the type of the inspection, and the area of the inspection in the step S 411  will now be explained. Steps S 1501  to S 1506  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
     By performing processing in this flowchart, the inspection apparatus  109  sets various inspection parameters such as the inspection area of a print image inspection, the inspection level thereof, etc. With reference to  FIG.  16   , an example of UI regarding inspection settings will now be explained. 
     In the step S 1501 , the inspection apparatus  109  determines whether the number of the feature points in the reference image is less than the predetermined number or not. If the number of the feature points is less than the predetermined number, the process proceeds to the step S 1502  in order to limit the range of selectable inspection levels. If the number of the feature points is not less than the predetermined number, the process proceeds to the step S 1505  in order to set the inspection area and the inspection level without limiting the range of selectable inspection levels. 
     In the step S 1502 , the inspection apparatus  109  limits the range of selectable inspection levels. In the present embodiment, a case where selection is available from among seven levels from “Level 1” to “Level 7” before the restriction will be described. 
     The inspection apparatus  109  limits the range of inspection levels selectable for the area of focus to three levels from “Level 1” to “Level 3”. The inspection apparatus  109  limits the range of inspection levels selectable for the standard inspection area to two levels from “Level 1” to “Level 2”. The inspection apparatus  109  limits the range of inspection levels selectable for the simple inspection area to “Level 1” only. 
     In the step S 1503 , the inspection apparatus  109  sets print image inspection areas. The method of setting print image inspection areas according to the present embodiment is as follows. First, the user presses the button  1021  labeled “Area-of-focus Setting”. Next, the user designates a regional range which the user wants to be inspected more rigorously in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as the area of focus  1005 . 
     To set a standard inspection area, the user presses the button  1022  labeled “Standard Inspection Area Setting”. Next, the user designates a regional range which the user wants to be inspected with a standard degree of rigorousness in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as the standard inspection area  1007 . 
     To set a simple inspection area, the user presses the button  1023  labeled “Simple Inspection Area Setting”. Next, the user designates a regional range which the user wants to be inspected simply in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as the simple inspection area  1006 . 
     To set an out-of-the-scope-of-inspection area, the user presses a button  1024  labeled “Exclusion Area Setting”. Next, the user designates a regional range which the user wants to be excluded from the scope of inspection in the page preview  1004 . The inspection apparatus  109  sets the designated regional range as the out-of-the-scope-of-inspection area  1008 . 
     In the step S 1504 , the inspection apparatus  109  sets detection items for which defect detection should be performed in the print image inspection, and the inspection level thereof, on a UI screen  1609 . 
     The illustrated example of the UI screen  1609  shows that “Level 3” has been selected as the inspection level for a spot defect by the user, and “Level 3” for a streak defect. 
     The foregoing is an explanation of processing regarding detailed settings made in the step S 411  for the inspection level of a print image inspection, the type of the inspection, and the area of the inspection. 
     As described above, by limiting a range of the levels of inspection settable in detailed settings of the method of inspection in a case where an image having a few feature points has been registered as the reference image, it is possible to reduce the occurrence of erroneous determination even in a case of low alignment precision. 
     In the foregoing embodiment, a method of switching the inspection mode for a case where the image has a few feature points has been described. However, in a case where a job containing a large number of pages (for example, 1,000 pages) is to be inspected, it is impractical to make inspection settings individually for all of these pages. A job of this kind contains pages having a few feature points and pages having many feature points in a mixed manner. In such a case, it is conceivable to register inspection operation for a case where common inspection settings and an image (page) having a few feature points have been registered and to carry out the inspection, rather than forcing the user to register inspection settings individually on a page-by-page basis. 
     In view of the above, in the present embodiment, the following method will be described: a method of presetting an inspection mode to be applied when, in a case where a job containing a large number of pages is to be inspected, an image having a few feature points has been registered as the reference image for a part of the job. In a case where a job containing a large number of pages is to be inspected, registration for every page will be needed if the method according to the first embodiment is employed. Using the method according to the third embodiment makes it possible to, just by performing registration processing once, carry out the inspection by performing inspection operation under common inspection settings for pages having many feature points and by performing, for pages having a few feature points, pre-registered inspection operation for a case where an image having a few feature points has been registered. Pre-registering the inspection mode makes it possible to reduce the burden of making individual settings on the user. 
     Only the differences from the foregoing embodiment will be described in detail below. 
     Overall Flow of Inspection Processing 
     With reference to the flowchart of  FIG.  17   , an overall flow according to the present embodiment from tasks performed before a start of inspection at the inspection apparatus  109  to execution of the inspection will now be explained. Steps S 1701  to S 1711  in the flowchart are implemented by reading and running a program stored in the HDD  255  by the CPU  238 . 
     Each processing in  FIG.  17    is performed by the inspection apparatus  109  in accordance with user operation via the client PC  103 . 
     First, in the step S 1701 , for example, the inspection apparatus  109  displays a screen illustrated in  FIG.  8    on the display unit  241  of the inspection apparatus  109  for the user to make settings about operation for a case where an image having a few feature points has been registered. 
     In the step S 1702 , the inspection apparatus  109  registers a reference image to be used as a reference for inspection OK determination. 
     In the step S 1703 , the inspection apparatus  109  extracts the positions of sheet vertices from the image captured by the image capturing unit  240 . In the present embodiment, the term “sheet vertices” means the four corners of the sheet of paper. 
     In the step S 1704 , based on the positions of the sheet vertices obtained in the S 1702 , the inspection apparatus  109  transforms the image into the shape of the sheet. 
     In the step S 1705 , the inspection apparatus  109  calculates feature points. 
     In the step S 1706 , the inspection apparatus  109  determines whether the number of the feature points extracted by executing the step S 1704  is less than a predetermined number or not. In a case where the number of the extracted feature points is determined to be not less than the predetermined number and thus where a sufficient number of feature points for image-to-image alignment has been extracted, the process proceeds to the step S 1708 . If the number of the extracted feature points is determined to be less than the predetermined number, the process proceeds to the step S 1707 . 
     In the step S 1707 , the inspection apparatus  109  stores the sheet vertices as alignment information into the memory  239 . 
     In the step S 1708 , the inspection apparatus  109  stores the feature points extracted in the step S 1704  as alignment information into the memory  239 . 
     Each page of the print job is stored in association with the alignment information into the memory  239 . 
     In the step S 1709 , the inspection apparatus  109  determines whether image reading for use as the reference image in the inspection has been completed for all of the pages contained in the print job or not. If the inspection apparatus  109  determines that the reading has been completed for all of the pages, the process proceeds to the step S 1710  for detailed settings of the method of inspection. If the inspection apparatus  109  determines that the reading has not been completed for all of the pages yet, the process returns to the step S 1702 , and the image reading continues. 
     In the step S 1710 , the inspection apparatus  109  sets detailed information such as the inspection level of a print image inspection, the type of the inspection, and the area of the inspection in accordance with user operation. 
     In the step S 1711 , in response to a print job for inspection commanded from the client PC  103 , the inspection apparatus  109  detects the conveyance of a sheet, scans the sheet by the image capturing unit  240 , and stores the scan image into the memory  239  of the inspection apparatus  109 . Then, the inspection apparatus  109  compares the scan image obtained by the scanning of the inspection job with the reference image registered in the step S 1702  to carry out the inspection thereof while using the inspection parameters set in the steps S 1701  and S 1710 . 
     As explained above, a method of presetting an inspection mode to be applied when, in a case where a job containing a large number of pages is to be inspected, an image having a few feature points has been registered as the reference image for a part of the job has been described. Pre-registering the inspection mode makes it possible to reduce the burden of making individual settings on the user. 
     OTHER EMBODIMENTS 
     While various examples and embodiments of the present disclosure have been disclosed, the spirit and scope of the present disclosure shall not be construed to be limited to any specific disclosure made herein. 
     Embodiment(s) of the present disclosure can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)), a flash memory device, a memory card, and the like. 
     While the present disclosure includes exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-183017, filed Nov. 10, 2021, which is hereby incorporated by reference herein in its entirety.