Patent Publication Number: US-9895918-B2

Title: Image forming apparatus, image forming system, and method for forming test patterns

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 from Japanese Patent Application No. 2015-256859 filed on Dec. 28, 2015. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The following description relates to aspects of an image forming apparatus, an image forming system, and a method for forming test patterns. 
     Related Art 
     Heretofore, as image forming apparatuses, serial printers such as inkjet printers and dot impact printers, and electrophotographic page printers such as laser printers and LED printers have been known. 
     Further, an image forming apparatus has been known that is configured to form test patterns on a sheet-shaped recording medium (e.g., a paper), in order to form a high-quality image by suppressing a conveyance distance error caused when the recording medium is conveyed. For instance, the image forming apparatus may be configured to form a first test pattern on a recording medium, then convey the recording medium over a predetermined distance, and thereafter form a second test pattern on the recording medium. In this case, a conveyance distance error caused by the conveyance of the recording medium may be determined based on a positional relationship between the first test pattern and the second test pattern. 
     SUMMARY 
     As methods for forming test patterns, for instance, the following first and second methods have been known. In the first method, after a first test pattern is formed on a recording medium, conveying the recording medium over a very short distance and forming a second test pattern on the recording medium are repeatedly performed. A conveyance distance error is determined based on a positional relationship (e.g., an overlap) between the first test pattern and the second test pattern formed each time the recording medium has been conveyed over the very short distance. 
     In the second method, a first test pattern along a main scanning direction is formed on a recording medium with upstream nozzles, which are positioned upstream of the other nozzles of a recording head (e.g., an inkjet head) in a conveyance direction of the recording medium. Then, after the recording medium is conveyed over a predetermined distance, a second test pattern inclined relative to the main scanning direction is formed on the recording medium with downstream nozzles, which are positioned downstream of the other nozzles of the recording head in the conveyance direction. For instance, the first test pattern may include a plurality of dot rows (e.g., pixel rows) each of which is formed parallel to the main scanning direction and arranged on a straight line parallel to the main scanning direction. The second test pattern may include a plurality of dot rows each of which is formed parallel to the main scanning direction and arranged on a straight line inclined relative to the main scanning direction. A conveyance distance error may be determined based on a positional relationship (e.g., an overlap) between the first test pattern and the second test pattern. 
     According to the first method, the second test pattern is formed by repeating the conveyance of the recording medium over the very short distance. Therefore, it takes a long period of time to complete all the procedure for forming the test patterns. Meanwhile, according to the second method, it is not possible to adjust the position of each second test pattern on the basis of a distance less than a dot pitch in a sub scanning direction (i.e., the conveyance direction of the recording medium). Thus, it is difficult to evaluate a conveyance distance error less than the dot pitch. 
     Aspects of the present disclosure are advantageous to provide one or more improved techniques for efficiently forming test patterns to accurately determine a conveyance distance error. 
     According to aspects of the present disclosure, an image forming apparatus is provided that includes a conveyor configured to convey a recording medium in a conveyance direction, a first image former configured to form an image on the recording medium by selectively placing pixels on lattice points of a two-dimensional lattice, the two-dimensional lattice including a plurality of lattice points arranged at intervals of a predetermined pitch in each of an X-axis direction and a Y-axis direction, the Y-axis direction being parallel to the conveyance direction, the X-axis direction intersecting the Y-axis direction, a second image former disposed downstream of the first image former in the conveyance direction, the second image former being configured to form an image on the recording medium by selectively placing pixels on lattice points of the two-dimensional lattice, and a controller configured to perform a first formation control process including controlling the first image former to form a first test pattern on the recording medium, the first test pattern including a plurality of figures arranged on a first virtual straight line extending in a first direction, the first direction being inclined relative to the X-axis direction and directed from a first arbitrary lattice point toward a first specific lattice point on the two-dimensional lattice, the first specific lattice point being an A1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the Y-axis direction, B1 being a value smaller than A1, perform a conveyance control process including controlling the conveyor to convey the recording medium downstream in the conveyance direction, and perform a second formation control process including in response to the recording medium being conveyed to a position where a second test pattern to be formed is allowed to intersect or be in proximity to the first test pattern formed on the recording medium, controlling the second image former to form the second test pattern on the recording medium, the second test pattern including a plurality of figures arranged on a second virtual straight line extending in a second direction, the second direction being inclined relative to each of the X-axis direction and the first direction and directed from a second arbitrary lattice point toward a second specific lattice point on the two-dimensional lattice, the second specific lattice point being an A2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the Y-axis direction, B2 being a value smaller than A2. 
     A conveyance distance error of the recording medium may be determined based on a positional displacement of an intersection between the first test pattern and the second test pattern from a reference point. The intersection may be interpreted in a broad sense. For instance, the intersection may include a point where the first test pattern and the second test pattern would intersect each other if the first test pattern were extended in the first direction, and the second test pattern were extended in the second direction. 
     As an angle between the first test pattern and the second test pattern is made smaller, the positional displacement of the intersection therebetween in the X-axis direction (i.e., the main scanning direction) becomes larger with respect to the same conveyance distance error in the Y-axis direction (i.e., the conveyance direction). However, a known image forming apparatus forms a first test pattern including a plurality of first dot rows each of which is formed parallel to the X-axis direction and arranged in the X-axis direction. Further, the known image forming apparatus forms a second test pattern including a plurality of second dot rows each of which is formed parallel to the X-axis direction and arranged in a particular direction of a vector (X, Y)=(A2, B2), where B2 is not equal to zero. The particular direction is inclined relative to the X-axis direction. Each first dot row, which includes a plurality of dots arranged in the X-axis direction, may be an example of the “first elemental figure” according to aspects of the present disclosure. Each second dot row, which includes a plurality of dots arranged in the X-axis direction, may be an example of the “second elemental figure” according to aspects of the present disclosure. The vector (X, Y) is directed from an arbitrary lattice point toward a specific lattice point on the two-dimensional lattice. The specific lattice point is an X-th lattice point from the arbitrary lattice point as a zeroth lattice point in the X-axis direction and is a Y-th lattice point from the arbitrary lattice point as a zeroth lattice point in the Y-axis direction. 
     When the first test pattern is formed in the above manner by the known image forming apparatus, the angle between the first test pattern and the second test pattern corresponds to an angle between the X-axis direction and the vector (X, Y)=(A2, B2). The angle is made smaller by setting an absolute value of B2 smaller and setting an absolute value of A2 larger. It is noted that a lower limit of the absolute value of B2 is one. The absolute value of B2 equal to one corresponds to a pixel pitch in the conveyance direction of the recording medium. Namely, according to the known image forming apparatus, due to an influence of the pixel pitch (i.e., a resolution) in the conveyance direction of the recording medium, it is impossible to set the angle between the first test pattern and the second test pattern to a desirably small angle. Thus, it is difficult to determine a minute conveyance distance error caused by conveyance of the recording medium. 
     According to aspects of the present disclosure, the first and second test patterns inclined relative to the X-axis direction are formed on the recording medium. Thus, when both of the first test pattern and the second test pattern are inclined relative to the X-axis direction, it is possible to set smaller the angle between the first test pattern and the second test pattern in comparison with the known image forming apparatus that forms one of the first and second test patterns to be parallel to the X-axis direction. 
     Thus, according to aspects of the present disclosure, it is possible to form test patterns for accurately determining the conveyance distance error caused by conveyance of the recording medium. Further, it is possible to efficiently form test patterns for accurately determining the conveyance distance error of the recording medium without repeating an operation of conveying the recording medium over a very short distance as performed by a known image forming apparatus. 
     According to aspect of the present disclosure, the first test pattern and the second test pattern may be formed as geometrical patterns each of which includes a plurality of pixels (e.g., dots) arranged in a terraced shape to be macroscopically or approximately a straight line inclined relative to the other. Further, the first test pattern and the second test pattern may be formed as geometrical patterns each of which includes a plurality of pixels (e.g., dots) arranged to be macroscopically or approximately a straight line with a uniform width. 
     According to aspects of the present disclosure, further provided is an image forming system that includes a printer configured to form a first test pattern and a second test pattern on a recording medium, a scanner configured to scan the recording medium with the first test pattern and the second test pattern formed thereon, and generate image data expressing a scanned image of the recording medium, and a controller coupled with the printer and the scanner. The printer includes a conveyor configured to convey the recording medium in a conveyance direction, a first image former configured to form an image on the recording medium by selectively placing pixels on lattice points of a two-dimensional lattice, the two-dimensional lattice including a plurality of lattice points arranged at intervals of a predetermined pitch in each of an X-axis direction and a Y-axis direction, the Y-axis direction being parallel to the conveyance direction, the X-axis direction intersecting the Y-axis direction, and a second image former disposed downstream of the first image former in the conveyance direction, the second image former being configured to form an image on the recording medium by selectively placing pixels on lattice points of the two-dimensional lattice. The controller is configured to perform a first formation control process including controlling the first image former to form the first test pattern on the recording medium, the first test pattern including a plurality of figures arranged on a first virtual straight line extending in a first direction, the first direction being inclined relative to the X-axis direction and directed from a first arbitrary lattice point toward a first specific lattice point on the two-dimensional lattice, the first specific lattice point being an A1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the Y-axis direction, B1 being a value smaller than A1, perform a conveyance control process including controlling the conveyor to convey the recording medium downstream in the conveyance direction, perform a second formation control process including in response to the recording medium being conveyed to a position where the second test pattern to be formed is allowed to intersect or be in proximity to the first test pattern formed on the recording medium, controlling the second image former to form the second test pattern on the recording medium, the second test pattern including a plurality of figures arranged on a second virtual straight line extending in a second direction, the second direction being inclined relative to each of the X-axis direction and the first direction and directed from a second arbitrary lattice point toward a second specific lattice point on the two-dimensional lattice, the second specific lattice point being an A2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the Y-axis direction, B2 being a value smaller than A2, perform a scanning control process including controlling the scanner to scan the recording medium with the first test pattern and the second test pattern formed thereon and to generate image data expressing a scanned image of the recording medium, and perform an error determining process including identifying a position of an intersection between the first test pattern and the second test pattern formed on the recording medium by analyzing the image data generated by the scanner, and determining a conveyance distance error caused by conveyance of the recording medium based on the identified position of the intersection between the first test pattern and the second test pattern. 
     According to aspects of the present disclosure, further provided is a method implementable on a processor coupled with an image forming apparatus including a conveyor configured to convey a recording medium in a conveyance direction, a first image former configured to form an image on the recording medium by selectively placing pixels on lattice points of a two-dimensional lattice, the two-dimensional lattice including a plurality of lattice points arranged at intervals of a predetermined pitch in each of an X-axis direction and a Y-axis direction, the Y-axis direction being parallel to the conveyance direction, the X-axis direction intersecting the Y-axis direction, and a second image former disposed downstream of the first image former in the conveyance direction, the second image former being configured to form an image on the recording medium by selectively placing pixels on lattice points of the two-dimensional lattice, the method including performing a first formation control process including controlling the first image former to form a first test pattern on the recording medium, the first test pattern including a plurality of figures arranged on a first virtual straight line extending in a first direction, the first direction being inclined relative to the X-axis direction and directed from a first arbitrary lattice point toward a first specific lattice point on the two-dimensional lattice, the first specific lattice point being an A1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B1-th lattice point from the first arbitrary lattice point as a zeroth lattice point in the Y-axis direction, and B1 is a value smaller than A1, performing a conveyance control process including controlling the conveyor to convey the recording medium downstream in the conveyance direction, and performing a second formation control process including in response to the recording medium being conveyed to a position where a second test pattern is allowed to intersect or be in proximity to the first test pattern formed on the recording medium, controlling the second image former to form the second test pattern on the recording medium, the second test pattern including a plurality of figures arranged on a second virtual straight line extending in a second direction, the second direction being inclined relative to each of the X-axis direction and the first direction and directed from a second arbitrary lattice point toward a second specific lattice point on the two-dimensional lattice, the second specific lattice point is an A2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the X-axis direction and a B2-th lattice point from the second arbitrary lattice point as a zeroth lattice point in the Y-axis direction, B2 being a value smaller than A2. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a block diagram schematically showing a configuration of a multi-function peripheral (hereinafter referred to as an “MFP”) in an illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 2  schematically shows a partial configuration, around a recording head, of a sheet conveyor of the MFP in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 3  is a flowchart showing a procedure of a test printing process to be executed by a controller of the MFP in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIGS. 4A, 4B, 4C, and 4D  show a process in which test patterns are printed on a step-by-step basis in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 5  is an enlarged view of a first test pattern in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 6  is an enlarged view of a second test pattern in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 7  is an illustration for showing how to detect a position of an intersection between the first test pattern and the second test pattern in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 8A  is an enlarged view showing a first test pattern and a second test pattern in a modification according to one or more aspects of the present disclosure. 
         FIG. 8B  shows a relationship between a density distribution (i.e., a density change) of the first and second test patterns in a main scanning direction (i.e., an X-axis direction) and a position of an intersection between the first and second test patterns in the main scanning direction, in the modification according to one or more aspects of the present disclosure. 
         FIG. 9  is an illustration for geometrically showing a relationship between a positional displacement of the intersection between the first and second test patterns in the main scanning direction and a conveyance distance error in a sub scanning direction (i.e., a Y-axis direction), in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIG. 10  is an illustration for showing a positional displacement of the intersection between the first and second test patterns from a reference point in the illustrative embodiment according to one or more aspects of the present disclosure. 
         FIGS. 11A, 11B, 11C, 11D, and 11E  shows test patterns in modifications according to one or more aspects of the present disclosure. 
         FIG. 12  schematically shows a configuration of a line inkjet printer in a modification according to one or more aspects of the present disclosure. 
         FIG. 13  is an illustration for showing a positional displacement of an intersection between first and second test patterns from a reference point in a known method. 
         FIG. 14  schematically shows a partial configuration, around a recording head, of a sheet conveyor of an MFP in a further modification according to one or more aspects of the present disclosure. 
         FIGS. 15A and 15B  are flowcharts showing a procedure of a test printing process to be executed by a controller of the MFP in the further modification according to one or more aspects of the present disclosure. 
         FIGS. 16A, 16B, 16C, and 16D  show a process in which test patterns are printed on a step-by-step basis in the further modification according to one or more aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and, unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. Aspects of the present disclosure may be implemented on circuits (such as application specific integrated circuits) or in computer software as programs storable on computer-readable media including but not limited to RAMs, ROMs, flash memories, EEPROMs, CD-media, DVD-media, temporary storage, hard disk drives, floppy drives, permanent storage, and the like. 
     (Illustrative Embodiment) 
     Hereinafter, an illustrative embodiment according to aspects of the present disclosure will be described with reference to the accompanying drawings. As shown in  FIG. 1 , a digital multi-function peripheral (hereinafter, simply referred to as an “MFP”)  1  of the illustrative embodiment includes a controller  10 , a printing unit  20 , a scanning unit  70 , and a user interface  90 . The controller  10  is configured to take overall control of the MFP  1  and perform processes for causing the MFP  1  to serve as a printer, an image scanner, and a copy machine. The controller  10  includes a CPU  11 , a ROM  13 , a RAM  15 , and an NVRAM  17 . 
     The CPU  11  is configured to perform processes in accordance with computer programs  13   a  stored in the ROM  13 . The RAM  15  is used as a work area when the CPU  11  is executing a computer program  13   a . The NVRAM  17  is a non-volatile memory configured to electrically rewrite data stored therein. For instance, the NVRAM  17  may include a flash memory and/or an EEPROM. The controller  10  further includes a communication interface (not shown) configured to communicate with an external device  3  (e.g., a personal computer). 
     The printing unit  20  is configured as an inkjet printer. Specifically, the printing unit  20  is configured to, when controlled by the controller  10 , form an image on a sheet Q. For instance, the printing unit  20  forms on a sheet Q an image based on data received from the external device  3  or image data expressing an image read by the scanning unit  70 . Further, the printing unit  20  is configured to, when controlled by the controller  10 , form on a sheet Q test patterns for determining a conveyance distance error caused when the sheet Q is conveyed. 
     The scanning unit  70  is configured as a flatbed scanner. Specifically, the scanning unit  70  is configured to, when controlled by the controller  10 , optically scan a document placed on a document table and transmit to the controller  10  image data expressing a scanned image of the document. The user interface  90  includes a display configured to display various kinds of information for users, and an input device configured to accept instructions from users. The input device may include mechanical key switches and/or a touch panel on the display. 
     Subsequently, the printing unit  20  will be described in detail. As shown in  FIG. 1 , the printing unit  20  includes a printing unit driver  30 , a recording head  40 , a carriage moving mechanism  51 , a CR motor  53 , a linear encoder  55 , a sheet conveyor  61 , a PF motor  63 , and a rotary encoder  65 . 
     The printing unit driver  30  is configured to control the recording head  40  to discharge ink droplets, control the carriage moving mechanism  51  to move a carriage  52  (see  FIG. 2 ), and control the sheet conveyor  61  to convey a sheet Q, in accordance with instructions from the controller  10 . The printing unit driver  30  may include an ASIC. 
     The recording head  40  is a known inkjet head. The recording head  40  is configured to, when controlled by the printing unit driver  30 , discharge ink droplets thereby forming an image on a sheet Q. The recording head  40  has a lower surface facing the sheet Q, and includes ink discharge nozzles disposed at the lower surface. Specifically, the recording head  40  includes a group N 0  of ink discharge nozzles arranged in a sub scanning direction. The sub scanning direction corresponds to a sheet conveyance direction. A main scanning direction is perpendicular to the sub scanning direction. The main scanning direction corresponds to a carriage moving direction (i.e., a normal direction of a flat surface on which  FIG. 2  is drawn). Hereinafter, the group N 0  of ink discharge nozzles may be referred to as a “nozzle group N 0 .” 
     The carriage moving mechanism  51  includes the carriage  52  carrying the recording head  40 . The carriage moving mechanism  51  is configured to move the carriage  52  along the main scanning direction (i.e., the normal direction of the flat surface on which  FIG. 2  is drawn). The CR motor  53  includes a direct-current motor for driving the carriage moving mechanism  51 . The CR motor  53  is controlled by the printing unit driver  30 . Namely, the printing unit driver  30  controls rotation of the CR motor  53  thereby implementing control for moving the carriage  52 . 
     The linear encoder  55  is configured to input pulse signals, which correspond to displacement of the carriage  52  in the main scanning direction, as encoder signals into the printing unit driver  30 . The printing unit driver  30  detects a position and a velocity of the carriage  52  in the main scanning direction based on the encoder signals from the linear encoder  55 , and performs feedback control of the position and the velocity of the carriage  52 . The printing unit driver  30  controls the recording head  40  in accordance with the movement of the carriage  52 , and causes the recording head  40  to intermittently discharge ink droplets while moving relative to the sheet Q in the main scanning direction. Thereby, an intended image is formed on the sheet Q. 
     The sheet conveyor  61  is configured to convey a sheet Q from a feed tray (not shown) to a discharge tray (not shown) via a recording area R 0  in which image formation is performed by the recording head  40 .  FIG. 2  schematically shows a partial configuration, around the recording head  40 , of the sheet conveyor  61 . As shown in  FIG. 2 , the sheet conveyor  61  includes a platen  611  below the recording head  40 . Further, the sheet conveyor  61  includes a conveyance roller  613 , a pinch roller  614 , a discharge roller  617 , and a spur roller  618 . The conveyance roller  613  and the pinch roller  614  are disposed to face each other in a position upstream of the platen  611  in the sheet conveyance direction. The discharge roller  617  and the spur roller  618  are disposed to face each other in a position downstream of the platen  611  in the sheet conveyance direction. 
     The conveyance roller  613  and the discharge roller  617  are connected with the PF motor via a transmission mechanism (not shown). In response to receiving a driving force from the PF motor  63 , the conveyance roller  613  and the discharge roller  617  rotate in synchronization with each other. The PF motor  63  includes a direct-current motor for driving the sheet conveyor  61 . 
     When a pickup roller (not shown) rotates, the sheet conveyor  61  separates sheets Q placed on the feed tray (not shown) on a sheet-by-sheet basis, and sequentially feeds the separated sheets Q between the conveyance roller  613  and the pinch roller  614 . When driven to rotate by the PF motor  63 , the conveyance roller  613  conveys a sheet Q fed from the feed tray downstream in the sheet conveyance direction indicated by a dashed arrow in  FIG. 2 . While pinching the sheet Q with the pinch roller  614 , the conveyance roller  613  conveys, by the rotation thereof, the sheet Q downstream in the sheet conveyance direction. 
     The sheet Q, which is being conveyed downstream in the sheet conveyance direction by the rotation of the conveyance roller  613 , passes over the recording area R 0  below the recording head  40  while being supported by the platen  611 . Then, the sheet Q is conveyed downstream in the sheet conveyance direction by the rotation of the discharge roller  617  while being pinched between the discharge roller  617  and the spur roller  618 . After passing between the discharge roller  617  and the spur roller  618 , the sheet Q is finally discharged onto the discharge tray (not shown). 
     The rotary encoder  65  may be disposed at a rotational shaft of the conveyance roller  613  or a rotational shaft of the PF motor  63 , or may be disposed on a power transmission path from the PF motor  63  to the conveyance roller  613 . The rotary encoder  65  is configured to input pulse signals, which correspond to rotation of the conveyance roller  613 , as encoder signals into the printing unit driver  30 . 
     Based on the encoder signals from the rotary encoder  65 , the printing unit driver  30  detects a rotational quantity, a rotational speed, and a rotational phase φ of the conveyance roller  613 . The rotational phase φ corresponds to a rotational angle φ (0≦φ&lt;2π) of the conveyance roller  613  within a range from zero to 2π when a single rotation of the conveyance roller  613  is expressed as 2π. 
     The controller  10  stores in the NVRAM  17  control parameters set according to an individual difference of the printing unit  20 . The controller  10  appropriately controls the printing unit  20  based on the control parameters. Specifically, based on the control parameters stored in the NVRAM  17 , the controller  10  sets for the printing unit driver  30  specific parameters that regulate operations of the printing unit driver  30 , and controls the printing unit driver  30  to operate in accordance with the specific parameters. Thus, the controller  10  adapts the operations of the printing unit driver  30  to the individual difference of the printing unit  20 , and thereby appropriately controls the printing unit  20 . 
     Based on the encoder signals from the linear encoder  55  and the rotary encoder  65 , the printing unit driver  30  takes control of the CR motor  53  and the PF motor  63  according to parameters set specifically for the CR motor  53  and the PF motor  63  by the controller  10 . In the illustrative embodiment, the controller  10  and the printing unit driver  30  cooperate with each other. Thereby, it is possible to implement ink discharge control for the recording head  40  to discharge ink droplets, carriage moving control for the carriage moving mechanism  51  to move the carriage  52  carrying the recording head  40 , and sheet conveyance control for the sheet conveyor  61  to convey the sheets Q. 
     Specifically, the control parameters stored in the NVRAM  17  include a particular control parameter that represents an association between the rotational quantity of the conveyance roller  613  and a sheet conveyance distance. Based on the particular control parameter, the controller  10  sets for the printing unit driver  30  the specific parameters that are adjusted to suppress control errors (e.g., a conveyance distance error caused when a sheet Q is conveyed) caused by the individual difference of the printing unit  20 . For instance, the controller  10  calculates a target rotational quantity of the conveyance roller  613  corresponding to a target sheet conveyance distance, and sets for the printing unit driver  30  a parameter that represents the calculated target rotational quantity of the conveyance roller  613 . Thereby, the conveyance roller  613  is controlled to convey the sheets Q in such a manner as to suppress a conveyance distance error caused by an eccentricity and/or an individual difference in shape of the conveyance roller  613 . 
     The controller  10  corrects the particular control parameter that represents the association between the rotational quantity of the conveyance roller  613  and the sheet conveyance distance based on a result of test pattern formation. The particular control parameter is initially set to a standard value that is determined without considering the individual difference, and is updated to a value according to the individual difference, based on the result of test pattern formation. 
     When receiving an instruction to print test patterns via the user interface  90  or from the external device  3 , the controller  10  performs a test printing process shown in  FIG. 3  in accordance with one or more programs  13   a  stored in the ROM  13 . For instance, when a user of the MFP  1  or an operator of a manufacturer of the MFP  1  operates the user interface  90  or the external device  3 , the instruction to print test patterns is issued. 
     When the test printing process is started, the controller  10  activates and controls the printing unit driver  30  to, while controlling the PF motor  63 , cause the sheet conveyor  61  to convey a sheet Q to an upstream end section of the recording area R 0  below the recording head  40  in the sheet conveyance direction (S 110 : Cueing). 
     Afterward, the controller  10  performs a first pattern forming process (S 120 ). In the first pattern forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form a first test pattern TP 11  on a portion of the sheet Q that is positioned in a first recording area R 1 , using a first nozzle group N 1  (S 120 ). The first recording area R 1  corresponds to a partial area of the recording area R 0  that is positioned under the first nozzle group N 1 . In other words, the first recording area R 1  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the first nozzle group N 1 . The first nozzle group N 1  corresponds to a group of nozzles included in the nozzle group N 0  that are positioned upstream of the other nozzles included in the nozzle group N 0  in the sheet conveyance direction. 
     The first test pattern TP 11  formed on the sheet Q has a geometrical pattern as exemplified in  FIG. 4A . Specifically, the first test pattern TP 11  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction. In the following description, for the sake of convenience in explaining the positions and the forms of test patterns, an X-Y rectangular coordinate system will be defined on a sheet surface. 
     The defined X-Y rectangular coordinate system has an X-axis along the main scanning direction and a Y-axis along the sheet conveyance direction (i.e., the sub scanning direction). The positive directions of the X-axis and the Y-axis may be arbitrarily defined. In the present example, the positive direction of the Y-axis is defined as a downstream direction along the sheet conveyance direction. Further, the positive direction of the X-axis is defined as a rightward direction with respect to the positive direction of the Y-axis. 
     Further, in the illustrative embodiment, a length or a distance in the Y-axis direction is defined on the basis of pixels. In other words, a unit length of the Y-axis is expressed with an interval (i.e., a pixel pitch) between adjacent two of pixels formable on the sheet Q in the sub scanning direction. Hereinafter, the pixel pitch may be referred to as a “dot pitch.” The dot pitch corresponds to a nozzle interval of the recording head  40  in the sub scanning direction. When a distance (Y 2 -Y 1 ) between a position Y 1  and a position Y 2  in the Y-axis direction is expressed as the value “3,” it denotes that the position Y 2  is three pixels away from the position Y 1  in the sub scanning direction. In other words, it represents that there exists a space of two pixels between a pixel in the position Y 1  and a pixel in the position Y 2 . 
     Further, in the illustrative embodiment, a length or a distance in the X-axis direction is defined on the basis of pixels. In other words, a unit length of the X-axis is expressed with an interval (i.e., a dot pitch) between adjacent two of the pixels formable on the sheet Q in the main scanning direction. When a distance (X 2 -X 1 ) between a position X 1  and a position X 2  in the X-axis direction is expressed as the value “2,” it denotes that the position X 2  is two pixels away from the position X 1  in the main scanning direction. In other words, it represents that there exists a space of one pixel between a pixel in the position X 1  and a pixel in the position X 2 . 
     An image formed on the sheet Q is configured with a plurality of pixels (dots) selectively placed on a plurality of points (X, Y) each of which is defined by a combination of arbitrary integer values in the X-Y rectangular coordinate system. In the X-Y rectangular coordinate system, a point (X, Y) defined by a combination of arbitrary integer values may be regarded as a lattice point. Namely, the image formed on the sheet Q is configured with a plurality of pixels selectively placed on a plurality of lattice points of a two-dimensional lattice that are two-dimensionally arranged at intervals of a constant pitch in each of the X-axis direction and the Y-axis direction. In the following description, a vector (X, Y) may be understood as a vector directed from a start pixel to an end pixel that is away from the start pixel by X pixels in the X-axis direction and Y pixels in the Y-axis direction. In other words, the vector (X, Y) may be understood as a vector directed from an arbitrary lattice point to a specific lattice point in the aforementioned two-dimensional lattice. It is noted that, when the arbitrary lattice point is defined as the zeroth lattice point in both of the X-axis direction and the Y-axis direction, the specific lattice point is the X-th lattice point in the X-axis direction and the Y-th lattice point in the Y-axis direction. 
     In a general inkjet printer, the resolution in the main scanning direction is higher than the resolution in the sub scanning direction. For instance, an inkjet printer has been known that has a resolution of 600 dpi in the main scanning direction and a resolution of 300 dpi in the sub scanning direction. In the illustrative embodiment, the MFP  1  has a resolution in the main scanning direction that is higher than a resolution in the sub scanning direction. Namely, a dot pitch DP 1  in the main scanning direction is shorter than a dot pitch DP 2  in the sub scanning direction. This denotes that a degree of freedom for test pattern formation in the X-axis direction is higher than a degree of freedom therefor in the Y-axis direction. 
     Hereinafter, a detailed explanation will be provided of a disposition and a shape of the first test pattern TP 11 . The first test pattern TP 11  formed on the sheet Q in S 120  is a test pattern in which a rectangular elemental  figure EL11  as a constituent of the test pattern is repeatedly formed to be arranged on a virtual straight line LN 1 . The virtual straight line LN 1  is inclined relative to the main scanning direction. Specifically, each elemental  figure EL11  includes a plurality of dots DT arranged linearly or in a rectangular shape as shown in  FIG. 5 . 
     In an example shown in  FIG. 5 , the first test pattern TP 11  is a geometrical pattern in which a plurality of uniform elemental  figures EL11  are arranged in a direction of a first vector (X, Y)=(3, −1) along the virtual straight line LN 1 . Each uniform elemental  figure EL11  includes six dots DT arranged linearly in the X-axis direction. It is noted that the virtual straight line LN 1  is only used for describing that the elemental  figures EL11  are arranged on the straight line LN 1 , but is not actually printed on the sheet Q. 
     In  FIG. 5 , each elemental  figure EL11  includes six dots DT arranged in the main scanning direction. Nonetheless, the number of dots DT included in the single elemental  figure EL11  may be an arbitrary number. In the first test pattern TP 11  exemplified in  FIG. 5 , the direction in which the elemental  figures EL11  are arranged is expressed as the direction of the first vector (X, Y)=(A1, B1), where |A1|=3, and |B1|=1. It is noted that |A1| represents the absolute value of A1, and that |B1| represents the absolute value of B1. Nonetheless, the value |A1| may be more than three. 
     For instance, the direction of the first vector (X, Y)=(A1, B1) may be set based on integers A1 and B1 that satisfy an equality “|A1|&gt;|B1|&gt;0.” The equality “|A1|&gt;|B1|&gt;0” denotes that an angle formed between the direction of the first vector and the X-axis direction is less than 45 degrees. In order to make the first test pattern TP 11  close to the X-axis, |A1| is preferred to be a large value, and |B1| is preferred to be equal to one. 
     After the first test pattern TP 11  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to rotate the conveyance roller  613  by a particular amount L 1  thereby conveying the sheet Q over the particular amount L 1  downstream in the sheet conveyance direction (S 130 ). Thereafter, when making a negative determination in S 140 , the controller  10  again performs the first pattern forming process (S 120 ) in which the recording head  40  is controlled to form another first test pattern TP 11  on the sheet Q. The process of conveying the sheet Q over the particular amount L 1  is carried out by controlling a rotational amount of the conveyance roller  613 . Therefore, an actual sheet conveyance distance in S 130  contains an error relative to the particular amount L 1 . 
     After S 130 , the controller  10  determines whether the first test pattern TP 11  first formed on the sheet Q has reached a second recording area R 2  of the recording area R 0  (S 140 ). Hereinafter, the first test pattern TP 11  first formed on the sheet Q may be simply referred to as a “head first test pattern TP 11 ” or a “first-formed first test pattern TP 11 ” to differentiate it from other first test patterns TP 11  to be subsequently formed on the sheet Q. The second recording area R 2  is an area in which a second test pattern TP 21  is formed. When determining that the head first test pattern TP 11  has not reached the second recording area R 2  (S 140 : No), the controller  10  goes to S 120 . Namely, until the first-formed first test pattern TP 11  reaches the second recording area R 2 , the controller  10  repeatedly performs the processes of making the negative determination in S 140  (S 140 : No), controlling the recording head  40  to form the first test pattern TP 11  on the sheet Q (S 120 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 1  (S 130 ). Meanwhile, when determining that the head first test pattern TP 11  has reached the second recording area R 2  (S 140 : Yes), the controller  10  goes to S 150 .  FIG. 4B  shows the first test patterns TP 11  that are formed at intervals of the distance L 1  (i.e., the particular amount L 1 ) in the Y-axis direction by repeating the steps S 120  and S 130 . 
     The particular amount L 1 , which corresponds to the formation interval of the first test patterns TP 11  in the Y-axis direction, is as long as a part of a length L 0  (see  FIG. 2 ) divided by an integer. As shown in  FIG. 2 , the length L 0  is a length between an upstream end of the first recording area R 1  and an upstream end of the second recording area R 2  in the sub scanning direction, within the recording area R 0 . Further, the particular amount L 1  is as long as a part of an outer circumferential length of the conveyance roller  613  divided by an integer. The outer circumferential length of the conveyance roller  613  corresponds to a sheet conveyance distance when the conveyance roller has made a single rotation. 
     In the illustrative embodiment, a conveyance distance error between the rotational amount of the conveyance roller  613  and the actual sheet conveyance distance depends on a rotational phase of the conveyance roller  613  in sheet conveyance. In the illustrative embodiment, in order to suppress an influence of the above conveyance distance error depending on the rotational phase, a conveyance distance error is determined at each of different rotational phases that are defined by dividing the outer circumferential length of the conveyance roller  613  into a plurality of sections. The formation of the first test patterns TP 11  at intervals of the particular amount L 1  in the Y-axis direction is for determining a conveyance distance error at each rotational phase of the conveyance roller  613 . 
     When determining that the first test pattern TP 11  first formed on the sheet Q has reached the second recording area R 2  (S 140 : Yes), the controller  10  goes to S 150 . In S 150 , the controller  10  performs a both-patterns forming process. 
     In the both-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional first test pattern TP 11  and a second test pattern TP 21  on the sheet Q (S 150 ). Specifically, in S 150 , the controller  10  controls the recording head  40  to form the first test pattern TP 11  on a portion of the sheet Q that is positioned in the first recording area R 1 , using the first nozzle group N 1 , and form the second test pattern TP 21  on a portion of the sheet Q that is positioned in the second recording area R 2 , using the second nozzle group N 2  (see  FIG. 4C ). The second recording area R 2  corresponds to a partial area of the recording area R 0  that is positioned under the second nozzle group N 2 . In other words, the second recording area R 2  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the second nozzle group N 2 . Among the nozzle group N 0 , the second nozzle group N 2  is positioned downstream of the first nozzle group N 1  in the sheet conveyance direction. 
       FIG. 4C  exemplifies the second test pattern TP 21  formed on the sheet Q. The second test pattern TP 21  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction at a different degree of inclination from the first test pattern TP 11 . Namely, the second test pattern TP 21  is macroscopically or approximately inclined relative to each of the main scanning direction and the first test pattern TP 11 .  FIG. 4C  shows a state where the second test pattern TP 21  is formed in the first-executed S 150  so as to intersect the head first test pattern TP 11  on the sheet Q. 
     Specifically, the second test pattern TP 21  is a test pattern in which a rectangular elemental  figure EL21  as a constituent of the test pattern is repeatedly formed to be arranged on a virtual straight line LN 2 . The virtual straight line LN 2  is inclined relative to the main scanning direction. Each elemental  figure EL21  includes a plurality of dots DT arranged linearly or in a rectangular shape as shown in  FIG. 6 , in the same manner as the first test pattern TP 11 . 
     In an example shown in  FIG. 6 , the second test pattern TP 21  is a geometrical pattern in which a plurality of uniform elemental  figures EL21  are arranged in a direction of a second vector (X, Y)=(2, −1) along the virtual straight line LN 2 . Each uniform elemental  figure EL21  includes four dots DT arranged linearly in the X-axis direction. It is noted that the virtual straight line LN 2  is only used for describing that the elemental  figures EL21  are arranged on the straight line LN 2 , but is not actually printed on the sheet Q. 
     In  FIG. 6 , each elemental  figure EL21  includes four dots DT arranged in the main scanning direction. Nonetheless, the number of dots DT included in the single elemental  figure EL21  may be an arbitrary number. In the second test pattern TP 21  exemplified in  FIG. 6 , the direction in which the elemental  figures EL21  are arranged is expressed as the direction of the second vector (X, Y)=(A2, B2), where |A2|=2, and |B2|=1. Nonetheless, the value |A2| may be more than two. 
     For instance, the direction of the second vector (X, Y)=(A2, B2) may be set based on integers A2 and B2 that satisfy an equality “|A2|&gt;|B2|&gt;0” and “(B1/A1)×(B2/A2)&gt;0” within such a range that the direction of the second vector is not parallel to the direction of the first vector. The equality “|A2|&gt;|B2|&gt;0” denotes that an angle formed between the direction of the second vector and the X-axis direction is less than 45 degrees. The equality “(B1/A1)*(B2/A2)&gt;0” denotes that the inclination of the virtual straight line LN 2  along the direction of the second vector has the same one of the positive and negative signs as the inclination of the virtual straight line LN 1  along the direction of the first vector. In order to make the second test pattern TP 21  close to the X-axis, |A2| is preferred to be a large value, and |B2| is preferred to be equal to one. Furthermore, in order to make small an angle at which the first test pattern TP 11  intersects the second test pattern TP 21 , |A2| is preferred to be close to |A1|. 
     After the second test pattern TP 21  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 1  downstream in the sheet conveyance direction (S 160 ). Thereafter, when making a negative determination in S 170 , the controller  10  again performs the both-patterns forming process (S 150 ). In both-patterns forming process (S 150 ), as shown in  FIG. 4D , the recording head  40  is controlled to form another first test pattern TP 11  on the sheet Q with the first nozzle group N 1  and form another second test pattern TP 21  with the second nozzle group N 2  to intersect the first test pattern TP 11  that has reached the second recording area R 2 . 
     After S 160 , the controller  10  determines whether a predetermined number of first test patterns TP 11  have been completely formed (S 170 ). When determining that the predetermined number of first test patterns TP 11  have not been completely formed (S 170 : No), the controller  10  goes to S 150 . Namely, until the predetermined number of first test patterns TP 11  are completely formed, the controller  10  repeatedly performs the processes of making the negative determination in S 170  (S 170 : No), controlling the recording head  40  to form the first test pattern TP 11  and the second test pattern TP 21  on the sheet Q (S 150 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 1  (S 160 ). 
     Meanwhile, when determining that the predetermined number of first test patterns TP 11  have been completely formed (S 170 : Yes), the controller  10  goes to S 180 . It is noted that, according to aspects of the present disclosure, the controller  10  may stop forming the first test patterns TP 11  at a point of time when the head first test pattern TP 11  reaches the second recording area R 2 . In this case, in response to making the affirmative determination in S 140  (S 140 : Yes), the controller  10  may go to S 180  without executing any of the steps S 150  to S 170 . 
     In S 180 , the controller  10  performs a second pattern forming process. In the second pattern forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form a second test pattern TP 21  on a portion of the sheet Q that is positioned in the second recording area R 2  with the second nozzle group N 2  (S 180 ). Namely, the recording head  40  is controlled to form the second test pattern TP 21  to intersect the first test pattern TP 11  that has reached the second recording area R 2 . 
     Afterward, the controller  10  determines whether the second test pattern TP 21  has been formed for each of all the first test patterns TP 11  (S 190 ). When determining that the second test pattern TP 21  has not been formed for each of all the first test patterns TP 11  (S 190 : No), the controller  10  goes to S 200 . Meanwhile, when determining that the second test pattern TP 21  has been formed for each of all the first test patterns TP 11  (S 190 : Yes), the controller  10  goes to S 210 . 
     In S 200 , the controller  10  controls the sheet conveyor  61  to convey the sheet Q over the particular amount L 1  in the same manner as executed in S 130 . Thereafter, the controller  10  goes to S 180 . Thus, until the second test pattern TP 21  is formed for each of all the first test patterns TP 11 , the controller  10  repeatedly performs the processes of making the negative determination in S 190  (S 190 : No), controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 1  (S 200 ), and controlling the recording head  40  to form the second test pattern TP 21  on the sheet Q (S 180 ). Then, when the second test pattern TP 21  has been formed for each of all the first test patterns TP 11  (S 190 : Yes), the controller  10  performs a sheet discharging process (S 210 ). 
     Specifically, in S 210 , the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey and discharge the sheet Q onto the discharge tray (not shown). Further, the controller  10  controls the user interface  90  to display, on the display of the user interface  90 , a message that prompts the user to place the sheet Q with the test patterns printed thereon on the document table of the scanning unit  70  and input a scanning instruction (S 220 ). Thereafter, the controller  10  waits until a scanning instruction is input through the user interface  90  (S 230 ). 
     When a scanning instruction has been input, the controller  10  performs a scanning process (S 240 ). Specifically, in S 240 , the controller  10  controls the scanning unit  70  to scan the sheet Q with the test patterns printed thereon, and acquires image data expressing the scanned image from the scanning unit  70 . 
     Further, the controller  10  calculates a positional displacement, from a reference point, of an intersection between each first test pattern TP 11  and the corresponding second test pattern TP 21  based on the image data acquired from the scanning unit  70 , and calculates a conveyance distance error of the sheet Q based on the calculated positional displacement (S 250 ). Based on the conveyance distance error calculated in S 250 , the controller  10  updates the particular control parameter (included in the control parameters stored in the NVRAM  17 ) that represents the association between the rotational quantity of the conveyance roller  613  and the sheet conveyance distance (S 260 ). Thereafter, the controller  10  terminates the test printing process shown in  FIG. 3 . 
     An explanation will be provided about how to calculate the conveyance distance error of the sheet Q. The controller  10  calculates a conveyance distance error of the sheet Q for a combination of each of the first test patterns TP 11  formed on the sheet Q and a corresponding one of the second test patterns TP 21  formed on the sheet Q, by performing the following process in S 250  based on the image data acquired from the scanning unit  70 . Then, based on the calculation results, the controller  10  updates the particular control parameter in S 260 . 
     Specifically, as shown in  FIG. 7 , with respect to each of the first test patterns TP 11  in the image data, the controller  10  slides a position of a rectangular window WN (indicated by a solid line) along the first test pattern TP 11  on a step-by-step basis of a predetermined amount (as indicated by alternate long and short dash lines). Then, the controller  10  calculates a density (e.g., a total area of the test patterns TP 11  and TP 21  per a particular area of the rectangular window WN) within the rectangular window WN in each position of the rectangular window WN. Thereby, the density in each position along each first test pattern TP 11  is calculated. The rectangular window WN may be defined as a rectangular window having a longitudinal direction along the sub scanning direction. A length of the rectangular window WN in the sub scanning direction may be set as a length including both of the first test pattern TP 11  and the second test pattern TP 21 . 
     As a total area of the test patterns TP 11  and TP 21  included in the rectangular window WN becomes smaller, the density becomes lower. Accordingly, a change of the density (hereinafter, which may be referred to as a “density distribution”) along the first test pattern TP 11  is likely to have a local minimum value at the intersection between the first test pattern TP 11  and the second test pattern TP 21 . In S 250 , the controller  10  identifies a position in the X-axis direction where the density distribution (i.e., the change of the density) along the first test pattern TP 11  has the local minimum value, as a position of the intersection between the first test pattern TP 11  and the second test pattern TP 21 . 
       FIG. 8A  shows a first test pattern TP 12  and a second test pattern TP 22  that are different from the first test pattern TP 11  and the second test pattern TP 21  shown in  FIGS. 4A-4D and 5-7 , respectively.  FIG. 8B  provides an upper graph showing a dot distribution of the first test pattern TP 12  and the second test pattern TP 22  and a lower graph showing a density distribution calculated to identify a position of the intersection between the first test pattern TP 12  and the second test pattern TP 22 , in association with each other. 
     As shown in  FIG. 8A , the first test pattern TP 12  is formed as a geometrical pattern including a plurality of elemental  figures EL12  arranged in a direction of a first vector (X, Y)=(9, −1). Each elemental  figure EL12  has two rows each of which includes nine dots DT arranged in the X-axis direction. Namely, each elemental  figure EL12  is a cluster of dots defined by nine pixels in the X-axis direction and two pixels in the Y-axis direction. The second test pattern TP 22  is formed as a geometrical pattern including a plurality of elemental  figures EL22  arranged in a direction of a second vector (X, Y)=(8, −1). Each elemental  figure EL22  has two rows each of which includes eight dots DT arranged in the X-axis direction. Namely, each elemental  figure EL22  is a cluster of dots defined by eight pixels in the X-axis direction and two pixels in the Y-axis direction. As understood from  FIG. 8B , a density distribution of the present example has a (local) minimum value at an intersection between the first test pattern TP 12  and the second test pattern TP 22 . The combination of the first test pattern TP 11  and the second test pattern TP 21  provides the same density distribution as the density distribution for the combination of the first test pattern TP 12  and the second test pattern TP 22 . 
     After identifying a position in the X-axis direction of the intersection between the first test pattern TP 11  and the second test pattern TP 21  based on the density distribution, the controller  10  calculates a positional displacement ΔX in the X-axis direction between the identified position of the intersection and the reference point. The reference point corresponds to a position of the intersection between the first test pattern TP 11  and the second test pattern TP 21  when the conveyance distance error of the sheet Q is zero. Positional information of the reference point may be stored in the NVRAM  17 . 
     In an upper area of  FIG. 9 , a white circle indicates an intersection between the virtual straight line LN 1  along the first test pattern TP 11  and the virtual straight line LN 2  along the second test pattern TP 21  when the conveyance distance error of the sheet Q is zero. The white circle corresponds to the reference point. In a lower area of  FIG. 9 , a black circle indicates an intersection between the virtual straight line LN 1  along the first test pattern TP 11  and the virtual straight line LN 2  along the second test pattern TP 21  when the second test pattern TP 21  is formed in a situation where a sheet conveyance distance is shorter than when the conveyance distance error is zero and where the sheet Q is positioned |ΔY| upstream in the sheet conveyance direction relative to a position of the sheet Q when the conveyance distance error is zero. As understood from positional relationships shown in  FIG. 9 , a relationship between the conveyance distance error ΔY in the sub scanning direction and the positional displacement ΔX between the intersection and the reference point in the main scanning direction is expressed as follows.
 
Δ Y=ΔX *(tan θ2−tan θ1)
 
In the above expression, tan θ1 corresponds to an inclination of the virtual straight line LN 1  along the first test pattern TP 11 , i.e., tan θ1=B1/A1. Further, tan θ2 corresponds to an inclination of the virtual straight line LN 2  along the second test pattern TP 21 , i.e., tan θ2=B2/A2. When ΔY is a positive value, it denotes that the sheet Q is over-conveyed by |ΔY| downstream in the sheet conveyance direction in comparison with when the conveyance distance error is zero. When ΔY is a negative value, it denotes that the sheet Q is under-conveyed by |ΔY| upstream in the sheet conveyance direction in comparison with when the conveyance distance error is zero. In the above expression, ΔX is a value on the X-axis having the unit length in the main scanning direction, and ΔY is a value on the Y-axis having the unit length in the sub scanning direction. When ΔX and ΔY are required to be expressed on the basis of inch unit, tan θ1 may be expressed as tan θ1=(B1*DP 2 ), and tan θ2 may be expressed as tan θ2=(B1*DP 2 )/(A2*DP 1 ).
 
     In S 250 , the controller  10  calculates the conveyance distance error ΔY of the sheet Q by substituting the calculated positional displacement ΔX in the expression ΔY=ΔX*(tan θ2−tan θ1). Thus, the controller  10  calculates the conveyance distance error ΔY of the sheet Q when the conveyance roller  613  is rotated such that the sheet Q is conveyed over the distance L 0 , for each first test pattern TP 11  (i.e., for each rotational phase of the conveyance roller  613 ). Based on the conveyance distance error ΔY, the controller  10  updates (corrects) the particular control parameter that represents the association between the rotational quantity of the conveyance roller  613  and the sheet conveyance distance for each rotational phase of the conveyance roller  613  (S 260 ). 
     As understood from the above expression, the absolute positional displacement |ΔX| of the intersection between the first test pattern TP 11  and the second test pattern TP 21  depending on the absolute conveyance distance error |ΔY| increases as an angle |Δθ|=|θ2−θ1| between the first test pattern TP 11  and the second test pattern TP 21  decreases. Furthermore, in general, as understood from that the differential of tan θ is 1/cos 2  θ, the inclination of tan θ decreases as θ decreases. Namely, when |Δ θ| is constant, a value |tan θ2−tan θ1| decreases as the angles θ1 and θ2 decrease. 
     Accordingly, in order to increase the absolute positional displacement |ΔX| of the intersection between the first test pattern TP 11  and the second test pattern TP 21  depending on the absolute conveyance distance error |ΔY|, it is preferred to decrease the angle |Δθ| between the first test pattern TP 11  and the second test pattern TP 21  in a state where the first test pattern TP 11  and the second test pattern TP 21  are inclined to be close to the main scanning direction (i.e., the X-axis direction). 
     Therefore, preferably, the first test pattern TP 11  and the second test pattern TP 21  may be formed in such a manner that |B1|=|B2|=1, that a difference between |A1| and |A2| is small, and that |A1| and |A2| are large values. 
     From a comparison between  FIGS. 10 and 13 , it is understood that the test pattern printing of the illustrative embodiment (see e.g.,  FIG. 10 ) causes a larger positional displacement of the intersection between the test patterns formed thereby with respect to a conveyance distance error of the sheet Q than a known test pattern printing method (see e.g.,  FIG. 13 ). In an upper area of  FIG. 10 , a white circle indicates an intersection between the first test pattern TP 11  and the second test pattern TP 21  as illustrative test patterns according to aspects of the present disclosure when there is no conveyance distance error caused by conveyance of the sheet Q. In a lower area of  FIG. 10 , a black circle indicates an intersection between the first test pattern TP 11  and the second test pattern TP 21  when a conveyance distance error of ΔY 1  (one pixel) in the sub scanning direction is caused by the conveyance of the sheet Q. In this situation, a positional displacement in the main scanning direction between the intersection indicated by the black circle and the reference point indicated by the white circle is defined as ΔXA. 
     In an upper area of  FIG. 13 , a white circle indicates an intersection between a first test pattern TP 10  and a second test pattern TP 20  as test patterns formed in the known test pattern printing method when there is no conveyance distance error caused by the conveyance of the sheet Q. In a lower area of  FIG. 13 , a black circle indicates an intersection between the first test pattern TP 10  and the second test pattern TP 20  when a conveyance distance error of ΔY 1  (one pixel) in the sub scanning direction is caused by the conveyance of the sheet Q. In this situation, a positional displacement in the main scanning direction between the intersection indicated by the black circle and the reference point indicated by the white circle is defined as ΔXB. 
     According to the known test pattern printing method, the first test pattern TP 10  is parallel to the main scanning direction. Therefore, since an angle formable between the first test pattern TP 10  and the second test pattern TP 20  is influenced by the dot pitch in the sub scanning direction, it is not possible to make so small the angle between the first test pattern TP 10  and the second test pattern TP 20 . Accordingly, when the conveyance distance error of the sheet Q is not large, the positional displacement of the intersection from the reference point is small. Thus, in such a case, it is difficult to accurately calculate the conveyance distance error based on the positional displacement. Further, the second test pattern TP 20  is not formed in a strictly straight line but in a terraced shape. Hence, even though the angle between the first test pattern TP 10  and the second test pattern TP 20  is made small, the intersection is not displaced from the reference point in the main scanning direction as long as the conveyance distance error of the sheet Q is less than a length of one pixel in the sub scanning direction. Thus, according to the known test pattern printing method, it is impossible to detect a conveyance distance error less than a dot pitch (e.g., a nozzle interval) in the sub scanning direction. 
     In contrast, according to the illustrative embodiment, both of the first test pattern TP 11  and the second test pattern TP 21  are inclined relative to the main scanning direction. Therefore, it is possible to reduce the angle between the first test pattern TP 11  and the second test pattern TP 21  without being so influenced by the dot pitch in the sub scanning direction. Accordingly, as understood from the comparison between  FIGS. 10 and 13 , according to the illustrative embodiment, it is possible to remarkably enlarge the positional displacement of the intersection in the main scanning direction with respect to the conveyance distance error of the sheet Q in the sub scanning direction in comparison with the known test pattern printing method. Thus, according to the illustrative embodiment, it is possible to accurately the conveyance distance error of the sheet Q in the sub scanning direction based on the positional displacement of the intersection in the main scanning direction. Further, according to the illustrative embodiment, even though the conveyance distance error of the sheet Q is less than a length of one pixel in the sub scanning direction, the intersection is displaced from the reference point in the main scanning direction. Thus, it is possible to accurately calculate such a small conveyance distance error. According to the known test pattern printing method, in order to detect a conveyance distance error less than the dot pitch in the sub scanning direction, the sheet Q needs to be finely conveyed. Nonetheless, according to the illustrative embodiment, it is possible to detect a conveyance distance error less than the dot pitch in the sub scanning direction without having to so finely convey the sheet Q. Thus, according to the illustrative embodiment, it is possible to accurately detect the conveyance distance error of the sheet Q while quickly and efficiently forming the test patterns. 
     The first test pattern TP 11  and the second test pattern TP 21  shown in  FIGS. 4A to 7  and the first test pattern TP 12  and the second test pattern TP 22  shown in  FIG. 8  are merely examples, and may be replaced with other test patterns. For instance, the first test pattern TP 11  may be replaced with the second test pattern TP 21 . The first test pattern TP 11  and the second test pattern TP 21  may be changed to test patterns exemplified in  FIGS. 11A to 11E . Each of  FIGS. 11A to 11E  shows a part of each of the test patterns TP 3 , TP 4 , TP 5 , TP 6  in an enlarged manner, in the same way as  FIGS. 5 and 6  show the test patterns TP 11  and TP 21 , respectively. The first test patterns TP 11  and TP 12  and the second test patterns TP 21 , and TP 22  may be changed to any of the test patterns TP 3 , TP 4 , TP 5 , TP 6 , and TP 7  exemplified in  FIGS. 11A to 11E , or may be changed to other test patterns. 
     The test pattern TP 3  shown in  FIG. 11A  is formed as a geometrical pattern in which a plurality of uniform elemental  figures EL3  are arranged in a direction of a vector (X, Y)=(4, −1). Each uniform elemental  figure EL3  includes seven dots DT arranged linearly in the X-axis direction. The test pattern TP 4  shown in  FIG. 11B  is formed as a geometrical pattern in which a plurality of uniform elemental  figures EL4  are arranged in a direction of the vector (X, Y)=(4, −1). Each uniform elemental  figure EL4  includes nine dots DT arranged linearly in the X-axis direction. 
     The test patterns TP 3  and TP 4  are similar to the test patterns TP 11  and TP 21  shown in  FIGS. 5 and 6 , but are different from them in the following points. The test pattern TP 11  shown in  FIG. 5  includes the elemental  figures EL11  each of which has a width of six pixels in the X-axis direction. The width of each elemental  figure EL11  in the X-axis direction is an integral multiple of the X-axis component A1=3 of the vector (X, Y)=(3, −1) that defines the direction in which the elemental  figures EL11  are arranged. On the other hand, the test pattern TP 3  shown in  FIG. 11A  includes the elemental  figures EL3  each of which has a width of seven pixels in the X-axis direction. The width of each elemental  figure EL3  in the X-axis direction is not an integral multiple of the X-axis component (i.e., 4) of the vector (X, Y)=(4, −1) that defines the direction in which the elemental  figures EL3  are arranged. Further, the test pattern TP 4  shown in  FIG. 11B  includes the elemental  figures EL4  each of which has a width of nine pixels in the X-axis direction. The width of each elemental  figure EL4  in the X-axis direction is not an integral multiple of the X-axis component (i.e.,  4 ) of the vector (X, Y)=(4, −1) that defines the direction in which the elemental  figures EL4  are arranged. The test pattern TP 21  shown in  FIG. 6  includes the elemental  figures EL21  each of which has a width of four pixels in the X-axis direction. The width of each elemental  figure EL21  in the X-axis direction is an integral multiple of the X-axis component A2=2 of the vector (X, Y)=(2, −1) that defines the direction in which the elemental  figures EL21  are arranged. 
     A particular test pattern (e.g., TP 3  and TP 4 ), which includes elemental figures each of which has a width that is not an integral multiple of the X-axis component of a vector (X, Y)=(A, B) to define the arrangement direction of the elemental figures, has particular portions as indicated by bold type arrows in  FIGS. 11A and 11B . Specifically, the particular portions of the particular test pattern have a different width in the sub scanning direction from the width of the other portions of the particular test pattern. Owing to the unevenness of the width of the particular test pattern in the sub scanning direction, the particular test pattern is formed in the shape of a macroscopically or approximately straight line with an uneven width. The width unevenness of the particular test pattern is likely to cause a minute fluctuation in the density distribution determined in S 250  and make it difficult to identify the local minimum value of the density corresponding to the intersection between the test patterns. 
     Accordingly, as shown in  FIGS. 5 and 6 , a test pattern is preferred to be formed to satisfy the following relationship among a vector (X, Y)=(A, B) to define an arrangement direction of elemental figures included in the test pattern, and a width of each elemental figure in the X-axis direction, and a width of each elemental figure in the Y-axis direction. Specifically, the test pattern is preferred to be formed as a geometrical pattern in which a plurality of rectangular or linear elemental figures are arranged in the arrangement direction defined by the vector (X, Y)=(A, B) at intervals of a distance corresponding to the vector (X, Y)=(A, B) (i.e., the elemental figures are arranged in such a manner that one elemental figure is away from another by A pixels in the X-axis direction and B pixels in the Y-axis direction). In this case, each rectangular or linear elemental figure is preferred to be formed as a cluster of dots having a count of pixels in the Y-axis direction corresponding to the Y-axis component B of the vector (X, Y)=(A, B) and a count of pixels in the X-axis direction that is an integral multiple of (e.g., may be one time as many as) the X-axis component A of the vector (X, Y)=(A, B). 
     Alternatively, as shown in  FIG. 8 , the test pattern may be formed as a geometrical pattern in which a plurality of rectangular or linear elemental figures are arranged in the arrangement direction defined by the vector (X, Y)=(A, B) at intervals of the distance corresponding to the vector (X, Y)=(A, B). Each rectangular or linear elemental figure is formed as a cluster of dots having a count of pixels in the X-axis direction equal to the X-axis component A of the vector (X, Y)=(A, B). The test patterns TP 12  and TP 22  shown in  FIG. 8  satisfy the above requirements. Therefore, in the test patterns TP 12  and TP 22  shown in  FIG. 8 , there is no unevenness of their width in the sub scanning direction as shown in  FIGS. 11A and 11B . Accordingly, it is possible to suppress a minute fluctuation in the density distribution and to accurately identify the local minimum value of the density corresponding to the intersection between the test patterns. 
     A test pattern TP 5  shown in  FIG. 11C  is formed as a geometrical pattern in which a plurality of elemental  figures EL5  are arranged in a direction of a vector (X, Y)=(3, −2). Each elemental  figure EL5  has two rows each including six dots DT arranged in the X-axis direction. 
     A test pattern TP 6  shown in  FIG. 11D  is formed as a geometrical pattern in which elemental  figures EL61  and elemental  figures EL62  are alternately arranged on a virtual straight line LN 6 . Each elemental  figure EL61  has a single row including seven dots DT arranged in the X-axis direction. Each elemental  figure EL62  has two rows each including seven dots DT arranged in the X-axis direction. In other words, the test pattern TP 6  shown in  FIG. 11D  may be interpreted as a geometrical pattern in which a plurality of non-rectangular elemental  figures EL6  are arranged in a direction of a vector (X, Y)=(8, −3) at intervals of a distance corresponding to the vector (X, Y)=(8, −3). Each non-rectangular elemental  figure EL6  is formed by integrating the different two elemental  figures EL61  and EL 62 . 
     A test pattern TP 7  shown in  FIG. 11E  is formed as a geometrical pattern in which different elemental  figures EL71 , EL 72 , EL 73 , EL 74 , and EL 75  are arranged in a direction of a vector (X, Y)=(4, −1). Each of the different elemental  figures EL71 , EL 72 , EL 73 , EL 74 , and EL 75  has a single row of seven dots formed by arranging two types of dots DT 1  and DT 2  different in size in the X-axis direction in a unique order. The dot size may be changed by changing the size of an ink droplet discharged from a nozzle. 
     As an example different from the aforementioned examples, a first test pattern and a second test pattern may be macroscopically or approximately formed as linear geometrical patterns (including patterns formed in the shape of a straight dashed line) that have mutually-different inclinations relative to the main scanning direction. Preferably, the first test pattern and the second test pattern may be formed in a straight line with a uniform width. Further preferably, the first test pattern and the second test pattern may be formed to intersect each other at a small angle and have a small inclination angle with respect to the main scanning direction. By using the first test pattern and the second test pattern formed as above, it is possible to accurately calculate the conveyance distance error of the sheet Q. 
     Hereinabove, the illustrative embodiment according to aspects of the present disclosure has been described. The present disclosure can be practiced by employing conventional materials, methodology and equipment. Accordingly, the details of such materials, equipment and methodology are not set forth herein in detail. In the previous descriptions, numerous specific details are set forth, such as specific materials, structures, chemicals, processes, etc., in order to provide a thorough understanding of the present disclosure. However, it should be recognized that the present disclosure can be practiced without reapportioning to the details specifically set forth. In other instances, well known processing structures have not been described in detail, in order not to unnecessarily obscure the present disclosure. 
     Only an exemplary illustrative embodiment of the present disclosure and but a few examples of their versatility are shown and described in the present disclosure. It is to be understood that the present disclosure is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For instance, according to aspects of the present disclosure, the following modifications are possible. 
     (Modifications) 
     Aspects of the Present Disclosure may be Applied to Line Inkjet Printers and Laser Printers. 
     Hereinafter, a modification according to aspects of the present disclosure will be described with reference to  FIG. 12 .  FIG. 12  shows a line inkjet printer  101  that includes a plurality of line inkjet heads  141 ,  142 ,  143 , and  144  as recording heads. In  FIG. 12 , elements provided with the other reference characters are configured substantially in the same manner as elements provided with the same reference characters in  FIG. 2 . Each of the line inkjet heads  141 ,  142 ,  143 , and  144  includes a nozzle group N 4  of ink discharge nozzles arranged in the main scanning direction. The line inkjet printer  101  is configured to intermittently discharge ink droplets, onto a sheet Q relatively moving along the sub scanning direction, from the nozzle groups N 4  each having the ink discharge nozzles arranged in the main scanning direction, thereby forming an image on the sheet Q without moving the recording heads along the main scanning direction. In the line inkjet printer  101  shown in  FIG. 12 , the line inkjet heads  141 ,  142 ,  143 , and  144 , each of which is a specific recording head for a corresponding color and disposed along the main scanning direction, are arranged in the sub scanning direction. When aspects of the present disclosure are applied to the line inkjet printer  101 , a first test pattern may be formed with at least one of the upstream line inkjet heads  141 ,  142 , and  143  that are positioned upstream of the line inkjet head  144  in the sheet conveyance direction (i.e., the sub scanning direction). Further, in this case, a second test pattern may be formed with at least one of the downstream line inkjet heads  142 ,  143 , and  144  that are positioned downstream of the line inkjet head  141  in the sheet conveyance direction. 
     As an example of laser printers, a laser printer (e.g., a tandem type laser printer) has been known that is configured to form a color image on a recording medium (e.g., a sheet Q or a transfer belt) by sequentially superposing one toner image of a specific color on another toner image of a different color on the recording medium. In the laser printer, when the toner images of different colors are transferred onto the recording medium, the toner images are required to be accurately superposed. The accurate superposing of the toner images needs accurate conveyance control for the recording medium. When aspects of the present disclosure are applied to the laser printer, it is possible to accurately calculate a conveyance distance error of the recording medium by forming and reading test patterns as exemplified in the aforementioned illustrative embodiment. Thus, it is possible to suppress the conveyance distance error of the recording medium and accurately superpose the toner images of different colors on the recording medium. 
     In the aforementioned illustrative embodiment, a position of an intersection between test patterns formed on a sheet Q is identified by analyzing the image data acquired by the scanning unit  70 . Nonetheless, the position of the intersection between the test patterns formed on the sheet Q may be identified by visual recognition by the user. In this case, coordinate information (e.g., a grid or scales) may be printed together with the test patterns. The user may input, into the MFP  1 , positional coordinates of the intersection identified through the visual recognition using the coordinate information printed together with the test patterns. In this case, in S 250 , the controller  10  may calculate the conveyance distance error of the sheet Q based on the positional coordinates of the intersection input via the user interface  90 . 
     Alternatively, a position of an intersection between test patterns formed on a sheet Q may be calculated as a position of an intersection between approximate straight lines determined through linear approximation of the test patterns. In the aforementioned illustrative embodiment, a first test pattern and a second test pattern are formed on a sheet Q to intersect each other on the sheet Q. Nonetheless, the first test pattern and the second test pattern may be formed on the sheet Q so as to be in proximity to each other without intersecting each other on the sheet Q. Namely, the first test pattern and the second test pattern may be formed on the sheet Q in such a manner that the first test pattern and the second test pattern would interest each other in an imaginary intersection outside the sheet Q if the first test pattern and the second test pattern were extended beyond the sheet Q. Further, in other words, the first test pattern and the second test pattern may be formed on the sheet Q in such a manner that a virtual straight line or an approximate straight line for the first test pattern intersects a virtual straight line or an approximate straight line for the second test pattern within a specified area on the two-dimensional lattice that may include a printable area on the sheet Q and may extend beyond the sheet Q. In this regard, however, it is noted that the accuracy for calculating the conveyance distance error becomes lower as the imaginary intersection between the first test pattern and the second test pattern becomes farther away from the printable area on the sheet Q within which the first test pattern and the second test pattern are actually formed. Therefore, the first test pattern and the second test pattern are preferred to be formed so as to be as close to each other as possible even though the first test pattern and the second test pattern do not intersect each other on the sheet Q. Further, the aforementioned two-dimensional lattice having lattice points to place pixels is not limited to a two-dimensional lattice with an X-axis direction and a Y-axis direction orthogonal to each other. In other words, the two-dimensional lattice may be a two-dimensional lattice in which an X-axis direction is not orthogonal to a Y-axis direction perpendicular to each other. 
     In the aforementioned illustrative embodiment, aspects of the present disclosure are applied to the MFP  1  as an example of an image forming apparatus. Nonetheless, aspects of the present disclosure may be applied to an image forming system. In other words, in  FIG. 1 , the element  1 , which is referred to as the MFP  1  in the aforementioned illustrative embodiment, may be configured as an image forming system  1 . In this case, the image forming system  1  may include a separate printer  30 , a separate scanner  70 , and a controller  10  coupled with the printer  30  and the scanner  70 . The printer  30  may be configured to form a first test pattern and a second test pattern on a sheet Q. The scanner  70  may be configured to scan the sheet Q with the first and second test patterns formed thereon, e.g., in an optical manner, and generate image data expressing a scanned image of the sheet Q. The controller  10  may be configured to control the printer  30  to form the first and second test patterns on the sheet Q, control the scanner  70  to scan the sheet Q, and identify a position of an intersection between the first and second test patterns formed on the sheet Q by analyzing the image data expressing the scanned image of the sheet Q scanned by the scanner  70 . Further, the controller  10  may be configured to correct a control parameter for controlling the sheet conveyor  61  to convey sheets Q so as to suppress a conveyance distance error caused when the sheet conveyor  61  conveys the sheets Q, based on the identified position of the intersection between the first and second test patterns. 
     Hereinafter, a further modification according to aspects of the present disclosure will be described with reference to  FIGS. 14, 15A, 15B, and 16A to 16D . In the following modification, elements having substantially the same configurations as exemplified in the aforementioned illustrative embodiment will be provided with the same reference numerals as used for the corresponding elements in the illustrative embodiment. Further, detailed explanations of those elements will be omitted. 
     In the aforementioned illustrative embodiment, as exemplified in  FIGS. 2, 3, and 4A to 4D , the controller  10  controls the recording head  40  to form the first test pattern TP 11  on a portion of the sheet Q that is positioned in the first recording area R 1 , using the first nozzle group N 1 , and form the second test pattern TP 21  on a portion of the sheet Q that is positioned in the second recording area R 2 , using the second nozzle group N 2 . Nonetheless, the group N 0  of ink discharge nozzles arranged in the sub scanning direction may include three or more nozzle groups for forming test patterns. For instance, in the present modification, as shown in  FIG. 14 , the group N 0  of ink discharge nozzles includes a first nozzle group NG 1 , a second nozzle group NG 2 , a third nozzle group NG 3 , and a fourth nozzle group NG 4 . In other words, in the present modification, the controller  10  controls the recording head  40  to form a first test pattern TP 10  on a portion of the sheet Q that is positioned in a first recording area RA 1 , using the first nozzle group NG 1 , form a second test pattern TP 20  on a portion of the sheet Q that is positioned in a second recording area RA 2 , using the second nozzle group NG 2 , form a third test pattern TP 30  on a portion of the sheet Q that is positioned in a third recording area RA 3 , using the third nozzle group NG 3 , and form a fourth test pattern TP 40  on a portion of the sheet Q that is positioned in a fourth recording area RA 4 , using the fourth nozzle group NG 4 . 
     In the present modification, when receiving an instruction to print test patterns via the user interface  90  or from the external device  3 , the controller  10  performs a test printing process shown in  FIGS. 15A and 15B  in accordance with one or more programs  13   a  stored in the ROM  13 . For instance, when a user of the MFP  1  or an operator of the manufacturer of the MFP  1  operates the user interface  90  or the external device  3 , the instruction to print test patterns is issued. 
     When the test printing process is started, the controller  10  activates and controls the printing unit driver  30  to, while controlling the PF motor  63 , cause the sheet conveyor  61  to convey a sheet Q to an upstream end section of the recording area R 0  below the recording head  40  in the sheet conveyance direction (S 310 : Cueing). 
     Afterward, the controller  10  performs a first pattern forming process (S 320 ). In the first pattern forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form a first test pattern TP 10  on a portion of the sheet Q that is positioned in the first recording area RA 1 , using the first nozzle group NG 1  (S 320 ). The first recording area RA 1  corresponds to a partial area of the recording area R 0  that is positioned under the first nozzle group NG 1 . In other words, the first recording area RA 1  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the first nozzle group NG 1 . The first nozzle group NG 1  corresponds to a group of nozzles included in the nozzle group N 0  that are positioned upstream of the other nozzles included in the nozzle group N 0  in the sheet conveyance direction. 
     The first test pattern TP 10  formed on the sheet Q has a geometrical pattern as exemplified in  FIG. 16A . Specifically, the first test pattern TP 10  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction. The first test pattern TP 10  of the present modification may be the same geometrical pattern as one of the test patterns exemplified in the aforementioned illustrative embodiment. For instance, the first test pattern TP 10  may be the same as the first test pattern TP 11  shown in  FIG. 5 . In this case, an elemental  figure EL10  (see  FIG. 16A ) of the first test pattern TP 10  may be identical to the elemental  figure EL11  shown in  FIG. 5 . 
     After the first test pattern TP 10  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to rotate the conveyance roller  613  by a particular amount L 2  thereby conveying the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 330 ). Thereafter, when making a negative determination in S 340 , the controller  10  again performs the first pattern forming process (S 320 ) in which the recording head  40  is controlled to form another first test pattern TP 10  on the sheet Q. The process of conveying the sheet Q over the particular amount L 2  is carried out by controlling a rotational amount of the conveyance roller  613 . Therefore, an actual sheet conveyance distance in S 330  contains an error relative to the particular amount L 2 . 
     After S 330 , the controller  10  determines whether the first test pattern TP 10  first formed on the sheet Q has reached the second recording area RA 2  of the recording area R 0  (S 340 ). Hereinafter, the first test pattern TP 10  first formed on the sheet Q may be simply referred to as a “head first test pattern TP 10 ” or a “first-formed first test pattern TP 10 ” to differentiate it from other first test patterns TP 10  to be subsequently formed on the sheet Q. The second recording area RA 2  is an area in which the second test pattern TP 20  is formed, within the recording area R 0 . When determining that the head first test pattern TP 10  has not reached the second recording area RA 2  (S 340 : No), the controller  10  goes to S 320 . Namely, until the first-formed first test pattern TP 10  reaches the second recording area RA 2 , the controller  10  repeatedly performs the processes of making the negative determination in S 340  (S 340 : No), controlling the recording head  40  to form the first test pattern TP 10  on the sheet Q (S 320 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 330 ). Meanwhile, when determining that the head first test pattern TP 10  has reached the second recording area RA 2  (S 340 : Yes), the controller  10  goes to S 350 . In this regard, however, as shown in  FIG. 14 , a length between an upstream end of the first recording area RA 1  and an upstream end of the second recording area RA 2  in the sub scanning direction may be equal to the particular amount L 2 . In this case, after the first test pattern TP 10  has been first formed on the sheet Q in S 320  (see  FIG. 16A ), when the sheet Q is conveyed over the particular amount L 2  in S 330 , the first-formed first test pattern TP 10  necessarily reaches the second recording area RA 2 . Thus, in this case, after S 330 , the controller  10  may go to S 350  without making the determination in S 340 . 
     In S 350 , the controller  10  performs a first-and-second-patterns forming process. In the first-and-second-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional first test pattern TP 10  and a second test pattern TP 20  on the sheet Q (S 350 ). Specifically, in S 350 , the controller  10  controls the recording head  40  to form the first test pattern TP 10  on a portion of the sheet Q that is positioned in the first recording area RA 1 , using the first nozzle group NG 1 , and form the second test pattern TP 20  on a portion of the sheet Q that is positioned in the second recording area RA 2 , using the second nozzle group NG 2  (see  FIG. 16B ). The second recording area RA 2  corresponds to a partial area of the recording area R 0  that is positioned under the second nozzle group NG 2 . In other words, the second recording area RA 2  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the second nozzle group NG 2 . Among the nozzle group N 0 , the second nozzle group NG 2  is positioned downstream of the first nozzle group NG 1  in the sheet conveyance direction. 
       FIG. 16B  exemplifies the second test pattern TP 20  formed on the sheet Q. The second test pattern TP 20  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction at a different degree of inclination from the first test pattern TP 10 . Namely, the second test pattern TP 20  is macroscopically or approximately inclined relative to each of the main scanning direction and the first test pattern TP 10 .  FIG. 16B  shows a state where the second test pattern TP 20  is formed in the first-executed S 350  so as to intersect the head first test pattern TP 10  (i.e., the first-formed first test patterns TP 10 ) on the sheet Q. Further, as shown in  FIG. 16B  (as well as  FIGS. 16C and 16D ), the first test patterns TP 10  are formed at intervals of the distance L 2  (i.e., the particular amount L 2 ) in the Y-axis direction. 
     The particular amount L 2 , which corresponds to the formation interval of the first test patterns TP 10  in the Y-axis direction, is as long as a part of a recording area length divided by an integer. The recording area length is defined as a length between the upstream end of the first recording area RA 1  and an upstream end of the fourth recording area RA 4  in the sub scanning direction, within the recording area R 0 . Further, the particular amount L 2  is as long as a part of the outer circumferential length of the conveyance roller  613  divided by an integer. The outer circumferential length of the conveyance roller  613  corresponds to a sheet conveyance distance when the conveyance roller has made a single rotation. 
     In the present modification, a conveyance distance error between the rotational amount of the conveyance roller  613  and the actual sheet conveyance distance depends on a rotational phase of the conveyance roller  613  in sheet conveyance. In the present modification, in order to suppress an influence of the above conveyance distance error depending on the rotational phase, a conveyance distance error is determined at each of different rotational phases that are defined by dividing the outer circumferential length of the conveyance roller  613  into a plurality of sections. The formation of the first test pattern TP 10  at intervals of the particular amount L 2  in the Y-axis direction is for determining a conveyance distance error at each rotational phase of the conveyance roller  613 . 
     For instance, in the present modification, an elemental  figure EL20  (see  FIG. 16B ) of the second test pattern TP 20  may be identical to the elemental  figure EL21  shown in  FIG. 6 . In this case, the second test pattern TP 20  may have the same geometrical pattern as the second test pattern TP 21  shown in  FIG. 6 . 
     After the second test pattern TP 20  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 360 ). Thereafter, when making a negative determination in S 370 , the controller  10  again performs the first-and-second-patterns forming process (S 350 ), in which the recording head  40  is controlled to form another first test pattern TP 10  on the sheet Q with the first nozzle group NG 1  and form another second test pattern TP 20  with the second nozzle group NG 2  to intersect the first test pattern TP 10  that has reached the second recording area RA 2 . 
     After S 360 , the controller  10  determines whether the head first test pattern TP 10  (i.e., the first-formed first test pattern TP 10 ) has reached the third recording area RA 3  of the recording area R 0  (S 370 ). The third recording area RA 3  is an area in which the third test pattern TP 30  is formed. It is noted that in S 370 , the controller  10  may determine whether the second-formed first test pattern TP 10  has reached the second recording area RA 2  of the recording area R 0 . When determining that the head first test pattern TP 10  has not reached the third recording area RA 3  (S 370 : No), the controller  10  goes to S 350 . Namely, until the first-formed first test pattern TP 10  reaches the third recording area RA 3 , the controller  10  repeatedly performs the processes of making the negative determination in S 370  (S 370 : No), controlling the recording head  40  to form the first test pattern TP 10  and the second test pattern TP 20  on the sheet Q (S 350 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 360 ). Meanwhile, when determining that the head first test pattern TP 10  has reached the third recording area RA 3  (S 370 : Yes), the controller  10  goes to S 380 . In this regard, however, as shown in  FIG. 14 , a length between the upstream end of the second recording area RA 2  and an upstream end of the third recording area RA 3  in the sub scanning direction may be equal to the particular amount L 2 . Namely, a length between the upstream end of the first recording area RA 1  and the upstream end of the third recording area RA 3  in the sub scanning direction may be twice as long as the particular amount L 2 . In this case, after the second test pattern TP 20  has been first formed to intersect the first-formed first test pattern TP 10  on the sheet Q in S 350  (see  FIG. 16B ), when the sheet Q is conveyed over the particular amount L 2  in S 360 , the first-formed first test pattern TP 10  necessarily reaches the third recording area RA 3 . Thus, in this case, after S 360 , the controller  10  may go to S 380  without making the determination in S 370 . 
     In S 380 , the controller  10  performs a first-to-third-patterns forming process. In the first-to-third-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional first test pattern TP 10 , an additional second test pattern TP 20 , and a third test pattern TP 30  on the sheet Q (S 380 ). Specifically, in S 380 , the controller  10  controls the recording head  40  to form the first test pattern TP 10  on a portion of the sheet Q that is positioned in the first recording area RA 1 , using the first nozzle group NG 1 . Concurrently, in S 380 , the controller  10  controls the recording head  40  to form the second test pattern TP 20  on a portion of the sheet Q that is positioned in the second recording area RA 2 , using the second nozzle group NG 2 , in such a manner that the second test pattern TP 20  intersects the first test pattern TP 10  that has reached the second recording area RA 2 . Furthermore, in S 380 , the controller  10  controls the recording head  40  to form the third test pattern TP 30  on a portion of the sheet Q that is positioned in the third recording area RA 3 , using the third nozzle group NG 3  (see  FIG. 16C ). The third recording area RA 3  corresponds to a partial area of the recording area R 0  that is positioned under the third nozzle group NG 3 . In other words, the third recording area RA 3  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the third nozzle group NG 3 . Among the nozzle group N 0 , the third nozzle group NG 3  is positioned downstream of the second nozzle group NG 2  in the sheet conveyance direction. 
       FIG. 16C  exemplifies the third test pattern TP 30  formed on the sheet Q. The third test pattern TP 30  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction.  FIG. 16C  shows a state where the third test pattern TP 30  is formed in the first-executed S 380  so as not to intersect the first test patterns TP 10  or the second test patterns TP 20  (i.e., so as to be spaced apart from the first test patterns TP 10  and the second test patterns TP 20  in the main scanning direction). 
     For instance, in the present modification, an elemental  figure EL30  (see  FIG. 16C ) of the third test pattern TP 30  may be identical to the elemental  figure EL11  shown in  FIG. 5 . In this case, the third test pattern TP 30  may have the same geometrical pattern as the first test pattern TP 11  shown in  FIG. 5 . Namely, in the present modification, the third test pattern TP 30  may have the same geometrical pattern as the first test pattern TP 10 . 
     After the third test pattern TP 30  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 390 ). Thereafter, when making a negative determination in S 400 , the controller  10  again performs the first-to-third-patterns forming process (S 380 ), in which the recording head  40  is controlled to form another first test pattern TP 10  on the sheet Q with the first nozzle group NG 1 , form another second test pattern TP 20  with the second nozzle group NG 2  to intersect the first test pattern TP 10  that has reached the second recording area RA 2 , and form another third test pattern TP 30  with the third nozzle group NG 3  to be spaced apart from the first test patterns TP 10  and the second test patterns TP 20  in the main scanning direction. 
     After S 390 , the controller  10  determines whether the third test pattern TP 30  first formed on the sheet Q has reached the fourth recording area RA 4  of the recording area R 0  (S 400 ). Hereinafter, the third test pattern TP 30  first formed on the sheet Q may be simply referred to as a “head third test pattern TP 30 ” or a “first-formed third test pattern TP 30 ” to differentiate it from other third test patterns TP 30  to be subsequently formed on the sheet Q. The fourth recording area RA 4  is an area in which the fourth test pattern TP 40  is formed. It is noted that in S 400 , the controller  10  may determine whether the head first test pattern TP 10  has reached the fourth recording area RA 4  of the recording area R 0 . Alternatively, the controller  10  may determine in S 400  whether the third-formed first test pattern TP 10  has reached the second recording area RA 2  of the recording area R 0 . When determining that the head third test pattern TP 30  has not reached the fourth recording area RA 4  (S 400 : No), the controller  10  goes to S 380 . Namely, until the first-formed third test pattern TP 30  reaches the fourth recording area RA 4 , the controller  10  repeatedly performs the processes of making the negative determination in S 370  (S 400 : No), controlling the recording head  40  to form the first test pattern TP 10 , the second test pattern TP 20 , and the third test pattern TP 30  on the sheet Q (S 380 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 390 ). Meanwhile, when determining that the head third test pattern TP 30  has reached the fourth recording area RA 4  (S 400 : Yes), the controller  10  goes to S 410 . In this regard, however, as shown in  FIG. 14 , a length between the upstream end of the third recording area RA 3  and the upstream end of the fourth recording area RA 4  in the sub scanning direction may be equal to the particular amount L 2 . In this case, after the third test pattern TP 30  has been first formed on the sheet Q in S 380  (see  FIG. 16C ), when the sheet Q is conveyed over the particular amount L 2  in S 390 , the first-formed third test pattern TP 30  necessarily reaches the fourth recording area RA 4 . Thus, in this case, after S 390 , the controller  10  may go to S 410  without making the determination in S 400 . 
     In S 410 , the controller  10  performs a first-to-fourth-patterns forming process. In the first-to-fourth-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional first test pattern TP 10 , an additional second test pattern TP 20 , an additional third test pattern TP 30 , and a fourth test pattern TP 40  on the sheet Q (S 410 ). Specifically, in S 410 , the controller  10  controls the recording head  40  to form the first test pattern TP 10  on a portion of the sheet Q that is positioned in the first recording area RAL using the first nozzle group NG 1 . Concurrently, in S 410 , the controller  10  controls the recording head  40  to form the second test pattern TP 20  on a portion of the sheet Q that is positioned in the second recording area RA 2 , using the second nozzle group NG 2 , in such a manner that the second test pattern TP 20  intersects the first test pattern TP 10  that has reached the second recording area RA 2 . Furthermore, in S 410 , the controller  10  controls the recording head  40  to form the third test pattern TP 30  on a portion of the sheet Q that is positioned in the third recording area RA 3 , using the third nozzle group NG 3 . Moreover, in S 410 , the controller  10  controls the recording head  40  to form the fourth test pattern TP 40  on a portion of the sheet Q that is positioned in the fourth recording area RA 4 , using the fourth nozzle group NG 4 , in such a manner that the fourth test pattern TP 40  intersects the third test pattern TP 30  that has reached the fourth recording area RA 4  (see  FIG. 16D ). The fourth recording area RA 4  corresponds to a partial area of the recording area R 0  that is positioned under the fourth nozzle group NG 4 . In other words, the fourth recording area RA 4  is an area of the recording area R 0  where the recording head  40  is allowed to perform image formation using the fourth nozzle group NG 4 . Among the nozzle group N 0 , the fourth nozzle group NG 4  is positioned downstream of the third nozzle group NG 3  in the sheet conveyance direction. 
       FIG. 16D  exemplifies the fourth test pattern TP 40  formed on the sheet Q. The fourth test pattern TP 40  is macroscopically or approximately a straight line that is slightly inclined relative to the main scanning direction at a different degree of inclination from the third test pattern TP 30 . Namely, the fourth test pattern TP 40  is macroscopically or approximately inclined relative to each of the main scanning direction and the third test pattern TP 30 .  FIG. 16D  shows a state where the fourth test pattern TP 40  is formed in the first-executed S 410  so as to intersect the head third test pattern TP 30  (i.e., the first-formed third test patterns TP 30 ) on the sheet Q. 
     For instance, in the present modification, an elemental  figure EL40  (see  FIG. 16D ) of the fourth test pattern TP 40  may be identical to the elemental  figure EL21  shown in  FIG. 6 . In this case, the fourth test pattern TP 40  may have the same geometrical pattern as the second test pattern TP 21  shown in  FIG. 6 . Namely, in the present modification, the fourth test pattern TP 40  may have the same geometrical pattern as the second test pattern TP 20 . 
     After the fourth test pattern TP 40  has been formed, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 420 ). Thereafter, when making a negative determination in S 430 , the controller  10  again performs the first-to-fourth-patterns forming process (S 410 ), in which the recording head  40  is controlled to form another first test pattern TP 10  on the sheet Q with the first nozzle group NG 1 , form another second test pattern TP 20  with the second nozzle group NG 2  to intersect the first test pattern TP 10  that has reached the second recording area RA 2 , form another third test pattern TP 30  with the third nozzle group NG 3 , and form another fourth test pattern TP 40  with the fourth nozzle group NG 4  to intersect the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     After S 420 , the controller  10  determines whether a predetermined number of first test patterns TP 10  have been completely formed (S 430 ). When determining that the predetermined number of first test patterns TP 10  have not been completely formed (S 430 : No), the controller  10  goes to S 410 . Namely, until the predetermined number of first test patterns TP 10  are completely formed, the controller  10  repeatedly performs the processes of making the negative determination in S 430  (S 430 : No), controlling the recording head  40  to form the first test pattern TP 10 , the second test pattern TP 20 , the third test pattern TP 30 , and the fourth test pattern TP 40  on the sheet Q (S 410 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 420 ). 
     Meanwhile, when determining that the predetermined number of first test patterns TP 10  have been completely formed (S 430 : Yes), the controller  10  goes to S 440 . In this regard, however, according to aspects of the present disclosure, the controller  10  may stop forming the first test patterns TP 10  at a point of time when the fourth test pattern TP 40  has been first formed on the sheet Q in S 410  (see  FIG. 16D ). In this case, after S 420 , the controller  10  may go to S 440  without making the determination in S 430 . 
     In S 440 , the controller  10  performs a second-to-fourth-patterns forming process. In the second-to-fourth-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional second test pattern TP 20 , an additional third test pattern TP 30 , and an additional fourth test pattern TP 40  on the sheet Q (S 440 ). Specifically, in S 440 , the controller  10  controls the recording head  40  to form the second test pattern TP 20  on a portion of the sheet Q that is positioned in the second recording area RA 2 , using the second nozzle group NG 2 , in such a manner that the second test pattern TP 20  intersects the first test pattern TP 10  that has reached the second recording area RA 2 . Furthermore, in S 440 , the controller  10  controls the recording head  40  to form the third test pattern TP 30  on a portion of the sheet Q that is positioned in the third recording area RA 3 , using the third nozzle group NG 3 . Moreover, in S 440 , the controller  10  controls the recording head  40  to form the fourth test pattern TP 40  on a portion of the sheet Q that is positioned in the fourth recording area RA 4 , using the fourth nozzle group NG 4 , in such a manner that the fourth test pattern TP 40  intersects the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     After S 440 , the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 450 ). Thereafter, when making a negative determination in S 460 , the controller  10  again performs the second-to-fourth-patterns forming process (S 440 ), in which the recording head  40  is controlled to form another second test pattern TP 20  with the second nozzle group NG 2  to intersect the first test pattern TP 10  that has reached the second recording area RA 2 , form another third test pattern TP 30  with the third nozzle group NG 3 , and form another fourth test pattern TP 40  with the fourth nozzle group NG 4  to intersect the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     After S 450 , the controller  10  determines whether the second test pattern TP 20  has been formed for each of all the first test patterns TP 10  (S 460 ). When determining that the second test pattern TP 20  has not been formed for each of all the first test patterns TP 10  (S 460 : No), the controller  10  goes to S 440 . Namely, until the second test pattern TP 20  is formed for each of all the first test patterns TP 10 , the controller  10  repeatedly performs the processes of making the negative determination in S 460  (S 460 : No), controlling the recording head  40  to form the second test pattern TP 20 , the third test pattern TP 30 , and the fourth test pattern TP 40  on the sheet Q (S 440 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 450 ). 
     Meanwhile, when determining that the second test pattern TP 20  has been formed for each of all the first test patterns TP 10  (S 460 : Yes), the controller  10  goes to S 470 . In S 470 , the controller  10  performs a third-and-fourth-patterns forming process. 
     In the third-and-fourth-patterns forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional third test pattern TP 30  and an additional fourth test pattern TP 40  on the sheet Q (S 470 ). Specifically, in S 470 , the controller  10  controls the recording head  40  to form the third test pattern TP 30  on a portion of the sheet Q that is positioned in the third recording area RA 3 , using the third nozzle group NG 3 . Further, in S 470 , the controller  10  controls the recording head  40  to form the fourth test pattern TP 40  on a portion of the sheet Q that is positioned in the fourth recording area RA 4 , using the fourth nozzle group NG 4 , in such a manner that the fourth test pattern TP 40  intersects the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     After S 470 , the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  downstream in the sheet conveyance direction (S 480 ). Thereafter, when making a negative determination in S 490 , the controller  10  again performs the third-and-fourth-patterns forming process (S 470 ), in which the recording head  40  is controlled to form another third test pattern TP 30  with the third nozzle group NG 3 , and form another fourth test pattern TP 40  with the fourth nozzle group NG 4  to intersect the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     After S 480 , the controller  10  determines whether a predetermined number of third test patterns TP 30  have been completely formed (S 490 ). When determining that the predetermined number of third test patterns TP 30  have not been completely formed (S 490 : No), the controller  10  goes to S 470 . Namely, until the predetermined number of third test patterns TP 30  are completely formed, the controller  10  repeatedly performs the processes of making the negative determination in S 490  (S 490 : No), controlling the recording head  40  to form the third test pattern TP 30  and the fourth test pattern TP 40  on the sheet Q (S 470 ), and controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 480 ). 
     Meanwhile, when determining that the predetermined number of third test patterns TP 30  have been completely formed (S 490 : Yes), the controller  10  goes to S 500 . In this regard, however, according to aspects of the present disclosure, the controller  10  may stop forming the third test patterns TP 30  at a point of time when the fourth test pattern TP 40  has been formed on the sheet Q in S 470 . In this case, after S 480 , the controller  10  may go to S 500  without making the determination in S 490 . 
     In S 500 , the controller  10  performs a fourth pattern forming process. In the fourth pattern forming process, the controller  10  controls, via the printing unit driver  30 , the recording head  40  to form an additional fourth test pattern TP 40  on the sheet Q (S 500 ). Specifically, in S 500 , the controller  10  controls the recording head  40  to form the fourth test pattern TP 40  on a portion of the sheet Q that is positioned in the fourth recording area RA 4 , using the fourth nozzle group NG 4 , in such a manner that the fourth test pattern TP 40  intersects the third test pattern TP 30  that has reached the fourth recording area RA 4 . 
     Afterward, the controller  10  determines whether the fourth test pattern TP 40  has been formed for each of all the third test patterns TP 30  (S 510 ). When determining that the fourth test pattern TP 40  has not been formed for each of all the third test patterns TP 30  (S 510 : No), the controller  10  goes to S 520 . Meanwhile, when determining that the fourth test pattern TP 40  has been formed for each of all the third test patterns TP 30  (S 510 : Yes), the controller  10  goes to S 530 . 
     In S 520 , the controller  10  controls the sheet conveyor  61  to convey the sheet Q over the particular amount L 2 . Thereafter, the controller  10  goes to S 500 . Thus, until the fourth test pattern TP 40  is formed for each of all the third test patterns TP 30 , the controller  10  repeatedly performs the processes of making the negative determination in S 510  (S 510 : No), controlling the sheet conveyor  61  to convey the sheet Q over the particular amount L 2  (S 520 ), and controlling the recording head  40  to form the fourth test pattern TP 40  on the sheet Q (S 500 ). Then, when the fourth test pattern TP 40  has been formed for each of all the third test patterns TP 30  (S 510 : Yes), the controller  10  goes to S 530 . 
     In S 530 , the controller  10  performs a sheet discharging process. In the sheet discharging process, the controller  10  controls, via the printing unit driver  30 , the sheet conveyor  61  to convey and discharge the sheet Q onto the discharge tray (not shown). Further, the controller  10  controls the user interface  90  to display, on the display of the user interface  90 , a message that prompts the user to place the sheet Q with the test patterns printed thereon on the document table of the scanning unit  70  and input a scanning instruction (S 540 ). Thereafter, the controller  10  waits until a scanning instruction is input through the user interface  90  (S 550 ). 
     When a scanning instruction has been input, the controller  10  performs a scanning process (S 560 ). Specifically, in S 560 , the controller  10  controls the scanning unit  70  to scan the sheet Q with the test patterns printed thereon, and acquires image data expressing the scanned image from the scanning unit  70 . 
     Further, the controller  10  calculates a positional displacement, from a reference point, of an intersection between each first test pattern TP 10  and the corresponding second test pattern TP 20  based on the image data acquired from the scanning unit  70 . The controller  10  also calculates a positional displacement, from a reference point, of an intersection between each third test pattern TP 30  and the corresponding fourth test pattern TP 40  based on the acquired image data. Then, the controller  10  calculates a conveyance distance error of the sheet Q based on the calculated positional displacements (S 570 ). Based on the conveyance distance error calculated in S 570 , the controller  10  updates the particular control parameter (included in the control parameters stored in the NVRAM  17 ) that represents the association between the rotational quantity of the conveyance roller  613  and the sheet conveyance distance (S 580 ). Thereafter, the controller  10  terminates the test printing process shown in  FIGS. 15A and 15B . 
     Further, according to the present modification, it is possible to determine whether the conveyance distance error calculated in S 570  contains an unacceptable amount of error caused by the scanning process. It is noted that, in the present modification, a sheet scanning direction for scanning the sheet Q in the scanning process (see S 560 ) is the same as the main scanning direction. Namely, in the present modification, as shown in  FIG. 16D , the intersections between the first test patterns TP 10  and the second test patterns TP 20  are positionally different from the intersections between the third test patterns TP 30  and the fourth test patterns TP 40  in the sheet scanning direction. For instance, as shown in  FIG. 16C , the third-formed first test pattern TP 10  and the first-formed third test pattern TP 30  are written in the same step, i.e., the first-executed S 380 . Further, as shown in  FIG. 16D , the third-formed second test pattern TP 20  that intersects the third-formed first test pattern TP 10  and the first-formed fourth test pattern TP 40  that intersects the first-formed third test pattern TP 30  are written in the same step, i.e., the first-executed S 410 . Namely, if the conveyance distance error calculated in S 570  merely contains a negligibly small amount of error caused by the scanning process, a first conveyance distance error calculated based on the intersection between the third-formed first test pattern TP 10  and the third-formed second test pattern TP 20  should be substantially identical to a second conveyance distance error calculated based on the intersection between the first-formed third test pattern TP 30  and the first-formed fourth test pattern TP 40 . Accordingly, by comparing the first conveyance distance error and the second conveyance distance error, it is possible to determine whether the conveyance distance error calculated in S 570  contains an unacceptable amount of error caused by the scanning process. For example, when there is not a significant difference between the first conveyance distance error and the second conveyance distance error, it may be possible to determine that the conveyance distance error calculated in S 570  does not contain an unacceptable amount of error caused by the scanning process. 
     Alternatively, the first conveyance distance error and the second conveyance distance error may be compared with a threshold value. In this case, when one of the first conveyance distance error and the second conveyance distance error is more than the threshold value, the error more than the threshold value may be regarded as an abnormal or invalid value, and may not be used for determining a final conveyance distance error in S 570 . 
     In the present modification, as shown in  FIGS. 16C and 16D , the third test patterns TP 30  and the fourth test patterns TP 40  are formed to be spaced apart from the first test patterns TP 10  and the second test patterns TP 20  in the main scanning direction. Nonetheless, the third test patterns TP 30  and the fourth test patterns TP 40  may not necessarily be formed to be spaced apart from the first test patterns TP 10  and the second test patterns TP 20  in the main scanning direction, as long as each test pattern and each intersection between two test patterns can be read in a discriminable manner in the scanning process. 
     A function of a single element in the aforementioned illustrative embodiment and modifications may be dispersedly provided to a plurality of elements. Functions of a plurality of elements may be integrally provided to a single element. Some of elements constituting the aforementioned illustrative embodiment and modifications may be omitted. At least a part of elements constituting the aforementioned illustrative embodiment and modifications may be added to or replaced with elements in another modification according to aspects of the present disclosure. 
     In the aforementioned illustrative embodiment, the first nozzle group N 1  of the recording head  40  may be an example of the “first image former” according to aspects of the present disclosure. The second nozzle group N 2  of the recording head  40  may be an example of the “second image former” according to aspects of the present disclosure. The controller  10  may be an example of the “controller” according to aspects of the present disclosure. A combination of the controller  10  and the printing unit driver  30  may also be an example of the “controller” according to aspects of the present disclosure. The processes in S 120  and S 150  may be examples of the “first formation control process” according to aspects of the present disclosure. The processes in S 130 , S 160 , S 200 , and S 210  may be examples of the “conveyance control process” according to aspects of the present disclosure. The processes in S 150  and S 180  may be examples of the “second formation control process” according to aspects of the present disclosure. The process in S 240  may be an example of the “scanning control process” according to aspects of the present disclosure. The process in S 250  may be an example of the “error determining process” according to aspects of the present disclosure. 
     In the modification exemplified in  FIG. 12 , the upstream line inkjet heads  141 ,  142 , and  143  may be examples of the “first image former” according to aspects of the present disclosure. The downstream line inkjet heads  142 ,  143 , and  144  may be examples of the “second image former” according to aspects of the present disclosure. 
     When aspects of the present disclosure are applied to a laser printer having a plurality of photoconductive bodies arranged in a conveyance direction of a sheet Q or a transfer belt, upstream two of the plurality of photoconductive bodies in the conveyance direction may be examples of the “first image former” according to aspects of the present disclosure. Downstream two of the plurality of photoconductive bodies in the conveyance direction may be examples of the “second image former” according to aspects of the present disclosure. 
     In the further modification exemplified in  FIGS. 14, 15A, 15B, and 16A-16D , the first nozzle group NG 1  and the third nozzle group NG 3  may be examples of the “first image former” according to aspects of the present disclosure. The second nozzle group NG 2  and the fourth nozzle group NG 4  may be examples of the “second image former” according to aspects of the present disclosure. The processes in S 320 , S 350 , S 380 , S 410 , S 440 , and S 470  may be examples of the “first formation control process” according to aspects of the present disclosure. The processes in S 330 , S 360 , S 390 , S 420 , S 450 , S 480 , S 520 , and S 530  may be examples of the “conveyance control process” according to aspects of the present disclosure. The processes in S 350 , S 380 , S 410 , S 440 , S 470 , and S 500  may be examples of the “second formation control process” according to aspects of the present disclosure. The process in S 560  may be an example of the “scanning control process” according to aspects of the present disclosure. The process in S 570  may be an example of an “error determining process” according to aspects of the present disclosure.