Patent Publication Number: US-8540342-B2

Title: Recording apparatus, method of controlling recording apparatus and computer readable recording medium

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2009-084155, which was filed on Mar. 31, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a recording apparatus having a liquid ejection head for ejecting a liquid, a method of controlling the recording apparatus and a computer readable recording medium storing a program. 
     A plurality of nozzles for ejecting ink droplets to a recording medium, such as a print sheet, are formed in an inkjet head provided in an inkjet printer. In such an inkjet head, viscosity of ink in the nozzles increases with elapse of a time, thereby sometimes causing a change in an ink ejection characteristic and an ejection failure. A hitherto known technique for preventing them is to produce image dots pertaining to an image on a recording medium in such a way that ink droplets are ejected from all nozzles before elapse of a predetermined time and to produce flushing dots on the recording medium by causing nozzles, which do not contribute to image production, to eject ink droplets. An increase in the viscosity of the ink in the nozzles can thereby be prevented without wasting the recording medium. 
     SUMMARY 
     According to the foregoing technique, in order to reduce visibility of flushing dots produced on a sheet, positions of the flushing dots are determined so as not to overlap each other or adjoin each other. However, according to the technique, positions of the flushing dots are previously determined, the probability of flushing dots being produced at the same locations on a plurality of sheets is high, so that flushing dots become prone to being noticeable. Meanwhile, when an attempt is made to store positions of flushing dots produced on one sheet or a plurality of sheets and individually determine positions of the respective flushing dots such that the flushing dots are not produced at the stored positions of the flushing dots in order to prevent production of flushing dots at the same positions on a plurality of sheets, amounts of arithmetic processing become enormous. 
     An object of the present invention is to provide a recording apparatus that can quickly determine positions of spare ejection dots through processing involving small amount of arithmetic processing and that can inhibit production of the spare ejection dots at the same positions on any recording medium. 
     In order to achieve the object, an exemplary embodiment of the present invention provides a recording apparatus comprising: 
     a conveyance mechanism which conveys a recording medium in a conveyance direction; 
     a liquid ejection head including a plurality of ejection ports which eject droplets toward the recording medium conveyed by the conveyance mechanism, to produce image dots, and which are arranged at intervals commensurate with a first resolution in a direction orthogonal to the conveyance direction; 
     an image data storage which stores image data showing, in a recording area defined in the recording medium, positions of image dots produced at intervals commensurate with the first resolution in the orthogonal direction and a second resolution in the conveyance direction; 
     a flushing area size storage which stores the number of virtual pixels arranged in the conveyance direction in a virtual flushing area in which a flushing area is represented in a virtual space, wherein the flushing area occupies a whole area of the recording area in the orthogonal direction and occupies at least a part of the recording area in the conveyance direction, and the flushing area includes a plurality of element rows having spatial elements arranged in the conveyance direction, the plurality of element rows being arranged in the orthogonal direction; 
     a standard flushing data storage which stores standard flushing data showing a standard flushing pattern within a standard area, wherein the standard area includes a plurality of elements rows having spatial elements arranged in the conveyance direction and has the same matrix shape as the virtual flushing area, and the standard flushing pattern includes flushing coordinate elements which are selected at least one from the spatial elements of each of the element rows of the standard area; 
     a reference area formation unit which repeatedly arranges the plural standard areas in at least one of the conveyance direction and the orthogonal direction to form a reference area; 
     an extraction unit which virtually arrange the virtual flushing area on an arbitrary position within the reference area, and selects the element row, in which the image dot is not included, from the element rows of the flushing area when the recording area is arranged on the flushing area, and extracts the coordinate element within the reference area in the selected element row as a flushing element; and 
     an ejection controller which controls the ejection ports to eject droplets based on the image data and the flushing element extracted by the extraction unit. 
     Further, the exemplary embodiment of the present invention provides a method of controlling a recording apparatus which includes a conveyance mechanism which conveys a recording medium in a conveyance direction; and a liquid ejection head including a plurality of ejection ports which eject droplets toward the recording medium conveyed by the conveyance mechanism, to produce image dots, and which are arranged at intervals commensurate with a first resolution in a direction orthogonal to the conveyance direction, the method comprising: 
     storing image data showing, in a recording area defined in the recording medium, positions of image dots produced at intervals commensurate with the first resolution in the orthogonal direction and a second resolution in the conveyance direction; 
     storing the number of virtual pixels arranged in the conveyance direction in a virtual flushing area in which a flushing area is represented in a virtual space, wherein the flushing area occupies a whole area of the recording area in the orthogonal direction and occupies at least a part of the recording area in the conveyance direction, and the flushing area includes a plurality of element rows having spatial elements arranged in the conveyance direction, the plurality of element rows being arranged in the orthogonal direction; 
     storing standard flushing data showing a standard flushing pattern within a standard area, wherein the standard area includes a plurality of elements rows having spatial elements arranged in the conveyance direction and has the same matrix shape as the virtual flushing area, and the standard flushing pattern includes flushing coordinate elements which are selected at least one from the spatial elements of each of the element rows of the standard area; 
     repeatedly arranging the plural standard areas in at least one of the conveyance direction and the orthogonal direction to form a reference area; 
     virtually arranging the virtual flushing area on an arbitrary position within the reference area; 
     selecting the element row, in which the image dot is not included, from the element rows of the flushing area when the recording area is arranged on the flushing area, and extracts the coordinate element within the reference area in the selected element row as a flushing element; and 
     controlling the ejection ports to eject droplets based on the image data and the extracted flushing element. 
     Further, the exemplary embodiment of the present invention provides a computer readable recording medium storing a program which causes a recording apparatus, which includes a conveyance mechanism which conveys a recording medium in a conveyance direction; and a liquid ejection head including a plurality of ejection ports which eject droplets toward the recording medium conveyed by the conveyance mechanism, to produce image dots, and which are arranged at intervals commensurate with a first resolution in a direction orthogonal to the conveyance direction, to perform: 
     storing image data showing, in a recording area defined in the recording medium, positions of image dots produced at intervals commensurate with the first resolution in the orthogonal direction and a second resolution in the conveyance direction; 
     storing the number of virtual pixels arranged in the conveyance direction in a virtual flushing area in which a flushing area is represented in a virtual space, wherein the flushing area occupies a whole area of the recording area in the orthogonal direction and occupies at least a part of the recording area in the conveyance direction, and the flushing area includes a plurality of element rows having spatial elements arranged in the conveyance direction, the plurality of element rows being arranged in the orthogonal direction; 
     storing standard flushing data showing a standard flushing pattern within a standard area, wherein the standard area includes a plurality of elements rows having spatial elements arranged in the conveyance direction and has the same matrix shape as the virtual flushing area, and the standard flushing pattern includes flushing coordinate elements which are selected at least one from the spatial elements of each of the element rows of the standard area; 
     repeatedly arranging the plural standard areas in at least one of the conveyance direction and the orthogonal direction to form a reference area; 
     virtually arranging the virtual flushing area on an arbitrary position within the reference area; 
     selecting the element row, in which the image dot is not included, from the element rows of the flushing area when the recording area is arranged on the flushing area, and extracts the coordinate element within the reference area in the selected element row as a flushing element; and 
     controlling the ejection ports to eject droplets based on the image data and the extracted flushing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of an inkjet printer of an embodiment of the present invention; 
         FIG. 2  is a cross sectional view of the inkjet head shown in  FIG. 1  taken along its widthwise direction; 
         FIG. 3  is a cross sectional view taken along line shown in  FIG. 2 ; 
         FIG. 4  is an enlarged view of an area enclosed by a dashed line shown in  FIG. 3 ; 
         FIG. 5  is a functional block diagram of a controller shown in  FIG. 1 ; 
         FIG. 6  is a schematic view of a bottom area representing a flushing pattern stored in a flushing data storage section shown in  FIG. 5 ; 
         FIGS. 7A and 7B  are schematic views of a bottom area and a standard area shown in  FIG. 6 ; 
         FIG. 8  is a partially enlarged view of the bottom area and the standard area Shown in  FIGS. 7A and 7B ; 
         FIG. 9  is a schematic view of a reference area produced by a reference area generation section shown in  FIG. 5 ; 
         FIG. 10  is a view showing operation of an extraction section shown in  FIG. 5 ; and 
         FIG. 11  is a flowchart showing operation procedures of a controller shown in  FIG. 5 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A preferred embodiment of the present invention is hereunder described by reference to the drawings. 
     As shown in  FIG. 1 , an inkjet printer  101  includes a parallelepiped housing  1   a . A sheet output section  31  is provided in an upper portion of the housing  1   a . An interior of the housing  1   a  is divided, in sequence from top, three spaces A, B, and C. Four inkjet heads  1  that respectively eject magenta ink, cyan ink, yellow ink, and black ink and a conveyance unit  20  are arranged in the space A. A sheet feed unit  1   b  removably attached to the housing  1   a  is disposed in the space B, and an ink tank unit  1   c  is disposed in the space C. In the embodiment, a sub-scan direction is a direction parallel to the conveyance direction in which a sheet P is conveyed by means of a conveyance unit  20 . A main scan direction is a direction that is orthogonal to the sub-scan direction and that is aligned to a horizontal plane. 
     A sheet conveyance path along which the sheet P is to be conveyed from the sheet feed unit  1   b  to the sheet output section  31  is formed in the inkjet printer  101  (as designated by an arrow of medium width shown in  FIG. 1 ). The sheet feed unit  1   b  includes a sheet feed tray  23  capable of housing a plurality of sheets P and a sheet feed roller  25  attached to the sheet feed tray  23 . The sheet feed roller  25  feeds the topmost sheet P among a plurality of sheets P stocked in a piled manner in the sheet feed tray  23 . The sheet P fed by the sheet feed roller  25  is fed to the conveyance unit  20  while being guided by guides  27   a  and  27   b  and nipped between a pair of feed rollers  26 . 
     The conveyance unit  20  includes two belt rollers  6  and  7 ; an endless conveyance belt  8  wrapped around the rollers so as to extend between the rollers  6  and  7 ; and a tension roller  10 . The tension roller  10  is downwardly forced while remaining in contact with an internal peripheral surface of a lower loop of the conveyance belt  8 , to thus impart tension to the conveyance belt  8 . The belt roller  7  is a drive roller and rotated in a clockwise direction in  FIG. 1  when imparted with drive force from a conveyance motor M through two gears. The belt roller  6  is a driven roller and rotated by rotation of the belt roller  7  in the clockwise direction in  FIG. 1  along with travel of the conveyance belt  8 . 
     An outer peripheral surface  8   a  of the conveyance belt  8  is subjected to silicon treatment and exhibits adhesiveness. A nip roller  4  is disposed at a position along the sheet conveyance path so as to oppose the belt roller  6  with the conveyance belt  8  sandwiched therebetween. The nip roller  4  presses the sheet P fed out of the sheet feed unit  1   b  against the outer peripheral surface  8   a  of the conveyance belt  8 . The sheet P pressed against the outer peripheral surface  8   a  is conveyed in a rightward direction in  FIG. 1  while held on the outer peripheral surface  8   a  by means of adhesiveness of the outer peripheral surface. 
     A separation plate  5  is disposed at a position on the sheet conveyance path where the separation plate opposes the belt roller  7  with the conveyance belt  8  sandwiched therebetween. The separation plate  5  separates the sheet P from the outer peripheral surface  8   a . The thus-separated sheet P is conveyed while guided by guides  29   a  and  29   b  and nipped by two feed roller pairs  28  and output to the sheet output section  31  from an opening  30  formed in the upper portion of the housing  1   a.    
     Four inkjet heads  1  are supported by the housing  1   a  through a frame  3 . The four inkjet heads  1  extend along the main scan direction and are arranged in parallel to each other along the sub-san direction. The inkjet printer  101  is a line-type color inkjet printer in which an ejection area extending in the main scan direction is formed. A lower surface of each of the inkjet heads  1  is an ejection surface  2   a  through which ink droplets are ejected. 
     A platen  19  is arranged in the loop of the conveyance belt  8  and is opposed to the four inkjet heads  1 . An upper surface of the platen  19  remains in contact with an internal peripheral surface of an upper loop of the conveyance belt  8  and supports the conveyance belt  8  from its inner peripheral side. The outer peripheral surface  8   a  of the upper loop of the conveyance belt  8  is opposed the lower surfaces of the inkjet heads  1 , namely, the ejection surfaces  2   a , in parallel to each other, whereby clearance of predetermined interval suitable for producing an image is created. The clearance makes up a portion of the sheet conveyance path. When the sheet P conveyed by the conveyance belt  8  passes by positions located immediately below the respective heads  1 , respective colors of ink are sequentially ejected toward an upper surface of the sheet P from the respective heads  1 , whereupon a desired color image is produced on the sheet P. 
     The respective inkjet heads  1  are connected to respective ink tanks  49  set in the ink tank unit  1   c  provided in the space C. The four ink tanks  49  store ink to be ejected by the corresponding ink jet heads  1 , respectively. Ink is supplied from each of the ink tanks  49  to the corresponding inkjet head  1  through a tube (not shown), or the like. 
     The inkjet heads  1  are now described in detail by reference to  FIGS. 2 and 3 . A lower housing  87  is omitted from  FIG. 3 . 
     As shown in  FIG. 2 , each of the inkjet heads  1  includes a reservoir unit  71 ; a head main body  2  including a flow channel unit  9  and an actuator unit  21 ; and a COF (Chip On Film: a flat flexible substrate)  50  that is connected at its one end to the actuator unit  21  and that is equipped with a driver IC  52 ; and a control substrate  54  to which the other end of the COF  50  is connected. The inkjet head  1  includes the reservoir unit  71 ; an upper housing  86  and the lower housing  87  that make up a box surrounding the flow channel unit  9 ; and a head cover  55  that encloses the control substrate  54  at a position above the upper housing  86 . 
     The reservoir unit  71  is a flow channel formation member that is fixed to an upper surface of the head main body  2  and that supplies the head main body  2  with ink. The reservoir unit  71  is a multilayered substance formed by stacking four mutually positioned plates  91  to  94 . An unillustrated ink inflow channel, the ink reservoir  72 , and ten ink outflow channels  73  are formed in the reservoir unit so as to mutually communicate with each other. Only one of the ink outflow channels  73  is shown in  FIG. 2 . The ink inflow channel is a channel into which ink flows from the ink tank  49 . The ink reservoir  72  temporarily stores an inflow of ink from the ink inflow channel. The ink outflow channel  73  is a flow channel through which ink flows from the ink reservoir  72  and that is in mutual communication with an ink supply port  105   b  formed in an upper surface of the flow channel unit  9 . Ink from the ink tank  49  flows into the ink reservoir  72  through the ink inflow channel, passes through the ink outflow channel  73 , and is supplied from the ink supply port  105   b  to the flow channel unit  9 . 
     An indentation  94   a  is formed in a lower surface of the plate  94 . The indentation  94  creates clearance  90  between the lower surface of the plate and an upper surface of the flow channel unit  9 . The four actuator units  21  on the flow channel unit  9  are arranged at equal intervals in the clearance  90  along the longitudinal direction of the flow channel unit  9 . In a side surface of the multilayered substance, four openings  90   a  of the clearance  90  are formed at equal intervals in a staggered pattern and along the longitudinal direction of the reservoir unit  71 . 
     Protuberances (areas other than the indentation  94   a ) on the lower surface of the plate  94  are adhered to the flow channel unit  9 . The ink outflow channels  73  are formed in the respective protuberances. 
     A neighborhood of one end of the individual COF  50  is connected to an upper surface of the corresponding actuator unit  21 . The COF  50  extends from the upper surface of the actuator unit  21  in a horizontal direction and passes through the opening  90   a . The COF thus passed through the opening is then curved and bent at substantially right angles in an upward direction. The thus-bent COF passes through a cutout  53  formed in an interior wall surface of the upper housing  86  and the lower housing  87  and is pulled to a position above the reservoir unit  71 . The COF  50  further extends in a leftward direction in  FIG. 2  at a position above the reservoir unit  71  and pulled to a position above the upper housing  86  through a slit  86   a  formed in the upper housing  86 . The other end of the COF  50  is connected to the corresponding control substrate  54  through a connector  54   a  at a position above the upper housing  86 . A driver IC  52  is mounted at an arbitrary position on the COF  50 . The driver IC  52  is affixed to the upper surface of the reservoir unit  71  and thermally coupled to the reservoir unit  71 . Heat given off by the driver IC  52  thereby propagates to the reservoir unit  71 , whereupon the driver IC  52  is cooled. On the other hand, ink in the reservoir unit  71  is heated, to thus hinder an increase in viscosity of ink. 
     The control substrate  54  is placed at a position above the upper housing  86  and controls actuation of the actuator unit  21  through the driver IC  52  of the COF  50 . The driver IC  52  is for generating a drive signal for actuating the actuator unit  21 . 
     The head main body  2  is now described with reference to  FIGS. 3 and 4 . Pressure chambers  110 , apertures  112 , and ejection ports  108 , which are located beneath the actuator unit  21  and which are to be drawn in broken lines, are drawn in solid lines in  FIG. 4  for the sake of explanation. 
     As shown in  FIG. 3 , the head main body  2  is a multilayered substance in which the four actuator units  21  are fixed to the upper surface  9   a  of the flow channel unit  9 . As shown in  FIGS. 3 and 4 , ink flow channels, including the pressure chambers  110 , are formed in the flow channel unit  9 . Each of the actuator units  21  includes a plurality of actuators assigned to the respective pressure chambers  110  and has a function of selectively imparting ejection energy to ink stored in the respective pressure chambers  110 . 
     The flow channel unit  9  assumes the shape of a rectangular parallelepiped having substantially the same planar shape as that of the plate  94  of the reservoir unit  71 . A total of ten ink supply ports  105   b  are formed in the upper surface  9   a  of the flow channel unit  9  in correspondence with the ink outflow channels  73  of the reservoir unit  71  (see  FIG. 2 ). As shown in  FIG. 3 , there are formed in the flow channel unit  9  a manifold flow channel  105  remaining in mutual communication with the ink supply ports  105   b , a sub-manifold  105   a  branched off from the manifold flow channel  105 , and a plurality of individual ink flow channels  132  branched off from the sub-manifold flow channel  105   a . As shown in  FIG. 1 , the ejection surfaces  2   a  are formed on a lower surface of the flow channel unit  9 , and as shown in  FIG. 4 , the plurality of ejection ports  108  are arranged in the ejection surfaces in a matrix pattern. The plurality of pressure chambers  110  are also arranged in a matrix pattern in the upper surface  9   a  of the flow channel unit  9  (i.e., the surface to which the actuator units  21  are fixed). The ejection ports  108  are arranged, along the main scan direction, at an interval of 600 dpi that is a resolution achieved in the main scan direction. 
     In the embodiment, sixteen rows of the pressure chambers  110  that are equally spaced along the longitudinal direction of the flow channel unit  9  are arranged in parallel to each other along a widthwise direction. The number of pressure chambers  110  included in each of the rows of pressure chambers becomes gradually smaller from a long side (a lower bottom side) to a short side (an upper bottom side) in correspondence with the outer shape (a trapezoidal shape) of the actuator unit  21  to be described later. The ejection ports  108  are also arranged correspondingly. 
     The flow channel unit  9  is a multilayered substance made by mutually positioning a plurality of metal plates made of stainless steel. Formed in the flow channel unit  9  are a plurality of individual ink flow channels  132  that extend from the manifold flow channel  105  to the sub-manifold flow channels  105   a  and from exits of the sub-manifold flow channels  105   a  to the ejection ports  108  through the pressure chambers  110 . 
     Ink flow in the flow channel unit  9  is now described. As shown in  FIGS. 3 and 4 , the ink supplied from the reservoir unit  71  into the flow channel unit  9  through the ink supply port  105   b  is distributed from the manifold flow channel  105  to the sub-manifold flow channels  105   a . The ink in the sub-manifold flow channels  105   a  flows into the individual ink flow channels and reaches the ejection ports  108  through the pressure chambers  110 . 
     The actuator units  21  are unimorph actuators. The unimorph actuator is made up of lead zirconate titanate (PZT)-based piezoelectric c sheet made of ceramic exhibiting ferroelectricity. Upon receipt of an input of a drive signal, the actuator unit  21  selectively imparts pressure (ejection energy) to the ink in a target pressure chamber  110 , thereby ejecting an ink droplet from the corresponding ejection port  108 . 
     The controller  16  is now described by reference to  FIG. 5 . The controller  16  includes a CPU (Central Processing Unit); EEPROM (Electrically Erasable and Programmable Read Only Memory) that rewritably stores a program to be executed by the CPU and data used for the program; and RAM (Random Access Memory) that temporarily stores data at the time of execution of the program. Respective operation parts making up the controller  16  are built as a result of these hardware parts and software in the EEPROM acting synergistically. As shown in  FIG. 5 , the controller  16  controls the entirety of the inkjet printer  101  and includes an image data storage section  41 , a flushing area size storage section  42 , a flushing data storage section  43 , a standard area formation section  44 , a reference area formation section  45 , an extraction section  46 , a head control section  47 , and a conveyance control section  48 . 
     The conveyance control section  48  controls a conveyance motor M of the conveyance unit  20  such that the sheet P is conveyed along a conveyance direction. 
     The image data storage section  41  stores image data pertaining to an image to be printed in a print area defined on the sheet P. The image data allocate volumes of ink droplets used for producing respective image dots, which make up an image, to the respective ejection ports  108  of the respective inkjet heads  1  at every print cycle. Ink droplets to be ejected from the ejection ports  108  in the present embodiment correspond to any selected from ink droplets having three types of volumes of ink droplets (large ink droplets, medium ink droplets, and small ink droplets). The print cycle corresponds to a period of time required to convey the sheet P only over a unit distance commensurate with a print resolution achieved in the conveyance direction. 
     The image data represent positions of virtual pixels (spatial elements), which are associated with image dots to be produced at intervals commensurate with a resolution for the main scan direction (a first resolution: a distance between the ejection ports  108  achieved in the main scan direction) and a resolution for the conveyance direction (a second resolution) with respect to the main scan direction and the conveyance direction, on a virtual sheet P′ (see  FIG. 10 ) that shows the print area of the sheet P in a data space. The virtual sheet P′ is formed as a result of a plurality of virtual pixels being arranged in a matrix pattern within a two-dimensional space defined by an axis extending along the main scan direction and another axis extending along the conveyance direction. A distance on the virtual sheet P′ between virtual pixels achieved in the conveyance direction is a distance on the sheet P commensurate with the resolution for the conveyance direction. A distance on the virtual sheet P′ between virtual pixels achieved in the main scan direction is a distance on the sheet P commensurate with the resolution for the main scan direction. The respective virtual pixels on the virtual sheet P′ are located at positions associated with any of the ejection ports  108  of the respective inkjet heads  1  with respect to the main scan direction. 
     A flushing area overlapping the print area is defined on the sheet P. The flushing area is an area where flushing dots are created through flushing (preliminary ejection) processing for ejecting ink before ink in the ejection ports  108  becomes degenerated. The flushing area is a range that occupies the entirety of the print area in the main scan direction and that occupies at least a portion of the print area in the conveyance direction. The flushing area can be arbitrarily determined by the user. In the present embodiment, the flushing area is assumed to match the print area of the sheet P. As shown in  FIG. 10 , a virtual flushing area F is a virtual area that depicts the flushing area in the data space. The virtual flushing area F is made up of a plurality of virtual pixels arranged in a matrix pattern in both the conveyance direction and the main scan direction. 
     In the embodiment, a length of the virtual flashing area F in the main scan direction may correspond to a length of an area, in which the plurality of ejection ports  108  are arranged, in the main scan direction. 
     The flushing area size storage section  42  stores the number of virtual pixels in the virtual flushing area F corresponding to the flushing area in the conveyance direction. As mentioned previously, the flushing area is determined in the form of the area that occupies at least a portion of the print area of the sheet P in the conveyance direction. Therefore, the number of virtual pixels in the virtual flushing area F in the conveyance direction is equal to or smaller than a quotient determined by dividing the length of the print area achieved in the conveyance direction by a unit distance commensurate with the resolution for the direction of the conveyance. A degree of freedom of setting of a data size pertaining to flushing is thereby increased, so that a storage capacity can be reduced. In the present embodiment, the flushing area matches the print area. The flushing area size storage section  42  stores, as the number of pixels, a quotient determined by dividing the length of the print area achieved in the conveyance direction by the unit distance commensurate with the resolution for the conveyance direction. 
     The flushing data storage section  43  stores flushing data for each color of ink used. Flushing data include data that show four base flushing patterns in correspondence with colors of ink. Each of the base flushing patterns is a layout pattern for a plurality of flushing candidate pixels achieved in a base area S 0 . The base flushing pattern corresponds to a layout of virtual pixels corresponding to positions where flushing dots can be produced. 
     The base area S 0  is a virtual area made as a result of a plurality of virtual pixels being arranged in a matrix pattern. The base area S 0  is made up of virtual pixels that are equal in number to the virtual flushing area F in the main scan direction and at least one flushing candidate pixel selected from the respective rows of virtual pixels in the conveyance direction. As will be described later, the standard area S is formed from the base area S 0 , and the base area S 0  can be said to be a unit virtual area where flushing candidate pixels are arranged in a flushing dot arrangement pattern. The flushing data storage section  43  stores the data pertaining to the base area S 0 , and the data include information about the number of virtual pixels making up the base area S 0  (achieved in both the main scan direction and the conveyance direction) and a unit arrangement location for a flushing candidate pixel in the base area S 0 . Detailed explanations are given to the flushing data stored in the flushing data storage section  43  by further referring to  FIGS. 6 and 7 . 
     As shown in  FIG. 6 , the base area S 0  is made up of a 4961×3508 matrix of virtual pixels  81 . The number of the virtual pixels  81  arranged in the base area S 0  along the main scan direction; namely, 4961, is a number equal to a result of the length of the print area of the sheet P in the main scan direction being divided by a unit distance (about 42 micrometers) commensurate with a resolution of 600 dpi for the main scan direction. The number of the virtual pixels  81  arranged in the base area S 0  along the conveyance direction; namely, 3508, is a number equal to a result of the length of the print area of the sheet P in the conveyance direction being divided by a unit distance (about 84 micrometers) commensurate with a resolution of 300 dpi for the conveyance direction. As mentioned above, in the present embodiment, the base area S 0  is a virtual area specified by the lowest resolution of 300 dpi (which will be described later) at least with respect to the conveyance direction. In the base area S 0 , 3508 virtual pixels  81  arranged along the conveyance direction make up one virtual pixel row  82 , and 4961 virtual pixel rows  82  arranged along the main scan direction make up the base area S 0 . 
     The inkjet printer  101  takes a resolution of 300 dpi as the lowest resolution for the conveyance direction and can print an image at any one of four resolutions that increase in steps of 100 dpi. A resolution of 600 dpi is the highest resolution for the conveyance direction at which the inkjet printer  101  can produce a print. A distance between adjacent virtual pixels  81  in the base area S 0  acquired with respect to the conveyance direction is equivalent to the unit distance commensurate with the resolution for the conveyance direction. A distance between adjacent virtual pixels  81  in the base area S 0  acquired with respect to the main scan direction is equivalent to the unit distance commensurate with the resolution for the main scan direction. 
     The virtual pixels  81  that are flushing candidate pixels are depicted by reference numerals  81   a  to  81   d , to thus be distinguished from virtual pixels that are not the flushing candidate pixels. The virtual pixel  81   a  represents a flushing candidate pixel pertaining to a black flushing pattern. The virtual pixel  81   b  represents a flushing candidate pixel pertaining to a magenta flushing pattern. The virtual pixel  81   c  represents a flushing candidate pixel pertaining to a cyan flushing pattern. The virtual pixel  81   d  represents a flushing candidate pixel pertaining to a yellow flushing pattern. 
     One virtual pixel row  82  in the base area S 0  is brought in correspondence with one ejection port  108 . Each of the virtual pixel rows  82  includes four virtual pixels  81   a  to  81   d  that are flushing candidate pixels. In each of the virtual pixel rows  82 , the virtual pixels  81   a  to  81   d  are placed at this time at mutually-different positions. 
     The present embodiment provides a configuration in which the flushing area matches the print area as mentioned above. The number of virtual pixels  81  (=3508) arranged in the base area S 0  along the conveyance direction is determined from the lowest resolution of 300 dpi at which the printer can produce a print. The flushing data storage area  43  stores the number of virtual pixels  81  arranged along the conveyance direction; namely, 3508, as the number of the virtual pixels  81  arranged in the base area S 0  along the conveyance direction. 
     As shown in  FIGS. 7 and 8 , the standard area formation section  44  produces a standard area S from the number of virtual pixels  81  arranged in the virtual flushing area F along the conveyance direction stored in the flushing area size storage section  42 , by enlarging the base area S 0  in the conveyance direction. The base area S 0  is enlarged at this time in such a way that the number of virtual pixels  81  arranged in the base area S 0  matches the virtual flushing area F with respect to the conveyance direction. 
     When the resolution required by image data is now assumed to be 300 dpi, the number of virtual pixels  81  arranged in the virtual flushing area F along the conveyance direction is equal to the number of virtual pixels  81  arranged in the base area S 0  along the conveyance direction. Accordingly, the standard area formation section  44  sets, as the standard area S, the base area S 0  where 4961 virtual pixels  81  are arranged along the main scan direction and where 3508 virtual pixels  81  are arranged along the conveyance direction. In this case, the standard area S has the same arrangement pattern as that of the base area S 0  in connection with the virtual pixels  81  and the flushing candidate pixels. 
     When the resolution required by image data is 600 dpi, the standard area formation section  44  first calculates a scale-up factor of 200% that is a ratio of 600 dpi to 300 dpi. On the basis of the base flushing pattern, the standard area formation section  44  doubles the number of virtual pixels in each of the virtual pixel rows of the base area S 0  except the flushing candidate pixels [processing for increasing the number of continual virtual pixel groups (which will be described later)]. Further, the standard area formation section  44  adds virtual pixels, which are equal in number to the flushing candidate pixels included in the virtual pixel row and which are not flushing candidate pixels, to each of the virtual pixel rows (processing for enlarging flushing candidate pixels themselves). Virtual pixels to be added are arranged; for instance, in such a way that each of the flushing candidate pixels is followed by one virtual pixel. When the scale-up factor is not an integral multiple as mentioned in connection with the foregoing embodiment, the number of virtual pixels included in the virtual pixel row is adjusted so as to obtain an integral multiple closest to a product resultant from multiplication of the scale-up factor. 
     Specifically, the standard area formation section  44  calculates a scale-up factor (130% in  FIGS. 7 and 8 ) that is a ratio of a resolution for the virtual flushing area F achieved in the conveyance direction to a resolution for the base area S 0  achieved in the conveyance direction (the lowest resolution achieved in the conveyance direction). In relation to each of the virtual pixel rows  82  of the base area S 0  in the conveyance direction, the standard area formation section  44  multiplies, by the scale-up factor, the number of continual virtual pixels  81  in the virtual pixel group consisting of one or a plurality of virtual pixels  81  that are defined by the virtual pixels  81   a  to  81   d  corresponding to flushing candidate pixels and that are not flushing candidate pixels, thereby calculating a new number of continual virtual pixels  81  in each of the virtual pixel groups. An integral closest to a product determined by multiplying the number of continual virtual pixels  81  by the scale-up factor is taken as a new number of continual virtual pixels  81 . The standard area formation section  44  inserts one or a plurality of virtual pixels  81  into a virtual pixel group in each of the virtual pixel row  82  in such a way that the calculated number of continual virtual pixels  81  that are not the flushing candidate pixels comes to a newly calculated number of continual virtual pixels, thereby forming the standard area S. The thus-formed standard area S is stored in the flushing data storage section  43 . Thus, the flushing data storage section  43  stores flushing data including four flushing patterns pertaining to the standard area S. 
     For instance, when four virtual pixels  81  are adjacently arranged in the virtual pixel row  82  as shown in  FIG. 8 , we have 4×130%=5.2; hence, an integer closest to the product is five. Therefore, one virtual pixel  81  is inserted into the virtual pixel group including four virtual pixels  81 . When only one virtual pixel  81  is arranged in the virtual pixel row  82 , we have 1×130%=1.3, and an integer closest to the product is one. Therefore, the virtual pixel  81  is not newly inserted in this case. When three virtual pixels  81  are arranged adjacently to each other, we have 3×130%=3.9, and an integer closest to the product is four. Therefore, one virtual pixel  81  is inserted into the virtual pixel group including the three virtual pixels  81 . When eight virtual pixels  81  are arranged adjacently to each other, we have 8×130%=10.4, and an integer closest to the produce is 10. Therefore, two virtual pixels  81  are inserted into the virtual pixel group including eight virtual pixels  81 . Even in this case, the standard area formation section  44  adds, to each of the virtual pixel group, virtual pixels that are equal in number to the flushing candidate pixels included in the virtual pixel row and that are not fluxing candidate pixels, as in the case where the scale-up factor is an integral multiple. The standard area formation section  44  finally brings the number of virtual pixels included in the virtual pixel row in agreement with the standard area S. Processing for increasing the number of continual virtual pixels and processing for enlarging flushing candidate pixels themselves have already been performed thus far. An excess or a deficiency of virtual pixels occurred at this time may also be collectively placed at the had of the virtual pixel row. Alternatively, the excess or deficiency of virtual pixels may also be placed at the end of the virtual pixel row. A flushing area matching the print area can be subjected to flushing processing. 
     The reference area formation section  45  forms a reference area Ref from the standard area S produced by the standard area formation section  44 , as shown in  FIG. 9 . In the reference area Ref, two standard areas S are adjacently arranged in both the main scan direction and the conveyance direction. The expression “adjacently arranged” signifies that the outermost virtual pixels  81  of each of the adjacent standard areas S are spaced apart from each other by a distance commensurate with the resolution for the conveyance direction and the resolution for an orthogonal direction. The virtual pixels  81  of the standard area S are repeatedly arranged twice in the reference area Ref in a line along the main scan direction and in units equal in number to the virtual pixels  81  arranged in the standard area S along the main scan direction. The virtual pixels  81  of the standard area S are also repeatedly arranged twice in the reference area Ref in a line along the conveyance direction and in units equal in number to the virtual pixels  81  arranged in the standard area S along the conveyance direction. The reference area Ref is thus made up of 9922 (=4961×2) virtual pixel rows arranged in the main scan direction, each of which includes 7016 (=3508×2) virtual pixels  81  arranged in the conveyance direction. 
     The extraction section  46  extracts flushing candidate pixels (flushing pixels) used for producing flushing dots from flushing candidate pixels (the virtual pixels  81   a  to  81   d ) included in the reference area Ref formed by the reference area formation section  45 . 
     Specifically, the extraction section  46  virtually arranges a virtual flushing area F at a randomly determined position in the reference area Ref in such a way that all pixels of the virtual flushing area F overlap the virtual pixels  81  of the reference area Ref.  FIG. 10  shows a state of the reference area achieved at this time. When respective image dots are associated with any pixel rows arranged along the conveyance direction, the extraction section  46  selects, in the virtual flushing area F, virtual pixel rows  84  not associated with the image dots. In an area where overlaps with the selected virtual pixel rows  84  are achieved, the extraction section  46  extracts, as flushing pixels for the virtual flushing area F, pixels overlapping the flushing candidate pixels (the virtual pixels  81   a  to  81   d ) in the reference area Ref. In the virtual flushing area F, there are actually produced the virtual area made up of only the virtual pixel rows  84  not associated with the image dots corresponds to the flushing area where flushing dots. Each of the virtual pixel rows  84  includes four flushing pixels in correspondence with the respective virtual pixels  81   a  to  81   d . The flushing data storage section  43  stores flushing data pertaining to an arrangement pattern of the flushing pixels achieved at this time as a flushing pattern. 
     The virtual flushing area F completely overlaps one standard area S or straddles two or four standard areas S within the reference area Ref. The reference area Ref is formed by arranging the four standard areas S so as to become adjacent to each other. Hence, even when the virtual flushing area F straddles two or four standard areas S, only one pixel overlapping any of the flushing candidate pixels (the virtual pixels  81   a  to  81   d ) is present in each of the virtual pixel rows arranged in the conveyance direction within the reference area Ref. Therefore, even when the virtual flushing area F is virtually arranged at any location within the reference area Ref, flushing pixels can be extracted from each of the virtual pixel rows  84  without fail. 
     The head control section  47  controls ejection of ink droplets from the ejection ports  108  of the inkjet head  1  through the control substrate  54 . The head control section  47  determines whether or not a time elapsed since purging-and-wiping operation (which will be described later) was performed or flushing was performed has exceeded a predetermined time T. When determined that the elapsed time has not exceeded the predetermined time T, the head control section  47  controls ejection of ink droplets from the ejection ports  108  of the respective ink jet heads  1  in such a way that only image dots pertaining to image data stored in the image data storage section  41  are formed on the sheet P without formation of the flushing dots on the sheet P. The predetermined time T is a time that is shorter than a period of time during which speed of an ink droplet ejected from the ejection port  108  decreases from a reference speed to a predetermined percentage of the reference speed as a result of the ink stored in the ejection ports  108  being degraded by drying, or the like. The predetermined time T is set to a time that is shorter than a period of time during which a change in image quality is visibly recognized. 
     When determined that the elapsed time exceeds the predetermined time T, the head control section  47  produces a logical OR between image pixels relevant to the image dots included in image data and flushing pixels extracted by the extraction section  46  (ejection data that are a logical OR between image data and flushing data stored in the flushing data storage section  43 ). The head control section  47  controls ejection of ink droplets from the ejection ports  108  of the respective inkjet heads  1  in such a way that the image dots included in the logical OR (the ejection data) and flushing dots corresponding to the flushing pixels are formed on the sheet P conveyed to the conveyance unit  20 . Ink droplets are ejected at least one from all of the ejection ports  108  of the respective inkjet heads  1  every time the predetermined time T elapses. 
     Operation procedures of the controller  16  are now described by reference to  FIG. 11 . As shown in  FIG. 11 , upon receipt of a print start command from a host computer, the controller performs purging-and-wiping operation (S 101 ). The purging-and-wiping operation means operation for forcefully supplying ink from an unillustrated ink supply pump to the respective inkjet heads  1 , purging (ejecting) ink from the respective ejection ports  108 , and wiping the ejection surfaces  2   a  by means of unillustrated wipers. It becomes possible to discharge ink having increased viscosity and impurities in the inkjet heads  1  to the outside by means of performing purging-and-wiping operation and to hold meniscuses formed in the ejection ports  108  in good condition. The head control section  47  resets an internal timer T 0  (S 102 ). 
     The head control section  47  subsequently determines whether or not the value of the internal timer T 0  has exceeded the predetermined time T (S 103 ). When determined that the value of the internal timer T 0  has not exceeded the predetermined time T (NO in S 103 ), the head control section  47  controls ejection of ink droplets from the ejection ports  108  of the ink jet head  1  in such a way that only image dots relevant to image data are produced on the sheet P (S 104 ). When the sheet P has finished undergoing printing, the controller  16  determines whether or not to subject the next sheet P to printing (S 105 ). When the next sheet P is subjected to printing (YES in S 105 ), processing again proceeds to S 103 , where the next sheet P undergoes printing. When the next sheet P is not subjected to printing (NO in S 105 ), processing pertaining to a flowchart shown in  FIG. 11  is completed. 
     When the head control section  47  determines that the value of the internal timer T 0  has exceeded the predetermined time T (YES in S 103 ), the standard area formation section  44  forms the standard areas S from the base area S 0 , and the reference area formation section  45  forms the reference area Ref from the standard areas S (S 106 ). The extraction section  46  randomly, virtually arranges the virtual flushing area F within the reference area Ref in such a way that all of the pixels of the virtual flushing area F overlap the virtual pixels  81  of the reference area Ref (S 107 ). Further, when the respective image dots are associated with any of the pixel rows arranged along the conveyance direction within the virtual flushing area F, the extraction section  46  extracts, as flushing pixels relevant to the virtual flushing area F, the pixels overlapping the virtual pixels  81   a  to  81   d  of the reference area Ref corresponding to the flushing candidate pixels within the range where there are overlaps with the virtual pixel rows  84  not associated with the image dots (S 108 ). The extraction section  46  generates flushing data at this time from the extracted flushing pixels and an arrangement pattern of the flushing pixels (the flushing pattern). Simultaneously, flushing data are stored in the flushing data storage section  43 . 
     The head control section  47  controls ejection of ink droplets from the ejection ports  108  of the respective inkjet heads  1  in such a way that the image dots included in image data and flushing dots corresponding to flushing pixels extracted by the extraction section  46  are formed on the sheet P conveyed to the conveyance unit  20  (S 109 ). The head control section  47  generates ejection data from the image data and flushing data in the flushing data storage section  43  and controls ejection of ink droplets in accordance with the ejection data. Ink droplets are ejected from all of the ejection ports  108  of the respective inkjet heads  1  at least once every time the predetermined time T elapses. When the sheet P has finished undergoing printing, the controller  16  determines whether to subject the next sheet P to printing (S 110 ). When the next sheet P is subjected to printing (YES in S 110 ), processing again proceeds to S 102 , and the internal timer TO is reset. Thus, the next sheet P is subjected to printing. When the next sheet P is not subjected to printing (NO in S 110 ), processing pertaining to the flowchart shown in  FIG. 11  is completed. 
     In the inkjet printer  101  of the present embodiment, the extraction section  46  can appropriately extract flushing pixels for the respective virtual pixel rows  84  even when the virtual flushing area F is virtually arranged at any location in the reference area Ref as mentioned above. Therefore, the virtual flushing area F is virtually arranged at a randomly-determined position within the reference area Ref, whereby it is possible to avoid formation of flushing dots at the same positions on all sheets P, to thus make the flushing dots less conspicuous. Positions of flushing dots can quickly be determined through processing involving small amount of arithmetic operation, such as virtually arranging the virtual flushing area F in the reference area Ref and extracting pixels overlapping the virtual pixels  81   a  to  81   d , which are the flushing candidate pixels, in the reference area Ref as flushing pixels for the virtual flushing area F within the range where there are overlaps with the virtual pixel rows  84  not associated with the image dots for the virtual flushing area F. 
     Flushing data stored in the flushing data storage section  43  include four flushing patterns compliant with the respective inkjet heads  1 . Four pixel rows for four flushing patterns placed at the same positions with respect to the main scan direction have the virtual pixels  81   a  to  81   d  that are flushing candidate pixels located at different positions with respect to the conveyance direction. When the flushing dots are formed, ink droplets ejected from the respective inkjet heads  1  do not arrive at the same locations in a record area on the sheet P. An increase in diameters of the flushing dots can thereby be prevented. 
     The extraction section  46  virtually arranges the virtual flushing area F at a randomly-determined position in the reference area Ref. Hence, positions where flushing dots are to be formed can readily be changed from one sheet P to another sheet P. 
     In addition, the reference area formation section  45  arranges two standard areas S adjacently to each other in both the main scan direction and the conveyance direction, thereby creating the reference area Ref. It is, therefore, possible to efficiently change positions where flushing dots are to be formed from one sheet P to another sheet P. 
     The standard area formation section  44  generates, from the base area S 0  stored in the flushing data storage section  43 , a plurality of standard areas S commensurate with mutually-different resolutions for the conveyance direction. It is not necessary to store the plurality of standard areas S in the flushing data storage section  43 , so that the storage capacity of the flushing data storage section  43  can be made small. Further, the flushing data storage section  43  stores the base area S 0  commensurate with the lowest resolution among the plurality of resolutions for the conveyance direction at which the inkjet printer  101  can produce a print, so that the storage capacity of the flushing data storage section  43  can be reduced to a much greater extent. 
     In relation to each of the virtual pixel rows  82  arranged in the base area S 0  along the conveyance direction, the standard area formation section  44  calculates a new number of continual pixels by multiplying, by a scale-up factor, the number of continual virtual pixels  81  in the pixel group that is grouped by the virtual pixels  81   a  to  81   d  and that includes one or a plurality of virtual pixels  81  that are not flushing candidate pixels. The standard area formation section  44  forms the standard areas S in such a way that the number of continual pixels in each of the pixel groups becomes equal to the calculated number of continual pixels. Therefore, the plurality of standard areas S commensurate with the plurality of resolutions for the conveyance direction can readily be formed. 
     Example Modification 
     Example modifications of the present invention are now described. In the foregoing embodiment, the flushing data storage section  43  is configured so as to store flushing data including flushing patterns for four colors black, magenta, cyan, and yellow having a plurality of flushing candidate pixels as pixels. The flushing data storage section, however, can store flushing data including only one flushing pattern. The extraction section at this time virtually arranges the virtual flushing area F, for each inkjet head  1 , at a randomly-determined position within the reference area Ref for the flushing data. Further, in the range where there are overlaps with the virtual pixel rows  84  not associated with the image dots of the virtual flushing area F, the extraction section extracts, as flushing pixels for the virtual flushing area F, pixels overlapping pixels that are flushing candidate pixels in the reference area Ref. 
     The essential requirement for the flushing data storage section  43  is to store flushing data including the flushing pattern pertaining to one inkjet head  1 , so that the storage capacity of the flushing data storage section  43  can be reduced. 
     The preferred embodiment of the present invention has been described thus far. However, the present invention is not limited to the foregoing embodiment and susceptible to various alterations within the scope of claims. In the foregoing embodiment, each of the pixel rows, which is made up of a plurality of pixels arranged in the conveyance direction for the four flushing patterns included in the flushing data stored in the flushing data storage section  43 , is configured so as to include, at mutually-different locations with respect to the conveyance direction, the virtual pixels  81   a  to  81   d  that are flushing candidate pixels. However, each of the pixel rows can have pixels, which are the flushing candidate pixels, at the same locations with respect to the conveyance direction. 
     In the foregoing embodiment, the extraction section  46  is configured in such a way that the virtual flushing area F is virtually arranged at a randomly-determined position in the reference area Ref. The extraction section can also be configured in such a way that the virtual flushing area F is virtually arranged at a position determined on the basis of a previously-stored pattern. The pattern can also change periodically. 
     In the foregoing embodiment, the reference area formation section  45  is configured so as to form the reference area Ref by arranging two standard areas S adjacently to each other in both the main scan direction and the conveyance direction. However, the reference area formation section can also be configured so as to form the reference area Ref by arranging an arbitrary number of standard areas S adjacently to each other in both the main scan direction and the conveyance direction. For instance, the reference area Ref can also be formed by arranging two standard areas S adjacently to each other in both the main scan direction and the conveyance direction. Alternatively, the reference area Ref can also be formed by letting the virtual pixels  81  of a standard area S repeatedly appear in at least the main scan direction or the conveyance direction and in units equal in number to the virtual pixels  81  arranged in the standard area S in the same direction, in such a way that the virtual pixels  81  arranged from one end of the standard area S become adjacent to virtual pixels  81  arranged toward the other end of the standard area S with respect to the same direction. 
     In the foregoing embodiment, the flushing data storage section  43  is configured so as to store the base area S 0  commensurate with the lowest resolution among the resolutions for the conveyance direction at which the inkjet printer  101  can produce a print. However, The flushing data storage section can also be configured so as to store the base area S 0  commensurate with another resolution for the conveyance direction. 
     In the foregoing embodiment, the standard area formation section  44  is configured so as to form the standard area S by means of inserting virtual pixels  81  into each of the pixel groups in each of the virtual pixel rows  82  arranged in the base area S 0  along the conveyance direction, in such a way that each of the pixel groups that is grouped by the virtual pixels  81   a  to  81   d  and that includes one or a plurality of virtual pixels  81  which are not flushing candidate pixels assumes a new number of continual pixels determined by multiplying, by a scale-up factor, the number of continual virtual pixels  81  in the pixel group. However, the configuration of the standard area formation section for forming the standard area S from the base area S 0  can be arbitrary. 
     In the foregoing embodiment, each of the virtual pixel rows  82  of the base area S 0  is configured so as to include one flushing candidate pixel in one inkjet head  1 . However, each of the virtual pixel rows  82  can also be configured so as to include two or more flushing candidate pixels in one inkjet head  1 . 
     In the embodiment, the number of pixels arranged in the virtual flushing area F along the conveyance direction is equal to a quotient obtained by dividing the length of the print area achieved in the conveyance direction by the distance equivalent to the resolution for the conveyance direction. However, the number of pixels arranged in the virtual flushing area F along the conveyance direction can also be smaller than the quotient. 
     In the embodiment, the flushing data storage section  43  stores a number equal to the number of the virtual pixels in the virtual flushing area F as the number of virtual pixels for the base area S 0  to be arranged in the main scan direction. The flushing data storage section can also store instead a number equal to or smaller than a quotient obtained by dividing the length of the flushing area (the print area) achieved in the main scan direction by a unit distance commensurate with the resolution for the main scan direction. When the standard area S is formed, the flushing data storage section  43  enlarges virtual pixels in such a way that a virtual pixel row repeatedly appears in units equal to the previously-stored number of virtual pixels to be formed in the base area S 0  with respect to the main scan direction. The flushing data storage section  43  performs enlargement processing until virtual pixel rows required to make up the virtual flushing area F are arranged. Small memory capacity is sufficient in this case. 
     In the foregoing embodiment, the base area S 0  is a virtual area defined at the lowest resolution for the conveyance direction. However, the base area S 0  must be specified at the highest resolution at least for the conveyance direction. The same also applies to the base area in connection with the other direction as well. However, in view of small memory capacity, it is much better to specify the base area at the smallest resolution for both directions. 
     In the foregoing embodiment, when the standard area formation section  44  forms a standard area S by enlarging the base area S 0 , addition of virtual pixels commensurate with enlargement of the respective flushing candidate pixels is performed by adding one virtual pixel so as to immediately follow each of the flushing candidate pixels. However, the virtual pixel can be placed at an arbitrary position, so long as the position is in the same virtual pixel row. For instance, in order to add one pixel corresponding to the flushing candidate pixel, the pixel can also be interposed between the flushing candidate pixel and the next flushing candidate pixel. The standard area S can thereby be formed in a pattern analogous to an arrangement pattern of flushing candidate pixels in the base flushing pattern. Further, all virtual pixels to be added can also collectively be arranged before the head or after the end of the virtual pixel row. 
     In the foregoing embodiment, the flushing area and the print area are configured so as to match each other. However, as mentioned previously, the essential requirement is that the flushing area should match the print area in the main scan direction and occupy a portion of the print area in the conveyance direction. All the standard area formation section  44  has to do is to enlarge each of the virtual pixel rows by multiplying, by a scale-up factor, the number of continual virtual pixels in the virtual pixel group made up of the virtual pixels that are not the flushing candidate pixels. The standard areas S are formed from the base area S 0  by means of simple processing. Processing for enlarging flushing candidate pixels themselves and processing for adjusting an excess or deficiency in virtual pixels in each of virtual pixel rows for letting the flushing area match the print area are not necessary at this time. 
     Although the base area S 0  and the standard areas S have been described as including only one of the virtual pixels  81   a  to  81   d  in each of the virtual pixel rows for each color, the plurality of virtual pixels  81   a  to  81   d  can also be included. 
     When the scale-up factor is not an integral multiple in the foregoing embodiment, processing for enlarging the area by use of an integral multiple closest to the value of the scale-up factor is performed. However, an integral multiple obtained by rounding up a value of a product may also be used. Alternatively, an integral value obtained by rounding down a value of a product may also be used. 
     The present invention is also applicable to a recorder that ejects a liquid other than ink. The present invention is not limited to the printer but is also applicable to a facsimile, a copier, and the like. Further, the present invention is also applicable to a computer readable recording memory storing a program which causes the recorder to function as described above. In the above exemplary embodiments, the EEPROM storing the program is employed as an example of the computer readable recording medium according to the invention. However, the computer readable recording medium according to the invention is not limited to the EEPROM. The computer readable recording medium according to the invention may be any computer readable recording medium, such as a hard disk, an optical disk (CD-ROM, DVD-ROM, etc.), flash memory and the like, storing the program.