Abstract:
An image forming device includes a feeding unit, a recording unit, and a controller. The feeding unit includes a first feeding member and a second feeding member disposed at a downstream of the first feeding member in a feeding direction. At least one of the first feeding member and the second feeding member feeds a recording medium in the feeding direction. The recording unit is disposed between the first feeding member and the second feeding member to eject a plurality of ink droplets onto the recording medium fed between the first feeding member and the second feeding member. A plurality of ink droplets ejected onto the recording medium forms a plurality of raster lines extending in a main-scanning direction orthogonal to the feeding direction. The controller controls the recording unit to eject a plurality of ink droplets so that a plurality of first raster lines formed within a first time span has a first resolution in the feeding direction, a plurality of second raster lines formed within a second time span has a second resolution in the feeding direction, and a plurality of third raster lines formed within a third time span including a banding timing at which a trailing edge of the recording medium gets away from the first feeding member has a third resolution higher than the first resolution in the feeding direction. The second time span follows the first time span and the third time span follows the second time span. The second resolution is higher than the first resolution and lower than the third resolution.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an image forming device, and more particularly to an image forming device which prevents a banding phenomenon caused by uneven feeding of a recording medium. 
         [0003]    2. Description of Related Art 
         [0004]      FIG. 1  is a side view showing a printer unit  1   a  included in a color inkjet printer as a conventional image forming device. The inkjet printer performs printing based on obtained data on a recording sheet using the printer unit  1   a . The printer unit  1   a  includes a feed roller  101 , a pinch roller  102 , a discharge roller  103 , spurs  104  and  105 , a line feed motor (not shown) for rotating the rollers  101  and  103 , a sheet sensor  106  for detecting the presence or absence of a recording sheet, a platen  108 , and an inkjet head  109 . In  FIG. 1 , a recording sheet led into the printer unit  1   a  is fed in a sheet feeding direction indicated by an arrow Z. 
         [0005]    The inkjet head  109  is mounted in a carriage  64  to be described later (see  FIG. 3 ) so as to move back and forth in a direction perpendicular to the sheet feeding direction Z (perpendicularly to the plane of  FIG. 1 ). Ink cartridges (not shown) each filled with one of the following colors of ink, namely cyan, magenta, yellow, and black, supply the inkjet head  109  with the ink. The inkjet-head  109  ejects the supplied ink from any one of the large number of nozzles formed on the inkjet head  109  onto the recording sheet, thereby printing an image. The inkjet head  109  has a plurality of nozzles for each color, at regular intervals in substantially parallel with the sheet feeding direction. The platen  108  is opposed to the inkjet head  109 . The recording sheet is fed between the platen  108  and the inkjet head  109 . 
         [0006]    In the printer unit  1   a , the feed roller  101  is provided upstream of the inkjet head  109  in the sheet feeding direction. The feed roller  101  and the pinch roller  102  facing the feed roller  101  sandwich the recording sheet therebetween, and then send the recording sheet forward by the rotation of the rollers. Passing through the feed roller  101 , the recording sheet is fed immediately below the inkjet head  109 . 
         [0007]    The discharge roller  103  is disposed downstream of the feed roller  101  and the inkjet head  109 . The discharge roller  103  and the spur  104  facing the discharge roller  103  also sandwich the recording sheet therebetween, and then send the recording sheet forward. The spur  104  is a rotational body with an uneven surface. The spur  104 , disposed above the discharge roller  103 , comes into contact with an ink image on a printed surface of the recording sheet. Since the ink image does not get dry immediately after printed, if the ink image contacts a roller having a large contact area before the image get dry, the printed image may be blurred, crinkled, or transferred to the roller, thereby degrading printing quality. Therefore, the spurs are employed in this conventional image forming device so as to reduce the contact area that contacts the printed surface of the recording sheet, thereby preventing printing quality from degrading. 
         [0008]    The feed roller  101  and the discharge roller  103  are driven by the line feed motor (not shown) as the drive source for transporting a recording sheet. This allows the feed roller  101  and the discharge roller  103  to rotate in the sheet feeding direction. The rollers  101  and  103 , together with the roller  102  and the spur  104  facing the rollers  101  and  103  respectively, sandwich the recording sheet therebetween, and then send the recording sheet forward. Note that the discharge roller  103  rotates faster than the feed roller  101 , while the feed roller  101  sandwiches the recording sheet stronger than the discharge roller  103 . Thus, the recording sheet disposed on the region opposing the inkjet head  109  is tautened, thereby preventing printing quality from degrading. 
         [0009]    Therefore, while the recording sheet is being sandwiched by the feed roller  101  and the pinch roller  102 , specifically until a trailing edge of the recording sheet passes through the nip between the feed roller  101  and the pinch roller  102 , the feed roller  101  continues to send the recording sheet forward. 
         [0010]    On the other hand, after the trailing edge of the recording sheet has passed through the nip between the feed roller  101  and the pinch roller  102 , the recording sheet is fed by the rotation of the discharge roller  103 . 
         [0011]    While the recording sheet is fed by the feed roller  101  and the pinch roller  102  sandwiching the recording sheet, the sheet feeding accuracy is maintained. However, when the recording sheet reaches a point where the recording sheet leaves the feed roller  101  and the pinch roller  102 , the trailing edge of the recording sheet is released from the pressure which has been applied on. This means that the sheet feeding accuracy becomes unstable due to the sheet being flipped, thereby causing a banding phenomenon. 
         [0012]    Japanese Patent Application Laid-Open Publication No. 11-207945 and Japanese Patent Application Laid-Open Publication No. 11-291506 propose increasing a resolution level in the region where a banding phenomenon occurs so as to prevent the banding phenomenon. Japanese Patent Application Laid-Open Publication No. 2005-138501 discloses the method of decreasing the feeding amount of a recording medium to prevent the occurrence of a banding phenomenon. 
         [0013]    However, although a banding phenomenon is prevented to some extent by these inventions, its banding pattern is still recognizable. 
         [0014]      FIGS. 2A and 2B  simulate how a banding phenomenon occurs. In these figures, each circle represents a dot formed with ink.  FIG. 2A  shows a case in which printing is performed while a recording sheet is fed by the feed roller  101  and the pinch roller  102  sandwiching the sheet. The dots are arranged systematically in a grid pattern. The expression “normal arrangement” in  FIG. 2  means that a raster line is formed with dots aligned in a direction perpendicular (orthogonal) to the sheet feeding direction (Z direction in the figure), and one raster line is disposed on another raster line in sequence in the feeding direction. 
         [0015]      FIG. 2B  shows a case in which printing is performed in normal arrangement when a recording sheet is sent forward suddenly to an extreme degree. In this case, a linear space appears as indicated by two parallel solid lines in a direction perpendicular to the sheet feeding direction. This phenomenon causes a white line to be recognizable in a direction perpendicular to the sheet feeding direction. 
       SUMMARY OF THE INVENTION 
       [0016]    In view of the above-described drawbacks, it is an objective of the present invention to provide an image forming device that prevents a banding phenomenon with a simple structure. 
         [0017]    In order to attain the above and other objects, the present invention provides an image forming device including a feeding unit, a recording unit, and a controller. The feeding unit includes a first feeding member and a second feeding member disposed at a downstream of the first feeding member in a feeding direction. At least one of the first feeding member and the second feeding member feeds a recording medium in the feeding direction. The recording unit is disposed between the first feeding member and the second feeding member to eject a plurality of ink droplets onto the recording medium fed between the first feeding member and the second feeding member. A plurality of ink droplets ejected onto the recording medium forms a plurality of raster lines extending in a main-scanning direction orthogonal to the feeding direction. The controller controls the recording unit to eject a plurality of ink droplets so that a plurality of first raster lines formed within a first time span has a first resolution in the feeding direction, a plurality of second raster lines formed within a second time span has a second resolution in the feeding direction, and a plurality of third raster lines formed within a third time span including a banding timing at which a trailing edge of the recording medium gets away from the first feeding member has a third resolution higher than the first resolution in the feeding direction. The second time span follows the first time span and the third time span follows the second time span. The second resolution is higher than the first resolution and lower than the third resolution. 
         [0018]    Another aspect of the present invention provides an image forming device including a feeding unit, a recording unit, and a controller. The feeding unit includes a first feeding member and a second feeding member disposed at a downstream of the first feeding member in a feeding direction. At least one of the first feeding member and the second feeding member feeds a recording medium in the feeding direction. The recording unit is disposed between the first feeding member and the second feeding member to eject a plurality of ink droplets onto the recording medium fed between the first feeding member and the second feeding member. A plurality of ink droplets ejected onto the recording medium forms a plurality of raster lines extending in a main-scanning direction orthogonal to the feeding direction. The controller controls the recording unit to eject a plurality of ink droplets so that a plurality of first lines formed within a first time span has a first resolution in the feeding direction, and a plurality of second raster lines formed within a second time span including a banding timing at which a trailing edge of the recording medium gets away from feeding by the first feeding member has a second resolution higher than the first resolution in the feeding direction. The second time span follows the first time span. The plurality of ink droplets ejected in the second time span forms a plurality of dots on the recording medium to be in a staggered pattern. 
         [0019]    Another aspect of the present invention provides an image forming device including a feeding unit, a recording unit, and, a controller. The feeding unit includes a first feeding member and a second feeding member disposed at a downstream of the first feeding member in a feeding direction. At least one of the first feeding member and the second feeding member feeds a recording medium in the feeding direction. The recording unit is disposed between the first feeding member and the second feeding member to eject a plurality of ink droplets onto the recording medium fed between the first feeding member and the second feeding member. A plurality of ink droplets ejected onto the recording medium forms a plurality of raster lines extending in a main-scanning direction orthogonal to the feeding direction. The recording medium includes a first region, a second region adjacent to the first region in the feeding direction, and a third region adjacent to the second region in the feeding direction without being adjacent to the first region. The third region has a banding region on which the recording unit records the plurality of ink droplets when a trailing edge of the recording medium gets away from the first feeding member. The controller controls the recording unit to eject a plurality of ink droplets so that a plurality of first raster lines formed on the first region has a first resolution in the feeding direction. A plurality of second raster lines formed on the second region has a second resolution in the feeding direction, and a plurality of third raster lines formed on the third region has a third resolution higher than the first resolution in the feeding direction. The second resolution is higher than the first resolution and lower than the third resolution. 
         [0020]    Another aspect of the present invention provides an image forming device including a feeding unit, a recording unit, and a controller. The feeding unit includes a first feeding member and a second feeding member disposed at a downstream of the first feeding member in a feeding direction. At least one of the first feeding member and the second feeding member feeds a recording medium in the feeding direction. The recording unit is disposed between the first feeding member and the second feeding member to eject a plurality of ink droplets onto the recording medium fed between the first feeding member and the second feeding member. A plurality of ink droplets ejected onto the recording medium forms a plurality of raster lines extending in a main-scanning direction orthogonal to the feeding direction. The recording medium includes a first region, and a second region adjacent to the first region in the feeding direction. The second region has a banding region on which the recording unit records the ink droplets when a trailing edge of the recording medium gets away from the first feeding member. The controller controls the recording unit to eject a plurality of ink droplets so that a plurality of first raster lines formed on the first region has a first resolution in the feeding direction, and a plurality of second raster lines formed on the second region has a second resolution higher than the first resolution. The plurality of ink droplets ejected on the second region forms a plurality of dots on the recording medium to be in a staggered pattern. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
           [0022]      FIG. 1  is a side view of a conventional printer unit; 
           [0023]      FIG. 2A  is a schematic diagram showing a conventional dot arrangement printed properly; 
           [0024]      FIG. 2B  is a schematic diagram showing a conventional dot arrangement printed when a skip occurs during the sheet feeding process; 
           [0025]      FIG. 3  is a block diagram showing the electric circuitry of a color inkjet printer including an image forming device according to a first embodiment of the present invention; 
           [0026]      FIG. 4A  is a schematic diagram showing an arrangement of dots formed on a recording sheet properly; 
           [0027]      FIG. 4B  is a schematic diagram showing an arrangement of dots formed on a recording sheet when a skip occurs during the sheet feeding process; 
           [0028]      FIG. 5A  illustrates a method of sorting dots formed in the interlace system according to the first embodiment; 
           [0029]      FIG. 5B  illustrates the dot storing method as a sequel to  FIG. 5A ; 
           [0030]      FIG. 6A  illustrates the dot sorting method as a sequel to  FIG. 5B ; 
           [0031]      FIG. 6B  illustrates the dot sorting method as a sequel to  FIG. 6A ; 
           [0032]      FIG. 7  is flow chart showing a dot sorting process according to the first embodiment; 
           [0033]      FIG. 8A  illustrates a dot sorting method according to a second embodiment; 
           [0034]      FIG. 8B  illustrates the dot sorting method as a sequel to  FIG. 8A ; 
           [0035]      FIG. 9  illustrates the dot sorting method as a sequel to  FIG. 8B ; 
           [0036]      FIG. 10  is a flow chart showing a dot sorting process according to the second embodiment; and 
           [0037]      FIG. 11  is a flow chart showing a raster resolution setting process in a mixed region according to the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    An image forming device according to preferred embodiments of the present invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description. 
         [0039]    In the following description, the expressions “front”, “rear”, “upper”, “lower”, “right”, and “left” are used to define the various parts when the image forming device is disposed in an orientation in which it is intended to be used. 
         [0040]    Hereinafter, a first preferred embodiment of a color inkjet printer  1  according to the present invention will be described with reference to the accompanying drawings.  FIG. 3  is a block diagram showing the electric circuitry of the color inkjet printer  1  briefly. The mechanism of a printer unit  1   a  as a main unit for printing is the same as shown in  FIG. 1 . 
         [0041]    A controller for controlling the color inkjet printer  1  is provided with a printer control circuit board  12  and a carriage board  13 . The printer control circuit board  12  includes a one-chip microcomputer (CPU)  32 ; a ROM  33  which stores various control programs to be run by the CPU  32  and fixed value data; a RAM  34  as a memory for storing various types of data temporarily; a flash memory  35 ; an image memory  37 ; and a gate array (G/A)  36 . 
         [0042]    The CPU  32 , which is a processor, generates a printing timing signal and a reset signal according to the control program pre-stored in the ROM  33 , and then transfers each of the signals to the gate array  36  (to be described later). The following devices are connected to the CPU  32 : an operation panel  45  which a user gives a direction for printing; a CR motor drive circuit  39  for driving a carriage motor (CR motor)  16  which operates a carriage  64  having an inkjet head  109 ; a LF motor drive circuit  41  which operates a line feed motor (LF motor)  40  for driving a feed roller  101 ; a sheet sensor  106 ; a linear encoder  43 ; and a rotary encoder  46 . These devices are controlled by the CPU  32 . 
         [0043]    The sheet sensor  106 , which is disposed upstream of the feed roller  101 , detects the presence or absence of a recording sheet. The sheet sensor  106  includes a lever  106   a  which turns around as comes into contact with a recording sheet (see  FIG. 1 ); and a photointerrupter  106   b  as a sensor unit which detects the lever  106   a  turning around (see  FIG. 1 ). The linear encoder  43  detects the moving amount of the carriage  64 . The amount encoded by the linear encoder  43  is detected by the photointerrupter (not shown) so that the reciprocating motion of the carriage  64  is controlled. The rotary encoder  46  detects the rotation amount of the feed roller  101 . The amount encoded by the rotary encoder  46  is detected by the photointerrupter (not shown) so that the feed roller  101  is controlled. Specifically, the rotary encoder  46  detects the actual position of the recording sheet fed by the feed roller  101  with a predetermined accuracy. 
         [0044]    The ROM  33  stores a print control program  33   a  for performing printing and a dot sorting process program  33   b  for arranging dot positions so as to prevent the occurrence of a banding phenomenon. The flash memory  35  has to store correction values for feeding a recording sheet accurately and for allowing the head to scan accurately. The flash memory  35  is reviewed for the values before its shipment. The CPU  32 , the ROM  33 , the RAM  34 , the flash memory  35  and the G/A  36  are connected through a bus line  47 . 
         [0045]    The G/A  36  outputs recording data (drive signal) for recording the image data stored in the image memory  37  on a recording sheet, based on the timing signal transferred from the CPU  32  and the image data; a transfer clock synchronizing to the recording data; a latch signal; a parameter signal for generating a basic drive waveform signal; and a discharging timing signal to be output cyclically. The signals each are transferred by the G/A  36  to the carriage board  13  including a head driver. 
         [0046]    The image data transferred from an external device such as a computer through an interface (I/F)  44  such as a USB interface is stored in the image memory  37  by the G/A  36 . The G/A  36  then generates a data reception interruption signal based on the data transferred from the computer or the like through the I/F  44 , and transfers the signal to the CPU  32 . The signals to be exchanged between the G/A  36  and the carriage board  13  are transferred through a harness cable connecting them. 
         [0047]    The head driver (drive circuit) mounted on the carriage board  13  drives the inkjet head  109 . The inkjet head  109  and the head driver are connected to each other by a flexible wiring board  19  on which a copper-foiled wiring pattern is formed on a polyimide film having a thickness of 50 to 150 μm. The head driver is controlled via the G/A  36  mounted on the printer control circuit board  12 , and applies a drive pulse signal having a waveform corresponding to a recording mode to a piezoelectric actuator included in the inkjet head  109  so that a predetermined amount of ink is ejected from the inkjet head  109 . 
         [0048]    Referring to  FIGS. 4A and 4B , a description is given for the arrangement of dots formed on a recording sheet by the color inkjet printer  1  according to the first embodiment.  FIGS. 4A and 4B  simulate the arrangement of dots formed on a recording sheet when forming an image at a resolution higher than normal value. Similarly to  FIG. 2  showing the conventional dot arrangement, each circle represents a dot formed with ink, and a sheet feeding direction is indicated by an arrow Z. 
         [0049]    In the present embodiment, as shown in  FIG. 4A , the one dot and one space are aligned alternately in a direction perpendicular to the sheet feeding direction Z to form dot lines (raster line). Each dot forming first raster line is formed at the position corresponding to each space in the adjacent second raster line, and the first raster line is disposed away from the second raster line at a distance equal to the half width of the raster line in the feeding direction Z. In other words, the dots forming first column are arranged at the positions deviated from the dots forming the adjacent second column, in the feeding direction Z.  FIG. 4A  shows a case in which a recording sheet is fed properly. Specifically, printing is performed while a recording sheet is fed by the feed roller  101  and the pinch roller  102  sandwiching the sheet. In this case, dots are arranged in a staggered layout with regularity. 
         [0050]      FIG. 4B  shows a staggered arrangement formed when a recording sheet is fed unevenly and sent forward suddenly to an extreme degree (a skip occurs during the sheet feeding process). In  FIG. 4B , two parallel solid lines are indicated virtually at the position of the linear space shown in  FIG. 2B . As shown in  FIG. 4B , the space does not appear linear when dots are formed at a staggered arrangement. Therefore, the dots thus arranged can prevent the occurrence of a banding phenomenon. 
         [0051]    Since the dots are arranged in the so-called staggered layout in the present embodiment, as shown in  FIG. 4B , even when a skip occurs during the sheet feeding process, the basic color of the unprinted portion of a recording sheet is not exposed in a straight line, but discretely. Thus, the banding pattern due to the skip becomes invisible. 
         [0052]    In  FIGS. 4A and 4B , the dots forming the first column are arranged at the positions deviated from the dots forming the adjacent second column in the feeding direction Z, and the dots forming the second column are arranged at the positions deviated from the dots forming adjacent third column. However, the dots forming the first column and the second column may not be deviated from each other in the feeding direction Z, while being deviated from the dots forming the third column in the feeding direction Z. Further, the dots forming the first column, the second column and the third column may not be deviated from each other in the feeding direction Z, while being deviated from the dots forming a fourth column adjacent to the third column. In short, one raster line is segmented into a plurality of sub-lines, and the segmented sub-lines are arranged so as to deviate from each other in their feeding directions. This prevents the occurrence of a banding phenomenon. 
         [0053]    Next, referring to  FIGS. 5A to 6B , a description is given for the method of sorting dots formed in the interlace system.  FIGS. 5A to 6B  show the following regions corresponding to the progress of raster processing: the region where dots are formed at a print resolution of 2400 dpi, and the region dots are formed at a print resolution of 4800 dpi. Dots are formed in sequence in the interlace system. Raster  33  as the last line shown in  FIG. 5A  is followed by raster  35  as the first line in  FIG. 5B . Raster  65  as the last line shown in  FIG. 5B  is followed by raster  67  as the first line shown in  FIG. 6A . Raster line  103  as the last line shown in  FIG. 6A  is followed by raster  104  shown in  FIG. 6B . The “raster number” means a line number assigned to each raster line as the dot line which is formed by the head moving in a main scanning direction, from the top toward the bottom of a recording sheet. In these figures, the description for the leading edge of a recording sheet is omitted, and a raster line around its middle portion is shown as raster  1 . 
         [0054]    The “path number” (path No.) provided above raster  1  in  FIG. 5A  is the order of the raster scanned by the head  109 . A black circle is given at the point of intersection of a given path number and a given raster line, which means a dot is formed by the path. If no dot is formed, a small point is given instead of the black circle. 
         [0055]    Referring to  FIGS. 5A and 5B , for example, in path  5 , the following seven raster lines are formed at the same time: raster  15  shown as a black circle of  5 - 1 ; raster  23  as a black circle of  5 - 2 ; raster  31  as a black circle of  5 - 3 ; raster  39  as a black circle of  5 - 4 ; raster  47  as a black circle of  5 - 5 ; raster  55  as a black circle of  5 - 6 ; and raster  63  as a black circle of  5 - 7 . Similarly, in path  6 , raster lines  29 ,  37 ,  45 ,  53 ,  61 ,  69 , and  77  are formed (see  FIG. 6A ). 
         [0056]    In the present embodiment, the head  109  is provided with 49 nozzles in the feeding direction Z. As indicated by a downward arrow, as one or more raster has been formed in one path, the recording sheet is fed in the opposite direction of the arrow by a width corresponding to seven raster lines. The recording sheet is thus feed by degrees, thereby forming each dot. 
         [0057]    In the region at a resolution level of 2400 dpi, only raster lines having an odd number are formed. On the right side of  FIGS. 5A to 6B , there is shown the arrangement of the dots formed on the recording sheet corresponding to the raster lines. 
         [0058]    As shown in  FIGS. 6A and 6B , in the range from raster  113  to  132 , resolution is set at a level of 4800 dpi, and dots are arranged in a staggered layout. Since a skip occurs while printing is performed in this section, the resolution is increased only within the section, and its dot arrangement is changed. As has been described above, since dots are arranged in a staggered layout, even when a skip occurs during the sheet feeding process, a banding phenomenon is prevented from occurring on an image to be formed. 
         [0059]    The path range where printing is performed with this staggered-arranged region is from path  9  to path  17 . Within the path range, the width of the recording sheet to be fed is reduced to half compared to the width for a resolution level of 2400 dpi; the resolution is set at a value of 4800 dpi; a raster line having an even number is formed in an even-numbered path; and a raster line having an odd number is formed in an odd-numbered path. For example, dots are formed along raster  118  and raster  126  for path  10  while dots are formed along raster  117 , raster  125 , and raster  133  for path  11 . The dots formed along an even-numbered raster line are arranged to deviate from the dots formed along an odd-numbered raster line in the feeding direction. 
         [0060]    In the region coming after the staggered-arranged region, the resolution returns to a level of 2400 dpi so as to form a raster line. 
         [0061]    Next, referring to  FIG. 7 , a description is given for the process performed by the dot sorting process program  33   b  run by the CPU  32 .  FIG. 7  is a flow chart showing the process performed by the dot sorting process program  33   b . In the dot sorting process, print data is input based on the assumption that printing is performed at a resolution level of 2400 dpi. For a region where the print data is to be printed at double resolution, a conversion is performed so that dots are arranged in a staggered layout. Since this process is required for each recording sheet, the process has to be repeated a plural number of times in order to perform printing on a plurality of recording sheets. 
         [0062]    First, the variable “i” representing a dot position for forming a raster line (dot order in the main scanning direction) is set to a value of “0”, and the variable “j” representing the raster order in the feeding direction is set to a value of “0” (S 1 ). Next, the input print data is obtained in sequence 8 bit by 8 bit, and a determination is made whether or not the raster having the 8 bit data belongs to the region where printing is performed at double resolution (S 2 ). In this embodiment, the information indicating that a record is to be produced at double resolution from which raster to which raster is pre-stored in the ROM  33 , and a determination is made whether or not the obtained data is recorded at double resolution based on the stored information. 
         [0063]    If the print data is to be recorded not at double resolution but at normal resolution (S 2 : No), the mask for the even-numbered raster is set to the hexadecimal number “FF” (the symbol “0x” attached in front of the number in the flowchart indicates that the number is represented in hexadecimal format), and the mask for the odd-numbered raster is set to the hexadecimal number “00” (S 3 ). Therefore, sorting is performed in the following manner. For the even-numbered raster, dots are formed on both the even-numbered and odd-numbered columns. For the odd-numbered raster, dots are formed on neither the even-numbered and odd-numbered columns. Note that even numbers and odd numbers assigned to the raster in  FIGS. 5A to 6B  are opposite to even numbers and odd numbers assigned to raster in  FIG. 7 . 
         [0064]    On the other hand, if the print data is to be recorded at double resolution (S 2 : Yes), the mask for the even-numbered raster is set to the hexadecimal number “AA”, and the mask for the odd-numbered raster is set to the hexadecimal number “55” (S 4 ). Therefore, sorting is performed in the following manner. For the even-numbered raster, dots are formed on the even-numbered columns while no dots are formed on the odd-numbered columns. For the odd-numbered raster, no dots are formed on even-numbered columns while dots are formed on the odd-numbered columns. 
         [0065]    When Step S 3  or S 4  is finished, the print data is multiplied by the set mask that is hexadecimal number, and the multiplied print data is stored in the image memory  37  (S 5 ). Next, a determination is made whether or not the value “i” has reached the value indicating that conversion has been performed up to the last dot of the raster (S 6 ). If not reached yet (S 6 : No), the value “i” is incremented (S 7 ), and the process returns to Step S 2  so that conversion is performed by obtaining the value of the next 8 bit composing a raster. If the value “i” has reached the value indicating that conversion has been performed up to the last dot of the raster (S 6 : Yes), a determination is made whether or not the raster variable “j” is equal to the value indicating the last raster of a page (S 8 ). If the variable “j” is not equal to the value indicating the last raster (S 8 : No), the variable “j” is incremented (S 9 ), and the process returns to Step S 2  so that conversion is performed for the next raster. If the raster variable “j” is equal to the value indicating the last raster (S 8 : Yes), which means that the all the steps in raster processing for this page has been finished, the dot sorting process has been completed. 
         [0066]    As has been described above in the first embodiment, in the printing region where a skip occurs during the sheet feeding process, resolution is set at a higher value, and its dot arrangement is changed into a staggered layout. This prevents the occurrence of a banding phenomenon such as the appearance of a linear space in spite of the skip occurrence, thereby achieving high quality printing. Further, in a region where no banding phenomenon occurs, it is possible to decrease its resolution while maintaining image quality, and to increase an image forming speed. Even if the discharge roller  103  rotates faster than the feed roller  101  while the feed roller  101  sandwiches the recording sheet stronger than the discharge roller  103 , such as the present embodiment, the occurrence of a banding phenomenon is prevented. 
         [0067]    Next, a description is given for a second embodiment. In the first embodiment, printing is performed at a higher resolution with dots arranged in a staggered layout in the printing region where a skip occurs during the sheet feeding process. However, since resolution changes drastically, that is, color changes in a direction perpendicular to the feeding direction, a slight line may appear around the boundary between the two regions. In the second embodiment, in order to prevent the appearance of such a line, resolution is changed gradually. Specifically, in the region being transition from the normal region to the region with a higher resolution, dots are formed with resolution increasing gradually. In the region being in transition from the region with the higher resolution to the normal region, dots are formed with resolution decreasing gradually. In the region where the resolution increases, dots are arranged in a staggered layout. Referring to  FIGS. 8A to 9 , the above situation will be described. In the second embodiment, the description for the structural points which are the same as in the first embodiment is omitted. Only different points are described here. 
         [0068]    Similarly to  FIGS. 5A to 6B  referred to in the first embodiment,  FIGS. 8A to 9  each show the path forming each raster and each dot arrangement corresponding to each raster. Raster  39  as the last line shown in  FIG. 8A  is followed to raster  40  as the first line shown in  FIG. 8B . Raster  73  as the last line shown in  FIG. 8B  is followed to raster  74  as the first line shown in  FIG. 9 . 
         [0069]    As shown in  FIGS. 8A and 8B , the range up to raster  5  is a region where dots are arranged at normal resolution. The range from raster  7  to raster  45  is a region where resolution gradually increases (first middle region). The range from raster  47  to raster  66  is a region where dots are arranged at double resolution (second middle region). The range from raster  67  to raster  105 , extending from  FIG. 8B  to  FIG. 9 , is a region where resolution gradually decreases (third middle region). 
         [0070]    In the first middle region and the third middle region, their resolution levels change according to the change of the ratio of the section having a higher resolution level and the section having a lower resolution level. Within the first middle region, the section from raster  7  to raster  13  shown as (A) and the section from raster  15  to raster  21  shown as (B), each have one raster line (raster  11  and raster  19 ) in which dots are arranged in a staggered layout at a higher resolution level. The rest raster lines arrange their dots at normal resolution. Next, the section from raster  23  to raster  29  shown as (C) and the section from raster  31  to raster  37  shown as (D) each have two raster lines (raster  25  and  27 /raster  33  and  35 ) in which dots are arranged in a staggered layout at a higher resolution level. The following section from raster  39  to raster  45  shown as (E) has three raster lines (raster  39 ,  41 , and  43 ) in which dots are arranged in a staggered layout at a higher resolution level. In other words, dots are arranged in the first middle region so as to increase the share for sections having a higher resolution level gradually toward the second middle region. 
         [0071]    On the other hand, in the third middle region, the section from raster  67  to raster  74  shown as (F) has three raster lines (raster  69 ,  71 , and  73 ) in which dots are arranged in a staggered layout at a higher resolution level. 
         [0072]    Next, the section from raster  75  to raster  81  shown as (G) and the section from raster  83  to raster  89  shown as (H) each have two raster lines (raster  77  and  79 /raster  85  and  87 ) in which dots are arranged in a staggered layout at a higher resolution level. Furthermore, the section from raster  91  to raster  97  shown as (I) and the section from raster  99  to raster  105  shown as (J) each have one raster line (raster  93  and raster 101 ) in which dots are arranged in a staggered layout at a higher resolution level. In other words, dots are arranged in the third middle region so as to decrease the share for sections having a higher resolution level gradually with distance from the second middle region. 
         [0073]    Referring now to  FIG. 10 , a description is given for the dot sorting process shown in  FIGS. 8A to 9 .  FIG. 10  is a flow chart showing the dot sorting process. Regarding this flow chart, the steps the same as in the flow chart of  FIG. 7  have the same step number, and their description is omitted here. 
         [0074]    In Step S 2 , if a determination has been made that the relevant raster line does not belong to the printing region at double resolution (S 2 : No), a determination is further made whether the raster line belongs to the first middle region or the third middle region, both of which are mixed regions arranging dots both at normal resolution and at double resolution (S 11 ). If a determination has been made that the raster line belongs to neither of the mixed regions (S 11 : No), which means that the raster line belongs to the region arranging dots at normal resolution, the process proceeds to Step S 3  so as to set a mask for converting dots into a normal arrangement. 
         [0075]    On the other hand, if the determination has been made that the raster line belongs to either of the mixed regions (S 11 : Yes), a determination is further made whether printing is to be performed at normal resolution or at double resolution for each raster line (S 12 ). This determination may be made, for example, by pre-storing the information to be based on, as to which raster line has to be printed at double resolution in the ROM  33 , or by computing. The determination by computing will be described later with reference to  FIG. 11 . Another determination is then made whether or not the raster line is to be printed at double resolution based on the previous determination result, if the determination has been made that the raster line is not to be printed at double resolution (S 13 : No), the process proceeds to Step S 3 . If the determination has been made that the raster line is to be printed at double resolution (S 13 : Yes), the process proceeds to Step S 4 . 
         [0076]    Referring next to  FIG. 11 , a description is given for the raster resolution determination by computing as Step S 12  in the flow chart of  FIG. 10 .  FIG. 11  is a flow chart showing the process of determining a raster resolution level. In this process, the mixed region is divided into N number of sub-regions in the feeding direction, and the i-th raster line within the k-th region is set either at normal resolution or at double resolution. In this setting method, in order to increase resolution gradually, raster lines are set to have a higher resolution level in the proportion of 2 raised to the (k−1)-th power/2 raised to the N-th power. For example, if the mixed region is divided into three sub-regions, one raster line out of eight raster lines is set to have a higher resolution level in the first sub-region; two raster lines out of eight raster lines are set to have a higher resolution level in the second sub-region; and four raster lines out of eight raster lines are set to have a higher resolution level in the third sub-region. 
         [0077]    In the other hand, in the region where its resolution decreases gradually from the double resolution value, if the region is similarly divided into three sub-regions, four raster lines out of eight raster lines are set to have a higher resolution level in the first sub-region adjacent to the double resolution region; two raster lines out of eight raster lines are set to have a higher resolution level in the next sub-region; and one raster line out of eight raster lines is set to have a higher resolution level in the sub-region adjacent to the normal resolution region. 
         [0078]    First, a determination is made whether or not the raster line to be processed belongs to the region (first middle region) being in transition from normal resolution to double resolution (S 21 ). If the determination has been made that the raster line belongs to the first middle region (S 21 : Yes), the following computation is performed: 
         [0000]      (i%2 N )%(2 (N−(k−1)) )  [expression 1] 
         [0079]    A determination is made whether or not the value obtained by the computation is zero (S 22 ). The notation “%” in the above expression is an operator to take the integer part of the quotient of the division. If the obtained value is zero (S 22 : Yes), the raster line is set to be printed at double resolution (S 24 ). If the obtained value is not zero (S 22 : No), the raster line is set to be printed at normal resolution (S 25 ). 
         [0080]    In the determination step S 21 , if the raster line to be processed does not belong to the first middle region (S 21 : No), the determination is made that the raster line belongs to the third middle region, and the following computation is performed: 
         [0000]      (i%2 N )%(2 (k) )  [expression 2] 
         [0081]    A determination is then made whether or not the value obtained by the computation is zero (S 23 ). If the obtained value is zero (S 23 : Yes), the raster line is set to be printed at double resolution (S 24 ). If the obtained value is not zero (S 23 : No), the raster line is set to be printed at normal resolution (S 25 ). 
         [0082]    As has been described above in the second embodiment, in the second middle region where a banding phenomenon occurs, dots are arranged at double resolution as well as in a staggered layout. By providing the first middle region being in transition from normal resolution to double resolution, and the third middle region being in transition from double resolution to normal resolution, it becomes possible to prevent the boundary between the region where dot are arranged in a staggered layout at double resolution, and the region where dots are normally arranged at normal resolution, from appearing as a line. 
         [0083]    Therefore, even when a skip occurs during the sheet feeding process, the occurrence of a banding phenomenon and the distortion of an image due to resolution change are prevented, thereby achieving high quality printing. Further, it is only necessary to switch between the normal resolution section and the double resolution for setting a resolution level, which means control over the image forming device becomes simple. 
         [0084]    While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
         [0085]    For example, although the above embodiments relate to the process performed by a color inkjet printer, the process may be performed by other devices including a multi-functional device and a facsimile machine. 
         [0086]    Although the above embodiments relate to the case in which printing is performed on a recording sheet as a recording medium, the present invention is not limited to paper. Examples of the applicable material include cloth and plastic. 
         [0087]    Although the second embodiment relates to the process in which raster resolution is set by computing in the mixed region, the present invention is not limited to this process. The following process is also applicable. The mixed region is divided into N number of sub-regions in the feeding direction. Within the first middle region, raster data is sorted in the proportion of k/(N+1) in the k-th sub-region. Within the third middle region, raster data is sorted in the proportion of (N+1−k)/(N+1) in the k-th sub-region. For example, if the first middle region is divided into seven sub-regions, one raster line out of eight raster lines in the first sub-region, two out of eight in the second sub-region, three out of eight in the third sub-region, similarly, seven out of eight are set to have double resolution in the seventh region adjacent to the double resolution region, for a staggered arrangement. 
         [0088]    Similarly, if third middle region is divided into seven sub-regions, seven raster lines out of eight raster lines in the first sub-region, six out of eight in the second sub-region, five out of eight in the third sub-region, similarly, one out of eight are set to have double resolution in the seventh region adjacent to the normal resolution region, for a staggered arrangement. 
         [0089]    Though the resolution gradually increases in the first middle region in the above embodiment, the resolution in the first meddle region is fixed to a resolution higher than the normal resolution and lower than the double resolution.