Patent Publication Number: US-7591521-B2

Title: Image forming apparatus and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-180857 filed Jun. 21, 2005, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus provided with a recording head with a plurality of nozzle arrays, for forming an image on an image-formed material, and more particularly, relates to an image forming apparatus provided with at least one long recording head composed of a plurality of short recording heads. 
     2. Description of the Related Art 
     For example, the recording head of the ink-jet type image forming apparatus has a tendency to multiply or lengthen a recording device (ink-jet nozzle) in order to meet the requirements of high-speed image formation (image recording). 
     As such an image forming apparatus, a structure provided with a so-called line head for disposing (forming) a recording device (ink-jet nozzle) across one side (width direction) of an image-formed material (recording medium) is known. 
     An image forming apparatus provide with a line head can form an image all over this image-formed material by relatively moving the image-formed material in the direction orthogonal to the ink-jet nozzle array direction of a line head (sub scan direction) and then jetting ink on the image-formed material from the ink-jet nozzle. Since the image forming apparatus provide with a line head needs neither the movement of a carriage nor the intermittent conveyance of the image-formed material, it can rapidly and easily form an image. 
     However, the line head has disadvantages that its cost is high compared with a short recording head, that its quality yield is bad, that its reliability is low and the like. 
     An image forming apparatus utilizing the advantages in cost of a short recording head, in its quality yield, in its reliability and the like by disposing a plurality of short recording heads in each of which a plurality of ink-jet nozzles is arrayed in one direction (main scan direction) in order to solve these problems is also known. 
     However, in the line head composed of such short recording heads, if there is a phase difference in pitch between nozzle arrays (improper pitch), striped optical density unevenness, white-wiping and the like are sometimes formed. 
     Patent reference 1 (Japanese Patent Application Publication No. 2002-144542) discloses am image recording method capable of recording a high-quality image without color/optical density unevenness or the like in an image recording apparatus in which one recording head (line head) is obtained by disposing a plurality of short (recording) heads. 
     The image recording method disclosed by patent reference 1 is described below with reference to  FIGS. 1A˜1C . 
     The recording head  10  shown in  FIG. 1A  comprises a plurality of short heads  12 A and  12 B. In this recording head  10 , a plurality of adjacent ink-jet nozzles  11 A of the short head  12 A and a plurality of adjacent ink-jet nozzles  11 B of the short head  12 B are disposed in such a way that a part of them overlaps when viewed from the sub scan direction. This joint area (overlapped area) corresponds to ink-jet nozzles  11 A- 1 ˜ 11 A- 3  on the short head side  12 A and ink-jet nozzles  11 B- 1 ˜ 11 B- 3  on the short head side  12 B. 
     In image record by the recording head  10  with such a structure, for example, as shown in the area “a” of  FIG. 1B , a high-optical density recording area occurs along the sub scan direction in the joint area of the ink-jet nozzles  11 A- 1 ˜ 11 A- 3  of the short head  12 A and the ink-jet nozzles  11 B- 1 ˜ 11 B- 3  of the short head  12 B, and a high-quality image cannot be recorded. 
     In such a case, a record control unit for controlling the recording head  10  to record an image, which is not shown in  FIG. 1 , determines the ink-jet nozzle  11 A- 2  of the short head  12 A and the ink-jet nozzle  11 B- 2  of the short head  12 B, which is shown by an one-dot chain line as one example, as a joint point and the use of the ink-jet nozzles  11 A- 1  and  11 B- 1  located further on the top end is stopped. 
     Since the space between the ink-jet nozzles  11 A- 2  and  11 B- 2 , which are the joint point is narrower than a proper pitch, the o optical density in the joint point becomes higher than a proper value, as shown in the area “a” of  FIG. 1A . In order to correct this, the record control unit stops the drive of one of the ink-jet nozzles  11  on every another line in the sub scan direction to record an image with proper optical density. In an example of an area “b” of  FIG. 1B , the drive of the ink-jet nozzle  11 B- 2  of the short head  12 B is stopped on every another line in the sub scan direction. 
     The image recording method of patent reference 1 can record a high-quality image without color/optical density unevenness and the like, by performing such control. 
     In this case, in an ink-jet recording head using a piezoelectric device (PZT), generally the amount of ink jetted from the ink-jet nozzle at the head end increases or decreases compared with that in an area other than the end, that is, a non-end area. In the case of an image recording apparatus with one recording head, even when there is a little change in optical density due to the change of the amount of ink jetted from a specific number of ink-jet nozzles on the end in such a phenomenon, a part whose optical density has changed becomes the end of an image recording area. Therefore, the optical density unevenness of a recorded image is not remarkable. However, in the case of an image recording apparatus with a line head obtained by adjacently disposing a plurality of recording heads, the joint part of adjacent recording heads becomes inner than the end of the image recording area. Therefore, when optical density unevenness occurs in this part, striped optical density unevenness, white-wiping and the like becomes remarkable in a recorded image. 
     Patent reference 2 (Japanese Patent Application Publication No. 2003-320647) discloses a method for visually reducing optical density unevenness due to the fluctuations of the jet amount of ink (ink jet volume) at the end of such a recording head. 
     The method of this patent reference 2 determines which an input image signal is, the end area signal or non-end area signal of a corresponding recording head. If it is determined that that it is the end area signal, an end area correction process is performed. If it is determined that that it is the non-end area signal, a non-end area correction process is performed. 
     In the end area correction process, the end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas. In the non-end area correction process, the non-end area is corrected in such a way that there is almost no difference in visual optical density between the non-end and end areas, and it is also corrected in such a way that a optical density value gradually decreases from the end toward the center. The method of patent reference 2 corrects the end and non-end areas and also reduces optical density unevenness. 
     However, although patent reference 1 discloses a method for improving the optical density unevenness of a nozzle array overlapped part between adjacent short heads, it does not disclose a method for improving unevenness due to a non-uniform recording characteristic between nozzles arrays. Therefore, only the method disclosed by patent reference 1 cannot improve unevenness due to this non-uniform recording characteristic. 
     Although patent reference 2 discloses a method for improving unevenness due to non-uniform ink jet volume from the ink-jet nozzles, the relative positions of short heads are adjusted and disposed in such a way that one ink-jet nozzle in the joint part of adjacent short heads can be matched with the other ink-jet nozzle in the joint part when viewed from the sub scan direction. 
     The recording devices (ink-jet nozzle) are formed at very fine intervals. For example, if its resolution is 300 dpi, the interval becomes 85 μm. In the method disclosed by patent reference 2, for example, a locating mechanism for locating a ink-jet nozzle of one short head and the ink-jet nozzle of the other adjacent short head at intervals of 85 μm with no error is needed, which incurs the cost-up of the apparatus. 
     The generation factor and degree of optical density unevenness due to the nozzle array overlapped part of this short head and those of optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles are different. 
       FIG. 2A  shows the recording optical density characteristic of a recording head (short head) in which the ink-jet nozzle at the nozzle array end is higher than that at non-end. 
       FIG. 2B  shows the recording optical density characteristic of a recording head (line head) in which the two short heads with the recording optical density characteristic shown in  FIG. 2A  are adjacently disposed in such a way that a part of their ink-jet nozzles are overlapped when viewed from the above-described sub scan direction. 
     Since as shown in  FIG. 2A , the optical density unevenness due to the non-uniform recording characteristic between ink-jet nozzles is due to the little structural or manufacturing error in the jet amount of ink (ink jet volume) between recording heads, its optical density change value becomes small and gentle. 
     Since as shown in  FIG. 2B , the optical density unevenness in the nozzle overlapped part of a recording head is due to a optical density change according to the phase difference in a ink-jet nozzle position between nozzle arrays of adjacent recording heads, its optical density change value becomes large and steep. 
     Therefore, in order to correct optical density unevenness due to these two factors, a sufficient correction effect cannot be obtained only by simply combining two methods (technologies) disclosed by patent references 1 and 2, and accordingly no high-quality image can be obtained. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming apparatus capable of making striped optical density unevenness, white-wiping and the like, due to the joint part of adjacent recording heads unremarkable and also correcting optical density unevenness due to the non-uniform recording optical density characteristic between short recording heads, without disposing short recording heads according to strict position adjustment in an image forming apparatus with one long recording head obtained by disposing a plurality of short recording heads. 
     The present invention presumes an image forming apparatus provided with a recording head obtained by overlapping and disposing a plurality of short recording heads each with ink-jet nozzles arrayed in one direction, for and jetting ink from the ink-jet nozzles, based on an image signal value and forming an image, which comprises an overlap correction control unit and a optical density characteristic correction control unit. 
     The overlap correction control unit controls the drive of ink-jet nozzles, according to the phase difference in a ink-jet nozzle position between overlapped short recording heads to correct the optical density of an image formed by the overlapped part of recording heads. 
     The optical density characteristic correction control unit operates independently of the overlap correction control unit and controls the drive of ink-jet nozzles, according to the recording optical density characteristic of the nozzle array of the ink-jet nozzles. 
     The present invention also includes the image formation method of the image forming apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B and  1 C roughly show the nozzle array overlap correction of patent reference 1; 
         FIG. 2A  shows the recording optical density characteristic of a single recording head.  FIG. 2B  shows the recording optical density characteristic in the case where two recording heads are overlapped and jointed; 
         FIG. 3  shows an example of the conceptual configuration of the image forming apparatus of the preferred embodiment; 
         FIG. 4  shows an example of the configuration of the image forming apparatus of the preferred embodiment; 
         FIG. 5  shows an example of the configuration in the case where the image forming apparatus of the preferred embodiment is of a full line type, and an example of the array of a nozzle array unit; 
         FIGS. 6A and 6B  explain the phase difference in a ink-jet nozzle position between nozzle arrays of recording heads; 
         FIGS. 7A and 7B  show the concept of an overlapped part in the case where no nozzle array overlap is corrected; 
         FIGS. 8A and 8B  show the concept of an overlapped part in the case where nozzle array overlap is corrected; 
         FIGS. 9A and 9B  show the concept of a formation dot which becomes the joint point of the nozzle array overlapped parts of recording heads; 
         FIGS. 10A ,  10 B and  10 C show the concept in the case where the nozzle array overlapped part of recording heads is corrected across a plurality of ink-jet nozzles; 
         FIGS. 11A ,  11 B and  11 C show examples of an error conversion distribution co-efficient; 
         FIG. 12  is a flowchart showing the process at the time of optical density correction parameter setting; 
         FIGS. 13A ,  13 B and  13 C show the optical density correction parameter determining process used for the nozzle array overlap correction; 
         FIGS. 14A ,  14 B and  14 C show how to correct optical density according to an image formation mode; 
         FIG. 15  shows an example of the configuration in the case where the image forming apparatus of the preferred embodiment is of a serial type; 
         FIG. 16  shows an example of the configuration of a carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage; and 
         FIG. 17  shows an example of the configuration of each carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are described in detail below with reference to the drawings. 
       FIG. 3  shows an example of the conceptual configuration of the image forming apparatus of the preferred embodiment. 
       FIG. 3  show examples of both the configurations of the image forming apparatus  30  in the case where it is of a serial type and of a full line type in one drawing. 
     The serial type image forming apparatus  30  shown in  FIG. 3  comprises a mounting table  32  provided with a moving mechanism for mounting and holding an image-formed material  31  or a moving mechanism for holding in such a way as to move the image-formed material  31  in the sub scan direction. In the upper section of the mounting table  32 , at least one of recording units  34 - 1 ˜ 34 -m (m=an integer of 2 or more), each with a plurality of nozzle arrays (short recording heads) and each with a plurality of ink-jet nozzles (ink jet outlets) formed, are disposed on a moving object  33 . The moving object  33  is movably supported by the moving mechanism, which is not shown in  FIG. 3 , and is moved in the main scan direction (Hx 0 ˜Hxn) and sub scan direction (Hy 0 ˜Hyn) or only in the main scan direction (Hx 0 ˜Hxn). In the case of this serial type image forming apparatus, an image is formed on the image-formed material  31  mounted on the mounting table  32  or held in such a way as to move in the sub scan direction while at least one of the recording units  34 - 1 ˜ 34 -m, disposed on the moving object  33  relatively, move only in the main scan direction or in both the main scan and sub scan directions. 
     The full line type image forming apparatus  30  shown in  FIG. 3  comprises a moving mechanism for mounting the image-formed material  31  and moving and carrying it in the sub scan direction, which is not shown in  FIG. 3 . In the upper section of this moving mechanism, which is not shown in  FIG. 3 , carriages  35  in which at least one of the recording units  36 - 1 ˜ 36 -m (m=integer of two or more) each with a plurality of nozzle arrays each composed of a plurality of ink-jet nozzles are oppositely deposited. In the case of this full line type image forming apparatus, an image is formed on the image-formed material  31  while the moving mechanism, which is not shown in  FIG. 3 , relatively moves the image-formed material  31  held in such a way as to move in the sub scan direction against at least one of the recording units  36 - 1 ˜ 36 -m. 
     In the carriage  35  indicated by a two-dot chain line in  FIG. 3 , a plurality of ink-jet nozzles of at least one of the recording units  36 - 1 ˜ 36 -m are disposed as a plurality of nozzle arrays (short recording heads) at least across the length of over the width in the main scan direction (Hx) of the image-formed material  31 . 
     In the case of this full line type image forming apparatus, an image is formed by moving the top end  31   a  in the sub scan direction (Hy) of the image-formed material  31  to My 0 ˜Myn by the moving mechanism, which is not shown in  FIG. 3 , and further jetting ink on the image-formed material  31  by at least one of the recording units  36 - 1 ˜ 36 -m while moving the back end  31   b  in the sub scan direction (Hy) of the image-formed material  31  to My 0 ˜Myn in  FIG. 3 . 
     As described above, in each image forming apparatus with a different configuration, the serial type image forming apparatus adjusts an image formation point (ink jet point) on the image-formed material  31  by changing jet timing for each of a plurality of nozzle arrays in at least one of the recording units  34 - 1 ˜ 34 -m while a control unit  37  controls to move the moving object  33  with at least one of the recording units  34 - 1 ˜ 34 -m in the main scan direction. 
     The full line type image forming apparatus adjusts an image formation point (ink jet point) on the image-formed material  31  by changing jet timing for each of a plurality of nozzle arrays in at least one of the recording units  36 - 1 ˜ 36 -m while a control unit  37  controls to move the image-formed material  31  in the sub scan direction by the moving mechanism, which is not shown in  FIG. 3 . 
     In the image forming apparatus  30  of the preferred embodiment, the control unit  37  comprises a optical density characteristic correction control unit and an overlap correction control unit, which are described later. When forming an image, the image forming apparatus  30  corrects an input image signal  41 , using an optical density characteristic correction control unit  46  and an overlap correction control unit  47 , based on an image formation mode set from a mode setting unit  38  by an operator or a correction value recorded in the control unit  37  as an already known value. 
       FIG. 4  shows an example of the configuration of the image forming apparatus of the preferred embodiment. 
     The image forming apparatus of the preferred embodiment shown in  FIG. 4  comprises a moving mechanism  43  including the above-described the mounting table  32  for supporting the moving object  33  in such a way as to move in a prescribed direction in addition to the moving object  33 , at least one the recording units  34 - 1 ˜ 34 -m, at least one the recording units  36 - 1 ˜ 36 -m, the control unit  37  and the mode setting unit  38 , which are shown in  FIG. 3 . 
     The at least one of the recording units  34 - 1 ˜ 34 -m or  36 - 1 ˜ 36 -m comprise at least one of the nozzle array driver circuits  50 - 1 ˜ 50 -m (m=integer of two or more) and a nozzle array units  51 - 1 ˜ 51 -m (m=integer of two or more). Then, at least one of the nozzle array units  51 - 1 ˜ 51 -m comprises a plurality of nozzle arrays (short recording heads). The control unit  37  comprises a plain memory  44 , a timing generation unit  45 , the optical density characteristic correction control unit  46 , and the overlap correction control unit  47 . The moving mechanism  43  comprises a moving mechanism driver unit  48  and a moving position information generating unit  49 . In the case of the serial type image forming apparatus, the moving mechanism further comprises the mounting table  32 . 
     The control unit  37  is composed of a CPU and like, and supervises and controls this entire image forming apparatus  30 . The control unit  37  controls the moving mechanism driver unit  48  of the moving mechanism  43 , processes a signal indicating the location of the moving object  33  notified by the moving position information generating unit  49 , instructs to control the timing generation unit  45 , stores an externally inputted the input image signal  41  in the plain memory  44  and so on. The control unit  37  also corrects the input image signal  41  by the optical density characteristic correction control unit  46  and the overlap correction control unit  47 . The optical density characteristic correction control unit  46  controls the drive of each ink-jet nozzle to determine the jet amount of ink, according to the recording optical density characteristic of nozzle arrays (short recording heads) constituting the nozzle array units  51 - 1 ˜ 51 -m. The overlap correction control unit  47  controls the drive of each ink-jet nozzle to determine the jet amount of ink, according to the phase difference in a ink-jet nozzle position between nozzle arrays (short recording heads). This detailed correction process is described later. 
     The moving mechanism driver unit  48  of the moving mechanism  43  is composed of a motor and the like. The moving mechanism driver unit  48  moves the moving object  33  only in the main scan direction, only in the sub scan direction or both in the main and sub scan directions to a prescribed position, based on the control/instruction of the control unit  37 . The moving position information generating unit  49  is composed of a rotary encoder and the like. The moving position information generating unit  49  notifies the control unit  37  of the amount of movement of the moving object  33  and its location as signals. The moving position information generating unit  49  can also comprise a high frequency generation circuit for multiplying by an integer the frequency of a pulse signal generated by the rotary encoder as requested. 
     The mode setting unit  38  is provided as a part of an input operation panel for the entire image forming apparatus  30 . Alternatively, it is provided as an individual input operation panel. By operating and instructing the mode setting unit  38 , the operator sets image formation modes, such as the correction function stoppage of the optical density characteristic correction control unit  46 , the correction function stoppage of the overlap correction control unit  47  and the like, in the image forming apparatus  30 . When a correction mode selected by the type of the image-formed material  31  or the like, the optical density characteristic correction control unit  46  and the overlap correction control unit  47 , which are described later, correct the input image signal  41 , based on this correction mode. 
     An imaging apparatus  42  shown in  FIG. 4  is an external apparatus provided with a function to detect the optical density of an image formed on the image-formed material  31 , such as a scanner connected to the image forming apparatus  30  or the like. The imaging apparatus  42 , for example based on the instruction from the control unit  37 , detects the optical density of the image formed on the image-formed material set in the imaging apparatus  42  and notifies the control unit  37  of the optical density. 
     Although in the preferred embodiment shown in  FIG. 4 , the image forming apparatus  30  and the imaging apparatus  42  are separated, the present invention is not limited to this configuration. The imaging apparatus  42  can also be provided inside the image forming apparatus  30  and adjustment information about image optical density unevenness and the like can also be calculated based on the detection result of the imaging apparatus  42 . 
     The optical density characteristic correction control unit  46  controls the drive of each ink-jet nozzle, based on a detection value notified by the imaging apparatus  42 , which is the external apparatus of the image forming apparatus  30 , according to a nozzle array optical density characteristic of each nozzle array (short recording head) to correct image optical density unevenness. The overlap correction control unit  47  controls the drive of each ink-jet nozzle, based on a detection value notified by the imaging apparatus  42 , according to the phase difference in the overlapped part of nozzle arrays (short recording heads) (relative position between the ink-jet nozzle in one nozzle array and the ink-jet nozzle in the other nozzle array) to correct image optical density unevenness. 
     These optical density characteristic correction control unit  46  and the overlap correction control unit  47  can be realized by dedicated hardware. Alternatively, they can be realized by a software method of executing a program by the CPU of the control unit  37 . If they are configured by hardware, it is preferable to configure each of them by different hardware in order to prevent the reciprocal influences of their correction processes. 
     The timing generation unit  45  determines each jet timing of each nozzle array (short recording head) of at least one of the nozzle array units  51 - 1 ˜ 51 -m, based on a position adjustment parameter predetermined according to the position information in the sub scan direction of each nozzle array to control/instruct the jet position adjustment of at least one of the recording units  34 - 1 ˜ 34 -m or  36 - 1 ˜ 36 -m. The recording units  34 - 1 ˜ 34 -m or  36 - 1 ˜ 36 -m forms an image for one line according to the input image signal  41  notified by the control unit  37  by forming an image by each nozzle array (short recording head) of the at least one of the nozzle array units  51 - 1 ˜ 51 -m in this jet timing. 
     Next, an example of the nozzle array unit array in the case of the full line type image forming apparatus of the preferred embodiment is described. 
     As shown in  FIG. 5 , in the carriage  35  fixed and deposited opposed to the moving mechanism  43 , the nozzle array units  51 - 1 ˜ 51 -m in which each color is made by jointing two short recording heads (in  FIG. 5 , m=4; four nozzle array units  51  of black (K) the nozzle array unit  51 - 1 , cyan (C) the nozzle array unit  51 - 2 , magenta (M) the nozzle array unit  51 - 3  and yellow (Y) the nozzle array unit  51 - 4 ) are disposed in the sub scan direction from the upper stream conveyance direction of the image-formed material  31  at prescribed intervals. In the nozzle array units  51 - 1 ˜ 51 -m, a plurality of short recording heads  51 A- 1 ,  51 B- 1 ˜ 51 A-m,  51 B-m (in  FIG. 5 , m=4; (K) short recording heads  51 A- 1  and  51 B- 1 , (C) short recording heads  51 A- 2  and  51 B- 2 , (M) short recording heads  51 A- 3  and  51 B- 3  and (Y) short recording heads  51 A- 4  and  51 B- 4 ) are disposed corresponding to the nozzle array units  51 - 1 ˜ 51 -m. Each pair of short recording heads  51 A- 1  and  51 B- 1 ˜ 51 A- 4  and  51 B- 4  comprises one to a plurality of nozzle arrays for each short recording head. 
     For example, a plurality of ink drops with the same volume is jetted from each ink jet outlet of the nozzle array units  51 - 1 ˜ 51 - 4 . Thus, the control unit  37  controls the number of ink drops jetted from the nozzle array units  51 - 1 ˜ 51 - 4  by the nozzle array driver circuits  50 - 1 ˜ 50 - 4  to adjust optical density gradation, that is, by a multi-drop method. 
     For example, if each jet outlet of the nozzle array units  51 - 1 ˜ 51 - 4  can jet seven ink drops at the maximum, zero to seven ink drops including the case where no ink is jetted are jetted on the image-formed material  31  and a dot corresponding to each number of ink drops is formed. Thus, the control unit  37  can control optical density gradation of eight grades including zero grade. 
     Each of the color the nozzle array units  51 - 1 ˜ 51 - 4  comprises a prescribed number of fluid jet outlets (ink-jet nozzles) across over the width in the main scan direction of the carried image-formed material  31 . For example, in (K) the nozzle array unit  51 - 1  made by jointing two short recording heads, a plurality of ink-jet nozzles at the end of the short recording head  51 A- 1  and a plurality of ink-jet nozzles at the end of the short recording head  51 B- 1  are jointed in such a way as to overlap when viewed from the sub scan direction (the nozzle array units  51 - 2 ˜ 51 - 4  also have the same structure as (K) the nozzle array unit  51 - 1 ). 
     As described above, if both ends of each nozzle array of each pair of short recording heads  51 A and  51 B are overlapped and disposed, the control unit  37  can selectively the input image signal  41  to each short recording head disposed in the carriage  35  to form an image. Therefore, the strict position adjustment in the nozzle array direction (main scan direction) of each short recording head can be omitted, thereby realizing an inexpensive position adjustment mechanism. 
     As a position adjustment mechanism for each of the nozzle array units  51 - 1 ˜ 51 - 4 , for example, a mechanical adjustment mechanism for finely rotating the nozzle array units  51 - 1 ˜ 51 - 4  can also be provided. 
     Next, an example of the configuration of the full line image forming apparatus  30  of the preferred embodiment is described. 
     As shown in  FIG. 5 , the moving mechanism  43  disposed opposed to the carriage  35  is disposed in the lower stream than an edge sensor  62  for detecting at least one end of the image-formed material  31  in the conveyance route of the image-formed material  31 . 
     An endless belt for forming a plurality of holes in the moving object  33  shown in  FIG. 3  is installed one roller  63   a  and the other roller  63   b  to rotate the other roller  63   b  by connecting it to the motor of the moving mechanism driver unit  48  and connect the rotary encoder of the moving position information generating unit  49  to the one roller  63   a . In the lower section of this endless belt, for example, an absorbing fan, which is not shown in  FIG. 5 , to absorb the image-formed material  31 . 
     The control unit  37  shown in  FIG. 3  places the image-formed material  31  carried from the upper stream of the moving mechanism  43  on the endless belt of the moving mechanism  43  and absorbs it. Then, the control unit  37  forms an image while moving the lower section of the short recording heads  51 A- 1 ,  51 B- 1 ˜ 51 A- 4 ,  51 B- 4  corresponding to color the nozzle array units  51 - 1 ˜ 51 - 4  disposed in the carriage  35 . 
     The control unit  37  converts the distance between the edge sensor  62  and the nozzle array of the short recording head  51 A- 1  corresponding to the color the nozzle array units  51 - 1 ˜ 51 - 4 , between the edge sensor  62 , the distance between the edge sensor  62  and the nozzle of the short recording head  51 B- 1 , the distance between the edge sensor  62  and the nozzle array of the short recording head  51 A- 2 , the distance between the edge sensor  62  and the nozzle array of the short recording head  51 B- 2 , the distance between the edge sensor  62  and the nozzle array of the short recording head  51 A- 3 , the distance between the edge sensor  62  and the nozzle array of the short recording head  51 B- 3 , the distance between the edge sensor  62  and the nozzle array of the short recording head  51 A- 4  and the distance between the edge sensor  62  and the nozzle array of the short recording head  51 B- 4  into the accumulated number of pulses of the rotary encoder and stores it in the non-volatile memory of the control unit  37  in advance. 
     Thus, the control unit  37  starts counting the pulse signals of the rotary encoder in the moving position information generating unit  49 , using the signal which the edge sensor has detected, for example, the top end of the image-formed material  31  carried on the conveyance route as trigger information. Then, the control unit  37  forms an image in each ink jet timing the counted number of the pulse signals of this rotary encoder and the accumulated number of pulses of the rotary encoder, based on the input image signal  41 , stored in advance in accordance with the nozzle array position of each of the short recording heads  51 A- 1 ,  51 B- 1 ˜ 51 A- 4 ,  51 B- 4  coincide with each other. 
     Next, the control method of the control unit of the image forming apparatus of the preferred embodiment is described in detail. 
     The input image signal  41  inputted from outside the image forming apparatus  30  is stored in the plain memory  44  as a 1˜n-lines of image signal (n=integer of two or more) under the control of the control unit  37 . In the case of the full line type image forming apparatus  30  of the preferred embodiment, this 1˜n lines of image signal corresponds to one line in the width direction of the recording area of the image-formed material  31 . 
     If the image forming apparatus  30  of the preferred embodiment comprises, for example, the color the nozzle array units  51 - 1 ˜ 51 - 4 , the plain memory  44  shown in  FIG. 5 , divides the 1˜n lines of image signal in relation with each nozzle array of the short recording heads  51 A- 1 ,  51 B- 1 ˜ 51 A- 4 ,  51 B- 4  and stores them. 
     The control unit  37  reads the 1˜n lines of the input image signal  41  stored in the plain memory  44  in synchronization with the count value of the moving position information generating unit  49 , generated by carrying the image-formed material  31  after for example, the edge sensor  62  has detected its top end, and inputs it to the optical density characteristic correction control unit  46 . 
     The optical density characteristic correction control unit  46  calculates a optical density correction signal by a process, which is described later, based on a optical density correction co-efficient set in advance according to the recording optical density characteristic of each ink-jet nozzle and the image signal value of each pixel formed on the image-formed material  31  by each ink-jet nozzle in the non-overlap part in which the ink-jet nozzles of the short recording heads do not overlap and which is jointed when viewed from the sub scan direction. 
     This optical density correction signal is a signal level quantization optical density correcting signal which can be inputted to each short recording head, and in each ink-jet nozzle of the nozzle array units  51 - 1 ˜ 51 - 4  of the preferred embodiment, it converts its optical density into eight gradation values of zero to seven grades. 
     It is preferable to diffuse a quantizing error caused by quantization conversion among peripheral pixels by an error diffusion method or the like. 
     If the image forming apparatus  30  of the preferred embodiment comprises, for example, the color the nozzle array units  51 - 1 ˜ 51 - 4  shown in  FIG. 5 , the image signal of each of 1˜n lines converted into a quantization optical density correction signal by the optical density characteristic correction control unit  46  is divided into image signals for each nozzle array and is inputted to the overlap correction control unit  47 . 
     Next, nozzle array overlap correction is described with reference to  FIGS. 6 ,  7  and  8 . 
     In the description of this nozzle array overlap correction, it is assumed that there is a phase difference between the ink-jet nozzle of one nozzle array in the overlapped part and the ink-jet nozzle on the other nozzle array when the one nozzle array and the other nozzle array are overlapped and adjacently disposed. 
     For example, when resolution is 300 dpi, the distance “a” between ink-jet nozzles of a recording head shown in  FIG. 6  is approximately 85 μm. The distance δ shown in  FIG. 6  indicates the phase difference in the main scan direction between the ink-jet nozzle of a recording head  71  and the ink-jet nozzle of a recording head  72 . In  FIG. 6A , this distance δ in a phase difference is the distance between the ink-jet nozzle  73  of the recording head  71  and the ink-jet nozzle  74  of the recording head  72 . In  FIG. 6B , this distance δ in a phase difference is the distance between the ink-jet nozzle  75  of the recording head  71  and the ink-jet nozzle  76  of the recording head  72 . 
     An image signal is divided and inputted to the slashed ink-jet nozzles of the recording heads  71  and  72  shown in  FIGS. 6A and 6B , and an image is formed by output dots formed by ink that is jetted from each ink-jet nozzle of the recording heads  71  and  72  (dot image). 
     No ink is jetted from each ink-jet nozzle not slashed in  FIG. 6 . 
     If the recording heads  71  and  72  are disposed in the position relationship shown in  FIG. 6A , the output dot formed on the image-formed material  31  is formed in the position relationship shown in  FIG. 7A . The main scan direction phase difference δ in this case has the relationship of δ&gt;a, and the distance between the output dot  81  of the ink-jet nozzle  73  and the output dot  82  of the ink-jet nozzle  74  increases. 
     Thus, in the nozzle array overlapped part, the optical density of the output dot formed on the image-formed material  31  decreases. Therefore, the optical density of the nozzle array overlapped part becomes lower than the optical density of the output dot of the ink-jet nozzle adjacent to the nozzle array overlapped part, and as a result, striped optical density unevenness occurs. 
     If the recording heads  71  and  72  are disposed in the position relationship shown in  FIG. 6B , the output dot formed on the image-formed material  31  is formed in the position relationship shown in  FIG. 7B . The main scan direction phase difference δ in this case has the relationship of δ&lt;a, and the distance between the output dot  81  of the ink-jet nozzle  73  and the output dot  82  of the ink-jet nozzle  74  decreases. 
     Thus, in the nozzle array overlapped part, the optical density of the output dot formed on the image-formed material  31  increases. Therefore, the optical density of the nozzle array overlapped part becomes higher than the optical density of the output dot of the ink-jet nozzle adjacent to the nozzle array overlapped part, and as a result, striped optical density unevenness occurs. 
     In this preferred embodiment, if one nozzle array and the other nozzle array are overlapped and adjacently disposed, optical density unevenness caused according to the phase difference between the ink-jet nozzle of the one nozzle array in the overlapped part and the ink-jet nozzle of the other nozzle array is corrected by controlling the jet of each ink-jet nozzle and changing the output dot formed on the image-formed material  31  by the overlap correction control unit  47 . 
     The change of an output dot includes at least one of the modification of a target ink-jet nozzle jetting ink and the modification of the diameter of a dot formed by ink jetted by the ink-jet nozzle. 
       FIG. 8A  shows an output dot formed on the image-formed material  31  after the nozzle array overlap correction when the main scan direction phase difference δ has the relationship of δ&gt;a, as shown in  FIG. 7A . 
     The output dot  86  shown in  FIG. 8A  is formed on the image-formed material  31  by the ink-jet nozzle  77  of the recording head  72  shown in  FIG. 6A . The amount of ink jetted when forming the output dot  86  is determined by multiplying the input image signal  41  by a optical density correction co-efficient according to the above-described main scan direction phase difference δ and quantizing this multiplied value into eight gradation values by the overlap correction control unit  47 . 
     In this way, in this preferred embodiment, the optical density unevenness of the nozzle array overlapped part can be corrected by adding the output dot  86  shown in  FIG. 8A  by the nozzle array overlap correction of the overlap correction control unit  47 . 
       FIG. 8B  shows an output dot formed on the image-formed material  31  after the nozzle array overlap correction when the main scan direction phase difference δ has the relationship of δ&lt;a, as shown in  FIG. 7B . 
     The output dot  86  shown in  FIG. 8B  is formed on the image-formed material  31  by the ink-jet nozzle  76  of the recording head  72  shown in  FIG. 6B . The amount of ink jetted when forming the output dot  84  is determined by multiplying the input image signal  41  by a optical density correction co-efficient according to the above-described main scan direction phase difference δ and quantizing this multiplied value into eight gradation values by the overlap correction control unit  47 . 
     In this way, in this preferred embodiment, the optical density unevenness of the nozzle array overlapped part can be corrected by changing (reducing) the dot diameter of the output dot  84 ′ shown in  FIG. 8B  by the nozzle array overlap correction of the overlap correction control unit  47 . 
     The above-described nozzle array overlap correction of the overlap correction control unit  47  is applied to several lines of image signals of a plurality of line image signals in the input image signal  41  (the above-described 1˜n lines of image signals). In the course of this nozzle array overlap correction, a quantizing error caused when quantizing the input image signal into eight gradation values is diffused into one subsequent line of image signal. 
       FIG. 9A  shows an output dot formed on the image-formed material  31  by a plurality of lines of image signals after the nozzle array overlap correction. 
     As shown in  FIG. 9A , the joint part of the output dots formed on this image-formed material  31  is formed linearly when viewed from the sub scan direction. 
     As shown in  FIG. 9B , the optical density unevenness of the joint part of the output dots formed on this image-formed material  31  can be made unremarkable by moving the joint part, for example, in the main scan direction at random. 
     This joint part of output dots can be at random when viewed from the sub scan direction, by moving the switch position between an image signal divided into one nozzle array of the overlap part and an image signal divided into the other nozzle array. 
     In the nozzle array overlap correction of this preferred embodiment, the optical density correction of the nozzle overlapped part is applied only to the one output dot corresponding to the joint part, and no quantizing error caused by this nozzle array overlap correction is diffused into an output dot adjacent to the output dot of the joint part. 
     In the optical density correction process of this preferred embodiment, the quantizing error caused by the nozzle array overlap correction is prevented from diffusing into outside the nozzle array overlap part by providing the optical density characteristic correction control unit  46  and the overlap correction control unit  47  in the former and later stages, respectively. 
     Thus, in the optical density correction process of this preferred embodiment, both optical density unevenness due to a nozzle array overlapped part, the change inclination of whose optical density value is large and steep and optical density unevenness due to a non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to the nozzle overlapped part, the change of whose optical density value is small and gentle can be corrected. 
     Although as described above, the optical density correction of a nozzle array overlapped part of this preferred embodiment is applied only to one output dot of a ink-jet nozzle corresponding to the above-described joint part, the application of the optical density correction is not limited to this ink-jet nozzle and can also be applied to a plurality of ink-jet nozzles. 
     In the optical density correction of a nozzle array overlapped part of this preferred embodiment, for example, as shown in  FIG. 10 , the optical density unevenness can also be corrected by multiplying the input image signal  41  inputted to a plurality of ink-jet nozzles corresponding to the nozzle array overlapped part by a optical density correction co-efficient and controlling it. 
       FIG. 10A  shows a optical density correction co-efficient multiplied to each ink-jet nozzle of one nozzle array (left side) in an overlapped part when the one nozzle array and the other nozzle array are overlapped and adjacently disposed.  FIG. 10B  shows a optical density correction co-efficient multiplied to each ink-jet nozzle of the other nozzle array (right side) in an overlapped part when the one nozzle array and the other nozzle array are overlapped and adjacently disposed. 
     As shown in  FIG. 10C , in the optical density correction of a nozzle array overlapped part of this preferred embodiment, the optical density unevenness can be corrected as a result, by forming an image after multiplying the input image signal  41  inputted to each ink-jet nozzle of one nozzle array (left side) in an overlapped part by the optical density correction co-efficient shown in  FIG. 10A  and multiplying the input image signal  41  inputted to each ink-jet nozzle of one nozzle array (right side) in the overlapped part by the optical density correction co-efficient shown in  FIG. 10B . 
       FIG. 1C  shows a nozzle array overlap correction area  93  to which nozzle array overlap correction is applied, by dotted lines  91  and  92 . In the course of this nozzle array overlap correction, a quantizing error caused when quantizing the optical density into eight gradation values is diffused into the input image signal  41  corresponding to this nozzle array overlap correction area  93 . 
     For example, as shown in  FIG. 11 , in the optical density correction of a nozzle array overlap part of this preferred embodiment, a plurality of error conversion/distribution co-efficients for diffusing a quantizing error from a processed pixel to a peripheral pixel is provided and used for separate purposes. 
       FIG. 11  shows a pixel X whose caused quantizing error is processed and an error conversion/distribution co-efficient ( FIG. 16 ) into its peripheral pixels. For example, a quantizing error caused by quantization is diffused into the pixel in which “7” is indicated at the ratio of 7/16. 
     The error conversion/distribution co-efficient K 1  shown in  FIG. 11A  is a normal diffusion co-efficient and is applied to a nozzle array overlap correction area  93  other than the area enclosed by dotted lines  91  and  92  shown in  FIG. 10C . 
     The error conversion/distribution co-efficient K 2  shown in  FIG. 11B  is designed to diffuse no error into the right side of the pixel X and is applied to a pixel on the right boundary of the nozzle array overlap correction area  93  enclosed by the dotted line  92  shown in  FIG. 10C . The error conversion/distribution co-efficient K 3  shown in  FIG. 11C  is designed to diffuse no error into the left side of the pixel X and is applied to a pixel on the left boundary of the nozzle array overlap correction area  93  enclosed by the dotted line  91  shown in  FIG. 10C . 
     As described above, in the optical density correction of a nozzle array overlapped part of this preferred embodiment, a quantizing error caused when quantizing the optical density into eight gradation values in its correction course can be prevented from diffusing into an area adjacent to the nozzle array overlap correction area  93  by restricting the diffusion range of a quantizing error. 
     Therefore, in the optical density correction process of this preferred embodiment, both optical density unevenness due to a nozzle array overlapped part, the change inclination of whose optical density value is large and steep and optical density unevenness due to a non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to the nozzle overlapped part, the change of whose optical density value is small and gentle can be corrected. 
     Next, the determination procedure of a optical density correction parameter composed of a optical density correction co-efficient and the like, which is set in advance in the optical density characteristic correction control unit  46  and the overlap correction control unit  47  in the optical density correction process of this preferred embodiment is described. 
     This optical density correction parameter is determined at the time of plant shipment, re-adjustment after exchanging the recording head of a nozzle array unit, of the image forming apparatus  30 . 
       FIG. 12  is a flowchart showing the determination procedure of the optical density correction parameter. 
     The image forming apparatus  30  reads a test image formed on the image-formed material  31  by the imaging apparatus  42  connected to the image forming apparatus  30  and determines this optical density correction parameter, based on its result. 
     The optical density correction process of this preferred embodiment is performed by setting this determined optical density correction parameter in the optical density characteristic correction control unit  46  and the overlap correction control unit  47  of the image forming apparatus  30 . 
     It is preferable to set this optical density correction parameter after setting basic image formation parameters, such as the main and sub scan direction positions of the test image formed on the image-formed material  31 , color overlap position information for piling several pieces of color ink on the same position of the image-formed material  31  in a prescribed pattern as a test image or the like. 
     In the optical density correction determining procedure shown in  FIG. 12 , firstly in step S 1 , the nozzle array optical density correction of the correction function by the optical density characteristic correction control unit  46  and the nozzle array overlap correction of the correction function by the overlap correction control unit  47  are stopped. 
     In the optical density correction parameter determining procedure of this preferred embodiment, the stoppage (off)/execution (on) of this correction function can also be switched by an operator operating the operation panel of the image forming apparatus  30  or the like. Alternatively, the image forming apparatus  30  can automatically switch between the stoppage (off)/execution (on) of this correction function when a optical density correction parameter determination mode is started. 
     Then, in step S 2 , a test image  1  for determining a nozzle array overlap correction parameter in the image forming apparatus  30  is formed on the image-formed material  31 . As described above, it is preferable for the test image  1  formed in step S 2  to be suited to detect the phase difference in a jet position between overlapped nozzle arrays of a nozzle array unit disposed in such a way that the respective ends of nozzles arrays of a plurality of recording heads overlap when viewed from the sub scan direction. 
     Therefore, as the test image, an image in which a reference distance between ink-jet nozzles can be measured or an image in which a phase difference can be estimated from optical density unevenness caused by the phase difference is used. 
     Since this test image  1  is used as a correction reference, an image is formed after the correction is stopped in step S 1 . 
       FIG. 13A  shows an example of a recording head with nozzle arrays having non-uniform forming characteristic (recording characteristic), in which images with different optical density are formed on the image-formed material  31  depending on the ink-jet nozzle position of a nozzle array. 
       FIG. 13B  shows the optical density of an image which the recording heads with non-uniform forming characteristic (recording characteristic) shown in shown  13 A form on the image-formed material  31  by nozzle array units disposed with an overlapped part while the optical density correction of this preferred embodiment is stopped. 
     The optical density value of the nozzle array overlapped part shown in  FIG. 13B  is large and its change inclination is steep. The test image  1  is used to determine a nozzle array overlap parameter for correcting the optical density of this nozzle array overlapped part. 
     Then, in step S 3 , the imaging apparatus  42  reads the test image  1  formed in step S 2 . 
     Then, in step S 4 , the phase difference in a ink-jet nozzle position between overlapped nozzle arrays is detected based on the reading result in step S 3  and a nozzle array overlap parameter, such as a optical density correction co-efficient or the like, is determined based on the phase difference. Then, the determined nozzle array overlap correction parameter is stored in the non-volatile memory of the overlap correction control unit  47 . 
     In this determination of the nozzle array overlap correction parameter in step S 4 , it is preferable to eliminate optical density unevenness due to the non-uniform forming characteristic (recording characteristic) for each nozzle array, in which the change of its optical density value is small and gentle in order to determine parameters for correcting only optical density unevenness due to the phase difference in a ink-jet nozzle position between overlapped nozzle arrays and to improve the accuracy of this nozzle array overlap correction parameter. 
     Then, in step S 5 , nozzle array overlap correction by the overlap correction control unit  47  is executed (made on) and nozzle array optical density correction by the optical density characteristic correction control unit  46  is stopped (made off). 
     Then, in step S 6 , a test image  2  for determining a nozzle array optical density correction parameter is formed on the image-formed material  31 . It is preferable for this test image  2  to be an image in which optical density unevenness due to a non-uniform forming characteristic (recording characteristic) for each nozzle array, such as a half-tone image formed by dots of one to seven grades or the like. 
     The test image  2  formed in step S 6  becomes the image whose optical density unevenness in a nozzle array overlapped part is corrected by the nozzle array overlap correction. 
     Then, in step S 7 , the imaging apparatus  42  reads the test image  2  formed in step S 6 . 
     Then, in step S 8 , a nozzle array optical density correction parameter is determined by the reading result in step S 7 . Then, the determined nozzle array optical density correction parameter is stored in the non-volatile memory of the optical density characteristic correction control unit  46 . 
     This nozzle array optical density correction parameter determined in step S 8  can improve its accuracy since it determines the parameter value based on an image whose optical density unevenness in a nozzle array overlapped part is corrected by the nozzle array overlap correction. 
     Next, the mode setting operation in the case where an operator forms an image in the optical density correction process of the image forming apparatus  30  in this preferred embodiment and the optical density correcting operation based on this mode setting process are described. 
     In the image forming apparatus  30  of this preferred embodiment, an operator can set an image forming mode suitable for a difference in the data of an image formed on the image-formed material  31  (the input image signal  41 ) or a difference in the material quality or the like of the image-formed material  31  in the mode setting unit  38 . 
     The image forming apparatus  30  of this preferred embodiment determines the respective optical density correction methods of the optical density characteristic control unit  46  and the overlap correction control unit  47 , based on the selected image forming mode to form an image. 
     If the operator does not set (select) the image forming mode, priority is given to a optical density correction process set as a default, which is described later, to correct a formed image. 
     Next, an operator-settable image forming mode is described. 
     The image forming apparatus  30  of this preferred embodiment has two types of modes; for example, a linear priority mode in which image data is character information, graphics or the like and a hue priority (optical density priority) mode in which image data is a picture or the like. 
     Since in the linear priority mode, priority is given to character information or the linearity of each line in graphics, no optical density correction process is performed. 
     Since in the hue priority (optical density priority) mode, priority is given to the reproducibility of hue for each ink color of, for example, black (K), cyan (C), magenta (M) and yellow (Y) and the hue of an overlapped color in the case where dots of each ink color is formed in the same position of the image-formed material  31 , a optical density correction process is performed. 
     The optical density correction processes in the case of forming an image are the same in improving the uniformity of image optical density. However, the optical density of the entire formed image has a tendency to decrease. 
     This is because a general optical density correction process reduces the output dots in a high-optical density part in such a way as to match image optical density in the high-optical density part of a formed image on the image-formed material  31  to that of the low-optical density part of the formed image on the image-formed material  31 , that is, image optical density in a part in which the diameter of an output dot jetted from a recording head is small and to make the image optical density uniform. 
     The reduced degree of optical density in this entire image varies depending on the difference in the material quality of the image-formed material  31 . In the image formation on a piece of ink-jet dedicated coated paper, since image optical density is kept high by the respective characteristics of this piece of ink-jet dedicated coated paper and ink, the image optical density is little influenced by the image optical density reduction in the optical density correction process. However, in the image formation on a piece of corrugated paper and a piece of cardboard, since the surface of a piece of corrugated paper and a piece of cardboard which is not especially coated for ink jet is applied and the soaked amount of ink increases, the image optical density is greatly influenced by the image optical density reduction in the optical density correction process. 
     However, since image data on a piece of corrugated paper and a piece of cardboard is often fairly large character data, little optical density unevenness is no problem. 
     In image formation on a piece of regular paper which is not especially coated for ink jet, the regular paper has an intermediate characteristic between a piece of ink-jet dedicated coated paper and a piece of corrugated paper/cardboard. 
     Therefore, in the image forming apparatus  30  of this preferred embodiment, an operator can set an image forming mode suitable for a difference in the data of an image formed on the image-formed material  31  (the input image signal  41 ) or a difference in the material quality or the like of the image-formed material  31  in the mode setting unit  38 . 
     Next, the optical density distribution (optical density unevenness) status after the image forming apparatus  30  of this preferred embodiment performs the respective optical density correction processes of the optical density characteristic correction control unit  46  and the overlap correction control unit  47 , based on the selected hue priority (optical density priority) mode is described with reference to  FIG. 14 . 
       FIG. 14A  shows the optical density distribution (optical density unevenness) in the case where an image is formed on the image-formed material  31  after applying only nozzle array overlap correction by the overlap correction control unit  47  to nozzle array units disposed in such a way that the respective ends of the nozzle arrays of a plurality of recording heads overlap when viewed from the sub scan direction. 
       FIG. 14B  shows an example of a optical density correction value (optical density correction parameter) added by the nozzle array optical density correction by the optical density characteristic correction control unit  46 . 
       FIG. 14C  shows the optical density distribution (optical density unevenness) in the case where an image is formed after the nozzle array optical density correction is applied using the optical density correction value shown in  FIG. 14B . 
     Of three types of optical density correction values shown in  FIG. 14B , one indicated by a solid line  101  is the inverse function of the optical density distribution (optical density unevenness) shown in  FIG. 14A , and by correcting the input image signal  41  according to this inverse function, the optical density distribution (optical density unevenness) indicated by a solid line  104  in  FIG. 14C  can be corrected. 
     Since there occurs almost no optical density unevenness in the optical density correction process indicated by this solid line  104 , it is suited to form a picture or the like on a piece of ink-jet dedicated coated paper. 
     Of the three types of optical density correction values shown in  FIG. 14B , one indicated by a dotted line  103  is a optical density correction value in the case where no nozzle array optical density correction is performed, and as shown by a dotted line  106  in  FIG. 14C , the input image signal  41  is formed without applying any process to it. The optical density correction process indicated by this dotted line  106  is greatly influenced by the image optical density reduction in the optical density correction process. Therefore, it is suited to form an image on a piece of corrugated paper, a piece of cardboard and the like. 
     Of the three types of optical density correction values shown in  FIG. 14B , one indicated by the dotted line  102  is the intermediate optical density correction value between the optical density correction value indicated by the solid line  101  and the optical density correction value indicated by the dotted line  103 , and an almost sufficient optical density correction effect can be obtained by little image optical density reduction in the optical density correction process. Therefore, it is suited to form an image on a piece of regular paper which is not especially coated for ink jet is applied. 
     As shown by the dotted line in  FIG. 14B , this intermediate optical density correction value can be obtained by the inverse function of the optical density distribution obtained by reading an image formed on the image-formed material  31  by the imaging apparatus  42  by a co-efficient with a specific size. 
     The image forming apparatus  30  of this preferred embodiment stores a plurality of optical density correction values, such as three types of optical density correction values indicated by the solid line  101  and dotted lines  102  and  103  in advance. 
     An operator can select a hue priority (optical density priority) mode corresponding to this selected contents and form an image to which proper nozzle array optical density correction is applied, by selecting and setting conditions, such as the type of image data (the input image signal  41 ), the type of the image-formed material  31  and the like, in the mode setting unit  38  of the image forming apparatus  30 , for selection/setting excluding the linear priority mode. 
     For the hue priority (optical density priority) mode selected by the operator, image quality, specifically, a correction value for the optical density correction process can also be selected when performing the optical density correction process. Alternatively, it can be selected which the optical density of an image to be corrected is, high or low. 
     The optical density correction by the image forming apparatus  30  of this preferred embodiment can also be realized not only in the case of the full line type image forming apparatus  30  but also the serial type image forming apparatus  30 . 
       FIG. 15  shows the simplified configuration of the serial type image forming apparatus and shows the simplified configuration of the serial type image forming apparatus in which the image-formed material  31  is not moved in the sub scan direction. 
       FIG. 15  also roughly shows a carriage  33 B by a two-dot chain line, in which the disposition form of color nozzle array units are different from that of a carriage  33 A, in addition to the carriage  33 A in which color nozzle array units are disposed in the main scan direction in parallel and at prescribed intervals in the moving object  33 . 
     In the serial type image forming apparatus  30  shown in  FIG. 15 , the carriages  33 A or  33 B are supported by a support member  118  in such a way to move back and forth in the upper section of the fixed and mounted image-formed material  31 . At one end of this support member  118 , one the moving mechanism  43  is constituted by the motor of one the moving mechanism driver unit  48  for rotating a pulley  115  and a rotary encoder of one the moving position information generating unit  49 . At the other end of the support member  118 , a pulley  116  is disposed opposed to the pulley  115  in the main scan direction. The pulleys  115  and  116  are installed in such a way as to move back and forth in the main scan direction, and are connected to the carriage  33 A or  33 B by a connecting unit  117   a.    
     Thus, the carriage  33 A or  33 B is moved back and forth in the main scan direction by the rotation of the motor of one the moving mechanism driver unit  48 , and also the moving position of the carriage  33 A or  33 B is notified to the control unit  37  of the image forming apparatus  30 . 
     In the vicinity of the other end of the support member  118 , a pulley  122  is disposed. In the position opposed to this pulley  122  in the sub scan direction, other the moving mechanism  43  is constituted by the motor of other the moving mechanism driver unit  48  for rotating a pulley  121  and the rotary encoder of other the moving position information generating unit  49  provided for the rotation shaft of this motor. In the pulleys  121  and  122 , an endless belt  123  is installed in such a way as to move back and forth in the sub scan direction and is connected to the other end of the support member  118  by a connecting unit  123   a.    
     Thus, the support member  118  is moved back and forth in the sub scan direction by the rotation of the motor of other the moving mechanism driver unit  48 , and also the rotary encoder of other the moving position information generating unit  49  notifies the control unit  37  of the image forming apparatus  30  of the moving position of the support member  118 . 
     If the image forming apparatus of the preferred embodiment is of a serial type, by adopting such a configuration, the serial type image forming apparatus  30  of the preferred embodiment can form an image by scanning the carriage  33 A or  33 B on the image-formed material  31  along a scan trace  125  while informing the control unit  37  of a moving position in the case where the carriage  33 A or  33 B is moved in the main and sub scan directions in the moving object  33 . 
       FIG. 16  shows an example of the configuration of a carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage. 
     In the carriage  33 A shown in  FIG. 16 , a (K) nozzle array unit  51 - 1 , a (C) nozzle array unit  51 - 2 , a (M) nozzle array unit  51 - 3  and a (Y) nozzle array unit  51 - 4  are disposed as color-corresponding nozzle array units in parallel in the main scan direction at prescribed intervals. The color the nozzle array units  51 - 1 ˜ 51 - 4  are composed of a pair of short recording heads  51 A- 1  and  51 B- 1 ˜ 51 A- 4  and  51 B- 4 , respectively. 
       FIG. 17  shows an example of the configuration of each carriage in the case where the image forming apparatus of the preferred embodiment is of a serial type, and an example of the array of the nozzle array units in the carriage. 
     In the carriage  33 B shown in  FIG. 16 , (K), (C), (M) and (Y) nozzle array units  51 - 1 ,  51 - 2 ,  51 - 3  and  51 - 4  ( 51 - 3  and  51 - 4  are not shown in  FIG. 17 ), are disposed as color-corresponding nozzle array units in the sub scan direction at prescribed intervals or at no intervals in parallel with the sub scan direction. The color the nozzle array units  51 - 1 ˜ 51 - 4  are composed of a pair of short recording heads  51 A- 1  and  51 B- 1 ˜ 51 A- 4  and  51 B- 4 , respectively. 
     As described so far, according to the image forming apparatus of this preferred embodiment, in the image forming apparatus comprising one long recording head by disposing a plurality of short recording heads, striped optical density unevenness due to the joint part of adjacent short recording heads, white wiping and the like can be made not remarkable and optical density unevenness due to the non-uniform recording optical density characteristic of the short recording heads can also be corrected when forming an image, without strictly adjusting the positions of the short recording heads. 
     According to the image forming apparatus of this preferred embodiment, since an quantizing error caused by nozzle array overlap correction is not diffused into outside the nozzle array overlapped part, both optical density unevenness due to the nozzle array overlapped part, whose optical density value is large and, the change inclination of whose optical density value is steep and optical density unevenness due to the non-uniform forming characteristic (recording characteristic) of a nozzle array adjacent to a nozzle array overlapped part, the change of whose optical density value is small and gentle can be corrected. 
     Although the nozzle array unit of this preferred embodiment comprises two nozzles arrays composed of a plurality of nozzles, it can also comprise three or more such array units. 
     The nozzle array unit of this preferred embodiment can also similarly comprise short recording heads obtained by jointing a plurality of recording heads with one or more nozzle arrays and a long recording head can be obtained by disposing a plurality of these short recording heads in one direction.