Patent Publication Number: US-8991963-B2

Title: Liquid discharging apparatus and liquid discharging method

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a liquid discharging apparatus and a liquid discharging method. 
     2. Related Art 
     An ink jet type printer for discharging ink to a medium to form an image has been developed. In such printers, there is a line head type printer which can form an image over the entire surface of the medium in a width direction by lining a plurality of nozzle rows in a direction intersecting with a transportation direction of the medium. 
     JP-A-2008-143065 discloses a technology of setting a total recording duty of recording elements in an overlapped portion to be higher than a recording duty of recording elements which are not in an overlapped portion. 
     In such a line head printer, an overlapped region of the nozzle rows is affected by an error due to an installation error of a head or a transportation error of a medium, and deviation of landing locations of ink discharged by an upstream nozzle and ink discharged by a downstream nozzle occurs. If such deviation of the landing locations occurs, a difference in gloss between the overlapped region and a non-overlapped region occurs, and lines may be noticed. Such a difference in gloss is not desirable and therefore the difference in gloss is required to be decreased as much as possible. That is, it is desirable to decrease the difference in gloss of an image between the overlapped region and the non-overlapped region of the nozzle rows. 
     SUMMARY 
     An advantage of some aspects of the invention is to decrease a difference in gloss of an image between an overlapped region and a non-overlapped region of nozzle rows. 
     According to an aspect of the invention, there is provided a liquid discharging apparatus including: a first nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction; a second nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction, and which is disposed by forming an overlapped region in which an end portion of one side in the predetermined direction is overlapped with an end portion of the other side in the predetermined direction of the first nozzle row; and a control unit which discharges liquid from the first nozzle row and the second nozzle row depending on input data of an image and a recording duty for discharging liquid, increases a recording duty in the overlapped region with respect to a recording duty in a non-overlapped region which is a region other than the overlapped region, and differentiates increased amounts of the recording duty in the overlapped region from each other depending on the input data. 
     Other aspects of the invention will be clear with the specification and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a block diagram of all configurations of a printer. 
         FIG. 2  is a schematic side view of a printer. 
         FIG. 3  is a diagram showing nozzle arrangement of a lower surface of a head unit. 
         FIG. 4  is a diagram illustrating pixels in which dots are formed by nozzles of a head unit. 
         FIG. 5  is a flowchart of a creation process of printing data of a comparative example. 
         FIG. 6  is a diagram showing allocation of half-tone processed data corresponding to an overlapped region to a nozzle row of an upstream head and a nozzle row of a downstream head. 
         FIG. 7  is a diagram showing recording duties of a first nozzle row and a second nozzle row. 
         FIG. 8  is a diagram showing a dot generation rate conversion table. 
         FIG. 9  is a flowchart of printing data creation of the embodiment. 
         FIG. 10  is a flowchart of a recording duty increasing process. 
         FIG. 11  is a first diagram showing an operation of replicating data in an overlapped region and multiplying a recording duty of each nozzle row by overlapped region data. 
         FIG. 12  is a diagram showing an example of a selected recording duty. 
         FIG. 13  is a first explanatory diagram of a recording duty. 
         FIG. 14A  is a diagram showing a dither mask and  FIG. 14B  is a diagram showing an operation of a half-tone process performed by a dither method. 
         FIG. 15  is a second diagram showing an operation of replicating data in an overlapped region and multiplying a recording duty by replicated region data. 
         FIG. 16  is a second explanatory diagram of a recording duty. 
         FIG. 17A  is an explanatory diagram of a spatial frequency property of a blue noise property and  FIG. 17B  is an explanatory diagram of a spatial frequency property of a green noise property. 
         FIG. 18  is a table showing quality of an image in an overlapped region. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     At least the following aspects will be made clear with the specification and the accompanying drawings. That is, there is provided a liquid discharging apparatus including: a first nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction; a second nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction, and which is disposed by forming an overlapped region in which an end portion of one side in the predetermined direction is overlapped with an end portion of the other side in the predetermined direction of the first nozzle row; and a control unit which discharges liquid from the first nozzle row and the second nozzle row depending on input data of an image and a recording duty for discharging liquid, increases a recording duty in the overlapped region with respect to a recording duty in a non-overlapped region which is a region other than the overlapped region, and differentiates increased amounts of the recording duty in the overlapped region from each other depending on the input data. 
     By doing so, since it is possible to differentiate the increased amounts of the recording duty from each other depending on the input data when increasing the recording duty in the overlapped region with respect to the recording duty in the non-overlapped region, it is possible to discharge the liquid with an appropriate increased amount for decreasing a difference in gloss based on the input data. It is possible to decrease the difference in gloss of an image between the overlapped region and the non-overlapped region of the nozzle rows. 
     In the liquid discharging apparatus, it is desirable that the increased amounts of the recording duty in the overlapped region be differentiated from each other depending on a liquid amount per unit area acquired based on the input data. 
     By doing so, since it is possible to differentiate the increased amounts of the recording duty from each other depending on a liquid amount per unit area, it is possible to acquire the increased amount of the recording duty depending on a filled degree of the liquid. 
     It is desirable that the increased amount of the recording duty with respect to a first liquid amount per unit area acquired based on first input data be an increased amount of the recording duty acquired based on second input data, and be greater than an increased amount of the recording duty with respect to a second liquid amount per unit area which is lower than the first liquid amount per unit area. 
     By doing so, since it is possible to set the increased amount of the recording duty when using the first liquid amount per unit area to be greater than the increased amount of the recording duty when using the second liquid amount per unit area, it is possible to increase the discharging amount of the liquid when the greater filled degree of dots is acquired. 
     It is desirable that the nozzles discharge liquid droplets having a plurality of sizes to form dots having a plurality of dot sizes, and the liquid be discharged from the nozzles based on dot data obtained by acquiring a corrected dot generation rate by multiplying the recording duty by the dot generation rate for each dot size and performing a half-tone process with respect to the corrected dot generation rate. 
     By doing so, it is possible to discharge the liquid to a location acquired by performing the half-tone process with respect to the corrected dot generation rate, to form the dots. 
     It is desirable that a mask having a blue noise property or a green noise property be used as a dither mask of the half-tone process. 
     By doing so, it is possible to cause the difference in gloss between the images in the overlapped region not to be noticed, by applying the dither mask considering human visual characteristics. 
     It is desirable that the liquid be transparent ink. 
     Since it is possible to decrease the difference in gloss between the overlapped region or the non-overlapped region of the nozzle rows, even in a case of using the transparent ink which easily generates the difference in gloss by irregular reflection, it is possible to decrease the difference in gloss to improve image quality. 
     It is desirable that the liquid be ultraviolet curable ink. 
     Since the ultraviolet curable ink is cured with ultraviolet irradiation after landing onto a medium, landing position deviation may easily occur and as a result the difference in gloss may easily occur, but according to the configurations described above, it is possible to decrease the difference in gloss between the overlapped region or the non-overlapped region of the nozzle rows. 
     In addition, at least the following aspect also will be made clear with the specification and the accompanying drawings. That is, there is provided a liquid discharging method for discharging liquid from a liquid discharging apparatus including a first nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction, and a second nozzle row in which nozzles for discharging liquid are lined up in a predetermined direction, and which is disposed by forming an overlapped region in which an end portion of one side in the predetermined direction is overlapped with an end portion of the other side in the predetermined direction of the first nozzle row, the method including: receiving input data of an image; increasing a recording duty in the overlapped region with respect to a recording duty in a non-overlapped region which is a region other than the overlapped region, and differentiating increased amounts of the recording duty in the overlapped region from each other depending on the input data, and discharging liquid from the first nozzle row and the second nozzle row depending on the input data and the increased recording duty. 
     By doing so, since it is possible to differentiate the increased amounts of the recording duty from each other depending on the input data when increasing the recording duty in the overlapped region with respect to the recording duty in the non-overlapped region, it is possible to discharge the liquid with an appropriate increased amount for decreasing a difference in gloss based on the input data. It is possible to decrease the difference in gloss of an image between the overlapped region and the non-overlapped region of the nozzle rows. 
     System Configuration 
     The embodiment will be described using an apparatus having a printing system in which a line head printer (hereinafter, printer  1 ) from ink jet printers and a computer  100  are connected to each other, as the liquid discharging apparatus. 
       FIG. 1  is a block diagram of all configurations of the printer  1  and  FIG. 2  is a schematic side view of the printer  1 . The printer  1  which receives printing data from the computer  100  which is an external device, controls each unit (transportation unit  20 , head unit  30 , and ultraviolet ray emitting unit  80 ) with a controller  10  and prints an image on a sheet S. In addition, a detector group  40  monitors a state in the printer  1  and the controller  10  controls each unit based on this detected result. 
     The controller  10  is a control unit for controlling the printer  1 . An interface unit  11  is for performing transmitting and receiving data between the computer  100  which is an external device and the printer  1 . A CPU  12  is an arithmetic processing unit for performing overall control of the printer  1 . A memory  13  is for securing an area for storing a program or a work area of the CPU  12 . The CPU  12  controls each unit with a unit control circuit  14  according to the program stored in the memory  13 . In the embodiment, the computer  100  is provided as an external device, but may be included in the printer  1  as an internal device. 
     The transportation unit  20  includes a transportation belt  21  and transportation rollers  22 A and  22 B, sends the sheet S to printable location, and transports the sheet S in a transportation direction at a predetermined transportation speed. The sheet S is fed onto the transportation belt  21 , and the sheet S on the transportation belt  21  is transported by rotating the transportation belt  21  by the transportation rollers  22 A and  22 B. Electrostatic adsorption or vacuum adsorption of the sheet S on the transportation belt  21  may be performed from a lower side. 
     In  FIG. 2 , a white ink head unit  30 W, a yellow ink head unit  30 Y, a magenta ink head unit  30 M, a cyan ink head unit  30 Cy, a black ink head unit  30 K, and a clear ink head unit  30 Cl are disposed from an upstream side of the sheet S in the transportation direction. In a case where an ink color is not particularly specified, only reference numeral “ 30 ” is denoted for the entire head units. 
     The head unit  30  is for discharging ink droplets to the sheet S and includes a plurality of heads  31 . The plurality of nozzles which are ink discharging units are provided on lower surfaces of the heads  31 . A pressure chamber (not shown) in which the ink is accommodated, and a driving element (piezoelectric element) for changing capacitance of the pressure chamber to discharge the ink are provided in each nozzle. 
     In the embodiment, the ink filled in the head unit  30  is ultraviolet curable ink (UV ink). 
     The printer  1  includes corresponding ultraviolet ray emitting units  80 W,  80 Y,  80 M,  80 Cy, and  80 K on a downstream side of the head unit  30  excluding the clear ink head unit  30 Cl. The ultraviolet ray emitting units are for temporarily curing the ink landed on the sheet S. An ultraviolet ray emitting unit  80 last is included on the most downstream side. The ultraviolet ray emitting unit  80 last is for completely curing the ink landed on the sheet S. The temporary curing is the curing of the surface of the ink droplet so that the ink on the sheet S does not flow, and the complete curing is the curing of the inside of the ink on the sheet S. 
     In the printer  1 , when the controller  10  receives the printing data, the controller  10  first sends the sheet S to an upper portion of the transportation belt  21 . After that, the sheet S is transported on the transportation belt  21  at a constant speed without stopping and faces nozzle surfaces of the heads  31 . The ink droplets are intermittently discharged from each nozzle based on the image data while the sheet S is transported under the head unit  30 . As a result, a dot row (hereinafter, also referred to as a “raster line”) along the transportation direction is formed on the sheet S and the image is printed. The image data is configured from a plurality of pixels disposed two-dimensionally, and each pixel (data) indicates whether or not to form the dot in a region (pixel region) on a medium corresponding to each pixel. 
     Nozzle Disposition 
       FIG. 3  is a diagram showing nozzle arrangement of a lower surface of the head unit  30 . The head unit  30  provided for each ink color has substantially the same configuration with each other. Herein, the clear ink head unit  30 Cl will be described as a representative. As shown in  FIG. 3 , in the head unit  30 , the plurality of heads  31  are disposed in a line in a sheet width direction intersecting the transportation direction, and end portions of each head  31  are disposed to be overlapped with each other. The heads  31 A and  31 B adjacent to each other in the sheet width direction are disposed to be shifted in the transportation direction (disposed in a so-called zigzag shape). 
     Among the heads  31 A and  31 B adjacent to each other in the sheet width direction, the head  31 A on the downstream side in the transportation direction is called a “downstream side head  31 A” and the head  31 B on the upstream side in the transportation direction is called an “upstream side head  31 B”. The heads  31 A and  31 B adjacent to each other in the sheet width direction are collectively called “adjacent heads”. 
       FIG. 3  is a projection view of the nozzles when they are seen from the top of the head. As shown in  FIG. 3 , nozzle rows for discharging the ink (herein, nozzle row for discharging clear ink Cl) are formed on the lower surface of each head  31 . Each nozzle row is configured with 358 nozzles (#1 to #358). The nozzles of each nozzle row are lined up at constant intervals (for example, 720 dpi) in the sheet width direction. In addition, smaller numbers are denoted sequentially from a left side in the sheet width direction with respect to the nozzles belonging to each nozzle row (#1 to #358). 
     The heads  31 A and  31 B lined up in the sheet width direction are disposed so that 8 nozzles on an end portion of the nozzle row of each head  31  are overlapped with each other. In detail, 8 nozzles (#1 to #8) on a left side end portion of the nozzle row of the downstream side head  31 A and 8 nozzles (#351 to #358) on a right side end portion of the nozzle row of the upstream side head  31 B are overlapped with each other, and 8 nozzles (#351 to #358) on a right side end portion of the nozzle row of the downstream side head  31 A and 8 nozzles (#1 to #8) on a left side end portion of the nozzle row of the upstream side head  31 B are overlapped with each other. As shown in the drawing, portions in which the nozzles are overlapped with each other in the adjacent heads  31 A and  31 B are called “overlapped region”. The nozzles (#1 to #8 and #351 to #358) belonging to the overlapped regions are called “overlapped nozzles”. 
     The respective head units  30  for each ink color are disposed so that the locations of the nozzles having the same nozzle number are overlapped (matched) with each other in the sheet width direction. For example, the nozzle #358 of the upstream side head  31 B of the black ink head unit  30 K and the nozzle #358 of the upstream side head  31 B of the clear ink head unit  30 C are disposed so as to be overlapped with each other in the sheet width direction. In the embodiment, the respective head units  30  for each ink color are disposed so that the locations of the nozzles having the same nozzle number are overlapped (matched) with each other in the sheet width direction, but they are not limited thereto, and the head units may be disposed so that the locations of the nozzles having the same nozzle number of the head units  30  with the different ink colors are shifted in the sheet width direction. In this case, it is possible to distribute the overlapped regions in the sheet width direction. 
     By disposing the plurality of heads  31  in the head unit  30  as described above, it is possible to line up the nozzles at equal intervals (720 dpi) over the entire area in the sheet width direction. As a result, it is possible to form the dot row in which the dots are lined up at equal intervals (720 dpi) over a sheet width length. 
       FIG. 4  is a diagram illustrating the pixels in which dots are formed by the nozzles of the head units. The drawing shows the nozzle row of the upstream side head  31 B and the nozzle row of the downstream side head  31 A. The pixels in which the dots are to be formed, are shown in a cell shape on a lower portion of the nozzles. In the drawing, a direction of hatching attached to each nozzle and a direction of hatching of pixels to which the nozzles are allocated for formation of dots, coincide with each other. As shown in the drawing, two nozzles perform the formation of dots in a shared manner in the overlapped region. 
     Creation Process of Printing Data of Comparative Example 
       FIG. 5  is a flowchart of a creation process of the printing data of a comparative example,  FIG. 6  is a diagram showing allocation of half-tone processed data corresponding to the overlapped region to the nozzle row (hereinafter, referred to as a first nozzle row) of the upstream side head  31 B and the nozzle row (hereinafter, referred to as a second nozzle row) of the downstream side head  31 A, and  FIG. 7  is a diagram showing the recording duties of the first nozzle row and the second nozzle row. Hereinafter, the creation process (comparative example) of the printing data for performing a printing method of the comparative example will be described. 
     In the printing method of the comparative example, the dots to be formed in the overlapped region are necessarily formed with any one of the overlapped nozzle of the first nozzle row (upstream side head  31 B) and the second nozzle row (downstream side head  31 A), for obtaining desirable image density. As shown in  FIG. 4 , for example, in a case where the image data is shown so as to form the dots in all pixels associated with the overlapped region, the dots are formed with respect to all pixels with any one of overlapped nozzle of the first nozzle row and the second nozzle row. The creation process of the printing data for performing such printing is shown below. Herein, the printing data is set to be created by a printer driver which is installed in the computer  100  connected to the printer  1 . 
     As shown in  FIG. 5 , when the image data is received from various application programs (S 102 ), the printer driver performs a resolution conversion process (S 104 ). The resolution conversion process is a process of converting the image data received from the various application programs to resolution used at the time of performing the printing on a medium S. The image data after performing the resolution conversion process is RGB data having 256 gradations (high gradation) represented by an RGB color space. Accordingly, the printer driver then converts the RGB data into MCK data corresponding to the ink of the printer  1  in a color conversion process (S 106 ). 
     Next, the printer driver performs a dot generation rate conversion process (S 108 ). 
       FIG. 8  is a diagram showing a dot generation rate conversion table. In the dot generation rate conversion process, the printer driver applies the gradation value of each pixel to the dot generation rate conversion table and performs conversion to generate dots having a certain dot size at corresponding degree of generation rate. For example, in a case where the input gradation value (hereinafter, may simply be referred to as the “gradation value”) is “180”, it is found that a generation rate of a large dot is approximately 40%, a generation rate of a medium dot is approximately 20%, and a generation rate of a small dot is approximately 20%. Herein, level data corresponding to the dot generation rate is shown. That is, the level data can be the dot generation rate obtained by replacing the dot generation rate to the 256 gradations. The fact that the level data is “100” when the dot generation rate is approximately 40%, is read from  FIG. 8 . 
     Such a dot generation rate conversion process is performed for each pixel. That is, the selected dot size and the level data (dot generation rate) of the size thereof are obtained for each pixel. 
     Next, the printer driver performs the half-tone process (S 110 ). In the half-tone process, the dither mask (may be referred to as a “dither matrix”) is applied to compare the level data described above and a value of a cell of the dither mask, and in a case of including the level data having a value greater than the value of the cell, the dot thereof is determined to be formed. In contrast, in a case of including the level data having a value equal to or less than the value of the cell, the dot thereof is determined not to be formed. With the half-tone process, the data showing generation or non-generation of the dot in each pixel for each dot size is obtained. 
     Next, the printer driver distributes the half-tone processed data to the overlapped nozzles (#351 to #358) of the first nozzle row and the overlapped nozzles (#1 to #8) of the second nozzle row in an image distribution process (S 112 ). The distribution is performed for each dot size. 
     The process from Step S 108  to Step S 116  are performed for each ink color of YMCK, and the same process is also performed for the white ink W and the clear ink Cl. 
     The drawing on the top of  FIG. 6  is data showing generation or non-generation of large dots after the half-tone process. Black squares represent pixels which form the large dots and white squares represent pixels which do not form the large dots. The data surrounded by a dashed-dotted line is the half-tone processed data which is allocated to the first nozzle row, and the data surrounded by a dotted line is the half-tone processed data which is allocated to the second nozzle row. The half-tone processed data which is surrounded by the lines in an overlapped manner is the half-tone processed data corresponding to the overlapped region. 
     The second drawings from the top of  FIG. 6  show data items distributed to the first nozzle row and the second nozzle row by the printer driver. However, the overlapped region data items surrounded by the dotted lines are data items allocated to both of the overlapped nozzles of the first nozzle row and the overlapped nozzles of the second nozzle row. Accordingly, if the data is maintained as the state shown in the second drawings from the top of  FIG. 6 , all of the dots formed by the overlapped nozzles of the first nozzle row and the dots formed by the overlapped nozzles of the second nozzle row are formed to be overlapped with each other. Therefore, the printer driver determines whether to form the dots showing the overlapped region data (half-tone processed data) by the overlapped nozzles of the first nozzle row or by the overlapped nozzles of the second nozzle row. Thus, a masking process (S 114 ) is performed using overlapped mask shown in the third drawings from the top of  FIG. 6 . 
     The masking process is performed by acquiring a logical product with the overlapped mask. That is, in a case where the pixel in black as the distribution data and the pixel in black in the overlapped mask are overlapped with each other in the pixel, the large dots are set to be formed in the pixel thereof. The overlapped mask used herein is generated based on the recording duty of  FIG. 7  and is a mask with the low dot generation rate as that in the end portion of the nozzle row. 
     After specifying the dot of the pixel to which the each nozzle row is allocated for formation, by the masking process (S 114 ) with respect to the overlapped region data as described above, the printer driver rearranges the image data items in a matrix shape in order to be transferred to the printer  1  by a rasterizing process (S 116 ). The printer driver transmits the data which is subjected to the processes are transmitted to the printer  1  with command data in accordance with the printing method. The printer  1  performs the printing based on the received printing data. 
     In such a so-called line head type printer, the overlapped region of the nozzle rows is affected by an installation error of the head or a transportation error of the medium, and deviation of the landing locations of the ink discharged by the upstream nozzle and the ink discharged by the downstream nozzle occurs. If such deviation of the landing locations occurs, the difference in gloss between the overlapped region and the non-overlapped region occurs, and lines may be seen. Such a difference in gloss is not desirable and therefore the difference in gloss is required to be decreased as much as possible. Accordingly, the difference in gloss between the overlapped region and the non-overlapped region is decreased with the embodiment described below. 
     Embodiment 
       FIG. 9  is a flowchart of printing data creation of the embodiment. If the image data is received from the application software (S 202 ), the printer driver in the computer  100  connected to the printer  1  performs the resolution conversion process (S 204 ), the color conversion process (S 206 ), and the dot generation rate conversion process (S 208 ), in the same manner as the creation process of the printing data of the comparative example. The processes from Step S 202  to Step S 208  are the same as those from Step S 102  to Step S 108  of  FIG. 5  described above, and thus the descriptions thereof will be omitted. 
     In the embodiment, the common dot generation rate conversion table is used for the overlapped region and the non-overlapped region. For example, the dot generation rate conversion table shown in  FIG. 8  in the comparative example described above may be used. However, it is not limited thereto, and different dot generation rate conversion tables may be used for the overlapped region and the non-overlapped region. 
     Next, the printer driver performs a recording duty increasing process (S 210 ). 
       FIG. 10  is a flowchart of the recording duty increasing process. In the recording duty increasing process, replication of the data in the overlapped region is initially performed (S 2102 ). 
       FIG. 11  is a first diagram showing an operation of replicating the data in the overlapped region and multiplying each recording duty by the overlapped region data. The drawing on the top of  FIG. 11  is a diagram showing the level data obtained by the dot generation rate conversion (S 210 ) described above. 
     The drawing on the top of  FIG. 11  shows the data of the generation rate of the large dots associated with the first nozzle row (nozzle row of the upstream side head  31 B) and the second nozzle row (nozzle row of the downstream side head  31 A). 1 square in the drawing corresponds to 1 pixel, and the number shown in the pixel is the level data of the large dots of the pixel. 
     Herein, for convenience of description, the value of the level data corresponding to the generation rate of the large dot is shown in each corresponding pixel, but by performing the dot generation rate conversion, the data items of the small dot and the medium dot are also generated. In addition, for convenience of description, the entire level data items of the large dots in each pixel are shown as “100” (input gradation value which is approximately “180” is used). 
     The level data items of the pixels surrounded with a thick line are the level data items corresponding to the overlapped region of the first nozzle row and the second nozzle row. As shown in the drawing, a direction corresponding to the sheet width direction is set as an X direction and a direction corresponding to the transportation direction is set as a Y direction. The printer driver replicates the overlapped region data. The result thereof is the data shown as the second drawing from the top of  FIG. 11 , and two overlapped region data items are arranged in the X direction. 
     The printer driver performs selection of the recording duty (S 2104 ). In the embodiment, the correction is performed by selecting the recording duty so that the level data described above is increased in the overlapped region. For the recording duty at that time, the recording duties different from each other depending on the discharging rate R in the overlapped region are employed. 
     Herein, a discharging rate R of the embodiment is defined as described below.
 
R=((number of times of recording large dots×(ink weight of large dots/ink weight of large dots)+(number of times of recording medium dots×(ink weight of medium dots/ink weight of large dots)+(number of times of recording small dots×(ink weight of small dots/ink weight of large dots))/(maximum number of times of recording which allows recording by 1 head per 1 square inch)×100
 
     Herein, the “number of times of recording” is the number of times of recording of dots with each dot size per 1 square inch. In the embodiment, since the printing with 720 dpi is performed, the 720×720 dots can be formed with 1 head per 1 square inch at most (this is defined as the maximum number of dots to be formed). The number of times of recording the dots with each dot size by 1 head per 1 square inch can be acquired by multiplying the maximum number of dots to be formed by the dot generation rate acquired from  FIG. 8 . 
     When the maximum value of the weight of the ink to be recorded in 1 pixel by 1 head is set to 100%, the discharging rate R corresponds to rate of the weight of the ink to be discharged by 1 head in the 1 pixel based on the dot generation rate acquired from  FIG. 8 , for the input gradation value with respect to 1 pixel. 
       FIG. 12  is a diagram showing an example of the selected recording duty.  FIG. 12  shows the recording duties corresponding to the plurality of discharging rates (R1&lt;R2&lt;R3&lt;R4&lt;R5). As described above, the recording duties which are different from each other for each discharging rate are prepared in advance. The increased amount of the recording duty is set to be higher as the discharging rate is high. 
     In the selection of the recording duty (S 2104 ), the printer driver acquires the discharging rate in the overlapped region. The recording duty corresponding to the discharging rate which is closest to the acquired discharging rate from the plurality of discharging rates R1 to R5 is selected ( FIG. 12 ). Herein, the embodiment is described by assuming that the closest discharging rate is R2. 
       FIG. 13  is a first explanatory diagram of the recording duty. The recording duty shown in  FIG. 13  is the recording duty when the discharging rate is R2, a dashed-dotted line shows the recording duty of the first nozzle row, and a dashed line shows the recording duty of the second nozzle row. 
     The recording duty changes depending on the locations of the overlapped nozzles. As shown in  FIG. 11 , in the recording duty of the first nozzle row, the recording duty is higher for the nozzle on the first nozzle row side (left side) among the overlapped nozzles and the recording duty is gradually decreased. In contrast, in the recording duty of the second nozzle row, the recording duty is lower for the nozzle on the first nozzle row side (left side) among the overlapped nozzles and the recording duty is gradually increased. In the overlapped region, the total of the recording duty of the first nozzle row and the recording duty of the second nozzle row is constantly the recording duty exceeding 100%, and the recording duty around the center is set to be highest ( FIG. 13 ). This is for preventing the decrease in the image quality because, in the overlapped region, the dot discharged from the first nozzle row and the dot discharged from the second nozzle row are overlapped with each other and the coverage with dots is low compared to that in the non-overlapped region. 
     The pixel (row) on the leftmost side of the original data in the overlapped region is the data allocated to the nozzle #351 of the first nozzle row, and the pixel (row) on the leftmost side of the replicated data in the overlapped region is the data allocated to the nozzle #1 of the second nozzle row. The recording duty of the nozzle #351 of the first nozzle row is set to 100%, the recording duty of the nozzle #1 of the second nozzle row is set to 5%, and the level data of the pixel before distribution is set to “100”. In this case, as shown in the lowermost portion of  FIG. 11 , the level data allocated to the nozzle #351 of the first nozzle row is set to “100” with 100% of the value of 100, and the level data allocated to the nozzle #1 of the second nozzle row is set to “5” with 5% of the value of 100. By doing so, the discharging amount of the ink in the overlapped region is set to be greater than the discharging amount of the ink in the non-overlapped region. 
     Next, the printer driver multiplies the recording duty of each nozzle row by the two overlapped region data items (S 2106 ). The data shown in the lowermost data of  FIG. 11  is the result obtained by multiplying the recording duty of each nozzle row by the overlapped region data (level data). 
     By doing so, when the multiplication process of the recording duty (S 2106 ) ends, the half-tone process (S 212 ) is performed for each nozzle row. 
       FIG. 14A  is a diagram showing the dither mask and  FIG. 14B  is a diagram showing an operation of the half-tone process performed by the dither method. The dither method is a method of determining formation or non-formation of the dots based on a magnitude relationship between a threshold value stored in the dither mask and the level data shown by each pixel. According to the dither method, it is possible to generate the dots at density corresponding to the level data shown by the pixel, for each unit area to which one dither mask is allocated. In addition, according to the dither method, it is possible to disperse and generate the dots by the setting of the threshold value of the dither mask and to improve a granularity of the image. 
     In particular, the dither mask used in the embodiment is a mask having a blue noise property or a green noise property. The blue noise property and the green noise property will be described later, but since a storage location of the threshold value is adjusted so as to generate a great frequency component in a high frequency range, it is possible to further cause the difference in gloss not to be noticed in the overlapped region by a visual sense of a person. 
       FIG. 14B  shows the location at which the dither mask (thick line) is associated with the non-overlapped region data and the overlapped region data of the first nozzle row and the second nozzle row. The printer driver associates the dither mask with the level data of the high gradation (256 gradations), and determines the formation or non-formation of the large dots by comparing a target pixel and a threshold value of the dither mask corresponding thereto. In the determination of the formation of the large dots, it is determined to form the large dots in a case where the level data of the target pixel is greater than the threshold value of the dither mask. 
     Herein, the example of the large dots has been described, but the same processes are also performed for the small dots and the medium dots. The dither mask shown in  FIG. 14A  is configured with 16 pixels×16 pixels, but a dither mask having another size may be used. In the embodiment, the half-tone process is performed by using the dither method, but another half-tone process such as an error diffusion method may be performed. 
     Lastly, the rasterizing process (S 214 ) is performed. The rasterizing process is the same process performed in the method of the comparative example described above. The printer driver transmits the data which is subjected to the processes are transmitted to the printer  1  with command data in accordance with the printing method. The printer  1  performs the printing based on the received printing data. 
     As described above, the recording duties which are different from each other depending on the input gradation value are used, and as an example, a case where the input gradation value is 237 and the level data of the large dot is 200 will be described. 
       FIG. 15  is a second diagram showing an operation of replicating data in the overlapped region and multiplying the recording duty of each nozzle row by the overlapped region data. Each explanatory diagram in  FIG. 15  is shown in the same manner as those in  FIG. 11 . Since the input gradation value is higher than in the case of  FIG. 11 , the level data of the large dot is also set to be 200 which is greater than in the case of  FIG. 11 . The discharging rate at that time is R4. Since the discharging rate R4 is a value higher than the discharging rate R2 described above, the recording duty which is higher than in the case of  FIG. 11  is used. 
       FIG. 16  is a second explanatory diagram of the recording duty.  FIG. 16  shows the recording duty when the discharging rate is R4. When comparing the recording duty of  FIG. 16  and the recording duty of  FIG. 13 , the recording duty of  FIG. 16  is constantly set to be higher than the recording duty of  FIG. 13 . 
     By doing so, as the input gradation value (or discharging rate corresponding thereto) is high (that is, density is high), a great amount of ink is discharged, and thus the filled degree of the dots is improved. The occurrence of difference in gloss due to insufficient dots filled in the pixels is suppressed. As described above, since the dither mask has the blue noise property or the green noise property, it is possible to further cause the difference in gloss not to be noticed in the overlapped region by a visual sense of a person. 
       FIG. 17A  is an explanatory diagram of a spatial frequency property of the blue noise property. In the dot disposition having the blue noise property, respective dots are disposed to be disordered or to be uniform. If the half-tone process is performed with respect to the input image having the constant gradation using the dither mask having the blue noise property, the frequency of the dot disposition has the following properties. 
     (a) A low frequency component is substantially or completely not included. 
     (b) A flat and smooth high frequency range is included. 
     (c) A main frequency which is generally shown by the following formula is included. 
     
       
         
           
             
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     Herein, D is a minimum distance between the dots, and g is a gray level (g=gradation value/255). 
       FIG. 17B  is an explanatory diagram of a spatial frequency property of the green noise property. In the dot disposition having the blue noise property, clusters of the dots are disposed to be disordered or to be uniform. If the half-tone process is performed with respect to the input image having the constant gradation using the dither mask having the green noise property, the frequency of the dot disposition has the following properties. 
     (a) A low frequency component is substantially or completely not included. 
     (b) The frequency decreases as the cluster of the dot increases. 
     (c) A main frequency which is generally shown by the following formula is included. 
     
       
         
           
             
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     Herein, D is a minimum distance between the dots, and g is a gray level (g=gradation value/255). 
       FIG. 18  is a table showing quality of the image in the overlapped region. The drawing shows the input gradation value in the overlapped region (expressed by percentages), and the increased amount of the ink discharging amount in the overlapped region at that time. In  FIG. 18 , “A” shows a case where the difference in gloss is not sensed by a visual sense of a person, “B” shows a case where the difference in gloss is slightly sensed by a visual sense of a person, and “C” shows a case where the difference in gloss is sensed by a visual sense of a person. 
     The increased amount of the ink in the overlapped region is shown from A 1  to A 8 , but the increased amount is set to become larger from A 1  to A 8 . As understood when referring to  FIG. 18 , when the input gradation value is low (when the discharging rate is low), the total increased amount in the overlapped region is set to a level about from A 1  to A 4  to not have a substantially increased amount of the ink, so that the difference in gloss is not sensed. 
     However, as the input gradation value increases (discharging rate also increases), the total increased amount in the overlapped region is also necessary to be gradually increased from A 5  to A 8  to set the difference in gloss to not be sensed. That is, it is difficult to decrease the difference in gloss by simply increasing the increased amount of the ink in the overlapped region. 
     Meanwhile, according to the printer  1  of the embodiment, since the increased amount of the ink in the overlapped region is variable depending on the input gradation value, it is possible to discharge the ink with the most appropriate discharging amount of the ink depending on the input gradation value to cause the difference in gloss not to be noticed. 
     In the embodiment, the recording duty is set to be selected based on the discharging rate of all areas of the overlapped region, but the overlapped region may be partitioned to a plurality of sections and the discharging rate for each section may be acquired. The recording duty selected based on the discharging rate of the section may be applied to the section. In addition, the overlapped region may be partitioned with a pixel unit and the recording duty may be selected based on the discharging rate for each pixel. In this case, the recording duty selected based on the discharging rate of the pixel is applied to the pixel. 
     Other Embodiment 
     The embodiment described above is for easily illustrating the invention and is not for limiting the invention. Modifications and improvements may be performed within a range not departing from a gist of the invention and equivalent materials as those in the embodiment may be included in the invention. Particularly, the following embodiment is included in the invention. 
     Printer 
     In the embodiment described above, the example of the printer (so-called line head printer) which forms an image by lining up the plurality of heads over the sheet width length and transporting the sheet under the fixed heads, is used, but it is not limited thereto. For example, the plurality of heads are lined up in a nozzle row direction so that end portions of the nozzle rows of the plurality of heads are overlapped with each other. A printer (so-called serial type printer) which alternately repeats an operation of forming an image while moving the plurality of heads in a direction intersecting with the nozzle row direction, and an operation of transporting the sheet with respect to the plurality of heads in the nozzle row direction, may be used. In this case, in the same manner as in the embodiment described above, it is possible to obtain the printing data by performing the half-tone process of the data obtained by multiplying the recording duty by the dot generation rate data (level data) for each dot size in the overlapped region in which the heads are overlapped with each other. 
     Liquid Discharging Apparatus 
     In the embodiment, the ink jet printer is used as the liquid discharging apparatus, but it is not limited thereto. The embodiment can be applied to any industrial apparatus which is not a printer, as long as it is a liquid discharging apparatus. For example, the invention can be applied to a printing apparatus for printing patterns on a fabric, a color filter manufacturing apparatus or a display manufacturing apparatus of an organic EL display, a DNA chip manufacturing apparatus for manufacturing a DNA chip by applying a solution containing dissolved DNA to a chip. 
     In addition, for the discharging method of the liquid, a piezoelectric method of applying voltage to a driving element (piezoelectric element) and expanding and contracting an ink chamber to discharge the liquid, or a thermal method of generating air bubbles in the nozzles using a heat generation element to discharge the liquid by the air bubbles may be used. The liquid is not limited to the liquid such as the ink and may be powder or the like. 
     The entire disclosure of Japanese Patent Application No. 2013-071622, filed Mar. 29, 2013 is expressly incorporated by reference herein.