Patent Publication Number: US-2011050763-A1

Title: Image processing apparatus and image processing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Provisional Application No. 61/238,755 filed on Sep. 1, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a technique for correcting the density of images printed on both sides of a sheet. 
     BACKGROUND 
     In recent years, from the viewpoint of effectively utilizing sheets for resource saving, a user tends to print images on both sides of a sheet (a recording medium) using an ink jet recording apparatus. The ink jet recording apparatus ejects ink from a head to print a desired image on the sheet. The ink adhering to the sheet penetrates from a first side on which the image is printed to a second side opposite to the first side according to characteristics of the sheet, characteristics of the ink, a printing ratio of the printed image, and the like. Thereafter, when the ink jet recording apparatus prints an image on the second side, in some case, the user feels it hard to see the image printed on at least one side. 
     Therefore, in the ink jet recording apparatus, since it is likely that printing quality falls depending on a type of print data, there is also a method of detecting attributes of the print data and correcting data other than text data (e.g., image data). Further, in the ink jet recording apparatus, there is also a method of detecting, when duplex printing is performed, an overlapping portion of print data and correcting the print data in only the overlapping portion. 
     However, in order to accurately align printing positions on both sides in correcting the print data only in the overlapping portion of the print data, the ink jet recording apparatus requires a mechanism for accurately conveying a sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary sectional view in a lateral direction of an ink jet recording apparatus according to a first embodiment; 
         FIG. 2  is an exemplary block diagram of the configuration of the ink jet recording apparatus and a host computer according to the first embodiment; 
         FIG. 3  is an exemplary flowchart for explaining processing for creating recording data according to the first embodiment; 
         FIG. 4  is an exemplary diagram for explaining density correction for the recording data according to the first embodiment; 
         FIG. 5  is an exemplary flowchart for explaining processing for creating recording data according to a second embodiment; 
         FIG. 6  is an exemplary flowchart for explaining processing for creating recording data according to a third embodiment; and 
         FIG. 7  is an exemplary flowchart for explaining processing for creating recording data according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image processing apparatus includes a detecting unit, a determining unit, and a correcting unit. The detecting unit detects an overlapping portion of a first image printed on a first side of a sheet and a second image printed on a second side opposite to the first side. The determining unit determines whether the detected overlapping portion of the first image and the second image is equal to or larger than a predetermined area. The correcting unit corrects, if the overlapping portion is equal to or larger than the predetermined area, the density of one of the first image and the second image. 
     A first embodiment is explained below with reference to the accompanying drawings.  FIG. 1  is a sectional view in a lateral direction of an ink jet recording apparatus  1  as an image processing apparatus according to the first embodiment. A first sheet cassette  100  stores a sheet bundle p including plural sheets (recording media). A second sheet cassette  101  stores a sheet bundle p′ including plural sheets of a size different from a size of the sheets stored in the first sheet cassette  100 . A first paper feeding roller  102  extracts sheets corresponding to a selected sheet size one by one from the top of the sheet bundle p stored in the first sheet cassette  100  and conveys the sheet to a first conveying roller pair  104  and a registration roller pair  106 . Similarly, a second paper feeding roller  103  extracts sheets corresponding to a selected sheet size one by one from the sheet bundle p′ stored in the second sheet cassette  101  and conveys the sheet to a second conveying roller pair  105 , the first conveying roller pair  104 , and the registration roller pair  106  in this order. 
     A conveyor belt  107  is endlessly laid over a driving roller  108  and two driven rollers  109 . Tension is applied to the conveyor belt  107  by the driving roller  108  and the two driven rollers  109 . Holes are opened at predetermined intervals on the surface of the conveyor belt  107 . A vacuum chamber  111  coupled to a fan  110  is set on an inner side of the conveyor belt  107  in order to attract the sheet to the conveyor belt  107 . 
     The driving roller  108  drives the conveyor belt  107  to convey the sheet from an upstream side on which the registration roller pair  106  is provided to a downstream side on which a first conveying roller pair  112  is provided. Ink jet recording heads configured to eject inks to the sheet according to print data are arranged in four rows above the conveyor belt  107 . An ink jet recording head  115 C configured to eject cyan (C) ink, an ink jet recording head  115 M configured to eject magenta (M) ink, an ink jet recording head  115 Y configured to eject yellow (Y) ink, and an ink jet recording head  115 Bk configured to eject black (Bk) ink are arranged in the ink jet recording apparatus  1  in order from the upstream side. The ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk are referred to as image forming units. In each of the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk, nozzles configured to eject ink droplets are arranged to extend over a width direction of the sheet at predetermined resolution. In other words, the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk are line-type heads in which not-shown plural nozzles are arrayed on lines. In the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk, the nozzles are arrayed in a direction orthogonal to a conveying direction of the sheet by the conveyor belt  107 . These nozzles are arrayed to be positioned a specified space apart from the sheet located on the conveyor belt  107 . 
     The ink jet recording head  115 C and an ink cartridge  116 C, the ink jet recording head  115 M and an ink cartridge  116 M, the ink jet recording head  115 Y and an ink cartridge  116 Y, and the ink jet recording head  115 Bk and an ink cartridge  116 Bk are respectively coupled by tubes  117 C,  117 M,  117 Y, and  117 Bk. The ink cartridge  116 C for cyan (C), the ink cartridge  116 M for magenta (M), the ink cartridge  116 Y for yellow (Y), and the ink cartridge  116 Bk for black (Bk) are respectively filled with inks of the colors. 
     A space between ends on the conveyor belt  107  side of the respective ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk and the surface of the sheet is kept constant by the vacuum chamber  111 . The respective ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk eject the inks of the colors in synchronization of timing when the sheet is conveyed from the registration roller pair  106  to the conveyor belt  107  and an operation for conveying the sheet by the conveyor belt  107 . A color image is formed in a desired position on the surface of the sheet by the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk. 
     The first conveying roller pair  112 , a second conveying roller pair  113 , and a third conveying roller pair  114  convey the sheet having an image formed on one side in full-color or the sheet having images formed on both sides in full-color by the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk to a discharge roller pair  126 . The discharge roller pair  126  discharges the sheet subjected to the image formation to a paper discharge tray  118 . The paper discharge tray  118  stores sheets one after another in a stacked state. 
     The ink jet recording apparatus  1  further includes a conveying roller pair  120 , a conveying roller pair  121 , a conveying roller pair  122 , a conveying roller pair  123 , a conveying roller pair  124 , and a conveying roller pair  125  in order to return the sheet from the downstream side to the upstream side when the ink jet recording apparatus  1  applies the duplex printing to the sheet. A first conveyance guide  131  is provided on the upstream side of the registration roller pair  106 . The first conveyance guide  131  is disposed to switch a conveyance route for the sheet. The direction of a guide tip of the first conveyance guide  131  is changed with a not-shown fulcrum as the center. A second conveyance guide  132  is provided on the downstream side of the second conveying roller pair  113 . The second conveyance guide  132  is disposed to switch the conveyance route for the sheet. The direction of a guide tip of the second conveyance guide  132  is changed with a not-shown fulcrum as the center. 
       FIG. 2  is a block diagram of a control system for the ink jet recording apparatus  1  and a host computer  5  connected thereto according to the first embodiment. The ink jet recording apparatus  1  includes a printer controller  201 , a communication I/F  202 , a conveying-unit control circuit  203 , a driving motor  204 , an image-forming-unit controller  205 , an attracting-unit control circuit  206 , a driving motor  207 , a conveyance-guide control circuit  208 , and a solenoid  209 . 
     The printer controller  201  is a microcomputer including a CPU  2011  as a processor and a memory  2012 . The CPU  2011  executes various kinds of arithmetic processing according to computer programs stored in the memory  2012 . The CPU  2011  outputs control signals to the respective units. The CPU  2011  communicates with, via the communication I/F  202 , the host computer  5  connected to the ink jet recording apparatus  1 . The CPU  2011  receives, from the host computer  5 , recording information and various kinds of attribute information concerning the recording information together with a print request input by a user. The recording information includes image data to be printed on a sheet and instruction information concerning whether printing is duplex printing or simplex printing. The attribute information is, for example, information concerning a type of an object of the image data, position information of the object of the image data, and color information of the object of the image data. The CPU  2011  converts the image data into recording data suitable for printing. 
     The conveying-unit control circuit  203  controls the driving motor  204 . The driving motor  204  drives the conveying roller pairs and the registration roller pair configured to convey the sheet during image formation. The image-forming-unit controller  205  controls image forming operations by the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk on the basis of the recording data. The attracting-unit control circuit  206  controls the driving motor  207 . The driving motor  207  drives the fan  110 . The conveyance-guide control circuit  208  controls the solenoid  209 . The solenoid  209  drives the first conveyance guide  131  and the second conveyance guide  132 . 
     Processing for creating recording data in the ink jet recording apparatus  1  is explained below.  FIG. 3  is a flowchart for explaining processing for creating recording data according to the first embodiment. The first embodiment is an example in which print density is corrected according to position information of image data printed on the front side of a sheet and image data printed on the rear side of the sheet. First, the CPU  2011  determines whether an instruction for the duplex printing is included in recording information requested to be printed (Act  101 ). 
     If the duplex printing is not instructed (i.e., when the simplex printing is instructed) (No in Act  101 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the simplex printing input in the host computer  5  (Act  102 ). The CPU  2011  converts image data of one side acquired from the recording information into recording data and temporarily stores the recording data in the memory  2012  (Act  103 ). The CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  104 ). If all the pieces of recording information are analyzed (Yes in Act  104 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  104 ), the CPU  2011  returns to Act  102  and continues the processing. 
     If the duplex printing is instructed (Yes in Act  101 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the duplex printing input in the host computer  5  (Act  105 ). The CPU  2011  acquires position information of image data of a front side (a first side) and a rear side (a second side) from the attribute information of the respective sides (Act  106 ). Act  106  is a detecting act for detecting an overlapping portion of the front side and the rear side. The CPU  2011  determines whether, when images are formed on the front side and the rear side of a sheet in the ink jet recording apparatus  1 , an overlapping portion of the front and the rear is equal to or larger than a predetermined area (Act  107 ). Act  107  is a determining act for determining whether the overlapping portion is equal to or larger than the predetermined area. If the overlapping portion is equal to or larger than the predetermined area (Yes in Act  107 ), the CPU  2011  acquires image data sizes of the overlapping portion of the front side and the rear side from the attribute information (Act  108 ). The CPU  2011  compares, concerning the image data sizes in the overlapping portion, the image data sizes of the rear side and the image data size of the front side and determines whether the former is smaller than the latter (Act  109 ). Act  109  is a front and rear comparing act. If the image data size of the rear side is smaller than the image data size of the front side (Yes in Act  109 ), the CPU  2011  corrects (reduces) the density of the image data of the rear side (Act  110 ). After correcting the image data of the rear side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  111 ). 
     If the image data of the front side is smaller than the image data of the rear side (No in Act  109 ), the CPU  2011  corrects (reduces) the density of the image data of the front side (Act  112 ). Acts  110  and  112  are density correcting acts for correcting density. After correcting the image data of the front side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  111 ). If the overlapping portion is not equal to or larger than the predetermined area (No in Act  107 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  111 ). 
     Thereafter, the CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  113 ). If all the pieces of recording information are analyzed (Yes in Act  113 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  113 ), the CPU  2011  returns to Act  105  and continues the processing. Thereafter, the CPU  2011  outputs the recording data stored in the memory  2012  to the image-forming-unit controller  205 . The image-forming-unit controller  205  controls the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk to form images on both the sides or the one side of the sheet on the basis of the recording data. The CPU  2011  temporarily stores, every time image data is converted into recording data in Act  111 , the recording data in the memory  2012  and outputs all recording data in one print request to the image-forming-unit controller  205  at a time. However, the CPU  2011  may output, every time image data is converted into recording data, the recording data to the image-forming-unit controller  205 . 
     Density correction for image data by the CPU  2011  is explained below. The CPU  2011  performs thinning-out processing for pixels in order to reduce the density of image data.  FIG. 4  is a diagram for explaining density correction for correcting the density of image data at a thinning-out ratio of 50%. In the image data, pixels in a portion where an image is formed by the ink jet recording apparatus  1  are corrected to pixels on which an image is not formed. The CPU  2011  can change the thinning-out ratio according to, for example, an area of an overlapping portion of the front and the rear. When the density correction for image data is performed, the CPU  2011  preferentially subjects black pixels to thinning-out processing. The image-forming-unit controller  205  controls ink amounts ejected from the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk to be reduced according to the thinning-out ratio. If the ink jet recording heads  115 C,  115 M,  115 Y, and  115 Bk are a multi-drop type, the image-forming-unit controller  205  may control the number of ink droplets to be reduced according to the thinning-out ratio. It is effective for the image-forming-unit controller  205  to perform control such that an image of image data having a smaller thinning-out ratio is formed on a sheet earlier. 
     In Acts  110  and  111 , the CPU  2011  corrects the density of the image data of the side having the smaller image data size to be reduced. However, the CPU  2011  may correct the density of the image data of the side having the larger image data size to be reduced. In this case, the ink jet recording apparatus  1  can save inks necessary for forming images. 
       FIG. 5  is a flowchart for explaining processing for creating recording data according to a second embodiment. The second embodiment is an example in which print density is corrected according to position information and types of image data printed on the front side of a sheet and image data printed on the rear side of the sheet. First, the CPU  2011  determines whether an instruction for the duplex printing is included in recording information requested to be printed (Act  201 ). If the duplex printing is not instructed (No in Act  201 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the simplex printing input in the host computer  5  (Act  202 ). The CPU  2011  converts image data of one side acquired from the recording information into recording data and temporarily stores the recording data in the memory  2012  (Act  203 ). The CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  204 ). If all the pieces of recording information are analyzed (Yes in Act  204 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  204 ), the CPU  2011  returns to Act  202  and continues the processing. 
     If the duplex printing is instructed (Yes in Act  201 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the duplex printing input in the host computer  5  (Act  205 ). The CPU  2011  acquires position information of image data of the front side and the rear side of a sheet from the attribute information of the respective sides (Act  206 ). The CPU  2011  determines whether, when images are formed on the front side and the rear side of the sheet in the ink jet recording apparatus  1 , an overlapping portion of the front and the rear is equal to or larger than a predetermined area (Act  207 ). If the overlapping portion is equal to or larger than the predetermined area (Yes in Act  207 ), the CPU  2011  acquires image data sizes of the overlapping portion of the front side and the rear side from the attribute information (Act  208 ). The CPU  2011  compares, concerning the image data sizes in the overlapping portion, the image data size of the rear side and the image data size of the front side and determines whether the former is smaller than the latter (Act  209 ). Act  209  is a front and rear comparing act. 
     If the image data size of the rear side is smaller than the image data size of the front side (Yes in Act  209 ), the CPU  2011  determines whether the image data of the rear side is character (text) data (Act  210 ). If the image data of the rear side is not character data (if the image data of the rear side is, for example, photograph data) (No in Act  210 ), the CPU  2011  corrects (reduces) the density of the image data of the rear side (Act  211 ). After correcting the image data of the rear side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  212 ). If the image data of the rear side is character data (Yes in Act  210 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  212 ). 
     If the image data of the front side is character data (Yes in Act  210 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  212 ). If the image data of the front side is smaller than the image data of the rear side (No in Act  209 ), the CPU  2011  determines whether the image data of the front side is character data (Act  213 ). Acts  210  and  213  are character image comparing acts. If the image data of the front side is not character data (No in Act  213 ), the CPU  2011  corrects (reduces) the density of the image data of the front side (Act  214 ). After correcting the image data of the front side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  212 ). If the overlapping portion is not equal to or larger than the predetermined area (No in Act  207 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  212 ). 
     Thereafter, the CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  215 ). If all the pieces of recording information are analyzed (Yes in Act  215 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  215 ), the CPU  2011  returns to Act  205  and continues the processing. Thereafter, the CPU  2011  outputs the recording data stored in the memory  2012  to the image-forming-unit controller  205  in the same manner as in the first embodiment. 
     If the image data is not character data in Acts  210  and  213 , the CPU  2011  does not correct the image data. However, the CPU  2011  may further determine, according to a type of character data, whether density correction is performed. For example, it is also possible that, if the character data is a thin-line character or a low-density character having density lower than predetermined density, the CPU  2011  does not perform density correction in order to give priority to identity of the character and, if the character data is a thick-line character or a high-density character, the CPU  2011  performs density correction because it is likely that image deterioration occurs. In Acts  211  and  214 , the CPU  2011  corrects the density of the image data of the side having the smaller image data size in the overlapping portion to be reduced. However, the CPU  2011  may correct the density of the image data of the side having the larger image data size to be reduced. In this case, the ink jet recording apparatus  1  can save inks necessary for forming images. 
       FIG. 6  is a flowchart for explaining processing for creating recording data according to a third embodiment. The third embodiment is an example in which print density is corrected according to position information and an average printing ratio of image data printed on the front side of a sheet and image data printed on the rear side of the sheet. First, the CPU  2011  determines whether an instruction for the duplex printing is included in recording information requested to be printed (Act  301 ). If the duplex printing is not instructed (No in Act  301 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the simplex printing input in the host computer  5  (Act  302 ). The CPU  2011  converts image data of one side acquired from the recording information into recording data and temporarily stores the recording data in the memory  2012  (Act  303 ). The CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  304 ). If all the pieces of recording information are analyzed (Yes in Act  304 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  304 ), the CPU  2011  returns to Act  302  and continues the processing. 
     If the duplex printing is instructed (Yes in Act  301 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the duplex printing input in the host computer  5  (Act  305 ). The CPU  2011  acquires position information of image data of the front side and the rear side of a sheet from the attribute information of the respective sides (Act  306 ). The CPU  2011  determines whether, when images are formed on the front side and the rear side of the sheet in the ink jet recording apparatus  1 , an overlapping portion of the front and the rear is equal to or larger than a predetermined area (Act  307 ). If the overlapping portion is equal to or larger than the predetermined area (Yes in Act  307 ), the CPU  2011  detects average printing ratios of the overlapping portion of the front side and the rear side from the attribute information (Act  308 ). The CPU  2011  compares, concerning the average printing ratios in the overlapping portion, the average printing ratios of the rear side and the average printing ratio of the front side and determines whether the former is smaller than the latter (Act  309 ). Act  309  is a printing ratio comparing act. 
     If the average printing ratio of the rear side in the overlapping portion is smaller than the average printing ratio of the front side in the overlapping portion (Yes in Act  309 ), the CPU  2011  corrects (reduces) the density of the image data of the rear side (Act  310 ). After correcting the image data of the rear side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  311 ). If the average printing ratio of the front side in the overlapping portion is smaller than the average printing ratio of the rear side in the overlapping portion (No in Act  309 ), the CPU  2011  corrects (reduces) the density of the image data of the front side (Act  312 ). After correcting the image data of the front side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  311 ). If the overlapping portion is not equal to or larger than the predetermined area (No in Act  307 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  311 ). 
     Thereafter, the CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  313 ). If all the pieces of recording information are analyzed (Yes in Act  313 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  313 ), the CPU  2011  returns to Act  305  and continues the processing. Thereafter, the CPU  2011  outputs the recording data stored in the memory  2012  to the image-forming-unit controller  205  in the same manner as in the first embodiment. 
     In Acts  310  and  312 , the CPU  2011  corrects the density of the image data of the side having the smaller average printing ratio in the overlapping portion to be reduced. However, the CPU  2011  may correct the density of the image data of the side having the larger average printing ratio to be reduced. In this case, the ink jet recording apparatus  1  can save inks necessary for forming images. In Act  309 , the CPU  2011  compares the densities of the image data on the basis of the average printing ratios. However, the CPU  2011  may compare the densities of the image data on the basis of maximum densities, ink amounts, or whether printing is monochrome printing or color printing. 
       FIG. 7  is a flowchart for explaining processing for creating recording data according to a fourth embodiment. The fourth embodiment is an example in which print density is corrected according to position information of image data printed on the front side of a sheet and image data printed on the rear side of the sheet and whether the image data is character data. First, the CPU  2011  determines whether an instruction for the duplex printing is included in recording information requested to be printed (Act  401 ). If the duplex printing is not instructed (No in Act  401 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the simplex printing input in the host computer  5  (Act  402 ). The CPU  2011  converts image data of one side acquired from the recording information into recording data and temporarily stores the recording data in the memory  2012  (Act  403 ). The CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  404 ). If all the pieces of recording information are analyzed (Yes in Act  404 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  404 ), the CPU  2011  returns to Act  402  and continues the processing. 
     If the duplex printing is instructed (Yes in Act  401 ), the CPU  2011  receives and analyzes recording information and attribute information concerning the duplex printing input in the host computer  5  (Act  405 ). The CPU  2011  acquires position information of image data of the front side and the rear side of a sheet from the attribute information of the respective sides (Act  406 ). The CPU  2011  determines whether, when images are formed on the front side and the rear side of the sheet in the ink jet recording apparatus  1 , an overlapping portion of the front and the rear is equal to or larger than a predetermined area (Act  407 ). If the overlapping portion is equal to or larger than the predetermined area (Yes in Act  407 ), the CPU  2011  determines whether both image data in the overlapping portion of the front side and the rear side are character data (Act  408 ). If both the image data in the overlapping portion are not character data (No in Act  408 ), the CPU  2011  compares, concerning image data size in the overlapping portion, image data size of the rear side and image data size of the front side and determines whether the former is smaller than the latter (Act  409 ). If the image data of the rear side is smaller than the image data of the front side (Yes in Act  409 ), the CPU  2011  corrects (reduces) the density of the image data of the rear side (Act  410 ). After correcting the image data of the rear side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  411 ). 
     If the image data of the front side is smaller than the image data of the rear side (No in Act  409 ), the CPU  2011  corrects (reduces) the density of the image data of the front side (Act  412 ). After correcting the image data of the front side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  415 ). If both the image data in the overlapping portion are character data (Yes in Act  408 ), the CPU  2011  compares thicknesses and sizes of characters in the character data in the overlapping portion of the front side and the rear side (Act  413 ). The CPU  2011  corrects (reduces) the density of the image data of the side having the larger thickness and size of the characters (Act  414 ). On the other hand, the CPU  2011  does not correct the density of the image data of the side having the smaller thickness and size of the character (No in Act  413 ). After correcting the image data of one side, the CPU  2011  converts the image data into recording data and temporarily stores the recording data in the memory  2012  (Act  415 ). If the overlapping portion is not equal to or larger than the predetermined area (No in Act  407 ), the CPU  2011  converts the image data of both the sides into recording data without correcting the image data and temporarily stores the recording data in the memory  2012  (Act  415 ). 
     Thereafter, the CPU  2011  determines whether all pieces of recording information in one print request are analyzed (Act  415 ). If all the pieces of recording information are analyzed (Yes in Act  415 ), the CPU  2011  ends the conversion into recording data. If not all the pieces of recording information are analyzed (No in Act  415 ), the CPU  2011  returns to Act  405  and continues the processing. Thereafter, the CPU  2011  outputs the recording data stored in the memory  2012  to the image-forming-unit controller  205  in the same manner as in the first embodiment. 
     In Acts  410  and  412 , the CPU  2011  corrects the density of the image data of the side having the smaller image data size in the overlapping portion to be reduced. However, the CPU  2011  may correct the density of the image data of the side having the larger image data size to be reduced. In this case, the ink jet recording apparatus  1  can save inks necessary for forming images. In Act  413 , the CPU  2011  compares the thicknesses and the sizes of the characters of the front side and the rear side. However, the CPU  2011  may compare densities or printing ratios of color characters of the front side and the rear side. As a result of comparing the front side and the rear side, first, the CPU  2011  may print the side on which the density is corrected to be reduced. 
     In the embodiments, if the overlapping portion in the duplex printing is equal to or larger than the predetermined area, the density of the image data of one of the surfaces is selectively reduced. However, the density of the image data of both the sides may be reduced to be lower than that during the simplex printing and the density of the image data of one of the sides may be further selectively reduced. In the embodiments, if the overlapping portion in the duplex printing is equal to or larger than the predetermined area, the density of the image data of both the sides may be reduced. In the embodiments, even if conditions (image data sizes, average printing ratios, etc.) are the same on the front side and the rear side in the overlapping portion in the duplex printing, the image data of arbitrary at least one side only has to be corrected. 
     The processing for creating recording data shown in  FIGS. 3 ,  5 ,  6 , and  7  in the embodiments may be performed by a CPU of the host computer  5 . In this case, the host computer  5  is equivalent to an image processing apparatus. According to the embodiments, it is possible to prevent image quality deterioration (e.g., a blur of an image due to show-through) in the duplex printing by the ink jet recording apparatus  1 . 
     In the disclosed embodiments, the processing can be accomplished by a computer-executable program, and this program can be realized in a computer-readable memory device. In the embodiments, the memory device, such as a magnetic disk, a flexible disk, a hard disk, an optical disk (CD-ROM, CD-R, DVD, and so on), an optical magnetic disk (MD and so on) can be used to store instructions for causing a processor or a computer to perform the processes described above. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.