Abstract:
An image forming system is provided. The image forming system includes an image forming apparatus configured to form a mark for detecting at least one of a position deviation and a density deviation and obtains a correction amount by detecting the mark, the image forming system comprising: an identification unit configured to identify an image characteristic of an image to be printed according to at least one of a positional relationship between colors, a gradation distribution and a resolution, and a selecting unit configured to select a choice from multiple choices containing at least two choices of: only position deviation; only density deviation; and both position deviation and density deviation, according to the image characteristic identified by the identification unit, wherein the image forming apparatus is configured to form the mark for obtaining the correction amount according to the choice selected by the selecting unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from Japanese Patent Application No. 2010-016721 filed on Jan. 28, 2010, the entire contents of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    Aspects of the present invention relates to an image forming system that comprises an image forming apparatus that forms a mark used for image adjustment and an information processing apparatus that sends print data to the image forming apparatus. 
       BACKGROUND 
       [0003]    A related-art image forming apparatus performs image adjustment to prevent producing position and density deviation of an image. As an example of a procedure of the image adjustment, the image forming apparatus forms a resist pattern that is a mark used for the position adjustment for each color, obtains a deviation amount (correction amount) between the resist pattern of a basic color and the resist pattern of a test color, and corrects the position deviation of the image of the test color according to the correction amount. In addition, for example, the image forming apparatus forms a density pattern which is a mark used for the density adjustment, obtains a deviation amount (correction amount) between the density of the density pattern and a target density, and corrects the density deviation of the test color according to the correction amount. 
         [0004]    However, it is well known that it takes time to obtain the correction amount used for the image adjustment, causing low printing efficiency. Furthermore, if the marks used for the image adjustment are frequently created, consumables (such as toner) will be wasted. In view of this problem, related-art discloses a technology in which whether the printing is monochrome printing or not is determined, and when it is determined that the printing is monochrome printing, the marks used for the image adjustment are not formed. 
         [0005]    However, the related-art image forming apparatus has the following problems. That is, there are times when necessity to perform the image adjustment is low, even if the printing is color printing. For example, for text image data in which there are some red characters (not mixed with another color) among black characters, even if the image is a color image, the necessity to correct the position deviation and the density deviation is low. However, the related-art image forming apparatus forms the marks used for the image adjustment as long as the printing is color printing. 
       SUMMARY 
       [0006]    Accordingly, it is an aspect of the present invention is to provide an image forming system and an image forming apparatus, which improve printing efficiency and prevent the waste of consumables. 
         [0007]    According to an exemplary embodiment of the present invention, there is provided an image forming system including an image forming apparatus configured to form a mark for detecting at least one of a position deviation and a density deviation and obtains a correction amount by detecting the mark, the image forming system comprising: an identification unit configured to identify an image characteristic of an image to be printed according to at least one of a positional relationship between colors, a gradation distribution and a resolution, and a selecting unit configured to select a choice from multiple choices containing at least two choices of: only position deviation; only density deviation; and both position deviation and density deviation, according to the image characteristic identified by the identification unit, wherein the image forming apparatus is configured to form the mark for obtaining the correction amount according to the choice selected by the selecting unit. 
         [0008]    According to another exemplary embodiment of the present invention, there is provided an image forming apparatus comprising: an obtaining unit configured to form a mark for detecting at least one of a position deviation and a density deviation and obtains a correction amount by detecting the mark, an identification unit configured to perform identification of an image characteristic of an image to be printed according to at least one of a positional relationship between colors, a gradation distribution and a resolution, and a selecting unit configured to select a choice from multiple choices containing at least two choices of: only position deviation; only density deviation; and both position deviation and density deviation, according to the image characteristic identified by the identification unit, wherein the obtaining unit is configured to form the mark for obtaining the correction amount according to the choice selected by the selecting unit. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a block diagram showing a configuration of an image forming system of an exemplary embodiment of the present invention; 
           [0010]      FIG. 2  is a schematic drawing showing a configuration of an image forming section of a MFP; 
           [0011]      FIG. 3  is a conceptual drawing showing a configuration of a mark sensor of the MFP shown in  FIG. 2 ; 
           [0012]      FIG. 4  is a flowchart showing a procedure of correction type determination process of a PC; and 
           [0013]      FIG. 5  is a flowchart showing a procedure of print execution process of the MFP. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Hereinafter, an exemplary embodiment in which an image forming apparatus and an image forming system of the present invention is implemented will be described in detail with reference to the drawings. The exemplary embodiment describes an image forming system, which includes a multi function peripheral (MFP) with a color printing function and a personal computer (PC) sending printing jobs to the MFP, as an example of the image forming system of the present invention. 
         [0015]    Overall Configuration of the Image Forming System 
         [0016]    As shown in  FIG. 1 , an image forming system  500  of the exemplary embodiment includes a MFP  100  (an example of the image forming apparatus) equipped with a color printing function and a PC  200  connected to the MFP  100  through network. In the image forming system  500 , a print job is sent from the PC  200  and printing according to the print job is performed in the MFP  100 . In the image forming system  500 , there can be several image forming apparatuses (the image forming apparatus is not limited to the MFP, and can be a printer, a copier, etc.) and PCs. Further, the image forming system  500  may also contain other information processing apparatuses (such as servers and scanners). 
         [0017]    The Configuration of the MFP 
         [0018]    Hereinafter, the schematic configuration of the MFP  100  will be described. The MFP  100  includes a control section  30 . The control section  30  includes a Central Processing Unit (CPU)  31 , a Read-Only Memory (ROM)  32 , a Random Access Memory (RAM)  33 , a nonvolatile RAM (NVRAM)  34 , an Application Specific Integrated Circuit (ASIC)  35 , a network interface  36  and a FAX interface  37 . Further, the control section  30  is electrically connected to an image forming section  10  that forms images on sheets, an image reading section  20  that reads an image of a copy, and an operation panel  40  that displays operation status and receives operations input by a user. 
         [0019]    Various control programs, settings, initial values, etc., are stored in the ROM  32  to control the MFP  100 . The RAM  33  is used as a work area from which various control programs may be read, or is used as a storage area where image data is temporarily stored. 
         [0020]    The CPU  31  stores the processing results in the RAM  33  or the NVRAM  34  according to the control programs read from the ROM  32  and signals sent from various sensors, and controls various elements of the MFP  100  via the ASIC  35 . 
         [0021]    The network interface  36  is connected to a network, to enable connection to the PC  200 . The FAX interface  37  is connected to a telephone line, to enable connection to a FAX apparatus. Data transmission between an external apparatus can be carried out through the network interface  36  and the FAX interface  37 . 
         [0022]    Configuration of the PC 
         [0023]    Hereinafter, the schematic configuration of the PC  200  will be described. The PC  200  includes a CPU  51 , a ROM  52 , a RAM  53 , a Hard Disc Drive (HDD)  54 , an operating section  55  including a keyboard and a mouse, a display section  56  including a liquid crystal display, and a network interface  58 . 
         [0024]    Further, the HDD  54  of the PC  200  includes an operating system (OS), a device driver controlling various devices, an application program including a print instruction function, etc. 
         [0025]    Configuration of the Image Forming Section of the MFP 
         [0026]    Hereinafter, the configuration of the image forming section  10  of the MFP  100  will be described with reference to  FIG. 2 . The image forming section  10  forms a toner image by electronic photography. The image forming section  10  includes a processing section  50  that transfers the toner image on the sheets, a fixing apparatus  8  that fixes the unfixed toner on the sheets, a paper feeding tray  91  that carries the sheets before the image is transferred, and a paper discharging tray  92  that carries the sheets after the image is transferred. The image reading section  20  is provided upper than the image forming section  10 . 
         [0027]    In the image forming section  10 , an S-shaped transport path  11  (the dashed line in  FIG. 2 ), via which the sheets placed in the paper feeding tray  91  that is located at a bottom portion of the image forming section  10  are conveyed to the paper discharging tray  92  located at a top portion of the image forming section  10 , is formed. The transport path includes a paper feeding roller  71 , a registration roller  72 , the processing section  50 , the fixing apparatus  8 , and a paper discharging roller  76 . 
         [0028]    The processing section  50  is capable of forming a color image, and includes processing sections corresponding to yellow (Y), magenta (M), cyan (C) and black (K) which are arranged respectively in parallel. Specifically, the processing section  50  includes a processing section  50 Y forming a yellow image, a processing section  50  M forming a magenta image, a processing section  50 C forming a cyan image and a processing section  50 K forming a black image. The processing sections  50 Y,  50 M,  50 C and  50 K are arranged at a certain distance from each other along the conveying direction of the sheets. 
         [0029]    In the processing section  50 , the surface of a photoconductor is uniformly charged by a charging apparatus. Then, the surface of the photoconductor is exposed to the light from an exposure apparatus  59 , and an electrostatic latent image of the image to be formed on the sheets is formed. Next, toner is supplied to the photoconductor through a developing apparatus. Thereby, the electrostatic latent image on the photoconductor is visualized as toner image. 
         [0030]    The image forming section  10  includes an exposure apparatus  59  that irradiates light to each processing section  50 Y,  50 M,  50 C and  50 K, a conveyor belt  7  that conveys the sheets to the transfer position of each processing section  50 Y,  50 M,  50 C and  50 K, and a mark sensor  61  that detects the resist patterns formed on the conveyor belt  7 . 
         [0031]    The conveyor belt  7  is an endless belt member that is stretched by conveyor rollers  73  and  74 . The conveyor belt  7  is made of resin, such as polycarbonate. The conveyor belt  7  moves circularly in a counterclockwise direction in  FIG. 2  by rotationally driving the conveyor roller  74 . The sheets placed on the belt  7  are thereby conveyed from the registration roller  72  to the fixing apparatus  8 . 
         [0032]    The image forming section  10  removes the sheets placed on the paper feeding tray  91  one by one, and conveys the sheet to the conveyor belt  7 . The toner image formed on the processing section  50  is transferred to the sheet on the belt conveyor  7 . In this case, when the printing is color printing, the toner image is formed on each processing section  50 Y,  50 M,  50 C, and  50 K. Each toner image is overlapped on the sheet. On the other hand, when the printing is monochrome printing, the toner image is only formed on the processing section  50 K, and is transferred to the sheet. After that, the sheet to which the toner image is transferred is conveyed to the fixing apparatus  8 , and the toner image is thermally fixed on the sheet. Then, the fixed sheet is discharged from the paper discharging tray  92 . 
         [0033]    The mark sensor  61  is located downstream from the processing section  50 Y,  50 M,  50 C and  50 K, and upstream from the fixing apparatus  8 . The mark sensor  61  detects the pattern used for the image adjustment, which is formed on the conveyor belt  7 . 
         [0034]    Specifically, as shown in  FIG. 3 , the mark sensor  61  is composed of two sensors, a sensor  61 R located on the right side of the width direction of the conveyor belt  7 , and a sensor  61 L located on the left side of the width direction of the conveyor belt  7 . Each sensor  61 R and  61 L is a reflective optical sensor which is a pair of a light emitting apparatus  62  (such as LED) and a light receiving apparatus  63  (such as phototransistor). In the mark sensor  61 , the light emitting apparatus  62  irradiates the light from the diagonal direction with respect to the surface of the conveyor belt  7  (dotted frame E in  FIG. 3 ), and the light receiving apparatus  63  receives the light. A mark used for the image adjustment can be detected according to the difference between the amount of light received when the mark used for the image adjustment  66  passes through and the amount of light directly received from the conveyor belt  7 . 
         [0035]    Image Adjustment of the MFP 
         [0036]    Hereinafter, the image adjustment of the MFP  100  will be described. As the image adjustment, in the MFP  100 , the position deviation correction to adjust the image position of each color and the density deviation correction to adjust the density of each color are performed. 
         [0037]    Hereinafter, the image adjustment will be briefly described by taking the position deviation correction for example. The position deviation correction can be divided into an obtaining process for obtaining the deviation amount (correction value) from a standard color, for each color, and a correction process for correcting the image according to the correction value. The obtaining process is, for example, executed according to an instruction from the print execution process described later. The correction process is, for example, executed each time when printing is performed. 
         [0038]    In the obtaining process, first, a resist pattern which is an image pattern used for the position deviation correction is formed by each processing section  50 Y,  50 M,  50 C and  50 K. Specifically, as shown in  FIG. 3 , the resist pattern includes a mark group. The mark group includes a mark  66 K formed by the processing section  50 K, a mark  66 C formed by the processing section  50 C, a mark  66 M formed by the processing section  50 M, and a mark  66 K formed by the processing section  50 K, which are arranged side by side in a sub-scanning direction. The mark group is hereinafter referred to as resist pattern  66 ″. 
         [0039]    The resist pattern  66  is formed at regular intervals along the sub-scanning direction (moving direction of the conveyor belt  7  shown in  FIG. 3 ). Each mark  66 K,  66 C,  66 M and  66 Y of the exemplary embodiment is a square bar shape, and is provided in a main scanning direction (the direction perpendicular to the sub-scanning direction). 
         [0040]    Position of each mark  66 K,  66 Y,  66 M and  66 C is detected based on binary signals output from the mark sensor  61 . The interval between each mark of the adjusting colors (such as marks  66 C,  66 M and  66 Y) and the mark of the standard color (such as mark  66 K) in the sub-scanning direction can thus be calculated. The interval between the mark of the standard color and the mark of the adjusting color changes according to the position deviation in the sub-scanning direction. Therefore, the deviation amount between the adjusting color and the standard color in the sub-scanning direction can be specified. 
         [0041]    In the correction process, the process conditions (for example, exposure position, and speed of the conveyor belt  7  and the photoconductor) of the adjusting color is adjusted according to the deviation amount obtained in the obtaining process, so that the image position of the adjusting color and the image position of the standard color match with each other. 
         [0042]    The above-described configuration of the resist pattern  66  is merely an example, and the present invention is not limited by the configuration. A common image pattern that is used in a position deviation correction can also be used as the resist pattern  66 . For example, an image pattern including a pair of bar-shaped marks, at least one of which inclines in a predetermined angle with respect to the line along the main scanning direction, can be used as the resist pattern. By using the above-described image pattern as the resist pattern, the position deviation amount of both the sub-scanning direction and the main scanning direction can be specified. 
         [0043]    The density deviation correction is similar to the position deviation correction. The density deviation correction is divided into an obtaining process that obtains a correction amount relative to a predetermined density, and a correction process that corrects the image according to the correction value. In the obtaining process, a density pattern which is an image pattern with density difference in the sub-scanning direction is formed by each processing section  50 Y,  50 M,  50 C and  50 K. The amount of light reflected from the density pattern is detected by a sensor. The sensor may be the sensor used for the position deviation correction or it may be another sensor. For example, in the exemplary embodiment, the amount of light reflected from the density pattern is detected by the sensor  61 L. The density is specified from the amount of the reflected light, and the difference between the density and the target density is calculated as the correction value. In the correction process, the processing conditions of each color (such as exposure intensity, exposure range, and developing bias) are adjusted according to the correction value, so as to maintain the target density. 
         [0044]    Management Operation of the Image Adjustment of the Image Forming System 
         [0045]    Hereinafter, the procedure of the image adjustment in the image forming system  500  will be described. In the image forming system  500 , the correction type which needs image adjustment is determined, and then, the obtaining process for each correction is performed according to the determination. Hereinafter, the process to execute the above operations will be described by dividing the process into a process by the PC  200  and a process by the MFP  100 . 
         [0046]    Correction Type Determination Process 
         [0047]    First, as a process executed by the PC  200 , the procedure of the correction type determination process (an example of identification unit and selection unit) that determines the correction type which needs the image adjustment will be described with reference to the flowchart shown in  FIG. 4 . The correction type determination process is performed by a printer driver included in the PC  200 . The correction type determination process is executed when the print instruction is received by the printer driver. 
         [0048]    First, various flags storing the correction types which need the image adjustment is turned off (S 100 ). In the exemplary embodiment, the correction type includes the position deviation correction and the density deviation correction, and a flag is provided for each correction type. Furthermore, a flag is provided between each colors (between cyan and magenta, between cyan and yellow, between cyan and black, between magenta and yellow, between magenta and black, and between yellow and black) in the position deviation correction, and a flag is provided for each color (cyan, magenta, yellow, black) in the density deviation correction. The initial state of each flag is off. When the correction is determined to be necessary according to image analysis results which will be described later, the initial state “off” changes to “on”. 
         [0049]    Next, the image characteristic of the image data to be printed is analyzed for each print page (S 101 ). In S 101 , at least the overlap ratio between colors, the adjacency ratio between colors and the gradation ratio are required. Each ratio will be described later in detail. 
         [0050]    Next, according to the analysis results in S 101 , whether the position deviation correction and the density deviation correction are necessary is determined (S 102 , S 103 , S 104  and S 105 ). In the exemplary embodiment, the following four items of the image properties are determined. 
         [0051]    First, whether the ratio of an area having color overlap is larger than a threshold value A is determined (S 102 ). The area having the color overlap means an area including a color mixture of at least two colors. Whether the ratio of the area having such color overlap in a single page is large or not is determined. The determination may also be performed based on the number of the pixels of the mixed area. The area having the color overlap may be limited to areas including a color mixture of three colors or more. The ratio may be a ratio of a color mixture that can be easily identified by human eyes, which is previously set. 
         [0052]    When the ratio of the area having the color overlap is larger than the threshold value A (S 102  is YES), the flag used for the position deviation correction and the flag used for the density deviation correction are both turned on (S 121 ). This is because, when the color mixture formed by overlapping the colors is output, if the colors don&#39;t overlap at the exact position, the output image will become unnatural. Further, if the density is not the exact density, the desired color will not be output. Therefore, the high accuracy of both the position and the density is required. Meanwhile, because each flag is prepared so as to correspond to each color, the flags corresponding to the colors of the color mixture are turned on. For example, when the overlap area of cyan and magenta is determined to be large, the flag for the position deviation correction between cyan and magenta and the flags for the density deviation correction of cyan and magenta are turned on. 
         [0053]    Second, whether the ratio of a contact area between different colors is larger than a threshold value B is determined (S 103 ). The contact between different colors means, for example, when a cyan color portion and a magenta color portion contact with each other in a round graph. Whether the length of the border generated by the contact between colors is large or not is determined. 
         [0054]    When the ratio of the contact area between different colors is larger than the threshold value B (S 103  is YES), the flag for the position deviation correction is turned on (S 131 ). This is because, when different colors contact each other, if the contact is not at the exact position, the border of the colors becomes unclear. Thus, high accuracy in position is required. Meanwhile, even if the density accuracy is low, the difference in colors can be identified. Therefore, high accuracy in density is not required. Only the flag corresponding to the adjacent colors is turned on. For example, when the adjacency between cyan and magenta is determined to be high, the flag for the position deviation correction between cyan and magenta is turned on. 
         [0055]    Third, whether the ratio of an area in which the color density changes gradually (gradation) is larger than the threshold value C is determined (S 104 ). For example, the gradation can be determined according to a length of a continuous image pixel, wherein density variation (0-255) between adjacent image pixels included in the continuous image pixel is small. This is because, in the gradation area, because the density variation between the adjacent image pixels is small, and because such image pixels become a continuous state, the gradation area can be distinguished from a non-gradation area. The determination may also be performed based on whether the ratio of the gradation area in a single page is large or not. 
         [0056]    When the ratio of the gradation area is larger than the threshold value C (S 104  is YES), the flag for the density deviation correction is turned on (S 141 ). This is because, when expressing the gradation, slight density change can not be expressed without exact density. Thus, the high accuracy of the density is required. Only the flag corresponding to the color forming the gradation is turned on. For example, when there are many cyan gradations, the flag for cyan density deviation correction is turned on. 
         [0057]    Fourth, whether the resolution is larger than the threshold value D is determined (S 105 ). When the resolution is larger than the threshold value D (S 105  is YES), the flag for the position deviation correction is turned on (S 151 ). This is because, for example, in a vector image file of the postscript, etc., the higher the resolution is, the higher the accuracy of the position of the print object is required. If the image to be printed is not a vector image file or a vector image file but position is not specified, it is not necessary to perform the determination of S 105 . 
         [0058]    The correction types which need the image adjustment are determined by S 102 , S 103 , S 104  and S 105 . That is, when the flag for the position deviation correction and the flag for the density deviation correction are both on, the position deviation correction and the density deviation correction are both needed to be performed. When the flag for the position deviation correction is on while the flag for the density deviation correction is off, only the position deviation correction is needed to be performed. When the flag for the position deviation correction is off while the flag for the density deviation correction is on, only the density deviation correction is needed to be performed. Meanwhile, when the flag for the position deviation correction and the flag for the density deviation correction are both off, both the position deviation correction and the density deviation correction are not performed. 
         [0059]    The processing order of S 102 , S 103 , S 104 , and S 105  is not limited to the above-described processing order. That is, the processing order is not important. Further, not all of the four items need to be performed, but at least one needs to be performed. 
         [0060]    After the position deviation correction or the density deviation correction is determined, the information of the flag for the position deviation correction and the flag for the density deviation correction are added to the print data (S 106 ). Next, the print data with the two flag information is sent to the MFP  100  (S 107 ). After S 107 , the process is terminated. 
         [0061]    Accordingly, the PC  200  determines whether the position deviation correction and the density deviation correction are necessary according to the image to be printed. Specifically, in the exemplary embodiment, the image characteristic of the image to be printed is identified, and whether both the position deviation correction and the density deviation correction are necessary, whether only the position deviation correction is necessary, whether only the density deviation correction is necessary, and whether neither the position deviation correction nor the density deviation correction is necessary, is determined. Then, the adjustment results are sent to the MFP  100 . 
         [0062]    Print Execution Process 
         [0063]    Hereinafter, as a process executed by the MFP  100 , the procedure of the print execution process (an example of the determination unit) that determines the execution of the obtaining process used for the image adjustment will be described with reference to the flowchart shown in  FIG. 5 . The print execution process is executed each time the print data is received. 
         [0064]    First, the flag information added to the print data is confirmed (S 201 ). Next, the on-off state of each flag is determined according to the flag information (S 202 , S 203 ). 
         [0065]    Specifically, whether the flag for the position deviation correction is on is determined (S 202 ). In S 202 , if at least one of the flags for the position deviation correction between each group of colors is on, the on state is determined. When the flag for the position deviation correction is on (S 202  is YES), whether a predetermined time has elapsed since the previous obtaining process for the position deviation correction was executed is determined (S 221 ). If the predetermined time has elapsed (S 221  is YES), the obtaining process is executed for each group of colors that the corresponding flags for the position deviation correction are on (S 222 ). Meanwhile, if the predetermined time has not elapsed (S 221  is NO), the obtaining process for the position deviation correction is not executed. This is because, when only a short time has elapsed since the previous obtaining process was executed, it is unlikely that the image quality changes significantly. Further, in the obtaining process, toner for is consumed for purpose other than printing, leading to low productivity. Therefore, when the predetermined time has not elapsed since the previous obtaining process for the position deviation correction was executed, the obtaining process is not executed due to the priority in effective use of toner and productivity. 
         [0066]    Next, whether the flag for the density deviation correction is on is determined (S 203 ). In S 203 , if at least one of the flags for the density deviation correction of each color is on, the on state is determined. When the flag for the density deviation correction is on (S 203  is YES), whether a predetermined time has elapsed since the obtaining process for the density deviation correction was executed is determined (S 231 ). If the predetermined time has elapsed (S 231  is YES), the obtaining process is executed for each color that the corresponding flags for the density deviation correction are on (S 232 ). Meanwhile, if the predetermined time has not elapsed (S 231  is NO), the obtaining process for the density deviation correction is not executed. 
         [0067]    The processing order of S 202  and S 203  is not limited to the above-described processing order. That is, the processing order may be reversed. Further, when the obtaining process is being executed in S 222  or S 232 , the next process will not be performed. In the exemplary embodiment, flag determination and image deterioration determination are executed in this order. However, this order can be reversed. After the flag determination, the print data is printed (S 204 ). After S 204 , the process is terminated. 
         [0068]    According to the above-described correction type determination process and the print execution process, even if the image to be printed is a color image, the obtaining process for each correction is not performed at times. For example, if a text image in which there are some red characters among black characters is the image to be printed, it is considered that the color overlap is low. In this case, the obtaining process of the position deviation correction and the obtaining process of the density deviation correction can be avoided. Further, for example, in an image only including characters and charts, the gradation ratio may be small. In this case, the obtaining process of the density deviation correction can be avoided. 
         [0069]    As described above in detail, the image forming system  500  identifies the image characteristic of the image to be printed, and forms necessary marks according to the image characteristic. Specifically, according to the positional relationship of the colors, gradation distribution and resolution of the image to be printed, the image forming system of the invention identifies whether high accuracy in image position is required, or whether high accuracy in density is required, and forms the marks corresponding to the required accuracy. Therefore, it is possible to avoid forming marks whose necessity is low. Thus, the reduction in productivity, due to forming marks whose necessity to perform the image adjustment is low, can be expected to be avoided. 
         [0070]    While the present invention has been shown and described with reference to the exemplary embodiment, the present invention is not limited to the exemplary embodiment. Various improvements and variations can be made without departing from the spirit and scope of the invention. For example, the image forming apparatus is not limited to the MFP, but can also be applied to apparatuses with print functions such as a printer and a copier. 
         [0071]    In addition, although an MFP with a color printing function is provided in the exemplary embodiment, the image forming apparatus of the present invention is not limited to a color printing apparatus. For example, the image forming apparatus can be applied to a monochrome printing apparatus, if the monochrome printing apparatus can form marks to perform the position deviation correction and the density deviation correction. 
         [0072]    In addition, although an image forming apparatus forming marks on the paper transport belt is provided in the exemplary embodiment, the present invention is not limited to this. For example, even if the image forming apparatus is an image forming apparatus with an intermediate transfer belt, the present invention can be applied when forming marks on the intermediate transfer belt. 
         [0073]    In addition, in the exemplary embodiment, although the PC  200  performs the image analysis and determines whether the position deviation correction and the density deviation correction are performed according to the results, the image analysis and the correction type determination can also be performed by the MFP  100 . That is, the correction type determination process of  FIG. 4  can be performed by the MFP  100 . In this case, in S 107 , print data with the flag information is sent to the image forming section  10 . Further, when the PC  200  sends the print data to the MFP  100  through a server, the correction type determination process of  FIG. 4  can be performed by the server. 
         [0074]    In addition, in the exemplary embodiment, although the image analysis is performed for each print page, the image analysis can be performed for each print job (multiple pages). 
         [0075]    In addition, in the exemplary embodiment, although the flags are prepared for each group of colors each color, the present invention is not limited to this. For example, the flag can be prepared for each correction type. In this case, if the position deviation correction between different colors is determined to be necessary for at least one of the group of colors, the obtaining processes for the position deviation correction is performed for all of the groups of colors. Further, if the density deviation corrections of the colors is determined to be necessary for at least one color, the obtaining processes for the density deviation correction is performed for all of the colors. 
         [0076]    In addition, in the exemplary embodiment, although the image deterioration is determined just before the obtaining process is executed (S 221 , S 231 ), the present invention is not limited to this. For example, the image deterioration can be determined before the flag information is confirmed (S 201 ). Further, if the PC  200  can obtain the image quality of the MFP  100  (for example, obtain the printed number), the image quality deterioration may be determined by the PC  200 . In the case, the image characteristic may be analyzed only when the image quality deterioration is determined. 
         [0077]    In addition, in S 221  and S 231 , although the elapsed time since the previous obtaining process was executed is used as a condition to determine the image quality deterioration, the present invention is not limited to this. For example, the print number and the used amount of toner since the previous obtaining process was executed can be used as the condition. Environmental conditions such as temperature change and moisture change can also be used. 
         [0078]    In addition, in the exemplary embodiment, although the MFP  100  receives the flag information and determines whether the obtaining process of each correction is to be executed according to the flag information, the invention is not limited to this. For example, the PC  200  may add various ratios, which are used in the determination in the correction type determining processes S 102 , S 103 , S 104  and S 105 , to the print data, and then the MFP  100  may determine executing the obtaining process of each image adjustment according to the ratios. 
         [0079]    In addition, in the exemplary embodiment, although whether to execute the obtaining process for each correction is determined by the state of the flag, a necessary factor for each correction, which is a required degree of the correction according to the image analysis results, may be prepared (for example, the initial value is zero, and the more necessary the correction according to the image analysis results, the larger the value. Or, the initial value is maximum, and the more necessary the correction is according to the image analysis results, the less the value). The execution of the obtaining process may be determined by comparing the factor with a threshold value. Further, the threshold value can be changed depending on the image quality deterioration. 
         [0080]    In addition, in the exemplary embodiment, although the density deviation correction is determined by the gradation ratio, it can also be determined by frequency distribution of gradation used in the image. For example, among the gradations expressed by 0 to 255, when the used gradation number is larger than a threshold value, the flag of the density deviation correction may be turned on.