Patent Publication Number: US-8526083-B2

Title: Image processing apparatus, method, and computer program product specifying a color difference between converted image data defined by a color space depending on different environmental lighting

Description:
This application is a divisional of U.S. patent application Ser. No. 12/641,922, filed Dec. 18, 2009, the contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image processing apparatus for processing an image depending on observation environment light and relates to an image processing method. 
     As described in Japanese Laid-Open Patent Application (JP-A) 2007-166402, it has been conventionally known the image processing apparatus for evaluating a color reproduction characteristic of an image output apparatus such as a printer on the basis of illumination information designated on a user interface (UI) by a user, as an example of an image data conversion in which the observation environment light was taken into account. 
     In the image processing apparatus, spectral data is obtained from an output image. Further, the image processing apparatus calculates the color reproduction characteristic of the image output apparatus under an observation environment, on the basis of the spectral data and the illumination information designated on the UI by the user. Then, the user determines, from the calculated colorimetric value, the color reproduction characteristic so as to satisfy a desired condition designated by the user under a plurality of pieces of the illumination information designated by the user on the UI. 
     JP-A 2006-254106 discloses a system for displaying a simulation image. In this system, as an example of an image processing user interface, when a print original is set on a multi-function machine, the multi-function machine reads the print original and obtains image data for simulation in the form of reduced-resolution printing image data. The thus-obtained data is transferred to a client PC. 
     The client PC displays a simulation image and at the same time, discriminates the presence or absence of an improper reproduction area in which a color difference between the simulation image and the original image in the printing by the multi-function machine is a threshold or more, on the basis of the data of the simulation image. 
     When the improper reproduction area is present, the improper reproduction area is clearly shown on the simulation image. Further, when change setting for a printing condition is designated by the user, the simulation image is displayed again depending on the printing condition subjected to the change setting. 
     When confirmation of the printing condition is made, the confirmed printing condition is sent to the multi-function machine and under the printing condition, printing on a recording sheet is effected. 
     However, generally, a print has a light source dependency by a type or composition of ink and a color appearing manner is changed depending on the type of illumination light in the observation environment of the print. However, in the case where the observation environment light is changed, the user cannot know an image area in which the change in color is large. Therefore, the user cannot know in advance the printing condition (such as the type of the printer, the type of sheet, or a printing mode) under which a degree of the change in color appearing manner depending on the observation environment light is small. 
     SUMMARY OF THE INVENTION 
     The present invention has been accomplished in view of the above-described circumstances. 
     A principal object of the present invention is to provide an image processing apparatus for presenting to a user an image area in which a degree of change in appearing color is large in the case where a light source with which an output image is observed is changed. 
     Another object of the present invention is to provide an image processing method of supplying the image data to be formed by the image forming apparatus to the image forming apparatus. 
     According to an aspect of the present invention, there is provided an image processing apparatus for supplying image data, to be formed by an image forming apparatus, to the image forming apparatus, the image processing apparatus comprising: 
     an obtaining unit for obtaining information on first environment light and second environment light different from the first environment light; 
     a converting unit for converting data, defined by a color space independent of the image forming apparatus, of the data into first image data defined by a color space dependent of the image forming apparatus depending on the first environment light and then for converting the first image data into second image data defined by a color space independent of the image forming apparatus depending on the second environment light; 
     a discriminating unit for discriminating whether or not a color difference between the first image data and the second image data is a predetermined value or more in an area in which the first image data and the second image data correspond to each other; and 
     a display controlling unit for controlling a display unit so that the image data is displayed on the display unit in an image area including an area in which the color difference is the predetermined value or more and including an area in which the color difference is less than the predetermined value. 
     According to another aspect of the present invention, there is provided an image processing method for supplying image data, to be formed by an image forming apparatus, to the image forming apparatus, the image processing method comprising: 
     a step of obtaining information on first environment light and second environment light different from the first environment light; 
     a step of converting data, defined by a color space independent of the image forming apparatus, of the image data into first image data defined by a color space dependent of the image forming apparatus depending on the first environment light and then for converting the first image data into second image data defined by a color space independent of the image forming apparatus depending on the second environment light; 
     a step of discriminating whether or not a color difference between the first image data and the second image data is a predetermined value or more in an area in which the first image data and the second image data correspond to each other; and 
     a step of displaying the image data is on a display unit in an image area including an area in which the color difference is the predetermined value or more and including an area in which the color difference is less than the predetermined value. 
     According to the present invention, it is possible to present to the user the image area in which the degree of change in appearing color is large with respect to a plurality of environment light sources with which the output image is observed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a hardware configuration of the image processing apparatus according to the present invention. 
         FIG. 2  is a schematic view showing a software configuration of the image processing apparatus. 
         FIG. 3  is a schematic view for illustrating a process performed by a printer driver. 
         FIG. 4  is a schematic view showing an example of a user interface in First Embodiment. 
         FIG. 5  is a block diagram showing an inner functional constitution of the image processing apparatus in First Embodiment. 
         FIG. 6  is a flow chart showing a procedure of a display process of a color difference area image in First Embodiment. 
         FIG. 7  is a schematic view showing a plurality of output characteristic tables stored in a database. 
         FIG. 8  is a schematic view showing an example of display in the case where a print color change intensity is weak. 
         FIG. 9  is a schematic view showing an example of display in the case where the print color change intensity is medium. 
         FIG. 10  is a schematic view showing an example of display in the case where the print color change intensity is strong. 
         FIG. 11  is a schematic view showing an example of a user interface in Second Embodiment. 
         FIG. 12  is a block diagram showing an inner functional constitution of the image processing apparatus in Second Embodiment. 
         FIG. 13  is a flow chart showing a procedure of a display process of a color difference area image in Second Embodiment. 
         FIG. 14  is a schematic view showing an example of a color difference area image displayed in Second Embodiment. 
         FIG. 15  is a schematic view showing an original image. 
         FIG. 16  is a schematic view showing an example of a user interface in Third Embodiment. 
         FIG. 17  is a flow chart showing a procedure of a display process of the color difference area image in Third Embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow, embodiments of the present invention will be specifically described with reference to the drawings. Incidentally, the same structural elements or members are represented by the same reference numerals or symbols, thus being omitted from redundant description. 
     In the image processing apparatus according to the present invention, first, image data (RGB data) defined in a standard RGB color space is input and then color correction is made in an appearance color space (e.g., uniform perceived color space CIELAB) to obtain image data (Lab data). 
     Next, the image processing apparatus converts the obtained image data (Lab data) which is independent of the device correspondingly to first environment light to be corrected (light to be corrected) to obtain device-dependent image data (which is image data converted into color area display which can be reproduced by the printer; hereinafter referred to as printer RGB data). The image data (printer RGB data) is converted on the appearance color space correspondingly to second environment light to obtain new image data (Lab data). The image processing apparatus displays, as an image area in which a degree of change in color is large, a color area in which a color difference between the thus-obtained new image data (Lab data as second image data) and original image data (Lab data as first image data) in the case where an observation environment light is changed from first environment light to second environment light. 
     As a result, the user can confirm the image area in which the color change due to the change in environment light is observed and a degree of color change intensity on a screen, thus capable of knowing that which color in which area is observed in a changed state in the resultant image in advance with respect to the observation environment light which is not an object to be corrected. As a result, the user can know in advance the type of printer, the type of sheet, the printing mode, and the like by which a degree of the change in color appearing manner with respect to the observation environment light is small. 
     An operation of the image processing apparatus according to the present invention will be described below. 
     First Embodiment 
       FIG. 1  is a schematic view showing a hardware configuration of the image processing apparatus according to the present invention. As the image processing apparatus  100 , information processing apparatus such as a personal computer is used. A CPU  101  is a central processing unit for executing an operating system, application programs, device drives, and the like to effect integrated control of respective portions in the image processing apparatus  100 . 
     A RAM  102  is random-access memory for providing a work area for the CPU  101 . A ROM  103  is a read-only memory for storing a boot program such as BIOS. An HDD  104  is a fixed storage such as a hard disk drive and stores not only computer programs such as the operating system and various driver programs but also image data such as a test pattern image data. 
     A communication interface (I/F) is an interface for performing data communication with an external device such as a device of USB, IEEE 1394, wired LAN, wireless LAN, or the like. A display interface (I.F)  106  controls a display device  109  connected externally or internally so that image information or the like is displayed on the display device  109 . 
     An operating portion  107  is an input device such as a pointing device or a key entry device. A printer  108  is an image forming apparatus of an ink jet type or a laser beam type in which the image is to be formed on media. The display device  109  displays the image data or the like and as the display device, a liquid crystal display or the like is used. 
       FIG. 2  is a schematic view showing a software configuration of the image processing apparatus  100 . With reference to  FIGS. 1 and 2 , flow during printout will be described. As the printer  108  for performing the printout, an ink jet print or the like which has been generally popularized can be used. During the printout of output image data, the printout is requested from an application  202  to an OS  201 . For example, in this case, drawing instructions are issued to the OS  201 . Of the drawing instructions, a portion constituted by a graphics drawing instruction corresponds to a graphics data portion of the output image and a portion constituted by an image drawing instruction corresponds to an image data portion of the output image. 
     The OS  201  receives the output request from the application and issued the drawing instructions to a printer driver  208  correspond to an output printer. The print driver  208  processes a print request and the drawing instructions which are input from the OS  201  and prepares print data capable of being printed by the printer  108  and then transfers the print data to the printer  108 . 
     For example, in the case where the printer  108  is a raster printer, the printer driver  208  successively executes image correction processing in response to the drawing instructions from the OS  201 . Then, the printer driver  208  successively rasterizes the drawing instructions in RGB 24-bit page memory. After all the drawing instructions are rasterized, the printer driver  208  converts the contents of the RGB 24-bit page memory into data format (e.g., CMYK data) for the image which can be printed by the printer  108  and transfers the data format to the printer  108 . 
       FIG. 3  is a schematic view for illustrating the processing performed by the printer driver  208 . An image correction processing portion  210  effects image correction processing with respect to color information contained in the drawing instructions input from the OS  201 . The image correction processing portion  210  converts, e.g., RGB color information into luminance/color difference signal and subjects the luminance signal to exposure correction processing and then inversely converts the corrected luminance/color difference signal into RGB color information. 
     A correcting processing portion  211  for the printer rasterizes the drawing instruction on the basis of the RGB color information which has been subjected to the image correction processing and forms a raster image in the RGB 24-bit page memory by the rasterization. Then, the correcting processing portion  211  effects color matching processing for performing color reproduction space mapping, color-separation processing into CMYK, gamma correction, half-tone correction, and the like. Thereafter, the correcting processing portion  211  forms CMYK data depending on color reproducibility of the printer with respect to each pixel to transfer printable image data to the printer  108 . 
     (User Interface) 
       FIG. 4  is a schematic view showing an example of a user interface in this embodiment. A user interface  400  displays a color difference area discrimination image on a display  401 . Further, the user interface  400  can also display a partly enlarged image. With respect to an original image shown in  FIG. 15 , display examples of the color difference area discrimination image are shown in  FIGS. 8 ,  9  and  10 , respectively. 
     The user can set the type of printer, the type of sheet, and the printing mode by using a display  402 , in which respective times are displayed in a pop-up menu system. The user can appropriately select and set the respective items. By each pop-up menu, a list of selected item is displayed by selecting an inverted triangle area, located at a right end of each rectangular area, with a pointer or the like. The user selects an objective item from the list of the selected item. In this embodiment, each pop-up menu functions as a receiving unit for receiving user&#39;s selection for an associated item. Incidentally, in this embodiment, all of the type of printer, the type of sheet, and the printing mode can be set but at least one of these pieces of information can also be set. 
     A display  403  is used in the case where light to be corrected and second environment light are set. Each item is, similarly as in the case of the display  402 , displayed as a list of selected item in the pop-up menu system. The user can select the item from the list and sets the selected item. As a type of environment light to be set, there are normal (standard) light (D50), high color rendering fluorescent lamp, three-waveform fluorescent lamp, and normal fluorescent lamp. 
     The user can adjust a degree of color difference discrimination tolerance by a slide bar  505  as a second receiving unit, so that an area with the degree of color difference discrimination tolerance designated by the user through the slide bar  404  is displayed on the display  401 . For example, as shown in  FIG. 4 , in the case where a “medium” value of print color change intensity is set as an initial value, a color difference area of a medium degree is displayed on the display  401  ( FIG. 9 ). Further, in the case where a “strong” value is set, the color difference area of a higher degree is displayed on the display  401  ( FIG. 8 ). Further, in the case where a “weak” value is set, the color difference area of a lower degree is displayed on the display  401  ( FIG. 10 ). When a button  405  is pushed down, the display of the user interface is completed. 
     Thus, in this embodiment, the user can visually confirm the area, in which the change in color depending on the change in environment light is observed, depending on the degree of the color change intensity. 
     (Flow of Processing) 
     The flow of the processing in this embodiment will be described based on  FIG. 6  with reference to  FIG. 5 .  FIG. 5  is a block diagram showing an inner functional constitution of the image processing apparatus  100 .  FIG. 6  is a flow chart showing a procedure of the display processing of the color difference area image in this embodiment. Each of steps in the flow chart is performed under control of the CPU  101  of the image processing apparatus  100 . 
     First, in a step S 401 , a printing condition input portion  309  of an output characteristic obtaining portion  305  obtains the information on the type of printer, the type of sheet, and the type of printing mode selected by the user through an operating portion  107  and then obtains an output characteristic table (described later) corresponding to the information from a database  311 . 
     In a step S 402 , an environment light type input portion  308  of the output characteristic obtaining portion  306  obtains the information on the type of first environment light selected by the user through the operating portion  107 . Further, from the output characteristic table obtained in the step S 401 , a color correction table and a conversion table corresponding to the first environment light are obtained. Here, the color correction table is a table used for effecting the color conversion from the normal RGB space to L*a*b* space, and the conversion table corresponding to the first environment light is a table used for effecting the color conversion from the L*a*b* space to printer RGB space (color space represented by RGB capable of being reproduced by the printer). 
     In a step S 403 , the environment light type input portion  308  of the output characteristic obtaining portion  305  obtains information on the second environment light selected by the user through the operating portion  107 . Further, from the output characteristic table obtained in the step S 401 , a conversion table corresponding to the second environment light is obtained. Here, the conversion table corresponding to the second environment light is a table used for effecting the color conversion from the printer RGB space to the L*a*b* space. 
     In a step S 404 , image data  301  is read from an HDD  104 . In the HDD  104 , a desired image data of the user is stored as the RGB data in advance. This image data may be input through an external interface or taken in through an image reading device such as a scanner. 
     In a step S 405 , a color conversion portion  304  effects the color correction processing of the image data  301  by using the color correction table obtained in the step S 402  to perform the color conversion from the normal RGB space to the L*a*b* space, thus obtaining Lab data of appearance color space. Further, the color conversion portion  304  effects the color conversion of the Lab data by using the conversion table obtained in the step S 402  to perform the color conversion from the L*a*b* space to the printer RGB space, thus obtaining the printer RGB data. 
     In a step S 406 , the color conversion portion  304  effects the color conversion of the printer RGB data obtained in the step S 405  by using the conversion table obtained in the step S 403 . As a result, the printer RGB space is color-converted into the L*a*b* space to obtain the Lab data of the appearance color space. Here, in order to differentiate the Lab data obtained in this step from that obtained the step S 405 , the Lab data obtained in this step is referred to as L′a′b′ data. 
     In a step S 407 , a color difference discrimination tolerance input portion  310  generates a color difference discrimination tolerance  307  depending on a designated value set by the user through the operating portion  107 . In this embodiment, the color difference discrimination tolerance  307  is used as a reference value. 
     In a step S 408 , a color difference discriminating portion  305  obtains a color difference between the Lab data obtained in the step S 405  and the L′a′b′ data obtained in the step S 406  with respect to the pixel in the image and compares the resultant color difference with the color difference discrimination tolerance  307  whether or not the color difference exceeds the color difference discrimination tolerance  307  to discriminate the color difference. 
     Here, the color difference is calculated according to the following equation (1):
 
Δ E*ab =SQRT[(Δ L *)×(Δ L *)+(Δ a *)×(Δ a *)+(Δ b *)×(Δ b *)]  (1)
 
     In the equation (1), SQRT[ ] represents the square root of [ ], and ΔL*, Δa* and Δb* represent differences in L*, a* and b*, respectively. 
     That is, in this embodiment, the color difference refers to a difference in each color component defined by the Lab space independent of the device. 
     In a step S 409 , the color difference area discrimination image forming portion  302  generates the color difference area discrimination image data  303  on the basis of the result of the comparison in the step S 408 . 
     In a step S 410 , the image processing apparatus  100  displays the color difference area discrimination image data  303  corresponding to the color difference exceeding the color difference discrimination tolerance on the display  401  of the display device  109 . That is, the display is made with respect to the pixel at which the color difference exceeds the color difference discrimination tolerance in the step S 408 . Here, the user can display the color difference area discrimination image data by changing the degree of the color difference through an operation of the slide bar  404  shown in  FIG. 4 .  FIGS. 8 ,  9  and  10  shows examples of the color difference area discrimination images displayed with different degrees of the color difference. 
     (Output Characteristic Database) 
     Next, the output characteristic table stored in the database  311  will be described.  FIG. 7  is a schematic view showing a plurality of output characteristic tables stored in the database  311 . As shown in the output characteristic tables are stored in the database  311  for each of the type of printer, the type of sheet (such as glossy paper (G.P.) or coated paper (C.P.)), the type of the printing mode. 
     Each of the output characteristic tables is constituted by including the color correction table and the conversion table set for each of the types of environment light. As described above, the color correction table is the table used for effecting the color conversion from the normal RGB space to the L*a*b* space. Further, the conversion table set for each of the types of environment light includes the table used for effecting the color conversion from the L*a*b* space to the printer RGB space and the table used for effecting the color conversion from the printer RGB space to the L*a*b* space. 
     As shown in  FIG. 7 , mutually different types of environment light are set. For example, the normal light (N.L.), the high color rendering fluorescent lamp (H.C.R.F.L.), and the three band fluorescent lamp (T.B.F.L.) are set. 
     The normal RGB color space described above is not limited to a particular RGB color space but may also be any color space such as Adobe RGB space or sRGB space. Further, in this embodiment, the CIELAB space is used as the appearance color space but any color appearance color space may also be used so long as the color space is a color appearance color space. For example, it is possible to use CIECAAM02 color space, CIECAM97s color space, and the like. 
     Further, in this embodiment, the conversion table into the appearance color space has been stored in the database in advance but such a constitution that spectral data of the output image is held as the output characteristic and the conversion table depending on the environment light is generated may also be employed. In this way, the image processing apparatus in this embodiment presents the area in which the degree of the color change due to the change in the observation environment light is large for each of the different output characteristics, so that the user can know the printing condition (the type of printer, the type of sheet, the type of printing mode, etc.) under which a degree of the change in color appearing manner depending on the type of the observation environment light. 
     Second Embodiment 
     Next, Second Embodiment of the present invention will be described. The hardware configuration, the software configuration and the database constitution in the image processing apparatus in this embodiment are similar to those described with reference to  FIGS. 1 to 3  and  7 . 
     (User Interface) 
       FIG. 11  is a schematic view showing an example of a user interface in this embodiment. A user interface  1100  displays a color difference area discrimination image on a display  1101 . Further, the user interface  1100  can also display a partly enlarged image. With respect to an original image shown in  FIG. 15 , a display example of the color difference area discrimination image is shown in  FIG. 14 . 
     The user can set the type of printer, the type of sheet, and the printing mode by using a display  1102 , in which respective times are displayed in a pop-up menu system. The user can appropriately select and set the respective items. By each pop-up menu, a list of selected item is displayed by selecting an inverted triangle area, located at a right end of each rectangular area, with a pointer or the like. The user selects an objective item from the list of the selected item. 
     A display  1103  is used in the case where light to be corrected and second environment light are set. Each item is, similarly as in the case of the display  1102 , displayed as a list of selected item in the pop-up menu system. The user can select the item from the list and sets the selected item. As a type of environment light to be set, e.g., there are normal (standard) light (D50), high color rendering fluorescent lamp, three-waveform fluorescent lamp, and normal fluorescent lamp. 
     The user can adjust the number of color difference on a scale from “SMALL” to “LARGE”. That is, the user determines the number of color difference scale as a plurality of reference values by a slide bar  1104 . A color-coded image providing different tone gradation levels within a range defined by the plurality of reference values determined by the slide bar  1104  is displayed on the display  1101  as shown in  FIG. 14 . 
     When a button  1105  is pushed down, the display of the user interface is completed. 
     Thus, in this embodiment, the user can visually confirm the area, in which the change in color depending on the change in environment light is observed, depending on the number of the color difference scale. 
     (Flow of Processing) 
     The flow of the processing in this embodiment will be described based on  FIG. 13  with reference to  FIG. 12 .  FIG. 12  is a block diagram showing an inner functional constitution of the image processing apparatus  100  in this embodiment.  FIG. 13  is a flow chart showing a procedure of the display processing of the color difference area image in this embodiment. Each of steps in the flow chart is performed under control of the CPU  101  of the image processing apparatus  100 . 
     First, in a step S 501 , a printing condition input portion  309  of an output characteristic obtaining portion  305  obtains the information on the type of printer, the type of sheet, and the type of printing mode selected by the user through an operating portion  107  and then obtains an output characteristic table (described later) corresponding to the information from a database  311 . 
     In a step S 502 , an environment light type input portion  308  of the output characteristic obtaining portion  306  obtains the information on the type of first environment light selected by the user through the operating portion  107 . Further, from the output characteristic table obtained in the step S 501 , a color correction table and a conversion table corresponding to the first environment light are obtained. Here, the color correction table is a table used for effecting the color conversion from the normal RGB space to L*a*b* space, and the conversion table corresponding to the first environment light is a table used for effecting the color conversion from the L*a*b* space to printer RGB space. 
     In a step S 503 , the environment light type input portion  308  of the output characteristic obtaining portion  305  obtains information on the second environment light selected by the user through the operating portion  107 . Further, from the output characteristic table obtained in the step S 401 , a conversion table corresponding to the second environment light is obtained. Here, the conversion table corresponding to the second environment light is a table used for effecting the color conversion from the printer RGB space to the L*a*b* space. 
     In a step S 504 , image data  301  is read from an HDD  104 . In the HDD  104 , image data taken in through a scanner or the like (not shown in  FIG. 1 ) is stored as the RGB data in advance. 
     In a step S 505 , a color conversion portion  304  effects the color correction processing of the image data  301  by using the color correction table obtained in the step S 502  to perform the color conversion from the normal RGB space to the L*a*b* space, thus obtaining Lab data of appearance color space. Further, the color conversion portion  304  effects the color conversion of the Lab data by using the conversion table obtained in the step S 502  to perform the color conversion from the L*a*b* space to the printer RGB space, thus obtaining the printer RGB data. 
     In a step S 506 , the color conversion portion  304  effects the color conversion of the printer RGB data obtained in the step S 505  by using the conversion table obtained in the step S 503 . As a result, the printer RGB space is color-converted into the L*a*b* space to obtain the Lab data of the appearance color space. Here, in order to differentiate the Lab data obtained in this step from that obtained the step S 505 , the Lab data obtained in this step is referred to as L′a′b′ data. 
     In a step S 507 , a color difference scale number input portion  312  generates a color difference scale number  313  depending on a slide bar value set by the user through the operating portion  107 . The processing in this embodiment is an example of a second determining means in the present invention. 
     In a step S 508 , a color difference discriminating portion  305  obtains a color difference between the Lab data obtained in the step S 505  and the L′a′b′ data obtained in the step S 406  with respect to the pixel in the image and discriminates the resultant color difference whether the color difference corresponds to which scale number determined by the color difference scale number  313 . 
     In a step S 509 , the color difference area discrimination image forming portion  302  generates the color difference area discrimination image data  303  on the basis of the result of the discrimination in the step S 508 . 
     In a step S 510 , the image processing apparatus  100  displays the color difference area discrimination image data  303  on the display  1101  of the display device  109 . Here, the user can display the color difference area discrimination image data by changing the color difference scale number through an operation of the slide bar  1104  shown in  FIG. 11 . 
     In this embodiment, the user sets the color difference scale number by the slide bar. This means that the user can set the plurality of reference values. Thus, by designating the plurality of reference values, ranges each determined by respective consecutive reference values are present. 
     On the other hand, by comparing the calculated color difference with the designated plurality of reference values (as an example of a second comparing means in the present invention), it is possible to determine that the color difference belongs to which range. As a result, for each pixel, the calculated color difference can be replaced with a discrete value corresponding to associate one of the plurality of reference values on the basis of the comparison result. Therefore, as shown in  FIG. 14 , the color difference can be displayed on the display  1101  of the display device with different tone gradation levels. 
     Third Embodiment 
     Next, Third Embodiment of the present invention will be described. The hardware configuration, the software configuration and the database constitution in the image processing apparatus in this embodiment are similar to those described with reference to  FIGS. 1 to 3  and  7 . 
     (User Interface) 
       FIG. 16  is a schematic view showing an example of a user interface in this embodiment. A user interface  1600  displays a color difference area discrimination image on a display  1601  with respect to four types of environment light such as normal light (N.L.) (D50), high color rendering environment light (H.C.R.F.L.), three wavelength fluorescent lamp (T.W.F.L.), and normal fluorescent lamp (N.F.L.). With respect to an original image shown in  FIG. 15 , display examples of the color difference area discrimination image are shown in  FIGS. 8 ,  9  and  10 , respectively. 
     The user can set the type of printer, the type of sheet, and the printing mode by using a display  1602 , in which respective times are displayed in a pop-up menu system. The user can appropriately select and set the respective items. By each pop-up menu, a list of selected item is displayed by selecting an inverted triangle area, located at a right end of each rectangular area, with a pointer or the like. The user selects an objective item from the list of the selected item. 
     A display  1603  is used in the case where light to be corrected is set. Each item is, similarly as in the case of the display  1602 , displayed as a list of selected item in the pop-up menu system. The user can select the item from the list and sets the selected item. As a type of environment light to be set, e.g., there are normal (standard) light (D50), high color rendering fluorescent lamp, three-waveform fluorescent lamp, and normal fluorescent lamp. 
     The user can adjust a degree of color difference discrimination tolerance by a slide bar  1604 , so that an area with the degree of color difference discrimination tolerance designated by the user through the slide bar  1604  is displayed on the display  1601 . For example, as shown in  FIG. 16 , in the case where a “medium” value of print color change intensity is set as an initial value, a color difference area of a medium degree is displayed on the display  1601  ( FIG. 9 ). Further, in the case where a “strong” value is set, the color difference area of a higher degree is displayed on the display  1601  ( FIG. 8 ). Further, in the case where a “weak” value is set, the color difference area of a lower degree is displayed on the display  1601  ( FIG. 10 ). When a button  1605  is pushed down, the display of the user interface is completed. 
     Thus, in this embodiment, the user can visually confirm the area, in which the change in color depending on the change in the four types of environment light (in addition to those in First Embodiment) is observed, depending on the degree of the color change intensity on four adjacent displays  1601 . 
     (Flow of Processing) 
     The flow of the processing in this embodiment will be described based on  FIG. 17  with reference to  FIG. 5 .  FIG. 17  is a flow chart showing a procedure of the display processing of the color difference area image in this embodiment. Each of steps in the flow chart is performed under control of the CPU  101  of the image processing apparatus  100 . 
     First, in a step S 601 , a printing condition input portion  309  of an output characteristic obtaining portion  305  obtains the information on the type of printer, the type of sheet, and the type of printing mode selected by the user through an operating portion  107  and then obtains an output characteristic table corresponding to the information from a database  311 . 
     In a step S 602 , an environment light type input portion  308  of the output characteristic obtaining portion  306  obtains the information on the type of first environment light selected by the user through the operating portion  107 . Further, from the output characteristic table obtained in the step S 601 , a color correction table and a conversion table corresponding to the first environment light are obtained. Here, the color correction table is a table used for effecting the color conversion from the normal RGB space to L*a*b* space, and the conversion table corresponding to the first environment light is a table used for effecting the color conversion from the L*a*b* space to printer RGB space. 
     In a step S 603 , the output characteristic obtaining portion  306  sets one of the four types of environment light as the second environment light. The second environment light is changed every processing from a step S 611  (described later). Examples of the environment light may include the normal light (D50), the high color rendering fluorescent lamp, the three wavelength (band) fluorescent lamp, and the normal fluorescent lamp. The information thereon is, e.g., held in a storing area of the RAM or the like in the form a list of the types of environment light and is set as the second environment light in the listed order. Further, from the output characteristic table obtained in the step S 601 , a conversion table corresponding to the second environment light is obtained. Here, the conversion table corresponding to the second environment light is a table used for effecting the color conversion from the printer RGB space to the L*a*b* space. 
     In a step S 604 , image data  301  is read from an HDD  104 . In the HDD  104 , image data taken a scanner or the like (not shown in  FIG. 1 ) is stored as the RGB data in advance. 
     In a step S 605 , a color conversion portion  304  effects the color correction processing of the image data  301  by using the color correction table obtained in the step S 602  to perform the color conversion from the normal RGB space to the L*a*b* space, thus obtaining Lab data of appearance color space. Further, the color conversion portion  304  effects the color conversion of the Lab data by using the conversion table obtained in the step S 602  to perform the color conversion from the L*a*b* space to the printer RGB space, thus obtaining the printer RGB data. 
     In a step S 606 , the color conversion portion  304  effects the color conversion of the printer RGB data obtained in the step S 605  by using the conversion table obtained in the step S 603 . As a result, the printer RGB space is color-converted into the L*a*b* space to obtain the Lab data of the appearance color space. Here, in order to differentiate the Lab data obtained in this step from that obtained the step S 605 , the Lab data obtained in this step is referred to as L′a′b′ data. 
     In a step S 607 , a color difference discrimination tolerance input portion  310  generates a color difference discrimination tolerance  307  depending on a value set by the user through the operating portion  107 . 
     In a step S 608 , a color difference discriminating portion  305  obtains a color difference between the Lab data obtained in the step S 605  and the L′a′b′ data obtained in the step S 606  with respect to each pixel in the image and discriminates the resultant color difference with the color difference discrimination tolerance  307  whether or not the color difference exceeds the color difference discrimination tolerance  307  to discriminate the color difference. 
     Here, the color difference is calculated according to the equation (1) described in First Embodiment. 
     In a step S 609 , the color difference area discrimination image forming portion  302  generates the color difference area discrimination image data  303  on the basis of the result of the discrimination in the step S 608 . 
     In a step S 610 , the image processing apparatus  100  displays the color difference area discrimination image data  303  corresponding to the color difference exceeding the color difference discrimination tolerance on the display  1601  of the display device  109 . That is, the display is made with respect to the pixel at which the color difference exceeds the color difference discrimination tolerance in the step S 608 . Here, the user can display the color difference area discrimination image data by changing the degree of the color difference through an operation of the slide bar  1604  shown in  FIG. 16 .  FIGS. 8 ,  9  and  10  shows examples of the color difference discrimination images displayed with different degrees of the color difference. 
     In a step S 611 , whether or not the environment light to be set as the second environment light is the last environment light listed in the list of the four types of environment light is judged. In the case where the environment light is not the last environment light but a subsequent environment light is present, the processing is returned to the step S 603 . On the other hand, in the case where the environment light is the last environment light and there is not subsequent environment light, the processing is completed. 
     As described above, according to this embodiment, the user can visually confirm the color change appearing area due to the change in the type of environment light of the four types of environment light (in addition to those in First Embodiment) depending on the degree of the color change intensity on four adjacent displays. 
     In the present invention, the case where the operating system (OS) or the like running on the computer performs a part or all of the actual processing on the basis of instructions by (image processing) program codes and thus the functions described in First to Third Embodiments are realized by the processing is also embraced. Further, the present invention is also applied to the case where the program code read from a memory medium is written in a memory provided to a function extending card inserted into the computer or a function extending unit connected to the computer. In this case, on the basis of instructions by the written program code, the CPU or the like provided in the function extending card or unit performs a part or all of the actual processing and thus the functions described in First to Third Embodiments are realized. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 326587/2008 filed Dec. 22, 2008, which is hereby incorporated by reference.