Abstract:
The purpose of this invention is to reproduce gradations of dark parts between devices with different ranges of reproducible brightness. The range of reproducible brightness by the printer is a range from L black  to 100, and this is narrow in relation to the range of brightness that is able to be reproduced by the digital camera (0 to 100). An image processing apparatus converts the input brightness L input  on the input brightness graph L input -graph to the output brightness L output  on the output brightness graph L output -graph. Accordingly, the low brightness gradations 0 to L black  of the input brightness L input  in relation to the input RGB values included in the range R1 become L black  to L1, the dark area gradations are reproduced.

Description:
BACK GROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to brightness gradation reproduction between devices with different ranges of reproducible brightness. 
   2. Description of the Related Art 
   A printing systems that have a digital camera and a printer have become popular. An image data shot by the digital camera is input to the printer and the printer prints images expressed by the image data in his printing systems Image data is expressed using the device dependent color space that is dependent on each device. For example, a digital camera expresses image data using the RGB color space that is the digital camera device dependent color space, and the printer expresses image data using the CMYK color space that is the printer device dependent color space. Image data expressed by the RGB color space is converted to image data expressed by the CMYK color space that is the printer device dependent color space and output to the printer. 
   The brightness range that is reproducible for each color space differs for each device. By correlating the reproducible brightness by the digital camera and the reproducible brightness by the printer, the gradations of the image data shot using the digital camera are reflected in the image output by the printer. 
   However, the range of reproducible brightness by the printer is narrower than the range of reproducible brightness by the digital camera, and of the reproducible brightness by the digital camera, all the brightness that is lower than the black point are converted to black points. The black point is the minimum value of the reproducible brightness by the printer. Because of that, there is a problem that low brightness gradations in image data are not able to be reproduced in the images printed by the printer. 
   The problem described above is not a problem specialized in cases when outputting image data shot using a digital camera to a printer to print an image, but a problem that occurs in the same way between devices for which the range of reproducible brightness by the device receiving the image data is narrower than the range of reproducible brightness by the device sending the image data. 
   This invention was carried on in view of these problems, and the purpose is to reproduce gradations of the part for which the image data has low brightness between devices for which the range of reproducible brightness by the device that receives the image data is narrower than the range of reproducible brightness by the device that sends the image data. 
   SUMMARY OF THE INVENTION 
   In order to address at least part of the problem described above, a first aspect of this invention provides an image processing apparatus that outputs input image data that is input from an input device to an output device, wherein the input device has a input brightness area that is reproducible by the input device, wherein the output device has a output brightness area narrower than the input brightness area and is reproducible by the output device. The image processing apparatus in the first aspect of the invention comprises: a first color conversion processing module that converts the input image data to first intermediate image data, wherein the input image data is expressed using a input device dependent color space that depends on the input device, wherein the first intermediate image data at least expresses the brightness of the input image data and is expressed by an device independent color space that does not depend on each device, a brightness conversion module that converts the first intermediate image data to second intermediate image data so that gradation of first brightness is expressed in the output brightness area, wherein the first brightness is, of the first intermediate data, at least lower than the minimum value of the output brightness area, and a second color conversion processing module that converts the second intermediate data to output image data to output to the output device, wherein the output image data is expressed using an output device dependent color space that depends on the output device. 
   According to the image processing apparatus in the first aspect of this invention, it is possible to reproduce in the output image data the gradations of input brightness that is lower than the minimum value of the output brightness area in the input image data. Therefore, it is possible to ensure the gradation of the dark part of image data output by output device. 
   A second aspect of this invention provides a gradation reproduction method. The method is implemented by an image processing apparatus that outputs input image data that is input from an input device to an output device, wherein the input device has a input brightness area that is reproducible by the input device, wherein the output device has a output brightness area narrower than the input brightness area and is reproducible by the output device the gradation reproduction method comprising the steps of, The gradation reproduction method in the second aspect of the invention comprises the steps of: converting the input image data to first intermediate image data, wherein the input image data is expressed using a input device dependent color space that depends on the input device, wherein the first intermediate image data at least expresses the brightness of the input image data and is expressed by an device independent color space that does not depend on each device, converting the first intermediate image data to second intermediate image data so that gradation of first brightness is expressed in the output brightness area, wherein the first brightness is, of the first intermediate data at least lower than the minimum value of the output brightness area, and converting the second intermediate data to output image data to output to the output device, wherein the output image data is expressed using an output device dependent color space that depends on the output device. 
   The image processing apparatus in the second aspect of this invention may provides the same action and effect as the image processing apparatus in the first aspect of this invention. The image processing apparatus in the second aspect of this invention may also be realized in a variety of ways in the same manners as the image processing apparatus in the first aspect of this invention. 
   These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  exemplary illustrates an example of an image output system in the first embodiment. 
       FIG. 2  exemplary illustrates a block chart describing the color conversion process in the first embodiment. 
       FIG. 3  exemplary illustrates an example of the contents of the input device profile in the first embodiment. 
       FIG. 4  exemplary illustrates an example of the contents of the output device profile of the first embodiment. 
       FIG. 5  exemplary illustrates a flow chart describing the printing process in the first embodiment. 
       FIG. 6  exemplary illustrates a brightness graph representing the brightness gradation in the first embodiment. 
       FIG. 7  exemplary illustrates a flow chart describing the brightness conversion process in the second embodiment. 
       FIG. 8  exemplary illustrates a brightness graph representing the brightness gradation in the second embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Following, embodiment aspects of the invention are described based on the following embodiments with references to the drawings. 
   A. First Embodiment 
   A1. System Configuration: 
     FIG. 1  exemplary illustrates an image output system  10  in the first embodiment. The image output system  10  has an image processing apparatus  20 , a printer  30 , and a digital camera  40 . The digital camera  40  and the image processing apparatus  20 , as well as the image processing apparatus  20  and the printer  30  are respectively connected locally via a USB cable. The image processing apparatus  20  converts the image data  50  so as to be reproduced in the image on which the gradations of the brightness of image data  50  obtained from the digital camera  40  is printed with printer  30 . The image processing apparatus  20  outputs the converted image data  50   a  to the printer  30 . 
   The image processing apparatus  20  is a typical personal computer equipped with a CPU, RAM, and ROM. The functional blocks of the image processing apparatus are illustrated in the  FIG. 1 . Each functional block is controlled by the CPU  100 . The memory  120  stores an image data obtaining module  130 , a color conversion module  140 , a profile storage area  150  and a printing module  160 . Each functional block is implemented using software. Each functional block may be constituted using hardware. The RAM  110  is readable/writable memory. The image data recording area  111  is constituted as part of the RAM  110 . 
   An input/output unit  170  sends and receives image data between the printer  30  and the digital camera  40 . 
   The image data obtaining module  130  records the image data  50  obtained from the digital camera  40  onto the image data recording area  111 . 
   An input device profile PF 1  and an output device profile PF 2  are stored in the profile storage area  150 . The input device profile PF 1  includes the color conversion information from the device dependent color space of the digital camera  40  that is the input device to the device independent color space. Also, the output device profile PF 2  includes the color conversion information from the device independent color space to the device dependent color space of the printer  30  that is the output device. 
   The color conversion module  140  uses the input device profile PF 1  and the output device profile PF 2  to convert the color space of the image data  50  from the device dependent color space that depends on the digital camera  40  as the input device to the device dependent color space that depends on the printer  30  as the output device. Converting the color space means converting each pixel value of the pixels that constitute the image data  50  expressed by the device dependent color space that depends on the digital camera  40  to each of the pixel values of the image data  50   a  expressed by the device dependent color space that depends on the printer  30 . The conversion process implemented by the color conversion module  140  is described later. 
   The printing module  160  converts the image data  50   a  converted by the color conversion module  140  to a format that is able to be interpreted by the printer  30 , and outputs the converted image data  50   a  that is converted by printing module  160  to the printer  30  via the input/output unit  170 . 
     FIG. 2  exemplary illustrates a block chart that describes the color conversion process implemented by the color conversion module  140  in the first embodiment. The color conversion module  140  has an input signal color conversion module  141 , a brightness conversion module  142 , and an output signal color conversion module  143 . The input signal color conversion module  141  correlates to the “first color conversion module” of this invention. The brightness conversion module  142  correlates to the “brightness conversion module” of this invention. The output signal color conversion module  143  correlates to the “second color conversion module” of the first embodiment. In the first embodiment, the device dependent color space of the digital camera  40  that is the input device is the RGB color space, and the pixel values of each pixel of the image data expressed by the RGB color space are expressed as (R, G, B). The device independent color space is the Lab color space and the image values of each pixel of the image data expressed by the Lab color spaces are expressed as (L, a, b). “L” represents brightness, and “a” and “b” represent chromaticity. The device dependent color space of the printer  30  that is the output device is the CMYK color space, and the pixel values of each pixel of the image data expressed by the CMYK color space are expressed as (C, M, Y, K). 
   The input signal color conversion module  141  implements color conversion processing from the device dependent color space of the input device, specifically, the digital camera  40 , to the device independent color space. In specific terms, the input signal color conversion module  141  obtains from the input device profile PF 1  the information for color conversion from the device dependent color space of the input device to the device independent color space. The input signal color conversion module  141  uses the obtained color conversion information to convert image data expressed using the RGB color space color space to image data expressed using the Lab color space. The color conversion information includes a color conversion table from the RGB color space to the Lab color space. The input signal color conversion module  141  uses the color conversion table to convert each pixel value of the image data  50  from (R, G, B) expressed using the RGB color space to (L, a, b) expressed using the Lab color space. It is also possible to include in the color conversion information an RGB-Lab color conversion matrix, for example. 
   The contents of the input device profile PF 1  are described with reference to the  FIG. 3 .  FIG. 3  exemplary illustrates of the contents of the input device profile PF 1  in the first embodiment. The input device profile PF 1  includes a header part PF 11 , device independent color space conversion information PF 12 , and input device color space information PF 13 . The header part PF 11  includes the device type indicating the type of input device, the model name, and the creation date and time of the input device profile PF 1 . The device independent color space conversion information PF 12  includes an RGB-Lab color conversion table as the color conversion information necessary for converting the image data  50  expressed using the device dependent color space (RGB color space) of the digital camera  40  that is the input device to the image data  50  expressed using the device independent color space (Lab color space). 
   The color space information PF 13  includes information relating to the color space that the digital camera, which is the input device, is able to reproduce. The input device profile PF 1  is stored in the memory  120  together with installation of the input device driver. The input device profile PF 1  is able to be supplied correlated to the image data  50 , for example. 
   The brightness conversion module  142 , in the Lab color space that is the device independent color space, converts the input brightness L of the image data  50  and calculates the output brightness L′ so that the gradation of the reproducible brightness by the input device is reproduced within the range of reproducible brightness by the output device. 
   Though an illustration is omitted in  FIG. 3 , the color conversion module  140  includes a color area mapping module to correlate of the input color area expressing the reproducible range of chromaticity by the input device and the output color area that expresses the range of reproducible chromaticity by the output device. The color area mapping module converts the chromaticity (a, b) of the image data  50  to the chromaticity (a′, b′), wherein the chromaticity (a, b) is expressed by the reproducible color area by the digital camera that is the input device, and the chromaticity (a′, b′) is expressed by the reproducible color area by the printer that is the output device. 
   The pixel values of the image data  50  are converted (L, a, b) to (L′, a′, b′) by the brightness conversion module  142  and the color area mapping module. 
   The output signal color conversion module  143  implements color conversion processing from the device independent space to the device dependent color space of the digital camera  40  that is the output device. In specific terms, the output signal color conversion module  143  obtains the color conversion information from the output device profile PF 2 . The output device profile PF 2  includes information to convert from the device independent color space of the output device to the device dependent color space. The output signal color conversion module  143  uses the obtained color conversion information to convert image data expressed using the Lab color space color space to image data expressed using the CMYK color space. The color conversion information includes a color conversion table from the Lab color space to the CMYK color space. The output signal color conversion module  143  uses the color conversion table to convert each pixel value of the image data  50  from (L′, a′, b′) expressed by the Lab color space to (C, M, Y, K) expressed by the CMYK color space. It is also possible to include in the color conversion information a Lab-CMYK color conversion matrix, for example. 
   The contents of the output device profile PF 2  are described using  FIG. 4 .  FIG. 4  exemplary illustrates an example of the contents of the output device profile PF 2  in the first embodiment. The output device profile PF 2  has the same constitution as the input device profile PF 1 , and consists of a header part PF 21 , device independent color space conversion information PF 22 , and input device color space information PF 23 . The header part PF 21  includes the device type that indicates the input device type, the model name, and the creation date and time of the output device profile PF 2 . The device independent color space information PF 22  includes the Lab-CMYK color conversion table as color conversion information necessary to confer the image data  50  expressed by the device independent color space (Lab color space) to image data  50   a  expressed by the device dependent color space (CMYK color space) of the printer  30  that is the output device. The color space information PF 23  includes information relating to the reproducible color space by the printer which is the output device. 
   A2. Printing Process: 
     FIG. 5  exemplary illustrates a flow chart for describing the printing process in the first embodiment. The image processing apparatus  20  starts this printing process with printing instructions from the user as the trigger. When the image processing apparatus  20  receives the printing instructions from the user, the image processing apparatus  20  reads the image data recorded in the image data recording area  111  (step S 10 ), implements input signal color conversion processing, and converts each pixel value (R, G, B) of the image data  50  expressed by the RGB color space to (L, a, b) expressed by the Lab color space (step S 11 ). 
   The image processing apparatus  20  implements color area and brightness conversion on each pixel value (L, a, b) of the image data  50  expressed by the Lab color space, and converts each pixel value to (L′, a′, b′) (step S 12 ). In the first embodiment, only the selected input brightness L input  that is the brightness of the achromatic pixels of the pixels of the image data  50  are converted to the output brightness L output  using the equation 3 shown below. 
   
     
       
         
           
             
               
                 
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       L max : Maximum value of brightness for the Lab color space 
       L black : Minimum value of the reproducible brightness by the printer 
       L input : selected input brightness 
       L output : Output brightness 
     
  
   Gradation change by the input brightness conversion process is described in reference to  FIG. 6 .  FIG. 6  exemplary illustrates a brightness graph  300  that expresses brightness gradation changes in the first embodiment. The horizontal axis of the brightness graph  300  represents the RGB value of the input image data  50 , and the vertical axis represents the output brightness L of the image data. For the input RGB values, each value (R, G, B) changes in sequence from (0, 0, 0) to (1, 1, 1) (2, 2, 2) . . . (255, 255, 255). The output brightness L output  has a range from 0 to 100. L black  that represents the minimum value of the reproducible brightness by the printer  30  which is the output device shows black points that are the darkest spots of the brightness. 
   The input brightness graph L input  shows the reproducible brightness by the digital camera  40  for the input RGB values. As shown in  FIG. 6 , as the input RGB value increases, the output brightness L gradually becomes higher, in other words, as the input RGB value increases, the brightness is higher. The output brightness graph Ls shows the reproducible output brightness Lout by the printer  30  when using the conventional method. The output brightness graph L output  is a graph representing the output brightness L output  calculated applying equation 3 in the first embodiment to the input RGB values. 
   The range of reproducible brightness by the printer  30  is a range from L black  to 100, and is narrower than the range of reproducible brightness by the digital camera  40  (0 to 100). Because of that, with the conventional brightness conversion processing, as shown in the output brightness graph Ls, when the RGB values of each pixel value of the input image data is included in the range R 1  from (0, 0, 0) to the P 1  (r, g, b), all of the input brightness L input  are converted to L black , and the gradations of the input brightness L input  are not reproduced with the output brightness L out . 
   In the first embodiment, by the brightness conversion module  142  implementing the brightness conversion process applying equation 3, the output brightness graph L output -graph is obtained as a graph representing brightness that is able to be reproduced by the printer  30 . By applying the equation 3, as shown by the arrow, the brightness conversion module  142  converts the input brightness L input  on the input brightness graph L input -graph to the output brightness L output  on the output brightness graph L output -graph. Specifically, the brightness conversion module  142  reproduces as L black  to L 1  the low brightness gradations from 0 to L black  of the input brightness L input  for the input RGB values included in the range R 1 . 
   According to the image processing apparatus of the first embodiment described above, by applying equation 3, even when the range of reproducible brightness by the printer that is the output device is narrower than the range of reproducible brightness by the digital camera that is the input device, it is possible to reproduce the gradations of the brightness of the image data shot using the digital camera in the image printed using the printer. Conventionally, of the image data brightness, brightness lower than the black points of the printer were all converted to printer black points, and with images output by the printer, low brightness gradations were not expressed, but with this invention, it is possible to express gradation of low brightness of image data shot using the digital camera for images output by the printer. 
   Also, equation 3 is applied not only to low brightness input brightness but to all the input brightness, so for reproducible brightness by the printer, it is possible to ensure gradation properties for the low brightness part and also to reproduce gradations smoothly for all the brightness. 
   B. Second Embodiment 
   With the first embodiment described above, by applying equation 3 to the achromatic input brightness, the gradation properties of the reproducible brightness by the printer are ensured. In the second embodiment, considering chroma saturation, the gradations of the brightness in a digital camera are reproduced in the range for which brightness is able to be reproduced by the printer. The system configuration in the second embodiment is the same as the system configuration in the first embodiment. 
   B1. Brightness Conversion Process: 
     FIG. 7  exemplary illustrates a flow chart for describing the brightness conversion process in the second embodiment. This brightness conversion process is implemented by the brightness conversion module  142 , and correlates to the process of step S 12  in  FIG. 5 . 
   The brightness conversion module  142  calculates the input chroma saturation C input  of each pixel value of the image data expressed using the Lab color space (step  30 ). The brightness conversion module  142  calculates the input chroma saturation C input  by applying the equation 4 below. 
   [Equation 4]
 
 C   input =√{square root over ( a   input   2   +b   input   2 )}  (Equation 4)
 
(Where a input  and b input  are the “a” and “b” values of the input pixels for the Lab color space)
 
   The brightness conversion module  142  determines whether the input chroma saturation C input  is lower than the chroma saturation threshold value C th  (step S 31 ). The chroma saturation threshold value C th  represents ⅔ of the lowest chroma saturation value for the outside edge of the chromaticity of the Lab color space. When the input chroma saturation C input  is lower than the chroma saturation threshold value C th  (step S 31 : Yes), the brightness conversion module  142  converts input brightness Linput to the output brightness L output  by in consideration of the chroma saturation brightness (step S 32 ). In specific terms, the brightness conversion module  142  calculates the output brightness L final  by applying the equation 5 below. Specifically, with a range for which the input chroma saturation C input  is from 0 to the chroma saturation threshold value C th , the output brightness L final  is calculated by varying the mixture ratio of the input brightness L input  and the output brightness L output . Note that L output  is the value calculated by applying equation 3 in the first embodiment, and correlates to the “tentative output brightness” for this invention. 
   
     
       
         
           
             
               
                 
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       C th : Chroma saturation threshold value for the Lab color space 
       L final : Output brightness after conversion 
     
  
   When the input chroma saturation C input  is the chroma saturation threshold value C th  or greater (step S 31 : No), the brightness conversion module  142  uses the input brightness L input  as is as the output brightness L final  applying equation 6 (step S 33 ). Specifically, with the range for which the input chroma saturation C input  is from the chroma saturation threshold value C th  to the chroma saturation maximum value, the input brightness L input  is used as the output brightness L final . 
   [Equation 6]
 
L final =L input   (Equation 6)
 
   Gradation reproduction considering chroma saturation is described with reference to  FIG. 8 .  FIG. 8  exemplary illustrates a brightness graph  400  representing brightness gradation changes in the second embodiment. The horizontal axis of the brightness graph  400  represents the input chroma saturation C input  of the image data  50 , and the vertical axis represents the brightness L of the image data. The input chroma saturation Cinput uses a value in the range from 0 to 60. The brightness L uses a value in the range from 0 to 100. L black  representing the minimum value of the reproducible brightness by the printer  30  that is the output device shows the darkest point of the brightness, in other words, black points. In the second embodiment, described is an example of a color for which the brightness is L 4  when the input chroma saturation C input  is 0. 
   The input brightness graph L input -graph shows the reproducible input brightness L input  by the digital camera  40  for the input chroma saturation C input . As shown in  FIG. 8 , with the digital camera  40 , as the input chroma saturation C input  increases from 0 to 60, the brightness L also gradually increases from L 4  to L 2  and as the input chroma saturation Cinput gradually decreases from 60 to 0, the brightness L increases from L 2  to 100. 
   The output brightness graph L output -graph shows the reproducible output brightness L output  by the printer  30  on the chroma saturation C input . The output brightness L output  is calculated applying equation 3 in the first embodiment. The output brightness graph L final -graph shows the reproducible output brightness L final  by the printer  30  on the input chroma saturation C input . The output brightness L final  is calculated applying equation 5 in the second embodiment. 
   When brightness conversion processing is implemented applying only equation 3 in the first embodiment, as shown in the output brightness graph L output -graph, as the chroma saturation increases, the output brightness L output  has a higher value than the input brightness Linput, with the result that and the brightness is not reproduced appropriately. 
   In the second embodiment, by the brightness conversion module  142  implementing brightness conversion processing applying equation 5, the output brightness graph L final -graph is obtained showing the output brightness Lfinal. As shown in the brightness graph  400 , as the input chroma saturation C input  increases from 0 to the chroma saturation threshold value Cth, the output brightness L final  increases from L black  to L 5  so that the gradations of the input brightness L input  are reproduced. As the input chroma saturation Cinput increases from the chroma saturation threshold value Cth to 60, the output brightness L final  uses the same value as the input brightness L input , and increases from L 5  to L 2 . Even the part for which the input chroma saturation Cinput becomes lower from 60 to the chroma saturation threshold Cth, the output brightness L final  uses the same value as the input brightness L input  and increases from L 2  to L 3 . As the input chroma saturation C input  decreases from the chroma saturation threshold value Cth to 0, the output brightness L final  increases from L 3  to 100. 
   In the second embodiment, when the input chroma saturation C input  is less than the chroma saturation threshold value Cth, the mixture ratio of the input brightness L input  and the output brightness L output  is varied to calculate the output brightness L final  and the condition branching midway in the calculation is simplified, but when the input chroma saturation C input  changes decreasingly from a maximum value of 60 to 0, the same as when the input chroma saturation C input  is the chroma saturation threshold value Cth or greater, it is also possible to use the input brightness L input  as the output brightness Lfinal. This is because in the range for which the brightness is high, it is possible for the printer  30  to reproduce the input brightness L input  without converting the input brightness L input . 
   According to the image processing apparatus in the second embodiment described above, in the low chroma saturation area, in other words, the low chroma saturation area near the achromatic axis, the ratio of the output brightness L output  is made higher than the input brightness L input  to mix the input brightness L input  and the output brightness L output , as the input chroma saturation C input  comes closer to the chroma saturation threshold value Cth, by making the ratio of the input brightness L input  higher than the output brightness L output  and mixing the input brightness L input  and the output brightness L output , it is possible to reproduce the gradations of the image data brightness with good precision. Also, in the high chroma saturation area, it is possible to use the input brightness as is as the output brightness, and it is possible to improve the precision of gradation reproduction. 
   C. Variation Embodiments 
   (1) In the first embodiment described above, the Lab color space was used as the device independent color space, but it is also possible to use the LUV color space or the YCrCb color space. It is acceptable as long as it is a device independent color space that uses brightness as one of the components. 
   (2) In the second embodiment described above, the chroma saturation threshold value Cth was set as ⅔ the chroma saturation value of the lowest chroma saturation value for the outside edge of the chroma saturation of the Lab color space, but it is also possible to use ½ the chroma saturation value of the lowest chroma saturation value, for example. The chroma saturation maximum value and the color area characteristics differ for each device, so it is preferable to set the chroma saturation maximum value and the chroma saturation threshold value Cth according to the device. 
   Above, various embodiments of the invention were described, but the invention is not limited to these embodiments, and it is clearly possible to have various constitutions in a range that does not stray from its key points. 
   The Japanese patent applications as the basis of the priority claim of this application are incorporated in the disclosure here of by reference:
         (1) Japanese Patent Application No. 2005-90765(filing data: Mar. 28, 2005).