Patent Publication Number: US-8111887-B2

Title: Image processing method, image processing apparatus and image processing program

Description:
This is a U.S. National Phase Application under 35 U.S.C. 371 of International Application PCT/JP2006/323169 filed on Nov. 21, 2006. 
     This Application claims the priority of Japanese Application No. 2005-356615, filed Dec. 9, 2005, the entire content of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an image processing method, image processing apparatus and image processing program, particularly to an image processing method, image processing apparatus and image processing program capable of executing an appropriate gradation processing according to image data. 
     BACKGROUND 
     In the field related to medical images, an image is captured by using a radioactive photographic film made of silver halide photosensitive material. In addition to this method, there is a method of obtaining radiographic image information using a photostimulable phosphor plate, without using a radiographic film. 
     Further, there is widespread use of the radiographic image input apparatus such as an X-ray computerized tomography (X-ray CT apparatus) and magnetic resonance imaging apparatus (MRI apparatus) other than the plain radiograph. 
     When such a medical image is used for diagnosis, in addition to the transparent recording medium placed on an illumination apparatus such as a viewing box (schaukasten), image information is often recorded on a reflective type recording medium and is examined in the form of a hard copy. 
     As described above, there are various forms of input and output method, and this requires appropriate gradation processing to be applied to each image data. Further, depending on the target region, there is a difference in the range of signal value to be emphasized. This calls for appropriate gradation processing. 
       FIG. 12  is a characteristic diagram illustrating an example of a gamma conversion processing characteristic as a gradation conversion processing. When the gamma value is smaller, there is an increase in the contrast of the low brightness, and this avoids possible black crushing (black compression). In the region of high brightness, however, white crushing tends to occur. If the gamma value is greater, there is an increase in the contrast of the high brightness, and this avoids possible white crushing. In the region of low brightness, however, black crushing tends to occur. 
     As described above, in the process of gamma conversion using any one gamma value, gradation processing that covers the overall brightness region cannot be implemented in the conventional art. 
     As a method for image processing intended for the aforementioned application, a combination of a plurality of steps of gamma conversion processing is proposed in the technique disclosed in the following Patent Document 1.
     Patent Document 1: Unexamined Japanese Patent Application Publication No. 2003-310602 (P.1, FIG. 1).   

     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     According to the method described in the Patent Document 1, the gamma value of about 0.1 is selected for the low brightness region, and the gamma value of about 2.0 is selected for the high brightness region, as shown in  FIG. 13 . This arrangement prevents black crushing from occurring in the region of low brightness, and white crushing does not occur in the image of high brightness region. 
     Thus, this arrangement appears to be preferable in the field of application wherein only the low brightness region and high brightness region are the target regions. However, in the intermediate brightness region connecting between the low and high brightness regions, the input/output characteristics are reversed. There is no accordance in the trend of the gradation conversion processing characteristics between the low and high brightness regions. This has caused the problem of deterioration in the gradation continuity. 
     Further, when the output brightness value alone is used for judgment, it can not be judged whether the relevant input was originally located in the low brightness region or in the high brightness region. This may lead to wrong diagnosis in the field of medical image. 
     The object of the present invention is to solve these problems and to perform appropriate improvement processing in each brightness region while gradation continuity is maintained. 
     Means for Solving the Problems 
     The following describes the characteristics of the present invention for solving the aforementioned problem: 
     1. An image processing method for applying image processing including gradation conversion to image data, the method comprising: 
     applying image processing to the image data based on each of a plurality of different gradation conversion processing characteristics, thereby generating a plurality of image-processed image data; 
     calculating synthesis ratios of a plurality of image-processed image data by referencing the image information of the image data prior to image processing; and 
     synthesizing a plurality of image-processed image data based on the aforementioned synthesis ratios, thereby generating image data to be outputted. 
     2. The image processing method described in Configuration 1, wherein the synthesis ratios are calculated for each target pixel by referencing a pixel value of the target pixel as the aforementioned image information. 
     3. The image processing method described in Configuration 1, wherein the synthesis ratios are calculated for each target pixel by referencing a pixel value of the target pixel subsequent to smoothing of the image data as the aforementioned image information. 
     4. The image processing method described in any one of Configurations 1 through 3, this method comprising determining a synthesis number M according to an image characteristic of image data on applying image processing to the image data based on each of a plural number M of different gradation conversion processing characteristics and generating a plural number M of image-processed image data, wherein synthesis ratios of the plural number M of image-processed image data are calculated, and the plural number M of image-processed image data are synthesized based on the synthesis ratios, thereby the image data to be outputted is generated. 
     5. The image processing method described in Configuration 4, wherein the aforementioned synthesis number M is determined by referencing the dynamic range of the image data as the aforementioned image characteristic. 
     6. An image processing apparatus for applying image processing including gradation conversion processing to image data, the image processing apparatus including; 
     an image processing section for applying image processing to the aforementioned image data based on each of a plurality of different gradation conversion processing characteristics, thereby generating a plurality of image-processed image data; 
     a synthesis ratio calculating section for calculating synthesis ratios of the plurality of image-processed image data by referencing the image information of the image data prior to image processing; and 
     a synthesis section for synthesizing the plurality of image-processed image data based on the aforementioned synthesis ratios, thereby generating image data to be outputted. 
     7. The image processing apparatus described in Configuration 6, wherein the aforementioned synthesis ratio calculating section calculates the aforementioned synthesis ratios for each target pixel by referencing a pixel value of the target pixel as the aforementioned image information. 
     8. The image processing apparatus described in Configuration 6, wherein the aforementioned synthesis ratio calculating section calculates the aforementioned synthesis ratios for each target pixel by referencing a pixel value of a target pixel subsequent to smoothing of the image data as the aforementioned image information. 
     9. The image processing apparatus described in any one of Configurations 6 through 8, wherein the image processing section determines a synthesis number M according to an image characteristic of image data when the image processing section applies image processing to the image data based on each of a plural number M of different gradation conversion processing characteristics and generates a plural number M of image-processed image data; 
     the synthesis ratio calculating section calculates synthesis ratios of the plural number M of image-processed image data; and 
     the synthesis section synthesizes the plural number M of processed data based on the aforementioned synthesis ratios, thereby generates the image data to be outputted. 
     10. The image processing apparatus described in Configuration 9, wherein the image processing section determines the synthesis number M by referencing a dynamic range of the image data as the image characteristics. 
     11. An image processing program comprising a routine for executing each processing method described in any one of the aforementioned Configurations 1 through 5. 
     Effects of the Invention 
     The present invention provides the following advantages: 
     (1) In the invention described in configuration 1, image processing is applied to the image data based on each of a plurality of different gradation conversion processing characteristics, thereby generating a plurality of image-processed image data; the synthesis ratios of a plurality of image-processed image data are calculated by referencing image information of the image data prior to image processing; and the plurality of image-processed image data is synthesized based on the aforementioned synthesis ratios, thereby image data to be outputted is generated. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (2) In the invention described in configuration 2, the synthesis ratios are calculated for each target pixel by referencing the pixel value of the target pixel as the aforementioned image information. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the pixel value, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (3) In the invention described in configuration 3, the synthesis ratios are calculated for each target pixel by referencing a pixel value of a target pixel subsequent to smoothing of the image data as the aforementioned image information. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics without being affected by abrupt change of noise and others included in image data, and it is also possible to ensure that appropriate and variation-free gradation processing are carried out in each brightness region while gradation continuity is maintained. 
     (4) In the invention described in configuration 4, a synthesis number is determined according to an image characteristic of image data when image processing is applied to the image data based on each of a plural number M of different gradation conversion processing characteristics and the plural number M of image-processed image data are generated; the synthesis ratios of the plural number M of image-processed image data are calculated; and the plural number M of image-processed image data is synthesized based on the aforementioned synthesis ratios, thereby an image data to be outputted is generated. Therefore the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the appropriate synthesis number and synthesis ratios according to the image characteristic, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (5) In the invention described in configuration 5, the aforementioned synthesis number M is determined by referencing the dynamic range of the image data as the aforementioned image characteristic. Therefore image is processed through synthesizing appropriate gradation conversion processing characteristics based on the appropriate synthesis number and synthesis ratios according to the dynamic range as the image characteristics, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (6) In the invention described in configuration 6, image processing is applied to the aforementioned image data based on each of a plurality of different gradation conversion processing characteristics, thereby generating a plurality of image-processed image data; the synthesis ratios of the plurality of image-processed image data are calculated by referencing the image information of the image data prior to image processing; and the plurality of image-processed image data is synthesized based on the aforementioned synthesis ratios, thereby image data to be outputted is generated. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (7) In the invention described in configuration 7, the aforementioned synthesis ratios are calculated for each target pixel by referencing a pixel value of a target pixel as the aforementioned image information. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the pixel value, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (8) In the invention described in configuration 8, the aforementioned synthesis ratios are calculated for each target pixel by referencing the pixel value of the target pixel subsequent to smoothing of the image data as the aforementioned image information. Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics without being affected by an abrupt change of noise and others included in image data, and it is also possible to ensure that appropriate and variation-free gradation processing are carried out in each brightness region while gradation continuity is maintained. 
     (9) In the invention described in configuration 9, a synthesis number M is determined according to an image characteristic of image data when image processing is applied to the image data based on each of the plural number M of different gradation conversion processing characteristics and the plural number M of image-processed image data are generated; the synthesis ratios of the plural number M of image-processed image data are calculated; and the plural number M of image-processed image data is synthesized based on the aforementioned synthesis ratios, thereby an image data to be outputted is generated. Therefore the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the appropriate synthesis number and synthesis ratios according to the image characteristic, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (10) In the invention described in configuration 10, the synthesis number M is determined by referencing a dynamic range of the image data as the image characteristic. Therefore image is processed through synthesizing appropriate gradation conversion processing characteristics according to the appropriate synthesis number and the synthesis ratios in according to the dynamic range as the image characteristic, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     (11) In the invention described in configuration 11, the routine is provided to implement the processing steps of the image processing method described in any one of the aforementioned Configurations (1) through (5). Therefore each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     Further, each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the pixel value, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     Further, each pixel of the image is processed through synthesizing appropriate gradation conversion processing characteristics without being affected by abrupt change of noise and others included in image data, and it is also possible to ensure that appropriate and variation-free gradation processing are carried out in each brightness region while gradation continuity is maintained. 
     Further, the image is processed through synthesizing appropriate gradation conversion processing characteristics based on the appropriate synthesis number M and synthesis ratios in response to image characteristic, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
     Furthermore, image is processed through synthesizing appropriate gradation conversion processing characteristics according to the appropriate synthesis number M and the synthesis ratios in response to the dynamic range as the image characteristics, and it is possible to ensure that appropriate gradation processing is carried out in each brightness region while gradation continuity is maintained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram representing an example of the functional configuration of the image processing apparatus as a first embodiment of the present invention; 
         FIG. 2  is a block diagram representing an example of a specific apparatus configuration of the image processing apparatus as a first embodiment of the present invention; 
         FIG. 3  is a block diagram representing an example of the functional configuration of the image processing apparatus as a first embodiment of the present invention, this configuration being characterized in that the synthesis number is variable; 
         FIG. 4  is a flow chart showing an example of the operation (procedures of the image processing method and image processing program) of the image processing apparatus as a first embodiment of the present invention; 
         FIG. 5  is a diagram showing an example of the gamma characteristics when M=10 in the image processing apparatus of the present embodiment of the invention; 
         FIG. 6  is a diagram showing an example of division wherein the brightness region of the original image is divided into M equal parts and the median value Im of each divided region is calculated, in the image processing apparatus of the present embodiment of the invention; 
         FIGS. 7(   a ) and  7 ( b ) are diagrams showing an example of the synthesized gradation conversion processing characteristic curves in the image processing apparatus of the present embodiment of the invention; 
         FIGS. 8(   a ),  8 ( b ) and  8 ( c ) are diagrams showing examples of the gradation conversion processing characteristic curves wherein a specific region is enhanced in the image processing apparatus of the present embodiment of the invention; 
         FIG. 9  is a block diagram representing an example of the functional configuration of the image processing apparatus as a second embodiment of the present invention; 
         FIG. 10  is a flow chart showing an example of the operation (procedures of the image processing method and image processing program) of the image processing apparatus as a second embodiment of the present invention; 
         FIG. 11  is a block diagram representing an example of the functional configuration of the image processing apparatus as a third embodiment of the present invention; 
         FIG. 12  is a diagram showing an example of the gradation conversion processing characteristic curves; and 
         FIG. 13  is a diagram showing another example of the gradation conversion processing characteristic curves. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               100  Image processing apparatus 
               101  CPU 
               102  ROM 
               103  RAM 
               110  Computing section 
               111  Control section 
               112  Image processing section 
               113  Gamma determining section 
               114  Synthesis number determining section 
               115  Synthesis ratio determining section 
               116  Synthesis section 
               117  Smoothing section 
               170  Operation section 
               180  Display section 
               190  HDD 
           
         
       
    
     BEST FORM OF EMBODIMENT OF THE PRESENT INVENTION 
     The following describes the details of the best form of the embodiment (hereinafter referred to as “embodiment”) with reference to drawings: 
     Referring to  FIG. 1 , the following describes the image processing method, image processing apparatus and image processing program of the present invention with reference to the first embodiment wherein the image processing program is applied to the image processing apparatus, and shows a specific example of the electrical configuration. 
     In the present embodiment, the operation of the image processing apparatus is based on the procedure of the image processing method, and the image processing program execution procedure represents the operation of the image processing apparatus or the processing procedure of the image processing method. 
     The image processing apparatus includes the following configurations:  FIG. 1  is a functional block diagram representing an example of the configuration of the major sections of the image processing apparatus in the present embodiment.  FIG. 2  is a block diagram representing an example of a specific circuit and apparatus constituting the image processing apparatus. The image processing apparatus is designed to implement the image processing method and to include various routines for image processing program. 
     Embodiment 1 
     The reference numeral  110  denotes a computing section for computation processing in the image processing apparatus  100  ( FIG. 2 ). In the image processing apparatus  100 , this computing section  110  applies image processing to the image data based on each of a plurality of different gradation conversion processing characteristics to generate a plurality of image-processed image data, and calculates synthesis ratios of a plurality of image-processed image data, by referencing the image information of the image data prior to image processing. The computing section  110  then synthesizes a plurality of image-processed image data based on the synthesis ratios, thereby generating the image data to be outputted. 
     In terms of functions, this computing section  110  includes at least an image processing section  112  for processing an image, a synthesis ratio determining section  115  for determining the synthesis ratios, and an image synthesis section  116  for synthesizing an image. 
     Based on each of a plurality of different gradation conversion processing characteristics (gamma characteristics), the image processing section  112  applies image processing to the image data to generate a plurality of image-processed image data. 
     By referencing the image information of the image data prior to image processing, the synthesis ratio determining section  115  determines the synthesis ratios of the plurality of image-processed image data. The image information of the image data in the sense in which it is used here refers to the pixel value of the target pixel. The image information of the image data also refers to the pixel value of the target pixel subsequent to smoothing. 
     Based on the aforementioned synthesis ratios, the synthesis section  116  synthesizes a plurality of image-processed image data, for each target pixel, and generates the image data to be outputted, while maintaining gradation continuity, wherein this image data has been subjected to appropriate gradation processing in each brightness region. 
     In  FIG. 2 , the image processing apparatus  100  includes the following components as hardware: 
     The computing section  110  including the image processing section  112 , synthesis ratio determining section  115  and image synthesis section  115  incorporates a CPU  101  as a control device for controlling each section, a ROM  102  for storing various forms of data and programs and a RAM  103  as a work area wherein image data is displayed and processed. 
     The reference numeral  170  denotes an operation section whereby a operator (user) inputs various forms of instruction into the image processing apparatus  100 , and  180  indicates the display section for displaying various statuses of the image processing apparatus  100 . The reference numeral  190  is an HDD wherein image data and various forms of data are stored. 
     The number of the gradation conversion processing characteristics and the number of the image-processed image data to be generated thereby are fixed in the aforementioned example. However, they can be designed as variable numbers. To be more specific, steps are taken to determine a synthesis number M according to the image characteristics of image data and to apply image processing to the image data based on each of the plural number M of different gradation conversion processing characteristics to generate the plural number M of image-processed image data. This is followed by the step of calculating the synthesis ratios of the plural number M of the image-processed image data and synthesizing the plural number M of the image-processed image data based on the synthesis ratios, thereby generating the image data to be outputted. 
     In this case, as shown in  FIG. 3 , the synthesis number determining section  114  determines the number to be synthesized M in response to the image characteristics of the image data such as a dynamic range. The gamma determining section  113  determines the plural number M of different gradation conversion processing characteristics. The image processing section  112  applies image processing to image data according to each of the plural number M of different gradation conversion processing characteristics (gamma characteristics), whereby a plurality of image-processed image data is generated. By referencing the image information of the image data prior to image processing, the synthesis ratio determining section  115  determines the synthesis ratios of the plural number M of the image-processed image data. Based on the aforementioned synthesis ratios, the synthesis section  116  synthesizes the plural number M of the image-processed image data, for each target pixel, and generates the image data to be outputted, while maintaining gradation continuity, wherein this image data has been subjected to appropriate gradation processing in each brightness region. 
     Referring to the flow chart of  FIG. 4 , the following describes the operation of the image processing apparatus  100  having the computing section  110  of the configuration shown in  FIG. 1  or  FIG. 3 , the procedure of the image processing method in the image processing apparatus  100 , and the execution procedure of the image processing program. 
     This image processing apparatus  100  receives colored or monochromatic image data from an image capturing apparatus (not illustrated) and others. The image processing apparatus  100  can receive image data directly from the image capturing apparatus, or can receive the image data stored in the image data storage apparatus. Further, it can be connected with the image capturing apparatus or image data storage apparatus directly or through the network. 
     In the first place, the image processing apparatus  100  receives colored or monochromatic image data from an image capturing apparatus or image storage apparatus (not illustrated) (S 1  in  FIG. 4 ). The image data received by the image processing apparatus  100  in this manner is stored in an HHD  190 . 
     In response to the width of the dynamic range of the image data having been received, the synthesis number determining section  114  determines the synthesis number M used to synthesize the image-processed image data. Here the plural number M refers to the synthesis number. However, it can be the number of different gradation conversion processing characteristics or the number of the image data processed according to different gradation conversion processing characteristics. Based on the plural number M having been determined, the gamma determining section  113  determines the gamma for the plural number M different gradation conversion processing characteristics (gamma characteristics) (S 2  in  FIG. 4 ). 
     To be more specific, as the dynamic range of the image data is wider, the difference in each synthesis value tends to be greater at the time of synthesis (to be described later). An abrupt change may occur between the adjustment pixels, depending on a change in pixel value. Thus, the synthesis number determining section  114  determines the plural number M in such a way that the value M will be greater as the dynamic range is wider. For example, numeral  3  through  10  can be assigned in response to the variation in the permissible dynamic range. 
       FIG. 5  shows an example of gamma characteristic when M=10 (0 through 9). Here M γ m &#39;s are prepared wherein n=0 through 9. These γ m &#39;s are defined by the following Mathematical Formula (1):
 
[Mathematical Formula 1]
 
γ m   =n·a+b   (1)
 
     wherein a&gt;0, b&gt;0, n is a positive integer, and m is 3 through M. 
     In this case, a plurality of different γ m &#39;s lie at an equally spaced interval. “a” indicates the magnitude of the interval, and “b” represents the minimum γ value. The minimum γ value is preferably smaller than 1. The interval “a” of different γ m &#39;s is preferably in the range from 0.1 through 0.5 to ensure that there will be no abrupt change subsequent to synthesis. Further, the aforementioned M is preferably determined so as to get such “a”. 
     Based on each of the plural number M of different gamma characteristics, the image processing section  112  applies image processing (gamma conversion processing) to the image data, and generates the plural number M of the image-processed image data (S 3  in  FIG. 4 ). 
     In the following description, “image-processed image data” will be referred to as “gamma converted image data”, and “the plural number M of the image-processed image data” will be referred to as “the plural number M of gamma converted image data”. 
     This image processing (gamma conversion processing) is preferably carried out, as shown in the following Formula (2). 
     
       
         
           
             
               
                 
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     wherein I max , I min  and I indicate the maximum brightness value of original image, the minimum brightness value of original image and the brightness value of the target pixel, respectively. 
     The synthesis ratios of the plural number M of gamma converted image data are calculated by reference to the image information of the image data prior to image processing (S 4  in  FIG. 4 ). The pixel value of the target pixel corresponds to the image information. 
     To put it specifically, the brightness region of the original image is divided into the plural number M of equal parts and the intermediate value I m  of each divided region is calculated.  FIG. 6  shows an example of division. 
     As shown in the following Formula (3), the synthesis ratio determining section  115  determines the synthesis ratios of the plural number M of gamma converted image data according to the brightness value I of each target pixel of the original image (S 4  in  FIG. 4 ). To put it another way, the synthesis ratio determining section  115  determines the synthesis ratios of the plural number M of gamma converted image data for each pixel. 
     
       
         
           
             
               
                 
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     where σ indicates a constant. 
     The constant σ in Formula (3) determines the number of the gamma converted image data wherein the weighing factor is increased at the time of synthesis. When this constant σ is greater, the range of synthesis is increased, and a great number of gamma converted image data are added to one another, with the result that a gentler curve is obtained without abrupt change. However, this includes the possibility of the contrast being reduced. To eliminate this possibility, it is preferred to assume the value σ shown in the following Formula (4). 
     
       
         
           
             
               
                 
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     When the constant σ and synthesis ratios have been determined by the synthesis ratio determining section  115 , the synthesis section  116  synthesizes the plural number M of gamma converted image data, as shown in the following Formula (5), thereby generating the image data to be outputted (S 5  in  FIG. 4 ). 
     
       
         
           
             
               
                 
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     The aforementioned procedure allows image processing to be carried out for each pixel by the synthesis of the appropriate gradation conversion processing characteristics. Thus, appropriate gradation processing can be carried out in each brightness region while the gradation continuity is maintained. 
     At the time of synthesis, the synthesis methods of the approximated gradation values are approximate to each other, and the gamma converted image data wherein the gamma values are approximate with each other have greater synthesis ratios. Accordingly, there is no abrupt change in the image data subsequent to synthesis, and the gradation conversion processing characteristic curve having been synthesized becomes smooth. 
       FIG. 7  is a diagram showing an example of the synthesized gradation conversion processing characteristic curve in the image processing apparatus of the present embodiment of the invention.  FIG. 7(   a ) is a synthesis curve wherein u is smaller.  FIG. 7(   b ) is a synthesis curve wherein σ is greater. If σ is smaller as a gradation conversion processing characteristic curve, there will be an abrupt inclination between the low and high brightness regions, as shown in  FIG. 7(   a ). If the σ is still smaller, gradation reversing will occur in the intermediate brightness region. In this case, some restriction is preferably imposed to eliminate the possibility of reversing. 
     If the σ is greater as the synthesized gradation conversion processing characteristic curve, a gentle inclination will be exhibited, as shown in  FIG. 7(   b ). There will be no abrupt inclination, as compared to  FIG. 7(   a ). An overall gentle characteristic can be observed. 
     As described above, adjustment of σ allows the inclination of the gradation conversion processing characteristic curve to be controlled within a desired input brightness region. 
     In the input brightness region to be emphasized, reducing of the σ will provide the gradation conversion processing characteristic curve wherein that region is emphasized. 
       FIG. 8(   a ) shows the characteristics wherein the low brightness region is emphasized,  FIG. 8(   b ) represents the characteristics wherein the intermediate brightness region is emphasized, and  FIG. 8(   c ) represents the characteristics wherein the high brightness region is emphasized. 
     As described above, adjustment of σ in a desired input brightness region ensures the inclination of the gradation conversion processing characteristic curve to be controlled in a desired input brightness region. 
     To put it another way, in the present specification, “appropriate gradation processing” can be defined as carrying out gradation conversion by implementing the characteristics wherein a desired input brightness region is emphasized, while the gradation continuity is maintained without allowing gradation reversing. 
     As shown in  FIG. 9 , the smoothing section  117  is arranged in the pre-stage of the synthesis ratio determining section  115  or in the pre-stages of the synthesis number determining section  114  and synthesis ratio determining section  115 . Based on the image data having been smoothed by the smoothing section  117  (S 4  in  FIG. 10 ), the synthesis number determining section  114  determines the synthesis number M and the synthesis ratio determining section  115  determines the synthesis ratios (S 5  in  FIG. 10 ). This procedure is also preferred. At least, it is preferred that the smoothing section  117  should be arranged in the pre-stage of the synthesis ratio determining section  115 , and, based on the image data smoothened by the smoothing section  117  (S 4  in  FIG. 10 ), the synthesis ratio determining section  115  should determine the synthesis ratios (S 5  in  FIG. 10 ). In this case, a Gaussian filter can be used in the smoothing section  117 . 
     To put it more specifically, the brightness region of the original image smoothed by the smoothing section  117  is divided into M equal parts, and the intermediate value Im of each divided region is calculated. After that, as shown in the following Formula (6), the synthesis ratio determining section  115  determines the synthesis ratios of the M gamma converted image data according to the brightness value of each target pixel off the original image having been smoothed. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
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     wherein σ indicates a constant. 
     The constant σ in Formula (6) determines the number of the gamma converted image data wherein the weighing factor is increased at the time of synthesis. When this constant σ is larger, the range of synthesis is increased, and a large number of gamma converted image data are added to one another, with the result that a gentler curve is obtained without abrupt change. However, this includes the possibility of the contrast being reduced. To eliminate this possibility, it is preferred to assume the value σ shown in the following Formula (7). 
     
       
         
           
             
               
                 
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                     Formula 
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     When the constant σ and synthesis ratio have been determined by the synthesis ratio determining section  115  in the aforementioned manner, the synthesis section  116  synthesizes plural number M of gamma converted image data according to the aforementioned Formula (5), and generates the image data to be outputted (S 6  in  FIG. 10 ). 
     According to the aforementioned procedure, image processing is carried out by the synthesis of the appropriate gradation conversion processing characteristics for each pixel. Thus, appropriate gradation processing can be carried out in each brightness region, while gradation continuity is maintained, without being affected by an abrupt change in the noise and other factors contained in the image data. 
     It is also possible to install an operation section  118  to perform various forms of operations and to input instructions, and a control section  111  so that the synthesis number M and value for σ can be set by the operator, as shown in  FIG. 11 . 
     In each of the aforementioned embodiments, gradation conversion processing (γ conversion processing) can be applied to either the brightness component or color component of the image data.