Abstract:
The purpose of the present invention is to provide an image display device, an image processing method, and a computer-readable medium which can perform proper color reproduction while saving memory capacity. The image display device of the present invention, which applies a desired image processing to an input image data so as to display an image, uses a first color corrector  110  to refer to a three-dimensional color correction table which matches a color characteristic of the image display device with a reference color characteristic based on a characteristic value of the image display device, and to apply desired color correction to the inputted image data. Then, the image display device uses second color correction means  120  to refer to a one-dimensional color correction table for correcting gradation according to an application circumstances, and applies desired color correction to the inputted image data.

Description:
BACKGROUND OF INVENTION 
   1. Field of Invention 
   The present invention relates to an image display device, an image processing method, a program, and a storage medium which apply desired color correction to an output image. 
   2. Description of the Related Art 
   Since a color reproduction area differs depending on the type of an image display device such as a projector, the colors of a displayed image may change. To prevent this problem, a processing referred to as a color matching is generally conducted to match a color characteristic of an image processing unit with a color characteristic of a general CRT monitor 
   When the image display device such as a projector is used, it is important to reproduce an image intended by an author even when an external circumstances changes. Especially, it is difficult to reproduce colors properly without considering a case where the brightness or the color of an external illumination, or the color of a plane of projection changes as changes of the external circumstances. 
   A color correction table is generally used for these color matching and correction according to the external circumstances. 
   However, it is difficult for the image display device such as a projector to hold a large quantity of data in the color correction table due to a restriction on a memory capacity. Namely, since the individual projectors largely differ from one another, it is necessary to store color correction tables matched to the individual machines. 
   SUMMARY OF INVENTION 
   The present invention is devised to solve the problem above, and has a purpose of providing an image display device, an image processing method, a program, and a storage medium which properly perform color reproduction while saving the memory capacity. 
   According to one aspect of the present invention, an image display device for applying a desired image processing on inputted image data, and displaying an image, includes: a 1st color correction unit for applying desired color correction to the inputted image data while referring to a three-dimensional color correction table so as to match a color characteristic of the image display device with a reference color characteristic based on a characteristic value of the image display device; and a 2nd color correction unit for applying desired color correction to the inputted image data while referring to a one-dimensional color correction table for correcting gradation in accordance with an application circumstances. 
   According to the thus constructed image display device for applying a desired image processing on inputted image data, and displaying an image, desired color correction is applied to the inputted image data by a 1st color correction unit while referring to a three-dimensional color correction table so as to match a color characteristic of the image display device with a reference color characteristic based on a characteristic value of the image display device. By the 2nd color correction unit, desired color correction is applied to the inputted image data while referring to a one-dimensional color correction table for correcting gradation in accordance with an application circumstances. 
   According to the present invention, the image display device may Her include a 1st rewriting unit for rewriting the three-dimensional color correction table so as to allow the 1st color correction unit to correct color tone in accordance with an application circumstances. 
   According to the present invention, the image display device may further include a 2nd rewriting unit for rewriting the three-dimensional color correction table so as to allow the 1st color correction unit to correct grid point data in the tree-dimensional color correction table based on the characteristic value. 
   According to the present invention, the one-dimensional color correction table for the 2nd color correction unit may be used for correction for a change of brightness of external illumination. 
   According to the present invention, the three-dimensional color correction table for the 1st color correction unit may be used for correction for a change of color on a plane of projection. 
   According to the present invention, the three-dimensional color correction table for the 1st color correction unit may be used for correction for a change of color of an external illumination. 
   According to the present invention, the image display device may further include a unit for entering the characteristic value. 
   According to the present invention, the image display device may be a projector. 
   According to the present invention, the 2nd rewriting unit may not rewrite the grid point data when the characteristic value is a reference characteristic value. 
   According to the present invention, if the inputted image data are converted to the outside of a color gamut when the desired image processing is applied while referring to the three-dimensional color correction table rewritten by the 1st rewriting unit, the inputted image data may be converted to the inside of the color gamut by reducing correction amounts while the ratio of changes of individual color components are maintained. 
   According to another aspect of the present invention, an image processing method for applying a image processing on image data inputted to a display device, includes: a 1st color correction step for applying desired color correction to the inputted image data while referring to a three-dimensional color correction table so as to match a color characteristic of the image display device with a reference color characteristic based on a characteristic value of the image display device; and a 2nd color correction step for applying desired color correction to the inputted image data while referring to a one-dimensional color correction table for correcting gradation in accordance with an application circumstances 
   According to another aspect of the present invention, a computer-readable medium has a program of instructions for execution by the computer to perform a image processing with respect to image data inputted to a display device, the program including: a 1st color correction processing for applying desired color correction to the inputted image data while referring to a three-dimensional color correction table so as to match a color characteristic of the image display device with a reference color characteristic based on a characteristic value of the image display device; and a 2nd color correction processing for applying desired color correction to the inputted image data while referring to a one-dimensional color correction table for correcting gradation in accordance with an application circumstances. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a function block diagram for an image processor in a projector according to an embodiment of the present invention; 
       FIG. 2  is a flowchart describing the operation of a 1st color corrector  110  and a 2nd color corrector  120  in the projector of an embodiment of the present invention; 
       FIG. 3  is a flowchart describing generation of a color correction table by the 1st color correction table generator  112 ; 
       FIG. 4  is a flowchart describing generation of LUT data stored in an LUT data storage unit  114 ; 
       FIG. 5  describes generation of the LUT data; 
       FIG. 6  describes correspondence between a color of a CRT and a color of the projector; 
       FIG. 7  is a flowchart describing calculation of correction amounts for a color of illumination in S 10  in  FIG. 3 ; 
       FIG. 8  is a flowchart describing calculation of correction amounts for a color of a screen in S 12  in  FIG. 3 ; 
       FIG. 9  is a flowchart describing conversion of values in a 3D-LUT in S 14  in  FIG. 3 ; 
       FIG. 10  is a flowchart describing generation/rewrite of a color correction table by a 2nd color correction table generator  150  in an embodiment of the present invention; 
       FIG. 11  is a flowchart describing calculation of a correction curve by the 2nd color correction table generator  150  in an embodiment of the present invention; 
       FIG. 12  is a graph showing γ curves in different circumstances 
       FIG. 13  is a graph showing normalized γ curves in different circumstances; 
       FIG. 14  is a graph showing a state in which normalized γ curves in different circumstances are overlapped together at a reference point Do; 
       FIG. 15  is a chart describing correction applied to an output characteristic after the correction; 
       FIG. 16  is a chart describing adjustment for the correction amount of the correction curve; 
       FIG. 17  is a chart showing a relationship between Dout and Din; 
       FIG. 18  describes rounding the correction curve; 
       FIG. 19  is a graph ( 1 ) showing an example of a correction curve obtained by changing the value of Do; 
       FIG. 20  is a graph ( 2 ) showing an example of a correction curve obtained by changing the value of Do; 
       FIG. 21  describes color correction by a 3rd color corrector  130 ; and 
       FIG. 22  is a flowchart describing the generation of a color correction table for the projector (processing in step  203  in  FIG. 2 ) according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following section describes a preferred embodiment of the present invention while referring to drawings. 
   First Embodiment 
   System Structure 
     FIG. 1  shows a function block diagram of an image processor  100  in a projector according to a first embodiment of an image display device of the present invention. The image display device of the present invention includes a CRT and an LCD display in addition to the projector 
   The image processor  100  in the projector according to the first embodiment of the present invention is provided with a 1st color corrector  110  for correcting color characteristic of the projector according to a surrounding environment and color matching based on a color correction table (LUT) generated by a 1st color correction table generator  112 , a 2nd color corrector  120  for correcting tone characteristic of the projector according to the surrounding environment based on a color correction table generated by a 2nd color correction table generator  150 , a 3rd color corrector  130  for adjusting an output characteristic of a liquid crystal light valve, and an L/V (light valve) drive unit  140  for driving a liquid crystal light valve so as to project an image for display. 
   In addition, the image processor  100  is provided with a γ value input unit  116  for entering a γ value for the projector, a LUT data storage unit  114  for storing data (converted values and LUT data) and grid point data, while they are associated with one another, in the color correction table for generating a three-dimensional color correction table (3D-LUT) for the color matching and the color characteristic correction according to the surrounding environment, a device characteristic storage memory  160  for storing output characteristic information of the project in a reference environment, an optical sensor  170  for measuring luminance of light from the screen on which the projector and the external illumination reflected, and a 1st color correction table generator  112  for generating the three-dimensional color correction table for the color matching and the color characteristic correction according to the surrounding environment based on the γ value entered from the γ input unit  116 , the data stored in the LUT data storage unit  114 , the color value measured by the optical sensor  170 , and the information stored in the device characteristic storage memory. 
   Further, the image processor  100  is provided with a 2nd color correction table generator  150  for generating a one-dimensional color correction table (1D-LUT) for correcting the tone characteristic according to the surrounding environment based on the color value measured by the optical sensor  170  and the information stored in the device characteristic storage memory. 
   In the projector of the first embodiment of the present invention, first, the 1st color corrector  110  applies the color matching and the color characteristic correction according to the surrounding environment to an image input signal supplied from a personal computer or the like while referring to the color correction table generated by the 1st color correction table generator  112 . Then, the 2nd color corrector  120  refers to the color correction table generated by the 2nd color correction table generator  150 , and then, applies tone characteristic correction according to the surrounding environment to the image signal applied with the color matching and the tone characteristic correction according to the surrounding environment. The 3rd color corrector  130  adjusts the image signal applied with the color correction while considering the output characteristic of the liquid crystal light valve. The L/V driving unit  140  derives the liquid crystal light valve based on this adjusted analog signal so as to project the image for display. 
   Processes Performed by the Image Processor  100   
   Processes such as a color correction table generation processing and an image processing performed by the image processor  100 , which will be described below, are carried out by executing an image processing program stored in a program storage unit (not shown) provided in the projector  20 . The program storage unit constitutes a medium which stores the image processing program. The image-processing program itself is also included in the scope of the present invention. 
   (1) Color Correction in 1st Color Corrector  110   
   The following section describes the operation of the 1st color corrector  110  in the projector according to an embodiment of the present invention while referring to  FIG. 2 . 
   First, before the projector of the present invention is used, the color correction table is generated, and then, is stored in the LUT data storage unit  114  in advance (step  203 ). The following section details the generation of the color correction table while referring to  FIG. 22 . 
   When the use of the projector according to the present invention is started, there is performed a color correction table generation/rewrite processing by the 1st color correction table generator  112  Cp  204 ). As to the color correction table generation/rewrite processing, it will be described below in detail with reference to  FIG. 3 . 
   After the color correction table generation/rewrite processing, there is made image display on the basis of the image signal color corrected by the 1st color corrector  110  and with reference to the rewritten color correction table (step  206 ). If the image display is not terminated (step  208 , No) and if a certain time has not elapsed from the end of the last-time color correction table generation/rewrite processing (step  210 , No), the state of image display in step  206  continues. On the other hand, if the display of image is not terminated (step  208 , No) and a certain time has elapsed from the termination of the last-time color correction table generation/rewrite processing (step  210 , Yes), the color correction table generation/rewrite processing is again performed to make the color tone correction according to the application circumstances with the lapse of time (step  204 ) and there is made image display (step  206 ). According to the present invention, since the color correction table is rewritten at every certain time to perform the color tone correction according to the application circumstances, an appropriate color reproduction is ensured even if the color of external illumination or the color of the plane of projection changes. 
   In the case where the display of image is terminated, for example by turning off a power supply of the projector (step  208 , Yes), the processing is ended. 
   (1-0) Color Correction Table Generation Processing 
   Next, with reference to  FIG. 22 , a description will be given about the color correction table generation processing (the processing in step  203  in  FIG. 2 ) which is performed by the projector according to an embodiment of the present Invention. 
   (1-0-1) Measuring Output Characteristic of Projector in Reference Circumstances 
   Before the 1st color correction table generator  112  rewrites the color correction table, tristimulus values of RGBK are measured in the reference circumstances in advance. The reference circumstances means a case where output light from the projector is projected on a reference screen in a dark room. 
   When a measured color is (R, G, B)=(255,0,0), the measured values are set to X R0 , Y R0 , and Z R0 , 
   when a measured color is (R, G, B)=(0,255,0), the measured values are set to X G0 , Y G0 , and Z G0 , 
   when a measured color is (R, G, B)=(0,0,255), the measured values are set to X B0 , Y B0 , and Z B0 , and 
   when a measured color is (R, G, B)=(0,0,0), the measured values are set to X K0 , Y K0 , and Z K0 . 
   An output characteristic matrix M 0  for the projector in the reference circumstances is obtained based on these measured values, and is stored in the device characteristic storage memory  160 . The output characteristic matrix M 0  for the projector in the reference circumstances is represented in the following equation. 
                   M   0     =     (             X   R0     -     X   K0               X   G0     -     X   K0               X   B0     -     X   K0                   Y   R0     -     Y   k0               Y   G0     -     Y   K0               Y   B0     -     Y   K0                   Z   R0     -     Z   K0               Z   G0     -     Z   K0               Z   B0     -     Z   K0             )             [     Expression   ⁢           ⁢   1     ]               
(1-0-2) Creating 3D-LUT for Color Matching in Reference Circumstances
 
   The 3D-LUT for matching the output characteristic of the projector with a predetermined color space is created, and-then, is stored in the LUT data storage unit  114  in advance. The method for creating this 3D-LUT is arbitrary, and the non-conversion 3D-LUT may be used when the matching is not required The output values of the 3D-LUT for the color matching are represented as:
 
{R(Rin, Gin, Bin), G(Rin, Gin, Bin), B(Rin, Gin, Bin)}
 
where Rin, Gin, and Bin are the entered value.
 
Example of LUT Data Generation
 
   The following section describes an example of generating the LUT data stored in the LUT data storage unit  114  while referring to  FIG. 4 . In this embodiment, a case where the color characteristic of the projector is matched with the color characteristic (the reference color characteristic) of the CRT is described 
   First, a correspondence between an input value R C G C B C ) for the CRT and a color coordinate (X C Y C Z C , L C *a C *b C *) of the output color is obtained (S 20 ). Correspondences for typical colors are obtained by actually presenting a color from the CRT, and then, measuring the presented color, and the correspondences for the remaining colors are obtained by interpolation calculation or the like. Then, a correspondence between an input value (R P G P B P ) for the projector and a color coordinate (X P Y P Z P , L P *a P *b P *) of the output color is obtained (S 22 ). In the same way, correspondences for typical colors are obtained by actually presenting a color from the projector, and then, measuring the presented color, and the correspondences for the remaining colors are obtained by interpolation calculation or the like. 
   Then, the output color (L P *a P *b P *) from the liquid crystal projector corresponding to the output color (L C *a C *b C *) from the CRT is determined (S 24 ). Usually, the same colors are associated with each other ((L C *=L P *, a C *=a P *, b C *=b P *)). However, when the projector cannot present the output color (T C *a C *b C *) from the CRT, a color which is relatively close to that color (a color having the same hue, and the minimum distance on the color coordinate, for example) is associated as shown in  FIG. 6 . 
   Then, R P G P B P  values corresponding to individual R C G C B C  values are obtained so as to create the LUT data based on the correspondences obtained in S 20  to S 26  as shown in  FIG. 5  (S 28 ). 
   In the present embodiment, it is assumed that the LUT data generated as described above, and the grid point data are stored in the LUT data storage unit  114  in advance. 
   (1-1) Generation and Rewrite of Color Correction Table by 1st Color Correction Table Generator  112   
   The following section describes generating and rewriting processes (processes in the step  204  in  FIG. 2 ) for the color correction table by the 1st color correction table generator  112  in the projector (processing in the step  204  in  FIG. 2 ) according to an embodiment of the present invention while referring to  FIG. 3 . 
   (1-1-1) Measuring Output Characteristic of Projector in Application Circumstances 
   First, the txistimulus values of RGBK of the projector are measured in the application circumstances for generating and rewriting the color correction table by the 1st color correction table generator  112 . 
   When a measures color is (R, G, B)=(255,0,0), the measured values are set to X R , Y R , and Z R , 
   when a measures color is CR, G, B)=(0,255,0), the measured values are set to X G , Y G , and Z G , 
   when a measures color is CR, G, B)=(0,0,255), the measured values are set to X B , Y B , and Z B , and 
   when a measures color is (R, G, B)=(0,0,0), the measured values are set to X K , Y K , and Z K . 
   An output characteristic matrix M for the projector in the application circumstances is obtained based on these measured values, and is stored in the device characteristic storage memory  160 . The output characteristic matrix M for the projector in the application circumstances is represented in the following equation. 
                 M   =     (             X   R     -     X   K               X   G     -     X   K               X   B     -     X   K                   Y   R     -     Y   k               Y   G     -     Y   K               Y   B     -     Y   K                   Z   R     -     Z   K               Z   G     -     Z   K               Z   B     -     Z   K             )             [     Expression   ⁢           ⁢   2     ]               
(1-1-2)
 
   After the output characteristic of the projector is measured in the application circumstances, first, correction amounts for the color of the illumination (S 10 ), and correction amounts for the color of the screen (the plane of projection) (S 12 ) are calculated in the process for generating and rewriting the color correction table by the 1st color correction table generator  112  as shown in  FIG. 3 . Then, the values in the three-dimensional color correction table are converted based on the result of these calculations (S 14 ). The individual processes in S 10  to S 14  are detailed later. 
   (1-1-2-1) Calculating Correction Amounts for Color of Illumination (S 10 ) 
   The following section describes calculation of the correction amounts for the color of the illumination in S 10  in  FIG. 3  while referring to  FIG. 7 . In this process, a change of an output color characteristic of the projector due to the color of the illumination is corrected by correcting offsets of the individual colors of R, G, and B of the projector. More specifically, a difference between the color of the illumination and white of the projector is obtained, and then the amounts of the offsets are adjusted by the difference. 
   As shown in  FIG. 7 , first, the color of the illumination is represented in the RGB of the projector in the calculation of the correction amounts for the color of the illumination in S 10  in  FIG. 3  (S 30 ). The color of the illumination (corresponding to a value when the black of the projector is measured in the application circumstances) is represented by the following equation when it is represented in a mixture of the R, G, and B of the projector: 
   
     
       
         
           
             
               
                 
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   Then, the R, G and B values for a white point in the 3D-LUT is read out, and luminance ratios of the R, G, and B values at the white point are obtained in the calculation of the correction amounts for the color of the illumination in S 10  in  FIG. 3  (S 32 ). The R, G, and B values at the white point, r W0 , g W0 , and b W0 , are:
 
 r   W0   ={R (255,0,0)/255} γ 
 
 g   W0   ={G (0,255,2)/255} γ 
 
 b   W0   ={B (0,0,255)/255} γ 
 
   γ is a value for representing an output gradation characteristic of the projector, and is entered from the γ value input unit  116 . 
   Based on the procedure above, offset correction amounts r 0 , g 0 , and b 0  are obtained in the calculation of the correction amounts of the illumination color in S 10  in  FIG. 3  ( 834 ). 
   As the following equation indicates, differences between r K , g K , and b K  and r W0 , g W0 , and b W0  are obtained so as to make them as the offset correction amounts. 
   
     
       
         
           
             
               
                 
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   In the equation, α 0  is a parameter for adjusting degree of applying the correction, and a proper value for it ranges from 0.0 to 0.5. 
   This is the end of the processing in S 10 . 
   (1-1-2-2) Calculating Correction Amounts for Screen Color (S 12 ) 
   The following section describes calculation of the correction amounts for the color of the screen in S 12  in  FIG. 3  while referring to  FIG. 8 . A change of an output color characteristic of the projector due to the color of the screen is corrected by correcting the gains of the individual RGB colors of the projector. Specifically, a gain, which is for representing the white of the projector in the reference circumstances by using the R, G, and B in the application circumstances, is obtained 
   In the calculation of the correction amounts for the color of the screen in S 12  in  FIG. 3 , as shown in  FIG. 8 , first, the color of the white of the projector (r W , g W , b W ) in the reference circumstances is represented by the R, G, and B of the projector in the application circumstance (S 40 ). Namely, luminance ratios of the R, G, and B are obtained if the luminance ratios of the R, G, and B reproduce the same color in the application circumstance with the white of the projector in the reference circumstances as follows: 
   
     
       
         
           
             
               
                 
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   Then, gain correction amounts r G , g G , and b G  are obtained in the calculation of the correction amounts for the color of the screen in S 12  in  FIG. 3  (S 42 ). The gain correction amounts are obtained as follows from r W , g W , and b W  and r W0 , g W0 , and b W0 .
 
 r   G =1+α G   {p   r /max( p   r   , p   g   , p   b )−1 }, p   r   =r   W0   /r   W 
 
 g   G =1+α G   {p   g /max( p   r   , p   g   , p   b )−1 }, p   g   =g   W0   /g   W 
 
 b   G =1+α G   {p   b /max( p   r   , p   g   , p   b )−1 }, p   b   =b   W0   /b   W 
 
   In the equation, α G  is a parameter for adjusting degree of applying the correction, and a proper value for it ranges from 0.5 to 1.0. 
   This is the end of the processing in S 12 . 
   (1-1-2-3) Converting Values in Three-dimensional Color Correction Table (3D-LUT) (S 14 ) 
   The following section describes conversion of the values in the 3D-LUT in S 14  in  FIG. 3  while referring to  FIG. 9 . In this processing, a correction corresponding to a change of the circumstances is added to the output values for the individual grid points of the 3D-LUT stored in the LUT? data storage unit  114  to create a new correction table. 
   As shown in  FIG. 9 , the output values are read out from the individual grid points in the 3D-LUT stored in the LUT data storage unit  114 , and are converted into luminance values in the conversion of the values in the 3D-LUT in S  14  in  FIG. 3  (S 50 ). The converted luminance values (r), (g), and (b) are:
 
 r={R ( R in,  G in,  B in)/255} γ 
 
 g={G ( R in,  G in,  B in)/255} γ 
 
 b={B ( R in,  G in,  B in)/255} γ 
 
   Then, circumstantial correction is applied in the conversion of the values in the 3D-LUT in S 14  in  FIG. 3  (S 52 ). The correction amounts obtained in S 10  and S 12  are used to correct the gain and the offset of the values in the 3D-LUT. The values r′, g′, and b′ after the correction are:
 
 r′=r   G   r+r   0 
 
 g′=g   G   g+g   0 
 
 b′=b   G   b+b   0 
 
   Then, a color outside the color gamut is processed in the conversion of the values in the 3D-LUT in S  14  in  FIG. 3  (S 54 ). 
   The values of r′, g′, and b′ obtained in S 52  may be outside the color gamut (r′&lt;0 or r′&gt;1, for example) after the correction. When the values after the correction are outside the color gamut, the correction amounts are adjusted such that they stay inside the color gamut. The adjusting of the correction amounts is performed while the R, G, and B are mutually correlated. Specifically, the following equations are used to process the color outside the color gamut. 
   
     
       
         
           
             
               
                 
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   Further, the output values from the 3D-LUT after the circumstantial correction are obtained in the conversion of the values in the 3D-LUT in S 14  in  FIG. 3  (S 56 ). The final output values R′(Rin, Gin, Bin), G′(Rin, Gin, Bin), and B′(Rin, Gin, Bin) from the 3D-LUT added with colons corresponding to the circumstantial change are:
 
 R ′( R in,  G in,  B in)=255( r ″) γ 
 
 G ′( R in,  G in,  B in)=255( g ″) γ 
 
 B ′( R in,  G in,  B in)=255( b ″) γ 
 
   This is the end of the processing in S 14 , and the procedure returns to processing in the step  206  in  FIG. 2 . 
   (2) Color Correction in 2nd Color Corrector  120  (Gradation Correction According to Application Circumstances) 
   The operation of the 2nd color corrector  120  in the projector according to an embodiment of the present invention is similar to the operation of the 1st color corrector  110  described while referring to  FIG. 2 . Thus, the following section describes the operation of the 2nd color corrector  120  while referring to  FIG. 2 . 
   When the operation of the projector of the present invention starts, the 2nd color correction table generator  150  generates/rewrites the color correction table (step  204 ). The following section details the generation/rewrite of the color correction table while referring to  FIG. 10 . It is possible to synchronously or independently conduct the generation/rewrite of the color correction table by the 1st color correction table generator  112  and the generation/rewrite of the color correction table by the 2nd color correction table generator  150 . 
   After generating/rewriting the color correction table, an image is displayed based on an image signal with color correction by the 2nd color corrector  120  while referring to the rewritten color correction table (step  206 ). When the image display is not terminated (step  208 , No), and a certain period has not passed since the last generation/correction of the color correction table (step  210 , No), the display state of the image in the step  206  continues. On the other hand, when the image display is not terminated (step  208 , No), and the certain period has passed since the last generation/correction of the color correction table (step  210 , Yes), the color correction table is generated/rewritten so as to correct the gradation according to the application circumstances as time elapses (step  204 ), and the image is displayed (step  206 ). With the present invention, since the color correction table is rewritten at a certain time interval to correct the gradation according to the application circumstances, proper color reproduction is enabled even if the brightness of external illumination changes. 
   Then, when the display of the image is terminated by turning off the projector (step  208 , Yes), the processing ends. 
   (2-1) Generation/rewrite of Color Correction Table by 2nd Color Correction Table Generator  150   
   The following section describes generation/rewrite of the color correction table by the 2nd color correction table generator  150  (processing in the step  204  in  FIG. 2 ) in the projector according to an embodiment of the present invention while referring to  FIG. 10 . 
   The projector (the image display device)  20  presents white (R=G=B=255 in gradation) in a dark room in the generation/rewrite of the color correction table, the optical sensor  170  measures the luminance of the reflected light from the screen  10 , and the result is stored in the device characteristic storage memory  160  in advance. 
   Then, the luminance of reflected light on the screen from the external illumination is measured while the projector is not providing the output (step  222 ). 
   Then, a correction curve is calculated (step  226 ). The following section details the calculation of the correction curve while referring to  FIG. 11 . Then, a new one-dimensional color correction table is generated based on the calculated correction curve Then, the newly generated one-dimensional color correction table replaces the one-dimensional color correction table referred from the color corrector  120  (step  228 ). 
   Calculating Correction Curve 
   The following section describes calculation of the correction curve by the color correction table generator  150  in the projector (processing in the step  226  in  FIG. 10 ) according to an embodiment of the present invention while referring to  FIG. 11 . The correction curve is obtained as follows based on the luminance of the reflected light on the screen for the white output from the projector, which is stored in the device characteristic storage memory  160 , and the measured value obtained in step  222  in  FIG. 10 . 
   In the correction curve calculation processing, γ curves are normalized in different circumstances (step  230 ). Correction curves in all of W (white), R (red), G (green), and B (blue) are the same curves, and therefore, in this embodiment a correction curve is calculated with respect to W as an example. γ curves in different circumstances (in a dark room and in an illuminated surround) are assumed as follows. The “γ” represents a gradation characteristic of the projector concerned. The value of γ is obtained by actually measuring the gradation characteristic of the projector concerned and it is suitable to use a average value as the value of γ. In this embodiment, γ is set equal to 2.2 as an example. 
   In a dark room:
 
 Fd ( D in)= Yw·D in γ   (1)
 
   In an illuminated surround:
 
 Fi ( D in)= Yw·D in γ   +Yi   (2)
 
   γ curves in different circumstances are shown in  FIG. 12 . 
   In the above equations, F represents a total luminance of lights reflected by the screen, Din represents normalized values 0–1 by normalizing digital input values (0–255 in gradation) of RGB, Yw represents the luminance of white d the projector, and Yi represents the luminance of illumination. Then, the equations (1) and (2) are normalized under the assumption that the eyes adapt themselves to luminances (Yw in a dark room, Yw+Yi in an illuminated surround) detected when the projector outputs white in different circumstances. That is, the equations (1) and (2) are normalized so that the luminance (Yw in a dark room, Yw+Yi in an illuminated surround) detected when the projector outputs white in each of different circumstances becomes 1. More specifically: 
   In a dark room:
 
 F′d ( D in)= Fd ( D in)/ Yw=D in γ   (3)
 
   In an illuminated surround:
 
 F′i ( D in)= Fi ( D in)/( Yw+Yi )=( Yw·Din   γ   +Yi )/( Yw+Yi )  (4)
 
   Normalized γ curves in different circustances are shown in  FIG. 13 . 
   Next, the γ curves are overlapped each other at a reference point Do (step  232 ). As shown in  FIG. 14 , F′d(Din) is shifted in parallel in F′ axis direction by only {F′i(Do)−F′d(Do)} so that F′d(Din) takes the same value as F′i(Din). More specifically: 
                     F   ″     ⁢     d   ⁡     (   Din   )         =         F   ′     ⁢     d   ⁡     (   Din   )         +     {         F   ′     ⁢     i   ⁡     (   Do   )         -       F   ′     ⁢     d   ⁡     (   Do   )           }                   =         F   ′     ⁢     d   ⁡     (   Din   )         -       F   ′     ⁢     d   ⁡     (   Do   )         +       F   ′     ⁢     i   ⁡     (   Do   )                       
If the equations (3) and (4) are used:
   F″d ( D in)= D in γ   ·Do   γ +( Yw·Do   γ   +Yi )/( Yw+Yi )  (5) 
   Then, using the equation (5), a correction curve is calculated (step  234 ). 
   Thus, in this embodiment, as shown in  FIG. 14 , a correction curve is formed so that an output value of the γ curve in an illuminated surround coincides with an output value of the γ curve in a dark room in the vicinity of the reference point Do. 
   Then, the change in color of output image, which depends on whether an external illumination is present or not, is diminished by correcting input gradation values so that a relative contrast (gradient of γ curve) in the vicinity of the reference point Do does not change depending on whether an external illumination is present or not. 
   The above can be expressed by the following equation:
 
 F″i ( D out)= F″d ( D in)  (6)
 
where Dout represents input gradation values after correction.
 
   Substitution of equations (4) and (5) for equation (6) gives:
 
( Yw·D out γ   +Yi )/( Yw+Yi )= D in γ   −Do   γ +( Yw·Do   γ   +Yi )/( Yw+Yi )
 
Thus,
 
 D out=[(1 +Yi/Yw ) D in γ −( Yi/Yw ) Do   γ ] 1/γ   (7)
 
   Note that since there is a limit for the luminance range for the output (0≦F″d(Din)≦1) as shown in  FIG. 15 , correction is applied so as to provide an output shown in  FIG. 15 . 
   Consequently, when Dout&lt;0, 
   Dout=0, and 
   when Dout&gt;1, 
   Dout=1. 
   The correction curve changes variously by changing the gradation Do which serves as a main factor in correcting a lowering of contrast caused by illumination. Generally, if the value of Do is small, there is obtained such a correction curve as shown in  FIG. 19 , and although the gradation in a low gray scale region is improved, the projection screen looks whitish, affording a light tone. On the other hand, if the value of Do is made large, there is obtained such a correction curve as shown in  FIG. 20 , in which the projection screen is blackish as a whole and the gradation change in the low gray scale region further decreases (what is called collapse of the low gray scale region becomes conspicuous). By setting the value of Do at an appropriate value, it is possible to make such a correction as contrast is most emphasized with little change in the entire brightness of projected image as compared with that before correction. As a result of experimental evaluation, it turned out that a Do value near a middle gray scale (0.25≦Do≦0.50 or so) was suitable. 
   Further, as shown in  FIG. 16 , the amount of correction can be adjusted by multiplying the amount of correction ΔF by α (0≦α≦1). This is for preventing an unnatural image reproduction caused by an excessive correction. The equation (7) of Dout in adjusting the amount of correction becomes as follows:
 
 D out=[(1 +αYi/Yw ) D in γ −(α Yi/Yw ) Do   γ ] 1/γ   (7′)
 
Therefore, multiplying the amount of correction by α eventually corresponds to multiplying the luminance of illumination, Yi, by α.
 
   It is preferable that the value of α be within the range of 0.8≦α≦1. 
   Then the correction curve is rounded (step  236 ). 
     FIG. 17  shows a relationship between Dout and Din represented by an equation (7) or equation (7′) in  FIG. 17 . As  FIG. 17  shows, though the correction curve is constituted so as to emphasize the contrast as a whole, since the gradation disappears around Dout=0 and Dout=1 on the color correction curve shown in  FIG. 17 , the disappearance of the gradation around Dout=0 and Dout=1 is prevented by rounding the correction curve. 
   1) Rounding for Reducing Correction Amount 
   First, to perform where Dout stays at 0 or 1 without gradation, the correction amount ΔD is reduced by converting the correction amount ΔD=Dout−Din as follows.
 
ΔD→ΔD−(ΔD) β   (8)
 
With this conversion, since the reduction of the correction amount increases as the correction amount becomes larger as shown in  FIG. 18 , the correction curve is rounded consequently. β in the equation (8) is a parameter representing the degree of the rounding, the rounding is not conducted when β=0, and Dout=Din when β=∞. A preferable value for β is about 1.5. A curve ( 1 ) in  FIG. 18  shows a relationship between Dout and Din when the rounding is applied to reduce the correction amount.
 
2) Rounding with Neighborhood Averaging
 
   Since a sharp angle remains on the correction curve ( 1 ) in  FIG. 18 , averaging neighborhood is conducted for individual points further Specifically, the gradation data are calculated as 33 points (Din×255=0, 8, 16, . . . , 255), and then, total of five points including previous and following two points are averaged at the individual points. As a result of the process above, a correction curve is generated without gradation where Dout stays at 0 or 1. 
   When the correction curve is calculated, four parameters comprising γ of the projector, a reference point Do, a correction amount α, and the parameter for rounding β are necessary. Various correction curves are generated with the same calculation method by adjusting these values. 
   (3) Color Correction in 3rd Color Corrector  130   
   The following section describes the color correction by the 3rd corrector  130  while referring to  FIG. 21 . 
   The output characteristic of the projector is set as shown in  FIG. 21(   a ), and an input/output characteristic of the liquid crystal panel is measured as shown in  FIG. 21(   b ). Then, a correspondence between the input signal and the input value to the liquid crystal panel is obtained based on the  FIGS. 21(   a ) and ( b ) as shown in  FIG. 21(   c ). 
   The 3rd color corrector  130  adjusts the input value to the liquid crystal panel while referring to a color correction table representing the correspondence between the input signal and the input value to the liquid crystal panel shown in  FIG. 21(   c ). The color correction tables are stored for the individual projectors in advance.