Patent Publication Number: US-8115820-B2

Title: Image processing apparatus and image processing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of operating an image obtained by a color image pickup apparatus, such as a film camera, a digital camera and a video camera, into a proper image, and a program and apparatus used for the method. The present invention relates more particularly to image processing that makes inconspicuous an originally nonexistent blur color distribution around a bright image. 
     2. Description of the Related Art 
     A color image pickup system generates originally nonexistent artifacts in blue, in red, and in purple as a blend of blue and red in a blur shape, around a bright part on an image due to a chromatic aberration of an imaging optical system. This blur is referred to as a color blur or a purple fringe. 
     A combination of plural lenses having different dispersions can optically reduce the chromatic aberration to some extent. However, as a small size of a digital camera progresses, high resolution image sensors (or image pickup devices) and smaller optical systems are increasingly demanded, and it is difficult to sufficiently reduce the chromatic aberration only by the optics. Therefore, the image processing is required to reduce an artifact. 
     The chromatic aberration is roughly classified into a lateral chromatic aberration (chromatic aberration of magnification) and a longitudinal (or axial) chromatic aberration. When the light emitted from a light source  10  images on a focal plane  12  via an imaging optical system  11 , as shown in  FIG. 1 , the lateral chromatic aberration appears as shifts of imaging positions of a B ray  13  having a wavelength of about 450 nm, a G ray  14  having a wavelength of about 550 nm, and a R ray  15  having a wavelength of about 650 nm in the image surface direction. On the other hand, when the light emitted from a light source  20  images on a focal plane  22  via an imaging optical system  21 , as shown in  FIG. 2 , the longitudinal chromatic aberration appears as shifts of imaging positions of the RBG rays  23 - 25  in the optical-axis direction. 
     As disclosed in U.S. Pat. No. 6,724,702, the lateral chromatic conversion of a digital imaging system of a primary color system can be corrected through a geometric conversion that applies a different deformation to each color plane of RGB. 
     On the other hand, the longitudinal chromatic aberration means, in an image that is focused with the G plane as a central wavelength of the visible light region, for example, that the image is out of focus with respect to the R and B planes that are ends of the visible light. This cannot be corrected by the geometric conversion although it is effective to the lateral chromatic aberration. The longitudinal chromatic aberration can be made inconspicuous by lowering the chroma of a color blur area as disclosed in Japanese Patent Laid-Open No. (“JP”) 2001-145117, or can be corrected by applying a different contour emphasis process to each color plane of RGB as disclosed in JP 2003-018407. 
     In addition, a color blur reduction process is available even in the motion picture imaging, as disclosed in JP 2006-135744, for a color blur generating portion by searching for it using the previous motion picture image frame, and by specifying a target region in this motion picture image frame to some extent. 
     However, a chroma lowering process disclosed in JP 2001-145117 is a process that degrades a color of a high brightness pixel down to a brightness level of another color. This process is effective in reducing unnaturalness by erasing a color of the color blur, but the original colors of a subject can sometimes be also influenced and the image is likely grayish irrespective of the existence of the color blur. 
     The process disclosed in JP 2003-018407 utilizes a deconvolution and its approximate contour emphasis process, but has a problem of a difficulty in obtaining a precise aberration function necessary to obtain a proper result. It is difficult to precisely recognize an aberration function corresponding to each of huge imaging conditions that include a zoom position, a stop value, a focus position, a size of each lens, and an assembly state. In addition, the deconvolution can be used only in the linear reaction area of an image sensor, and cannot reduce a color blur around the saturation pixel. 
     When each of these prior art chromatic aberration reduction processes is applied to a motion picture image obtained by taking a moving high brightness subject, discontinuous processes of a time series of an application and a non-application of the chromatic aberration reduction process generate unnatural motion picture images, bringing a sense of discomfort to a viewer. 
     The color blur reduction process disclosed in JP 2006-135744 utilizes the previous motion picture image frame and reduces a calculation load, but does not have an effect of mitigating a sense of discomfort caused by the discontinuous color blur reduction processes. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image processing apparatus and an image processing method which provide image processing that can effectively reduce a color blur that would otherwise appear around a bright part in an image due to a chromatic aberration of an imaging optical system, and that can reproduce original colors. The present invention is also directed to an image processing apparatus and an image processing method which can reduce unnaturalness generated by discontinuous color blur reduction processes between frame images in the motion pictures. 
     An image processing apparatus according to one aspect of the present invention configured to perform a process to reduce a color blur in a color motion picture image utilizing a first frame image and a second frame image that has been obtained after the first frame image includes a memory configured to store a first color blur reduction amount used for the process to the first frame image, an estimator configured to calculate a color blur estimation amount in the second frame image through a spatial operation to the second frame image, and a determination part configured to calculate a second color blur reduction amount used for the process to the second frame image so that a difference between the first color blur reduction amount and the color blur estimation amount can fall within a predetermined range. 
     An image processing apparatus according to another aspect of the present invention configured to perform a process to reduce a color blur in a color motion picture image utilizing a first frame image and a second frame image that has been obtained after the first frame image includes a memory configured to store a first pixel value used for the process to the first frame image, a first estimator configured to calculate a color blur estimation amount in the second frame image through a spatial operation to the second frame image, a second estimator configured to calculate an estimation pixel value obtained through the process by using the color blur estimation value to the second frame image, and a determination part configured to calculate a second pixel value output as a result of the process to the second frame image so that a difference between the first pixel value and the color blur estimation amount can fall within a predetermined range. 
     An image processing method according to another aspect of the present invention configured to perform a process to reduce a color blur in a color motion picture image utilizing a first frame image and a second frame image that has been obtained after the first frame image includes the steps of storing a first color blur reduction amount used for the process to the first frame image, calculating a color blur estimation amount in the second frame image through a spatial operation to the second frame image, and calculating a second color blur reduction amount used for the process to the second frame image so that a difference between the first color blur reduction amount and the color blur estimation amount can fall within a predetermined range. 
     An image processing method according to still another aspect of the present invention configured to perform a process to reduce a color blur in a color motion picture image utilizing a first frame image and a second frame image that has been obtained after the first frame image includes the steps of storing a first pixel value used for the process to the first frame image, calculating a color blur estimation amount in the second frame image through a spatial operation to the second frame image, calculating an estimation pixel value obtained through the process by using the color blur estimation value to the second frame image, and calculating a second pixel value output as a result of the process to the second frame image so that a difference between the first pixel value and the color blur estimation amount can fall within a predetermined range. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory view of the lateral chromatic aberration. 
         FIG. 2  is an explanatory view of the longitudinal chromatic aberration. 
         FIG. 3  is a block diagram showing a structure of a color image pickup apparatus according to a first embodiment of the present invention. 
         FIG. 4  is a graph showing a spectroscopic transmittance of a color filter. 
         FIG. 5  is a view showing an arrangement of the color filter. 
         FIG. 6  is a view showing the image intensity at the boundary between the brightness and the darkness. 
         FIG. 7  is a flowchart showing a process of the color blur remover according to the first embodiment. 
         FIG. 8  is an explanatory view of a profile of a high brightness subject. 
         FIGS. 9A-9C  each is an explanatory view showing a time change of the profile. 
         FIGS. 10A-10C  each is a view for explaining an effect of restricting a color blur estimation amount according to the first embodiment. 
         FIG. 11  is a view for explaining a change on a chromaticity diagram due to a color blur reduction. 
         FIG. 12  is a flowchart of a color blur remover according to a second embodiment of the present invention. 
         FIGS. 13A-13C  each is a view for explaining an effect of restricting the B plane intensity after the color blur removal according to the second embodiment. 
         FIG. 14  is a view for explaining an effect of changing a color blur estimation amount through a spatial operation step according to a third embodiment of the present invention. 
         FIGS. 15A-15C  each is a view for explaining an effect of limiting a restraint of the color blur estimation amount according to a fourth embodiment of the present invention when the estimation step switches a color blur estimation amount. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawings, a description will be given of a preferred embodiment of the present invention. 
     First Embodiment 
       FIG. 3  shows a color image pickup apparatus  100  that utilizes an image processing method or is mounted with an image processing apparatus according to a first embodiment of the present invention. 
     The color image pickup apparatus  100  includes an imaging optical system  110 , an image sensor  120 , an AD converter  130 , a demosaic part  140 , a color blur remover  150 , a sight corrector  160 , a compression part  170 , and a recorder  180 . 
     Although a view field (imaging area or subject) shown in  FIG. 3  and an R (red) ray  91 , a G (green) ray  92 , and a B (blue) ray  93  are not the elements of the color image pickup apparatus  100 , they are illustrated for description purposes. 
     In  FIG. 3 , the taken view field  90  is imaged on the image sensor  120  via the imaging optical system  110 . In general, an imaging optical system in a color image pickup apparatus is provided with a certain chromatic aberration correction. The longitudinal chromatic aberration of the imaging optical system  110  of this embodiment is well corrected for the R and G wavelength regions, and the longitudinal chromatic aberration leaves for the B wavelength region. Thus, other aberrational corrections can be improved and the image pickup apparatus can be made smaller by degrading the correction criteria of the longitudinal chromatic aberration in the B wavelength region. 
     The image sensor  120  is a single-plate color image sensor having a general primary color filter system. The primary color filter system includes three types of color filters  41 ,  42 ,  43  having main transmission bands near 650 nm, 550 nm, and 450 nm, as shown in  FIG. 4 , and images color planes corresponding to the RGB bands. 
     The single-plate color image sensor spatially arranges these color filters for each pixel, as shown in  FIG. 5 , and each pixel can provide the intensity of only a single color plane. Therefore, a color mosaic image is output from the image sensor  120 . 
     A three-plate type color image sensor may be used, which splits the incident light into the RGB wavelength regions using a color splitting prism (not shown) and images the light having a different wavelength with a separate image sensor. In this case, the demosaic part  140  becomes unnecessary. 
     The AD converter  130  converts a color mosaic image output as an analog voltage from the image sensor  120 , into digital data suitable for the subsequent image processing. 
     The demosaic part  140  interpolates a color mosaic image, and generates a color image in which all pixels have RGB color information. A variety of methods are proposed for the interpolation approach from a simple linear interpolation to a complicated approach introduced in E. Chang, S. Cheung, and D. Pan, “Color filter array recovery using a threshold-based variable number of gradients,” Proc. SPIE, vol. 3650, pp. 36-43, January 1999. The interpolation approach of this embodiment can use any one of these methods or another method. 
     This embodiment uses a RGB primary color system for a color filter of the image sensor  120 , but may use a color filter of a complementary color system, which provides a color image composed of the RGB color planes through the color conversion process. 
     In a color image generated by the demosaic part  140 , the resolution of the B (blue) plane is lower than each of the resolutions of the R (red) plane and the G (green) plane due to the chromatic aberration of the imaging optical system  110 . Therefore, as shown in  FIG. 6 , an image  62  of the B plane is dimmer than an image  61  of the R or G plane and causes a color blur  63  like a blue fringe around the bright part. 
     The color blur remover  150  removes (or reduces) a color blur, such as this blue fringe, from a color image. A color blur removal (or reduction) process by the color blur remover  150  will be described in detail later. 
     The sight corrector  160  mainly processes a color image so as to improve its appearance of the image. This image correction includes, for example, a tone curve (gamma) correction, a chroma emphasis, a hue correction, and an edge emphasis. 
     As the last process, the compression part  170  compresses the corrected image using a method, such as JPEG, and reduces a size used for recording. 
     The color image that has undergone the compression process is recorded by the recorder  180  in a recording medium, such as a hard disc drive, a DVD, a magnetic tape, and a flash memory. 
     Those processing components from the image sensor  120  to the recorder  190  may actually include separate devices or a single microprocessor. 
     Referring now to a flowchart shown in  FIG. 7 , a description will be given of a process or an image processing method by the color blur remover  150 . The color blur remover  150  includes a spatial operation step  151 , an estimation step  153 , an area determination step  152 , a comparison limitation step  157 , an excessive removal restraint step  154 , a removal step  155 , and an image process value storage step  156 . 
     The above steps  151 - 157  can be regarded as elements of the color blur remover  150 . The image process value storing step  156  forms a memory, and the spatial operation step  151  and estimation step  153  forms an estimator. The comparison limitation step  157  forms a determination part. 
     This embodiment sets the B plane to a color plane as a target of a color blur removal, and the G plane to a reference plane that is referred to in the color blur removal. 
     The following description refers to, in a continuous frame pixel group forming a color motion picture image obtained by imaging, a frame image that has been previously processed (although it might not be just previously processed) as a first frame image, and a frame image that is to be processed later (this time) as a second frame image. The B plane intensity represents a pixel value of the B plane. 
       FIG. 8  shows a profile  81  of the typical B plane, and a profile  82  of the typical G plane of a high brightness subject. Since R and G form the same profile, a description thereof will be omitted. In the top in  FIG. 8 , an abscissa axis denotes a section of an image, and an ordinate axis denotes the intensity. A high brightness subject  83  that exceeds a saturation level of an image sensor is provided at the center. The blurred light is generated due to the flare and the aberration of the imaging optical system  110  from the high brightness subject  83 , and forms an intensity distribution around the high brightness subject  83 . This intensity distribution depends upon the intensity of the high brightness subject  83 , and weakens like an exponential function as a distance from the high brightness subject  83  increases. The intensity distribution range of the G plane is smaller than that of the B plane. 
     When the image sensor  120  images the high brightness subject  83  shown at the top of  FIG. 8 , an image at the bottom of  FIG. 8  is obtained. Since the image sensor  120  cannot measure the intensity equal to or greater than the saturation level, a white saturation area  84  in which G and B are saturated is formed and the white saturation area  84  is greater than the high brightness subject  83 . Formed around the white saturation area  84  are a light blue saturation area  85  in which B is saturated and G is attenuated, a light blue saturation area  86  in which both B and G are attenuated, and a blue area  87  in which G disappears and only B has an intensity. 
     Among them, the bluishness of the light blue saturation area  85 , light blue saturation area  86 , and blue area  87  brings a unnatural sense. If the blur of B is similar to G, the blur is recognized as a color of the high brightness subject  83  or a natural blur. 
     As discussed, a blur is a useful image expression that indicates the brightness of the high brightness subject  83  beyond the saturation level of the image sensor  120 . 
     This embodiment estimates a B blur amount (or calculates an estimation blur amount) in a color image (second frame image) by using a slope of a profile of the B plane. 
     When ∇B is a brightness slope map to the B plane and ∇G is a brightness slope map to the G plane, the spatial operation step  151  calculates ∇R and ∇B by a spatial operation using the following equations: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     G(x+1, y) and B(x+1, y) are values of the G and B planes for a pixel next to the target pixel at the right side. 
     G(x−1, y) and B(x−1, y) are values of the G and B planes for a pixel next to the target pixel at the left side. 
     G(x, y+1) and B(x, y+1) are values of the G and B planes for a pixel next to the target pixel at the bottom side. 
     G(x, y−1) and B(x, y−1) are values of the G and B planes for a pixel next to the target pixel at the top side. 
     The estimation step  153  estimates the extra intensity of the B plane that causes a color blur for each pixel of the color image (second frame image). While the estimation approach differs according to whether or not B is saturated, the estimation step  1  ( 158 ) and estimation step  2  ( 159 ) calculate two types of color blur estimation amounts E 1  and E 2  in order to handle both cases. 
     In the light color area  86  and the blue area  87  in which B is not saturated, the estimation step  1  ( 158 ) calculates the color blur estimation amount E 1 . The estimation step  1  ( 158 ) calculates the color blur estimation amount E 1  by multiplying an absolute value of the B&#39;s brightness slope ∇B by a coefficient k 1  where k 1  is a positive value and can be about 3.
 
 E   1   =k   1   ·|∇B|   EQUATION 2
 
     On the other hand, the B&#39;s brightness slope becomes 0 in the light blue saturation area  85  in which B is saturated, and no brightness slope before the saturation is obtained. The estimation step  2  ( 159 ) calculates the color blur estimation amount E 2  for such an area. The estimation step  2  ( 159 ) calculates the color blur estimation amount E 2  by using the G&#39;s brightness slope ∇G and the following equation, where k 2  is a positive value and can be about 3.
 
 E   2   =k   2   ·|∇G|   EQUATION 3
 
     The area determination step  152  nonlinearly converts the intensity of the B plane, and generates a saturation degree S. The saturation degree S represents whether the B is saturated, and becomes 1 where the B&#39;s intensity is saturated, and 0 where the B&#39;s intensity is small. E 1  or E 2  calculated by the estimation step  153  is selected by using the saturation degree S: 
     
       
         
           
             
               
                 
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     The comparison limitation step  157  subtracts the previous color blur removal amount Ep (which is a first color blur reduction amount used for the color blur reduction process to the first frame image) stored by the image process value storing step  156  from the color blur estimation amount E. When the post-subtraction value (difference) falls within a predetermined range from a lower limit value  11  to a upper limit value  12 , the color blur estimation amount E (or a second color blur reduction amount used for the color blur reduction process to the second frame image) is supplied to the excessive removal restraint step  154 . 
     When the value after the color blur removal amount Ep is subtracted from the color blur estimation amount E is smaller than the lower limit value  11 , a sum (second color blur reduction amount) of the color blur removal amount Ep and the lower limit value  11  is supplied to the excessive removal restraint step  154 . When the value after the color blur removal amount Ep is subtracted from the color blur estimation amount E is greater than the upper limit value  11 , a sum (second color blur reduction amount) of the color blur removal amount Ep and the upper limit value  12  is supplied to the excessive removal restraint step  154 . 
     Thus, this embodiment restricts the color blur estimation amount E for the second frame image (second color blur reduction amount) so that a difference between the color blur estimation amount E and the color blur removal amount Ep (or the first color blur reduction amount) used for the process to the first frame image can fall within the predetermined range. 
     As described later, the excessive removal restraint step  154  of this embodiment corrects a color blur estimation amount (second color blur reduction amount) E, and calculates a color blur removal amount E′ that is to be actually applied to a color image as a second frame image. However, the present invention includes both the color blur estimation amount E calculated by the comparison limitation step  157  and the color blur removal amount E′ in the “second color blur reduction amount used for the color blur reduction process to the second frame image.” The excessive removal restraint step  154 , which will be described later, is not necessary provided. 
     The absolute values of the above lower limit value  11  and upper limit value  12  may be set greater as a distance to the high brightness subject reduces. The lower limit value  11  and the upper limit value  12  may be set approximately equally or differently to some extent. 
       FIGS. 9A-9C  show images obtained through imaging of the flashing high brightness subject  901 , a profile  902  of the typical B plane, and a profile  903  of the G plane. Since R and G form the same profile, a description thereof will be omitted. 
     In each of the tops of  FIGS. 9A-9C , an abscissa axis denotes a section on an image, and an ordinate axis denotes the intensity. As shown in  FIG. 9B , the blurred light is generated around the high brightness subject  901  due to the flare and the aberration of the imaging optical system  110 , and forms an intensity distribution. This intensity distribution depends upon the intensity of the high brightness subject  901 , and weakens like an exponential function as a distance from the high brightness subject  901  increases, causing a blue blur area  904  and a light blue blur area  907 . 
     Assume that the high brightness subject  901  changes with time in order of time T 1  ( FIG. 9A ), time T 2  ( FIG. 9B ), and time T 3  ( FIG. 9C ), above or below the saturation level of the image sensor  120 . Then, the blue blur area  904  and the light blue blur area  907  disappear as in  FIG. 9A , appear as in  FIG. 9B , or reduce as in  FIG. 9C . 
       FIGS. 10A ,  10 B and  10 C show time changes of the B plane intensity, the color blur estimation amount, and the B plane intensity after the color blur reduction, at a point  905  near the thus-changing high brightness subject  901 . 
     As shown in  FIG. 10A , as the light intensity of the high brightness subject  901  increases, the B plane intensity at the point  905  starts increasing ( 1001 ) and finally reaches the saturation level ( 1002 ). The light intensity of the high brightness subject  901  further increases, and becomes a constant value ( 1003 ), but the B plane intensity is saturated and constant during this period. As the light intensity of the high brightness subject  901  decreases ( 1004 ), the B plane intensity becomes less than the saturation level ( 1005 ), further decreases, and becomes constant at the point  905  ( 1006 ). 
     During this period, as shown in  FIG. 10B , according to the prior art ( 1007 ) that does not limit a color blur estimation amount, a color blur estimation amount at the point  905  suddenly changes due to switching ( 1002 ,  1005 ) between the estimation step  1  ( 158 ) and the estimation step  2  ( 159 ) by the area determination step  152 . On the other hand, according to this embodiment ( 1008 ), the color blur estimation amount E gently changes so that the difference between the color blur estimation amount E and the color blur estimation amount used for the previous process to the first frame image can fall within the predetermined change. 
     As a result, as shown in  FIG. 10C , according to the prior art ( 1009 ), the B plane intensity after the color blur removal at the point  905  suddenly changes at the time of switching ( 1002 ,  1005 ), and causes a sense of discomfort to a viewer. On the other hand, according to this embodiment ( 1010 ), the B plane intensity at the point  905  after the color blur removal gently changes and is less likely to bring a sense of discomfort to a viewer. 
     The excessive removal restraint step  154  further corrects the color blur estimation amount E supplied from the above comparison limitation step  157 , and calculates a color blur removal amount E′ that is used as a color blur amount to be actually removed. The color blur amount estimated by the estimation step  153  is pursuant to a certain model, and does not necessarily accord with the actual blur amount. For example, the blur of the light detected by the same B plane changes between the light with the wavelength of 450 nm and the light with the wavelength of 400 nm, but the estimation step  153  does not consider this. When the removal amount is excessively small, a bit of bluishness leaves even after the removal. 
     On the other hand, when the removal amount is excessively large, B is excessively reduced in comparison with the gray background and becomes yellow-green. The latter is particularly unnatural, and brings a sense of discomfort to a viewer. Accordingly, the excessive removal restraint step  154  limits the color blur removal only in a certain hue range. 
     Therefore, a chroma of a pixel is calculated. Assume the following equation for each of the RGB plane intensities. 
     
       
         
           
             
               
                 
                   
                     
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                   EQUATION 
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       FIG. 11  shows the chroma coordinate ab plane. Blue is located in the fourth quadrant, and moves like a dotted arrow  321  in the upper left direction when the estimation E is removed from the B plane intensity. When a starting point of an arrow is a chroma before the removal, and a tip is a chroma after the estimation amount E is removed. 
     When the hue range in which the color blur removal works is limited to a&gt;0 and b&lt;0, B&gt;0.22R+0.68G and B&gt;−1.84R+3.30G. 
     The step  154  sets E′=0 to the pixel that does not satisfy the condition so as to remove the pixel from the removal target. Thereby, these pixels do not change due to the removal step  155 , and the pixel value is not affected by the color blur removal part  150 . In other words, only the pixel that satisfies the above condition becomes the removal target. 
     Moreover, even for the pixel that satisfies the condition, the color blur removal amount E′ is set to E′=min(E, B−(0.22R+0.68G), B−(−1.84R+3.30G)) and supplied to the removal step  155 . The chroma change due to the blur removal corresponding to this color blur removal amount E′ remains in the fourth quadrant as shown by a solid arrow  322  in  FIG. 11 . Thereby, the removal step  155  prevents a decrease of B beyond the hue limitation range. 
     The removal step  155  subtract the color blur removal amount E′ from the B plane intensity, and sets a new B plane intensity to B=B−E′. 
     The image process value storing step  156  stores the above color blur removal amount E′ as Ep, and prepares for the subsequent process (to the frame image obtained after the second frame image). 
     The color image in which the B plane is corrected is supplied to the sight corrector  160  as an output of the color blur remover  150 . 
     This embodiment thus effectively removes a blue blur of a flashing or moving high brightness subject, and provides a natural color image that does not cause a sense of discomfort. 
     In the accompanying imaging optical system  110 , a limitation of the longitudinal chromatic aberration of the B wavelength range can be mitigated, and another aberrational correction can be improved and the image pickup apparatus can be made smaller. 
     Second Embodiment 
     Next follows a description of a second embodiment according to the present invention. The first embodiment restricts the color blur estimation amount E, whereas this embodiment restricts the B plane intensity after the color blur estimation amount is removed (or the image pixel value of the second frame image after the color blur reduction). Therefore, the process performed by the color blur remover  150  in the color image pickup apparatus shown in  FIG. 3  is different from the first embodiment. 
     Referring to a flowchart of  FIG. 12 , a description will be given of a process of the color blur remover  150  of this embodiment. The color blur remover  150  includes a spatial operation step  1151 , an estimation step  1153 , an area determination step  1152 , a removal estimation step  1158 , a comparison limitation step  1157 , an excessive removal restraint step  1154 , a removal step  1155 , and an image process value storing step  1156 . 
     The above steps  1151  to  1158  can be regarded as the elements of the color blur remover  150 . The image process value storing step  1156  forms a memory, and the spatial operation step  1151  and the estimation step  1153  form a first estimator. The removal estimation step  1158  forms a second estimator, and the comparison limitation step  1157  forms a determination part. 
     The spatial operation step  1151 , the estimation step  1153 , the area determination step  1152 , the excessive removal restraint step  1154 , and the removal step  1155  are similar to those of the first embodiment. However, the excessive removal restraint step  1154  and the removal step  1155  deal with the B plane intensity as an estimation pixel value rather than the color blur estimation amount. The estimation step  1153  is provided with the estimation step  1  ( 2158 ) and the estimation step  2  ( 2159 ), which are similar to those described for the first embodiment. 
     The removal estimation step  1158  subtracts the color blur estimation amount E obtained at the estimation step  1153  from the B plane intensity of the color image as a second frame image, and calculates the B plane intensity as the estimation pixel value after the color blur reduction. 
     The comparison limitation step  1157  subtracts the B plane intensity Bp stored in the image process value storing step  1156  (or a first pixel value obtained as a result from the color blur reduction process to the first frame image) from the B plane intensity as the above estimation pixel value. When this subtracted value (difference) falls within a predetermined range from the lower limit value  13  to the upper limit value  14 , the B plane intensity as this estimation pixel value is supplied as a second pixel value to the excessive removal restraint step  1154 . 
     When the subtracted value is lower than the lower limit value  13 , the B plane intensity (second pixel value) as a sum of the lower limit value  13  and the B plane intensity Bp is supplied to the excessive removal restraint step  1154 . When the subtracted value exceeds the upper limit value  14 , the B plane intensity (second pixel value) as a sum of the upper limit value  14  and the B plane intensity Bp is supplied to the excessive removal restraint step  1154 . 
     Thus, this embodiment restricts the B plane intensity (second pixel value) output after the color blur removal to the second frame image so that a difference between the estimated B plane intensity (estimation pixel value) obtained after the color blur removal to the second frame image and the B plane intensity (first pixel value) Bp obtained by the color blur removal to the first frame image can fall within the predetermined range. 
     Similar to the first embodiment, even in this embodiment, the excessive removal restraint step  1154  corrects the B plane intensity as the second pixel value, and calculates a pixel value of an actually output color image. However, the present invention includes the B plane intensity calculated by the comparison limitation step  1157  with the B plane intensity corrected by the excessive removal restraint step  1154  in the “second pixel value output as a result of the color blur reduction process to the second frame image.” The excessive removal restraint step  1154  is not necessarily provided. 
     The absolute values of the above lower limit value  13  and upper limit value  14  may be set greater as a distance to the high brightness subject reduces. The lower limit value  13  and upper limit value  14  may be set approximately equally or differently to some extent. 
     Like  FIGS. 9A-9C ,  FIGS. 13A ,  13 B, and  13 C show time changes of the B plane intensity, the color blur estimation amount, and the B plane intensity after the color blur reduction, at the point  905  near the thus brightness-changing high brightness subject  901 . 
     As shown in  FIG. 13A , as the light intensity of the high brightness subject  901  increases, the B plane intensity at the point  905  starts increasing ( 1301 ) and finally reaches the saturation level ( 1302 ). The light intensity of the high brightness subject  901  further increases, and becomes a constant value ( 1303 ), but the B plane intensity is saturated and constant. As the light intensity of the high brightness subject  901  decreases ( 1304 ), the B plane intensity also becomes less than the saturation level ( 1305 ), further decreases, and becomes constant at the point  905  ( 1306 ). 
     During this period, as shown in  FIG. 13B , the color blur estimation amount at the point  905  suddenly changes due to switching ( 1302 ,  1305 ) between the estimation step  1  ( 2158 ) and the estimation step  2  ( 2159 ) by the area determination step  1152 . 
     As a result, as shown in  FIG. 13C , according to the prior art ( 1307 ), the B plane intensity after the color blur removal at the point  905  suddenly changes at the time of switching ( 1302 ,  1305 ), and causes a sense of discomfort to a viewer. On the other hand, according to this embodiment ( 1308 ), the B plane intensity after the color blur removal at the point  905  gently changes and is less likely to bring a sense of discomfort to a viewer. 
     The image process value storing step  1156  stores the B plane intensity as Ep, and prepares for the subsequent process (to the frame image obtained after the second frame image). 
     This embodiment thus effectively removes a blue blur of a flashing or moving high brightness subject, and provides a natural color image that does not cause a sense of discomfort. 
     In addition, the color image pickup apparatus of this embodiment can restrain the chromatic aberration of the wavelength range corresponding to at least one color plane through the optical system and another chromatic aberration through image processing, obtaining a proper image through the combination. 
     Third Embodiment 
     Next follows a description of a third embodiment according to the present invention. This embodiment discusses an example that changes, based on a spatial operation result, the lower and upper limit values used to restrict the color blur estimation amount described in the first embodiment. 
     A point  906  shown in  FIGS. 9A-9C  is closer to the high brightness subject  901 , and has a greater color blur estimation amount than the point  905 . When the lower and upper limit values used to restrict the color blur estimation amount are made constant in an image, as understood from a comparison between the B plane intensity  1401  before the color blur removal and the B plane intensity  1402  after the removal shown in  FIG. 14 , a color blur can be sufficiently removed from the point  905  having a smaller color blur than the upper limit value. However, the color blur cannot be sufficiently eliminated from the point  906  having a color blur that exceeds the upper limit value, and may bring a sense of discomfort to a viewer. 
     The brightness slopes ∇G and ∇B (or the spatial operation values) calculated by the spatial operation step  151  increase due to a distance from the high brightness subject  901  and the brightness of the high brightness subject  901 . 
     This embodiment estimates the distance from the high brightness subject  901  and the brightness of the high brightness subject  901  based on the brightness slopes ∇G and ∇B, and changes the upper and lower limit values used to restrict the color blur estimation amount. In other words, this embodiment changes the upper and lower limit values based on the result of the spatial operation. Thereby, the limitation of the color blur estimation amount at the point  906  near the high brightness subject  901  can be mitigated. Thus, similar to the point  905 , as shown in the B plane intensity  1403  after the color blur removal, the color blur can be sufficiently removed and an image that does not bring a sense of discomfort to the viewer can be provided. 
     This embodiment thus effectively removes the blue blur of the flashing or moving high brightness subject, and provides a natural color image that does not cause a sense of discomfort. 
     The upper and lower limit values in the second embodiment may be similarly changed based on the spatial operation. 
     Fourth Embodiment 
     Next follows a description of the fourth embodiment according to the present invention. This embodiment discusses an example that removes the color blur from the relatively quickly moving or flashing high brightness subject without bringing a sense of discomfort by comparing and limiting the color blur estimation amount only when the area determination step  152  switches the color blur estimation amount. 
       FIG. 15A  shows time changes of the B plane intensity before the color blur removal. As illustrated, when the B plane intensity at the point  905  abruptly changes due to the comparatively fast turning-on  1501  and turning-off  1502  of the high brightness subject  901 , the color blur estimation amount changes with time as shown by  1503  in  FIG. 15B  due to the limitation to the color blur estimation amount. As a result, as shown by  1505  in  FIG. 15C , the B plane intensity after the color blur removal abruptly changes, bringing a sense of discomfort to a viewer. 
     This embodiment restricts the color blur estimation amount only when the area determination step  152  switches the estimation step  1  ( 158 ) and the estimation step  2  ( 159 ). Thereby, when the high brightness subject  901  comparatively quickly flashes, the color blur estimation amount can follow this and be calculated as shown by  1504  in  FIG. 15B . As a result, as shown by  1506  in  FIG. 15C , the B plane intensity after the color blur removal gently changes, bringing an image that has no sense of discomfort to a viewer. 
     Thus, this embodiment can effectively remove the bluishness of the flashing or moving high brightness subject, and provide a natural color motion picture image that does not bring a sense of discomfort. 
     In addition, the above first to fourth embodiments discuss the color image pickup apparatus that includes all components from the imaging optical system  110  to the recorder  180 . However, an image processing apparatus (such as a personal computer) having at least the color blur remover  150  may be formed as an apparatus separate from the color image pickup apparatus. In this case, a color image taken by the color image pickup apparatus and stored in a recording medium, such as a semiconductor memory and a magnetic/optical disc, may be input into the image processing apparatus. In addition, the color image pickup apparatus and the image processing apparatus may be connected to each other via a cable, a radio LAN, etc. so as to send an image from the color image pickup apparatus to the image processing apparatus. 
     A supplemental description will be given of each step described in the above embodiments. 
     The spatial operation step refers to a removal-target color plane and a reference plane, and operates an intensity slope of each of the removal-target color plane and the reference plane. The reference plane is a color plane having an intensity of a wavelength region in which a chromatic aberration of the imaging optical system is well corrected, or a plane indicative of the brightness, and the reference plane is generally the G or Y plane. 
     The estimation step outputs a value depending upon the intensity (or brightness) slope, or may simply output a value proportional to the intensity slope. In addition, the estimation step may switch an estimation method of the color blur estimation method, selects one of a plurality of color blur estimation amounts, or interpolate the plurality of color blur estimation amounts, depending upon whether the brightness saturation pixel exists in the removal-target color plane. 
     The removal step subtracts the image intensity of the removal-target color plane. The color plane that serves as a removal target can be the R or B plane, but may be a color plane U/V indicative of a corresponding hue. 
     The excessive removal restraint step may be provided prior to the removal step so as to prepare for the excess or deficiency of the color blur estimation amount. The excessive removal restraint step sets a removal target only to pixels in a certain color range so that they are maintained in a color range after the removal. The excessive removal restraint operation may be implemented within a predetermined hue angle. 
     In addition, the removal target may be set only to an image range in which the intensity of the removal-target color plane is greater than the intensity of the reference plane. When the estimation amount runs short, a bit of color blur leaves due to the removal deficiency. When the estimation amount is excessively large, a hue inverts at a portion from which the color blur is removed due to the excessively removal. According to the experiments of the instant inventors, an image of an excessive removal is likely to bring a sense of more unnaturalness than an image of a removal deficiency. In addition, when a color blur is left with a few removal deficiencies, the original brightness and color of the subject can be easily discerned. 
     The comparison limitation step of the first embodiment supplies the color blur estimation amount to the next process when the difference between the color blur estimation amount of the motion picture image frame (second frame image) B and the color blur removal amount used to process the stored motion picture image frame (first frame image) A falls within the predetermined range. When the color blur estimation amount exceeds the upper limit of the predetermined range, a sum of the upper limit value and the color blur removal amount used for the frame A is set to a color blur estimation amount for the next process. In addition, when the color blur estimation amount becomes less than the lower limit of the predetermined range, a sum of the lower limit value and the color blur estimation amount used for the frame A is set to a color blur estimation amount for the next step. 
     The comparison limitation step of the second embodiment supplies a pixel value after the color blur estimation amount is removed when the difference between a pixel value after the color blur estimation amount is removed from the motion picture image frame B and a pixel of the stored motion picture image frame A falls within the predetermined range. When the pixel value exceeds the upper limit of the predetermined range, a sum of the upper limit value and the pixel value of the motion picture image frame A is set for the next process as a pixel value of the motion picture image frame B after the color blur estimation amount is removed. In addition, when the pixel value becomes less than the lower limit of the predetermined range, a sum of the lower limit value and the pixel value of the motion picture image frame A is set for the next process as a pixel value of the motion picture image frame B after the color blur estimation amount is removed. 
     The above predetermined range may be changed based on the spatial operation result. At an area that is estimated to have a large brightness slope and a large color blur amount, the above predetermined range is changed based on the spatial operation result so as to remove (reduce) the color blur. 
     In addition, the spatial operation step may search for the high brightness subject in the image, and the estimation step may estimate the color blur amount based on a distance from the high brightness subject. In addition, the spatial operation step may search for the high brightness subject in the image, and the estimation step may calculate a blur image of the high brightness subject from a point spread function determined by the imaging optical system, thereby estimating a color blur amount. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2007-298281, filed on Nov. 16, 2007, which is hereby incorporated by reference herein in its entirety.