Patent Publication Number: US-8542292-B2

Title: Flicker correction device, flicker correction method and flicker correction program

Description:
TECHNICAL FIELD 
     The present invention relates to a flicker correction technique for correcting a periodic change (hereinafter referred to as flicker) in an output signal of an imaging element, which change is caused by an illumination light source powered by an AC power supply. 
     BACKGROUND ART 
     Generally, in an illumination light source (such as an electric bulb, a fluorescent lamp, and a mercury lamp) powered by an AC power supply, lightness and darkness are repeated according to the change in luminance of the light source occurring at a double frequency of the AC power supply frequency. When under such illumination, a subject image is captured by an imaging element in which the exposure time for each line begins sequentially or by an imaging element in which the exposure time for each point begins sequentially, periodic horizontal stripes, so-called flickers, are caused in the image signal outputted from the imaging element. The period of the periodic horizontal stripes can be obtained from the drive frequency of the imaging element and the power supply frequency. 
     For example, since a common fluorescent lamp powered by a 50 Hz AC power supply repeats light emission at 100 Hz, the horizontal scanning (hereinafter referred to as line) frequency in the NTSC system is 15.75 kHz, and hence lightness and darkness are repeated at every 157.5 (= 1/100 sec) lines. Further, since a least common multiple between the field period ( 1/59.94 sec) and the illumination flicker period ( 1/100 sec) is 1/20 sec, the same light and dark pattern is formed every 1/20 sec, that is, about every 3 fields. 
     Further, since a common fluorescent lamp powered by a 60 Hz AC power supply repeats light emission at 120 Hz, lightness and darkness are repeated at every 131.3 (= 1/120 sec) lines. Further, since the deviation between the field period ( 1/59.94 sec) and the illumination flicker period ( 1/120 sec) is 16.67 μsec, the field period and the flicker period coincide with each other every 500 fields, and hence the flickers appear as horizontal stripes moving very slowly on the screen. The deterioration of image quality due to such illumination flicker becomes an obstacle to viewing. 
     In order to cope with such problem, a technique is disclosed in Patent Literature 1, in which line average values are stored in a memory for each one third phase of the period between subsequent coincidences between the field period and the flicker period, and in which a reference value, from which the influence of flicker is removed, is created by averaging the average values stored in the memory, and flicker correction is performed on the basis of the reference value. Particularly, in the NTSC system affected by flicker of 60 Hz, a flicker reduction device, which is hardly affected by movement of a subject, is realized in such a manner that, when the subject is stationary, flicker correction is performed, and that, when the subject is moving, the flicker correction is not performed. 
     CITATION LIST 
     [Patent Literature ] 
     
         
         [Patent Literature 1] Japanese Patent Laid-Open No. 2009-81684 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the conventional method, the memory storing the line average values is refreshed every one third phase of the period between subsequent coincidences between the field period and the flicker period. Particularly, in the NTSC system affected by flicker of 60 Hz, a reference value, from which the influence of flicker is removed, is created by obtaining three average values of an average value delayed by 167 field period, an average value delayed by 334 field period, and an average value delayed by 500 field period. For this reason, there is a problem that, when there is a movement of the subject during the 500 field period, a correct reference value cannot be calculated and hence flicker cannot be detected, and there is also a problem that, even when the subject is stationary, it takes at least the 500 field period or longer until whether flicker exists or not is determined for the first time. 
     The present invention has been made to solve the above-described problems of the conventional method. An object of the present invention is to provide a flicker correction device, a flicker correction method, and a flicker correction program, each of which determines whether flicker exists or not, and corrects the flicker in a short time. 
     Solution to Problem 
     To this end, a flicker correction device according to the present invention is a flicker correction device for reducing 60 Hz flicker of an image captured at the field frequency of the NTSC system using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, and is configured by including: a line average value calculation section which calculates an average value for each line during the validity period of an input image signal; a storage section which stores, in the unit of one frame period, the average values obtained by the line average value calculation section; a movement detection section which detects movement from an output change between the average value obtained by the line average value calculation section, and the one-field preceding field average value stored in the storage section; a difference value calculation section which calculates a difference value between the average value obtained by the line average value calculation section, and the n-field preceding field (n: an integer multiple of 3) average value obtained in the storage section; a division value calculation section which calculates a division value by dividing the average value obtained by the line average value calculation section, by the difference value obtained by the difference value calculation section; a flicker component extraction section which extracts a flicker frequency from the result of the division value calculation section; a flicker determination section which determines whether flicker exists or not, and whether the flicker frequency is 60 Hz or not, from the results of the flicker component extraction section and of the movement detection section; a flicker correction gain calculation section which calculates a flicker correction gain synchronized with the flicker frequency from the result of the flicker determination section; and a flicker correction gain multiplication section which multiplies the image signal by the correction gain obtained by the flicker correction gain calculation section. 
     With this configuration, a flicker component, from which the influence of light and shade, and the like, of the subject is removed, can be extracted at high speed, and hence suitable flicker correction can be performed for various subjects. 
     Further, a flicker correction method according to the present invention is a flicker correction method for reducing flicker of an image captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, and is configured to include: a line average value calculation step of calculating an average value for each line during the validity period of an input image signal; a storage step of storing, in the unit of one frame period, the average values obtained in the line average value calculation step; a movement detection step of detecting movement from an output change between the average value obtained in the line average value calculation step, and the one-field preceding field average value stored in the storage step; a difference value calculation step of calculating a difference value between the average value obtained in the line average value calculation step, and the n-field preceding field (n: an integer multiple of 3) average value obtained in the storage step; a division value calculation step of calculating a division value by dividing the average value obtained in the line average value calculation step, by the difference value obtained in the difference value calculation step; a flicker component extraction step of extracting a flicker frequency from the result obtained in the division value calculation step; a flicker determination step of determining whether flicker exists or not, and whether the flicker frequency is 60 Hz or not, from the results obtained in the flicker component extraction step and in the movement detection step; a flicker correction gain calculation step of calculating a flicker correction gain synchronized with the flicker frequency from the result obtained in the flicker determination step; and a flicker correction gain multiplication step of multiplying the image signal by the correction gain obtained in the flicker correction gain calculation step. 
     With this configuration, a flicker component, from which the influence of the light and shade, and the like, of the subject is removed, can be extracted at high speed, and hence suitable flicker correction can be performed for various subjects. 
     Further, a flicker correction program according to the present invention is a flicker correction program for reducing flicker of an image captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, and is a flicker correction program for making a computer execute the following steps including: a line average value calculation step of calculating an average value for each line during the validity period of an input image signal; a storage step of storing, in the unit of one frame period, the average values obtained in the line average value calculation step; a movement detection step of detecting movement from an output change between the average value obtained in the line average value calculation step, and the one-field preceding field average value stored in the storage step; a difference value calculation step of calculating a difference value between the average value obtained in the line average value calculation step, and the n-field preceding field (n: an integer multiple of 3) average value obtained in the storage step; a division value calculation step of calculating a division value by dividing the average value obtained in the line average value calculation step, by the difference value obtained in the difference value calculation step; a flicker component extraction step of extracting a flicker frequency from the result obtained in the division value calculation step; a flicker determination step of determining whether flicker exists or not, and whether the flicker frequency is 60 Hz or not, from the results obtained in the flicker component extraction step and in the movement detection step; a flicker correction gain calculation step of calculating a flicker correction gain synchronized with the flicker frequency from the result obtained in the flicker determination step; and a flicker correction gain multiplication step of multiplying the image signal by the correction gain obtained in the flicker correction gain calculation step. 
     Advantageous Effects of Invention 
     The present invention provides an excellent effect that, even when the phase of the field period is slightly deviated from the phase of the flicker period, the flicker correction can be performed at high speed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a flicker correction device according to an embodiment of the present invention. 
         FIG. 2  shows an example of an image in which flicker is generated. 
         FIG. 3  shows line average values of the image in  FIG. 2 . 
         FIG. 4  shows line average values of three fields. 
         FIG. 5  shows results obtained by dividing the line average values of present field by the average values for the three fields. 
         FIG. 6  shows difference values between the line average values of the present field and the line average values of the n-field preceding field. 
         FIG. 7  shows results obtained by dividing the difference values by the line average values of the present field. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, a flicker correction device for reducing flicker of an image captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially will be described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a configuration of a flicker correction device according to an embodiment of the present invention. In  FIG. 1 , reference numeral  100  denotes a line average value calculation section, reference numeral  101  is a one-field preceding field line average value storage section, reference numeral  102  denotes a two-field preceding field line average value storage section, reference numeral  103  denotes an n-field preceding field line average value storage section, reference numeral  104  denotes a three-field average value calculation section, reference numeral  105  denotes a difference value calculation section, reference numeral  106  denotes a first division value calculation section, reference numeral  107  denotes a second division value calculation section, reference numeral  108  denotes a 50 Hz flicker component extraction section, reference numeral  109  denotes a 60 Hz flicker component extraction section, reference numeral  110  denotes a movement detection section, reference numeral  111  denotes a microcomputer, reference numeral  112  denotes a flicker determination section, reference numeral  113  denotes a flicker correction gain calculation section, reference numeral  114  denotes a flicker correction gain multiplication section, and reference numeral  115  denotes a delay section. 
     The line average value calculation section  100  calculates an average value of each line during the validity period of an input image signal. 
     Each of the one-field preceding field line average value storage section  101  and the two-field preceding field line average value storage section  102  can store line average values for one field and outputs the line average values by delaying the line average values by the one field period. Similarly, the n-field preceding field line average value storage section  103  can store line average values for one field and outputs the line average values by delaying the line average values by arbitrary n-field periods. 
     The three-field average value calculation section  104  calculates an average value of an output y 0  of the line average value calculation section  100 , an output y- 1  of the one-field preceding field line average value storage section  101 , and an output y- 2  of the two-field preceding field line average value storage section  102 . That is, the three-field average value calculation section  104  calculates an average value of line average values at the same line position for each set of three fields, and thereby creates a reference value from which the influence of flicker is removed. 
     The first division value calculation section  106  divides the line average value outputted from the line average value calculation section  100  by the average value outputted from the three-field average value calculation section  104 , and thereby obtains a flicker signal from which the light and shade of the subject is removed. 
     The difference value calculation section  105  subtracts an n-field preceding field line average value outputted from the n-field preceding field line average value storage section  103  from the line average value outputted from the line average value calculation section  100 . 
     The second division value calculation section  107  divides the difference value outputted from the difference value calculation section  105  by the average value outputted from the line average value calculation section  100 , and thereby obtains a flicker signal from which the light and shade of the subject is removed. 
     The 50 Hz flicker component extraction section  108  frequency-converts the flicker signal calculated by the first division value calculation section  106 , and thereby extracts only the frequency area of the 50 Hz flicker components. 
     The 60 Hz flicker component extraction section  109  frequency-converts the flicker signal calculated by the second division value calculation section  107 , and thereby extracts only the frequency area of the 60 Hz flicker components. 
     The movement detection section  110  compares the difference between the line average value outputted from the line average value calculation section  100  and the average value outputted from the one-field preceding field line average value storage section  101  with a threshold value, and thereby detects movement of the subject. 
     The flicker determination section  112  determines whether flicker exists or not, and whether the flicker frequency is 50 Hz or 60 Hz, on the basis of the detection result of the movement detection section  110 , and the results of the 50 Hz flicker component extraction section  108 , and of the 60 Hz flicker component extraction section  109 . 
     On the basis of the result of the flicker determination section  112 , the flicker correction gain generation section  113  generates a sinusoidal wave synchronized with the flicker frequency, by superposition of trigonometric functions. 
     The flicker correction multiplication section  114  reduces flicker of an inputted image by multiplying the image signal delayed and adjusted by the delay section  115  by the flicker correction gain. 
     The delay section  115  adjusts the delay amount corresponding to the period until the correction gain is generated for the input image signal. 
     The operations of the flicker correction device according to the present embodiment configured as described above will be described below. Note that it is obvious that the contents of the operations are based on a flicker correction method for reducing flicker of an image captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, and that each of the operations (steps) can be carried out by using a computer. Further, such program is a flicker correction program for reducing flicker of an image captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, and is a flicker correction program for making the computer execute each of the following operation (steps). 
     When an image signal, captured by using an imaging element in which the exposure time for each line begins sequentially or an imaging element in which the exposure time for each point begins sequentially, is inputted into the flicker correction device shown in  FIG. 1 , the image signal is inputted into the line average value calculation section  100  and the delay section  115 . The line average value calculation section  100  calculates an average value for each line during the validity period of the input image signal. The calculated average value y 0  is inputted into the one-field preceding field line average value storage section  101 , the n-field preceding field line average value storage section  103 , the three-field average value calculation section  104 , the difference value calculation section  105 , the first division value calculation section  106 , the second division value calculation section  107 , and the movement detection section  110 . 
     The one-field preceding field line average value storage section  101  stores the inputted line average value y 0 , and outputs an average value y- 1  delayed by one field period. The average value y- 1  outputted from the one-field preceding field line average value storage section  101  is inputted into the two-field preceding field line average value storage section  102 , the three-field average value calculation section  104 , and the movement detection section  110 . The two-field preceding field line average value storage section  102  stores the inputted line average value y- 1  and outputs an average value y- 2  delayed by one field period. Thereby, the line average values y 0 , y- 1 , and y- 2  are inputted into the three-field average value calculation section  104 . The three-field average value calculation section  104  calculates, for each same line, an average value of the average values of the three fields on the basis of the inputted line average values y 0 , y- 1 , and y- 2 , and thereby creates a reference value from which the influence of flicker is removed. 
     The average value calculated by the three-field average value calculation section  104  is inputted into the first division value calculation section  106 . The first division value calculation section  106  divides the line average value outputted from the line average value calculation section  100  by the average value outputted from the three-field average value calculation section  104 , and thereby outputs a flicker signal from which the light and shade of the subject is removed. 
     On the other hand, the difference value calculation section  105  subtracts an n-field preceding field line average value outputted from the n-field preceding field line average value storage section  103  from the line average value outputted from the line average value calculation section  100  and inputs the subtraction result into the second division value calculation section  107 . The second division value calculation section  107  divides the difference value outputted from the difference value calculation section  105  by the average value outputted from the line average value calculation section  100 , and thereby calculates a flicker signal from which the light and shade of the subject is removed. 
     The above described operations will be described with reference to the drawings. 
       FIG. 2  shows an example of an image in the case where, under illumination of a common fluorescent lamp powered by an AC power supply (fp=50 Hz), a white chart is photographed by using an imaging device of the NTSC system in which the vertical scanning frequency (field frequency) is 59.94 Hz, and in which the horizontal scanning frequency is 15.75 kHz. 
       FIG. 3  shows a plot of line average values of the image in  FIG. 2 . As shown in  FIG. 2 , the line average values become a signal like the flicker component signal modulated by the light and shade of the subject. 
     The y-line average value Ys (Y, n) in the n frame is expressed by the following expression from the relationship between the power supply frequency and the drive frequency of the imaging element. 
     
       
         
           
             
               
                 
                   
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     Here, Is (y) denotes a DC component of y-line (corresponding to the light and shade of the subject), A denotes the magnitude of flicker, f h  denotes the horizontal scanning frequency, f v  denotes the vertical scanning frequency, and f p  denotes the power supply frequency. 
     From this expression, it can be seen that as for the y line, a horizontal stripe appears at a line period of f h /(2×f p ), and that as for the n frame, the phase is shifted by (2π×2×f p )/f v  per one field. 
     That is, when f p =50 Hz, a horizontal stripe appears at a line period of f h /(2×f p )=15.75 kHz/100=157.5 Hz, and the phase is shifted per one field by (2π×2×f p )/f v =2π×100/59.94 Hz≈π×(1+⅔). Thus, it is seen from the characteristics of trigonometric functions that the flicker at the time of f p =50 Hz is circulated every three fields. 
     Similarly, since when f p =60 Hz, a horizontal stripe appears at a line period of f h /(2×f p )=15.75 kHz/120=131.25 Hz, and also the phase is shifted per one field by (2π×2×f p )/f v =2π×120/59.94 Hz≈2π×(2+ 1/500). Thus, it is seen from the characteristics of trigonometric functions that the flicker at the time of f p =60 Hz is circulated every 500 fields. 
       FIG. 4  shows a plot of line average values of the present field at the time of f p =50 Hz, a plot of line average values of the one-field preceding field, a plot of line average values of the two-field preceding field, and a plot of average values, each of which is obtained by averaging, for each same line, the line average values of these three fields. As described above, since the line average value is circulated every three fields, the light and shade of the subject, from which the influence of flicker is removed, can be extracted by calculating, for each same line, an average value of the line average values of the three fields. 
       FIG. 5  shows a result obtained by dividing the line average values of the present field by the above-obtained line average values of the three fields. As shown in  FIG. 5 , it is possible to extract a flicker signal from which the light and shade of the subject is removed. 
       FIG. 6  shows a plot of line average values of the present field at the time of f p =60 Hz, a plot of line average values of the n-field preceding field, and a plot of difference values between the line average values of these two fields. 
       FIG. 7  shows a result obtained by dividing the above-obtained difference values of two fields by line average values of the present field. As shown in  FIG. 7 , it is possible to extract a flicker signal from which the light and shade of the subject is removed. 
     The extraction of the flicker signal, from which the light and shade of the subject is removed, will be described using expressions. 
     The line average values in n 1  frame and the line average values in n 2  frame are expressed by the following expression. 
     
       
         
           
             
               
                 
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                                     ( 
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                                         ( 
                                         y 
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                                     · 
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                                     sin 
                                   
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                                   ⁢ 
                                   
                                     θ 
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                             = 
                               
                             ⁢ 
                             
                               
                                 
                                   
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                                     A 
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                                     A 
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                                     2 
                                   
                                 
                               
                             
                           
                         
                       
                       
                         
                           
                             = 
                               
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                                 A 
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                                   ( 
                                   
                                     
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                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         θ 
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                                       ⁢ 
                                       
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                                 1 
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                                     A 
                                     · 
                                     sin 
                                   
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                                   ⁢ 
                                   
                                     θ 
                                     2 
                                   
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In flicker due to a common fluorescent lamp, since A &lt;&lt;1, the above expression: ([2]=[1])/[2] can be approximated by the following expression, and thereby a flicker signal, from which the influence of the light and shade of the subject is removed, can be extracted.
 
≈A(sin θ 2 −sin θ 1 )  [Expression 4]
 
     In order to avoid the influence of 50 Hz flicker, n is usually set to a value, such as 9, 12, 15 and 18, which is a multiple of 3 and which enables a certain amount of line average value difference between the fields to be secured. Thereby, a 60 Hz flicker signal can be extracted by at least about ten frames. 
     The flicker signal outputted from the first division value calculation section  106  is inputted into the 50 Hz flicker component extraction section  108 . The 50 Hz flicker component extraction section  108  frequency-converts the inputted flicker signal, to extract only the frequency area of the 50 Hz flicker components. The flicker components extracted by the 50 Hz flicker component extraction section  108  are inputted into the flicker determination section  112 . 
     Further, the flicker signal outputted from the second division value calculation section  107  is inputted into the 60 Hz flicker component extraction section  109 . The 60 Hz flicker component extraction section  109  frequency-converts the inputted flicker signal, to extract only the frequency area of the 60 Hz flicker components. The flicker components extracted by the 60 Hz flicker component extraction section  109  are inputted into the flicker determination section  112 . 
     The movement detection section  110  compares a difference between the line average value outputted from the line average value calculation section  100 , and the average value outputted from the one-field preceding field line average value storage section  101  with a threshold value, so that the existence or absence of movement of the subject is inputted into the flicker determination section  112 . 
     On the basis of the detection result of the movement detection section  110 , and the results of the 50 Hz flicker component extraction section  108  and of the 60 Hz flicker component extraction section  109 , the flicker determination section  112  adjusts the flicker components according to the existence or absence of movement, and determines whether flicker exists or not, and whether the flicker frequency is 50 Hz or 60 Hz. Thereby, the flicker components based on the determination results are inputted into the flicker correction gain generation section  113 . 
     On the basis of the results of the flicker determination section  112 , the flicker correction gain generation section  113  generates a sinusoidal wave synchronized with the flicker frequency, by superposition of trigonometric functions, and inputs the generated sinusoidal wave into the flicker correction multiplication section. 
     The flicker correction multiplication section  114  multiplies the image signal delayed and adjusted by the delay section  115  by a flicker correction gain. The signal subjected to flicker correction is outputted from the flicker correction multiplication section  114 . 
     As described above, with the flicker correction device according to the present embodiment, it is possible to provide a device, a method, and a program, each of which, even when the phase of the field period is slightly deviated from the phase of the flicker period, detects the flicker frequency and performs adjustment at high speed. 
     [Industrial Applicability ] 
     As described above, the device, method and program for flicker correction according to the present invention, can detect the flicker frequency and perform adjustment at high speed even when the phase of the field period is slightly deviated from the phase of the flicker period. Thus, the device, method and program for flicker correction according to the present invention are useful in the field of camera systems, such as a movie camera, an in-vehicle camera, and a portable camera, in which the photographing location is not fixed, and in which flicker correction for different frequencies is required. 
     Reference Signs List 
     
         
           100  Line average value calculation section 
           101  One-field preceding field line average value storage section 
           102  Two-field preceding field line average value storage section 
           103  N-field preceding field line average value storage section 
           104  Three-field average value calculation section 
           105  Difference value calculation section 
           106  First division value calculation section 
           107  Second division value calculation section 
           108  50 Hz flicker component extraction section 
           109  60 Hz flicker component extraction section 
           110  Movement detection section 
           111  Microcomputer 
           112  Flicker determination section 
           113  Flicker correction gain calculation section 
           114  Flicker correction gain multiplication section 
           115  Delay section