Patent Publication Number: US-7221408-B2

Title: Adaptive contrast enhancement method for video signals based on time-varying nonlinear transforms

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to video processing, and more particularly to video signal enhancement. 
   BACKGROUND OF THE INVENTION 
   The development of modern digital video technology has brought significant enhancement in the video quality for consumers, such as in DVD players and in digital TVs (DTV) compared to the analog TV systems. However, such digital video systems only enhance the video quality in terms of signal to noise ratio (SNR) and resolution, without regard to other important issues relating to video enhancement. Such issues include contrast enhancement, brightness enhancement, and detail enhancement. Generally, video enhancement processes comprise a collection of techniques that seek to improve the visual appearance of video when displayed. This primarily includes gray level and contrast manipulation, noise reduction, edge crispening and sharpening. Compared to image restoration, video or image enhancement methods neither increase the inherent information content in the data nor require mathematical modeling. The basic principle of video enhancement is to manipulate a given sequence of images so that their appearance on display media can be improved. Because quantifying the criteria for enhancement is difficult, conventional video enhancement techniques are empirical and require interactive procedures to obtain satisfactory results. 
   Among the techniques for video enhancement, contrast enhancement is important because it plays a fundamental role in the overall appearance of an image to human being. A human being&#39;s perception is sensitive to contrast rather than the absolute values themselves. Hence, it is natural to enhance the contrast of an image in order to provide a good looking image to human beings. 
   Contrast enhancement involves considering the overall appearance of a given image rather than local appearances such as edge crispening or peaking. There are conventional models of contrast enhancement, and some examples include the root law and the logarithmic law. Image enhancement by contrast manipulation has been performed in various fields of medical image processing, astronomical image processing, satellite image processing, infrared image processing, etc. For example, histogram equalization is a useful method in X-ray image processing because it enhances the details of an X-ray image significantly to e.g. detect tumors easily. 
   Although several conventional methods for contrast enhancement exist, their primary application is limited to still images. Direct applications of such methods to moving images results in visual artifacts such as unnatural appearance or over-enhancement. Hence, such methods are not suitable for consumer products such as TV whose primary content is a sequence of images. Therefore, there is a need for a contrast enhancement method that is applicable to a video sequence which inherently has time-varying characteristics. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention addresses the above needs. It is an object of the present invention to provide an adaptive contrast enhancement (ACE) method and apparatus which provide a natural enhancement in accordance with the time-varying characteristics of a video sequence. 
   In one embodiment, an ACE method according to the present invention includes the steps of specifying the characteristics of a time varying video sequence and performing a nonlinear transform over the input video sequence to enhance mainly the contrast of the input. As such, first, a probability density function (PDF) of a time varying input video sequence is computed and then some predetermined video parameters relating to contrast is extracted from the PDF. Based upon the extracted video parameters, a nonlinear transform function is then constructed and updated as a look up table (LUT), which is synchronized with the associated video picture or field SYNC signal. The transform LUT is then applied to the input video to provide an enhanced video output signal. 
   An example method for adaptive contrast enhancement, according to the present invention includes the steps of: (i) obtaining a time varying video signal including a plurality of temporally ordered digital pictures, each one of the digital pictures represented by a set of samples, each one of the samples having a gradation level in a range from a lower limit C to an upper limit U; (ii) constructing a contrast enhancement transform including at least a first transform function and a second transform function by: for a first one of the digital pictures, selecting a first value for the samples with gradation values between the lower limit C and the upper limit U; dividing the set of the samples representing the first one of the digital pictures into a first portion having samples with a first mean value not greater than the first value and a second portion having samples with a second mean value not less than the first value; selecting the first transform function based on a distribution of the samples with the first mean value in the first portion, wherein the first transform function is based on a first enhancement function that is a varying function of gradation level, equaling zero at the lower limit C, equaling zero at the first value, and having only one local maxima in a range from the lower limit C to the first value; and selecting the second transform function in dependence on a distribution of the samples with the second mean value in the second portion, wherein the second transform function is based on a second enhancement function that is a varying function of gradation level, equaling zero at the first value, equaling zero at the upper limit U, and having only one local maxima in a range from the first value to the upper limit U; and (iii) enhancing contrast of a digital picture by applying the contrast enhancement transform to a set of samples representing the digital picture. 
   In another embodiment, the present invention provides an adaptive contrast enhancement device for enhancing a time varying video signal including a plurality of temporally ordered digital pictures, each one of the digital pictures represented by a set of samples, each one of the samples having a gradation level in a range from a lower limit C to an upper limit U. The adaptive contrast enhancement device comprises: (i) a distribution estimator that determines the number of samples in a picture, having gradation values; (ii) a mean estimator that: for a first one of the digital pictures, determines a first value for the samples with gradation values between the lower limit C and the upper limit U; for a first portion of the samples, determines a first mean value not greater than the first value, and for a second portion of the samples, determines a second mean value not less than the first value; and (ii) a contrast enhancer that enhances contrast of the digital picture by applying a contrast enhancement transform to a set of samples representing the digital picture, the contrast enhancer including a first transform function and a second transform function, wherein: the first transform function is based on a distribution of the samples with the first mean value in the first portion, and the second transform function in dependence on a distribution of the samples with the second mean value in the second portion. 
   As such, the present invention provides a contrast enhancement method and device that are applicable to a video sequence which inherently has time-varying characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures where: 
       FIG. 1A  is a block diagram of an embodiment of a device for performing the adaptive contrast enhancement method according to the present invention; 
       FIG. 1B  is a block diagram of another embodiment of a device for performing the adaptive contrast enhancement method according to the present invention; 
       FIG. 2  shows an example of a transform that can be used to increase the dynamic range of samples having gradation levels within a certain range; 
       FIG. 3  shows an example of a transform that can be used to increase the dynamic range of samples having gradation levels within another range; 
       FIGS. 4A–C  show examples of enhancement functions; 
       FIG. 5A  shows an example of the enhancement functions ƒ l (x) and ƒ u (x); 
       FIG. 5B  shows a plot of ƒ l (x)·g l (m l ) and ƒ u (x)·g u (m u ); 
       FIG. 5C  shows a plot of then transform function that is obtained using the terms ƒ l (x)·g l (m l ) and ƒ u (x)·g u (m u ); 
       FIG. 6  shows an example flowchart of an embodiment of a adaptive contrast enhancement method according to the present invention; and 
       FIG. 7  shows another block diagram of an embodiment of a device for performing the adaptive contrast enhancement method according to the present invention. 
   

   Like reference characters refer to like elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
   As noted above, an Adaptive Contrast Enhancement (ACE) method according to the present invention specifies the characteristics of a time varying video sequence and performs a nonlinear transform over the input video sequence to enhance mainly the contrast of the input. Referring to  FIG. 1A , an example architecture of an ACE apparatus (device)  10  implementing an example ACE method according to the present invention, is shown. In a functional block  14 , a probability density function (PDF) of a time varying input video sequence is computed and then some predetermined video parameters relating to contrast is extracted from the PDF. Based upon the extracted video parameters, a nonlinear transform function is then constructed and updated as a look up table (LUT) in the block  14 , which is synchronized with the associated video picture or field SYNC signal. The transform LUT is then applied to the input video to enhance the input signal in a functional block  16 . Therefore, the primary steps of the ACE method are the extraction of the video parameters relating to the contrast of the input video signal and the construction of the transform function, described further below. 
   In the example of  FIG. 1A , a memory device  12  is used to delay the input video for one frame/field period to apply the constructed transform to the video frame/field used for the transform construction. The incoming picture is stored in the memory  10  while the transform LUT is constructed using parameters obtained from the picture. The memory  10  is provided to delay the input video for one frame or field period so that the transform LUT can be applied to the picture that was used to construct the transform LUT, as shown in the functional block  14 . Alternatively, as shown in another example ACE apparatus  20  of  FIG. 1B , the memory device can be removed from the architecture because a video sequence typically has a high correlation in temporal direction, and therefore, in most applications, the LUT transform that is constructed from one picture can be applied to the subsequent picture in the video sequence. 
   The example ACE method according to the present invention is now described in more detail. In this description, I n (·) denotes a picture (fame, or, field) of an incoming video sequence at time instant n, wherein the picture I n (·) comprises samples to be enhanced whose values are, in general, from the gradation levels {C,C+1, . . . , U}, where C is associated with the black gradation level and U is associated with the white gradation level in a video system. The determination of the values of C and U is based on each particular application (e.g., C=0, U=255, etc.). The range represented by {C,C+1, . . . , U} can be narrower than the real dynamic range of the input picture system. 
   The average brightness, or, the mean of the input video picture I n (·) is denoted as m, and h(x) denotes the PDF of I n (·), where h(x) represents the number of the samples in I n (·) whose gradation level equals to x. It is preferable to use the mean m, since good results are obtained with this parameter, however, it should be understood that another value deviating from the mean m could be used instead. Conceptually any one of a number of values between C and U could be used, however, it is preferable to use the mean or a value very close to the mean so that the average brightness will not be changed, or at least will not be changed significantly. 
   A value 
           N   =       ∑     x   =   C     U     ⁢     h   ⁡     (   x   )               
represents the total number of samples in the input video picture I n (·) to be enhanced. Then the mean of those samples can be computed as
 
           m   =       ∑     x   =   C     U     ⁢     x   ·       h   ⁡     (   x   )       /     N   .                 
Or, simply the mean can be computed by summing up the gradation levels of the samples in the input video picture to be enhanced and dividing by N.
 
   Based on the mean, m, two parameters m l  and m u  are computed which are associated with the mean of samples lower than or equal to the mean m and the mean of samples greater than or equal to the mean m, respectively, wherein: 
   
     
       
         
           
             
               
                 
                   
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                             ( 
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                         ∑ 
                         
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                           C 
                         
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                       ⁢ 
                       x 
                     
                   
                 
                 ⁢ 
                 
                   
 
                 
                 ⁢ 
                 and 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
           
             
               
                 
                   m 
                   u 
                 
                 = 
                 
                   
                     
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                         x 
                         = 
                         m 
                       
                       U 
                     
                     ⁢ 
                     
                       
                         h 
                         ⁡ 
                         
                           ( 
                           x 
                           ) 
                         
                       
                       ⁢ 
                       x 
                     
                   
                   
                     
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                         x 
                         = 
                         m 
                       
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                 ( 
                 2 
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   such that C≦m l ≦m and m≦m u ≦U. The parameters m l  and m u  roughly indicate how the corresponding samples are distributed in the regions (C,m) and (m,U), respectively. In one example, the values m u  and m l  represent the mean brightness of the sub-images (the first is the sub-image which can virtually comprise of the samples less than or equal to the mean, and the second is the sub-image which can virtually comprise of the samples greater than or equal to the mean, respectively). For example, m l ≈m implies that the samples in the region (C,m) are mostly distributed near to the mean m, whereas m l ≈C implies that the samples in the region (C,m) are mostly distributed near to C. Similarly, m u ≈m implies that the samples in the region (m,U) are mostly distributed near to the mean m, whereas m u ≈U implies that the samples in the region (m,U) are mostly distributed near to U. The value of m can be selected as desired depending on application (e.g., m=128). 
     FIG. 2  shows an example representation of a transform, wherein x denotes the input gradation level of the input sample and y denotes the transformed output. In this example, the gradation values in the region (p,k) are mapped to the values in the region (p,k′). Hence, the dynamic range, D in , for the samples whose gradation levels are in the region (p,k) is increased to D out  as a result of the transform. The overall contrast of the image is enhanced if the input picture has more samples in the region (p,k) than in the region (k,q) because the dynamic range for the samples in the region (p,k) has been increased. 
   Therefore, the example transform illustrated in  FIG. 2  is suitable to enhance the contrast when the samples are distributed more in the region (p,k) region than in the region (k,q). However, if more samples are distributed in the region (k,q) than in the region (p,k), then another example transform function shown in  FIG. 3  can increase the overall contrast of the picture. 
   Further, ACE can be performed according to the present invention by combining the example transforms of  FIGS. 2–3  in accordance with the sample distribution of the input image. The transform function is adjusted adaptively in accordance with the distribution of the gradation levels in the input picture in order to increase the overall contrast of the picture. An example transform ψ(x) for the ACE according to the present invention is represented by: 
   
     
       
         
           
             
               
                 
                   ψ 
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                 = 
                 
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                                   ⁡ 
                                   
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                           ⁢ 
                           
                             C 
                             ≤ 
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                             ≤ 
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                           otherwise 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   wherein ƒ l (x) and ƒ u (x) are enhancement basis functions, and g l (m l ) and g u (m u ) are adaptive gain adjusting functions. The enhancement basis functions ƒ l (x) and ƒ u (x) determine the general characteristics of the enhancement, whereas the adaptive gain adjusting functions g l (m l ) and g u (m u ) control the manner and degree of enhancement associated with the input picture. 
   The following constraints apply to the enhancement basis functions: 
   ƒ l (x) is positive and only defined in region (C,m) and ƒ u (x) is positive and only defined in region (m,U),
 
ƒ l ( C )=ƒ l ( m )=0, and ƒ u ( m )=ƒ u ( U )=0,  (5)
 
ƒ l (x) has a local maxima in (C,m) and ƒ u (x) has a local maxima in (m,U),  (6)
 
   Arbitrary enhancement basis functions can be used so long as they meet the constraints given in relations (4)–(6) above. For example,  FIGS. 4A–C  show three example plots of the lower enhancement basis function, ƒ l (x), satisfying the constraints given in relations (4), (5) and (6) above. 
   The followings are constraints apply to the adaptive gain adjusting functions:
 
 g   l ( C )=1, and  g   l ( m )=−1,  (7)
 
 g   u ( m )=1, and  g   u ( U )=−1,  (8)
 
g l (m l ) is a monotonically decreasing function with respect to m u ,  (9)
 
g u (m u ) is a monotonically decreasing function with respect to m u ,  (10)
 
   wherein the value of g l (m l ) changes from 1 to −1 as m l  varies from C to m and the value of g u (m u ) changes from 1 to −1 as m u  varies from m to U. The gain parameters adjust the enhancement functions in accordance with the sample distribution as discussed in relation to  FIGS. 2–3  above. Further, the negative sign of g l (m l ) and g u (m u ) revert the curvature direction of the enhancement basis functions ƒ l (x) and ƒ u (x). 
   If the enhancement basis functions ƒ l (x) and ƒ u (x) are negative in the regions (C,m) and (m,U), respectively, the conditions given in relations (7)–(10) are changed, respectively, to:
 
 g   l ( C )=−1, and  g   l ( m )=1,  (11)
 
 g   u ( m )=−1, and  g   u ( U )=1,  (12)
 
g l (m l ) is a monotonically increasing function with respect to m l ,  (13)
 
g u (m u ) is a monotonically increasing function with respect to m u .  (14)
 
   Referring back to relation (3) above, the input gradation levels in (C,U) are changed as g l (m l )·ƒ l (x) if x∈(C,m) and as g u (m u )·ƒ u (x) if x∈(m,U). The transform given in relation (9) maps m to m because it is required that ƒ l (m)=ƒ u (m)=0, keeping the mean brightness of the transformed picture without significant change compared to the mean brightness of the input picture. 
   The characteristics of the transform in relation (3) change from picture to picture as the characteristics or the gradation level distribution of the video signal changes from picture to picture. Specifically, the values of the parameters m, m l  and m u  vary from picture to picture, and so does the final transform ψ(x) in relation (3). This provides the adaptive nature of an example contrast enhancement method according to the present invention. 
   Referring to  FIG. 5A  an example of the enhancement function ƒ u (x) which satisfies the conditions given in relations (4)–(6), is illustrated. Further,  FIG. 5B  shows an example plot of the term ƒ u (x)·g u (m u ) of relation (3), wherein e.g. g u (m u )=−0.25. And,  FIG. 5C  is an example of a complete plot of the transform ψ(x), for m≦x≦u, in relation (3) to be applied to the input video. 
   Referring to  FIG. 6  an example flowchart of the steps of the above embodiment of the contrast enhancement method of the present invention is shown. Specific functional forms of ƒ l (x), ƒ u (x), g l (m l ) and g u (m u ) that satisfy the respective conditions in relation (4) through (10) predetermined. Generally, the contrast enhancement method based on the transform given in relation (3) further includes the steps of: 
   Computing the PDF, h(x), of the incoming picture I n (·) (step  22 ); 
   Computing the mean, m, and the values m l  and m u  (step  24 ); 
   Computing the gain functions g l (m l ) and g u (m u ) (step  26 ); 
   Using the transform in relation (3) to construct the transform LUT (step  27 ); and 
   Applying the LUT to the incoming video input signal to generate an enhanced video signal (step  28 ). 
   As noted above, the LUT update can be synchronized with a picture SYNC signal, wherein the LUT is applied to transform the input picture that was used to construct the transform and stored in a picture memory (e.g.,  FIG. 1A ). Or, the LUT can be applied to the next input picture if no memory is incorporated (e.g.,  FIG. 1B ). This process is a pixel by pixel operation which outputs ψ(x) for the input pixel gradation level x. 
   In the above, the example ACE method is based upon the enhancement functions combined with the adaptive gain adjustment, depending on the sample distribution of the incoming picture. Any arbitrary function for ƒ l (x), ƒ u (x), g l (m l ) and g u (m u ) can be used as long as they are subject to the conditions given in relations (4)–(10) above. 
   In another example ACE method according to the present invention, a second order polynomial is used as one specific embodiment of the enhancement functions used in relation (3), wherein:
 
ƒ l ( x )= K   l ( x−C )( m−x ),  (15)
 
ƒ u ( x )= K   u ( m−x )( x−U ),  (16)
 
   where K l  and K u  are pre-determined constants. These enhancement functions satisfy the constraints in relations (4)–(6). Accordingly, the transform in relation (3) an be represented as: 
   
     
       
         
           
             
               
                 
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                           otherwise 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 17 
                 ) 
               
             
           
         
       
     
   
   The choices of the gain functions, g l (m l ) and g u (m u ), are versatile, satisfying the constraints given in relations (11)–(14). Different gain functions result in different characteristics of ψ(x) and, hence, said gain function choices can be varied depending on specific application. A simple example choice of g l (m l ) and g u (m u ) can be:
 
 g   l ( m   l )=2( m   l   −C )/( m−C )−1 and  g   u ( m   u )=2( m   u   −m )/( U−m )−1.  (18)
 
   Direct applications (using constants for K l  and K u ) of the transform given in relation (17) may result in the gray inversion problem, depending on the value of m where the gray inversion implies, wherein ψ(x 1 )&gt;ψ(x 2 ) for some x 1 &lt;x 2 . 
   Such gray inversion problem can be prevented with an algebraic manipulation such as: 
   
     
       
         
           
             
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                 ) 
               
             
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   wherein the above values K l  and K u  are bounded such as: 
   
     
       
         
           
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   As such, for the boundary values of K l  and K u , the transform given in relation (17) becomes: 
   
     
       
         
           
             
               
                 
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                           otherwise 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 19 
                 ) 
               
             
           
         
       
     
   
   Furthermore, depending on applications, the transform can be represented by: 
   
     
       
         
           
             
               
                 
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                                   ⁡ 
                                   
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                                       l 
                                     
                                     ) 
                                   
                                 
                                 · 
                                 
                                   
                                     
                                       ( 
                                       
                                         x 
                                         - 
                                         C 
                                       
                                       ) 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         m 
                                         - 
                                         x 
                                       
                                       ) 
                                     
                                   
                                   
                                     m 
                                     - 
                                     C 
                                   
                                 
                               
                             
                             , 
                           
                         
                       
                       
                         
                             
                           ⁢ 
                           
                             C 
                             ≤ 
                             x 
                             ≤ 
                             m 
                           
                         
                       
                     
                     
                       
                         
                           
                             x 
                             + 
                             
                               α 
                               · 
                               
                                 
                                   g 
                                   u 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     m 
                                     u 
                                   
                                   ) 
                                 
                               
                               · 
                               
                                 
                                   
                                     ( 
                                     
                                       m 
                                       - 
                                       x 
                                     
                                     ) 
                                   
                                   ⁢ 
                                   
                                     ( 
                                     
                                       x 
                                       - 
                                       U 
                                     
                                     ) 
                                   
                                 
                                 
                                   U 
                                   - 
                                   m 
                                 
                               
                             
                           
                           , 
                         
                       
                       
                         
                           m 
                           ≤ 
                           x 
                           ≤ 
                           U 
                         
                       
                     
                     
                       
                         
                             
                           ⁢ 
                           
                             x 
                             , 
                           
                         
                       
                       
                         
                             
                           ⁢ 
                           otherwise 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 20 
                 ) 
               
             
           
         
       
     
   
   where α is a pre-determined gain to adjust the overall degree of enhancement. Note that ψ(x)=x when α=0 in relation (20), meaning no change. 
     FIG. 7  shows a block diagram of another example ACE apparatus  30 , implementing an example ACE method, according to the present invention. Such an ACE apparatus is for a contrast enhancement method based on the transform in relation (3), wherein specific functional forms of ƒ l (x), ƒ u (x), g l (m l ) and g u (m u ) satisfying the respective conditions disclosed in relations (4) through (10) above, are pre-determined. In this example, the ACE apparatus comprises: A memory device  12 ; A PDF estimator block  32  that computes the PDF, h(x), for an input picture I n (·) video signal; A mean compute block  34  that computes the mean, m, and m l  and m u  values based on the PDF; A transform compute block  36  that computes g l (m l ) and g u (m u ) and ψ(x); and a LUT transform block  38  that constructs said transform LUT and applies the LUT to the incoming video input signal to generate enhanced video output signal, as described. The update can be synchronized with a picture SYNC signal as shown. 
   The incoming picture is stored in the memory  10  while the transform LUT is constructed using parameters obtained from the picture. As noted above, the memory  12  is provided to delay the input video for one frame or field period so that the transform LUT can be applied to the picture that was used to construct the transform LUT, as shown in functional block  16 . A video sequence typically has a high correlation in the temporal direction, and therefore, in most applications, the LUT transform that is constructed from one picture can be applied to the subsequent picture in the video sequence. As shown in  FIG. 1B  above, the incoming picture is not stored in a memory while the transform LUT is constructed using the parameters that are obtained from the incoming picture. The transform that had been constructed from the previous picture in the video sequence is applied to this incoming picture. Similarly, the transform that is being constructed from this incoming picture will be applied to the subsequent picture in the video sequence. Applying the transform LUT to the input picture is a pixel by pixel operation that outputs ψ(x) for the input pixel gradation level x. In both embodiments, the LUT transform can be updated in a manner that is synchronized with a picture SYNC signal. 
   The various components of the arrangements in  FIGS. 1A–B and 7  can be implemented in many ways known to those skilled in the art, such as for example, as program instructions for execution by a processor, as logic circuits such as ASIC, etc. The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.