Abstract:
A method for noise suppression in images of an image sequence, where an iteratively better adapted noise suppression can be ensured for images of the image sequence, in particular in a rising number of images in the image sequence. The method for noise suppression includes a low-pass filter algorithm:
 
 p    i,j   n+1 =α i,j   n+1 * p   i,j   n +( 1− α i,j   n+1 )*Q i,j   n+1  
 
with an attenuation function α i,j   n+1 =α i,j   n+1 (α 0   i,j   n+1 ,Γ i,j   n+1 ).

Description:
This application is a national stage of International Application No.: PCT/DE2012/100272, which was filed on Sep. 7, 2012, and which claims priority to DE 10 2011 054 056.3 which was filed on Sep. 29, 2011, and which are both herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for noise suppression in images of an image sequence. 
     2. Description of the Background Art 
     It is known in the field of digital image processing that characteristic picture disturbances occur in accordance with the properties of the sensors used for image acquisition. Digital images recorded under unfavorable lighting conditions or having a high proportion of dark image areas exhibit a relatively high noise level. Filtering methods which improve the signal-to-noise ratio are generally used in order to reduce the dominating photon noise present in the images. As a rule, for this purpose a time filter is applied to the regions of the images in which no movement is present. A detection of moved image contents is carried out on the basis of the change in intensity determined for each separately considered pixel. It is assumed that a pixel is in movement when the fluctuation in intensity between images of a sequence overshoots a threshold value which is directly related to the standard deviation of the noise. These pixels determined as being in motion are not filtered, or are filtered only slightly. Processing with a rigid recursive time filter as described in patent U.S. Pat. No. 6,314,160 B1, is applied to the pixels determined as static or immobile. The use of a recursive low-pass as a filter base limits the maximum averaging length over an image sequence. The method therefore has a limited quality of noise suppression and a slow transient response. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to find how an iteratively better adapted noise suppression can be ensured for images of an image sequence, in particular given a rising number of images. 
     This object is achieved according to the invention by a method for noise suppression which provides a low-pass filter algorithm:
 
 P   i,j   n+1 =α i,j   n+1   *P   i,j   n +(1−α i,j   n+1 )* Q   i,j   n+1  
 
with an attenuation function α i,j   n+1 =α i,j   n+1 (α 0   i,j   n+1 ,Γ i,j   n+1 ). For each pixel, the attenuation function α i,j   n+1  is dependent, firstly, on an attenuation factor α 0   i,j   n+1  and, secondly, on a movement measure Γ i,j   n+1 . The movement measure Γ i,j   n+1  is dependent on a grayscale value difference dQ i,j   n+1 =|P i,j   n −Q i,j   n+1 | between filtered and unfiltered pixel, a prescribable difference threshold dif_swl, and on an amplitude of the grayscale value Q i,j   n+1  of the pixel.
 
     The attenuation factor α 0   i,j   n+1  is adapted to the duration of the immobility Γ i,j   n+1 ≈0 of the grayscale value of a pixel. Furthermore, a lower limiting value α 0   min  and an upper limiting value α 0   max  are defined for the attenuation factor α 0   i,j   n+1 . In addition, the recursive filtering of unmoved pixels (Γ i,j   n+1 ≈0) has an attenuation factor approaching α 0   max , and the recursive filtering of moved pixels (Γ i,j   n+1 ≈1) with high grayscale value dynamics has an attenuation factor approaching α 0   min . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be explained in more detail below with the aid of exemplary embodiments and the accompaning drawings which are given by way odf illustration only, wherein: 
         FIG. 1  is a flowchart of an exempaly method of the invention, and 
         FIG. 2  is a chart of the weighting of the attenuation factor according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In one variant of the method, a filter algorithm with a recursive averaging is applied to each pixel of an image from a sequence. The low-pass filter used for this purpose acts iteratively with an attenuation function α i,j   n+1 , adapted for the pixel, on the grayscale value difference dQ i,j   n+1  from the grayscale value Q i,j   n+1  of the pixel of a currently recorded image and the grayscale value P i,j   n  of the pixel of a previously recorded and already filtered image. The attenuation function acts in dependence on an attenuation factor α 0   i,j   n+1  adapted for the pixel, and the movement measure Γ i,j   n+1 .The value of the movement measure Γ i,j   n+1  is governed by the grayscale value difference dQ i,j   n+1  and is influenced by a threshold value dif_swl to be prescribed, and a weighting factor γ i,j   n+1 . This weighting factor γ i,j   n+1  is dependent on the grayscale value αQ i,j   n+1  and a reference grayscale value Q 0 . The adaptation of the attenuation factor α 0   i,j   n+1  is likewise performed as a function of the movement measure Γ i,j   n+1  and the value of the already iterativeiy adapted attenuation factor α 0   i,j   n , which was used to filter the previous image. 
     The invention is based on the finding that moved image contents can be detected when the grayscale value of the dynamic image contents stands out from the grayscale value of the static image contents. The difference thus produced is therefore higher than the difference which occurs between noisy pixels in a static image section. This distinction can be used to process dynamic image contents with a different noise suppression than the static ones. The inventive filter algorithm can be used for iterativeiy amplifying the attenuating filter action in noisy, static image areas and reducing it in dynamic image areas or for admitting no attenuation at all. 
     The decision as to whether a pixel is to be classified as dynamic or static is performed using the movement measure Γ i,j   n+1 . It is defined by: 
     
       
         
           
             
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                 n 
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             ⁢ 
             
               { 
               
                 
                   
                     
                       = 
                       
                         
                           
                             ⅆ 
                             
                               Q 
                               
                                 i 
                                 , 
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                                 n 
                                 + 
                                 1 
                               
                             
                           
                           dif_swl 
                         
                         * 
                         
                           γ 
                           
                             i 
                             , 
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                             n 
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                       [ 
                       
                         
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                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ⅆ 
                             
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                                 i 
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                             γ 
                             
                               i 
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                         &lt; 
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                       = 
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                       [ 
                       otherwise 
                       ] 
                     
                   
                 
               
             
           
         
       
     
     The threshold value dif_swl and the weighting factor γ i,j   n+1  are used to weight the grayscale value difference dQ i,j   n+1 . The threshold value dif_swl is prescribed, and the weighting factor γ i,j   n+1  is calculated as follows: 
     
       
         
           
             
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             ⁢ 
             
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                             ⁢ 
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                               n 
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                             Q 
                             
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                         &gt; 
                         
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                       = 
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                         ⁢ 
                         
                             
                         
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     The noise of a pixel is dependent on the signal strength of its grayscale value. Lighter pixels have a stronger noise performance than darker ones do. Consequently, the brightness of the grayscale value of a pixel is taken into account by the use of the reference grayscale value Q 0 . An advantageous value of Q 0  is at 30% of the maximum grayscale value GW max . The threshold value dif_swl can assume values between 0 and GW max , and determines the separation between moved and unmoved pixels. The value of this threshold value dif_swl therefore influences the strength of the edge smearing on moved image contents. 
     The attenuation function α i,j   n+1  is dependent substantially on the grayscale value difference dQ i,j   n+1 , the signal strength Q i,j   n+1  and the attenuation factor α 0   i,j   n+1 : 
     
       
         
           
             
               
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             ⁢ 
             
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                               ⁢ 
                               
                                   
                               
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                     ⁢ 
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                     ⁢ 
                     
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                     ⁢ 
                     
                         
                     
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                           ~ 
                         
                         
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     An iterative adaptation of the attenuation factor α 0   i,j   n+1  in the direction of a stronger attenuation effect over the duration of the immobility P i,j   n+1 ≈P i,j   n  of a pixel Q i,j   n+1  leads to an improved noise suppression. In the case of unmoved pixels, an advantageous adaptation of the attenuation factor is given by the following sequence:
 
α0={½, ⅔, ¾, ⅘, ⅚, 6/7, . . . }
 
     Given lasting immobility, the attenuation factor α 0   i,j   n+1  is increased iteratively, and so it approaches the value 1, and the attenuation function α i,j   n+1  therefore enables a maximum noise suppression at this pixel. 
     The attenuation factor α 0   i,j   n+1  is therefore dependent on the attenuation factor α 0   i,j   n , which was used to filter the pixel of the previous image, and on the movement measure Γ i,j   n+1  of a pixel of the current image. It is evident that the attenuation factor α 0   i,j   n+1  is increased for a small movement measure Γ i,j   n+1 , and reduced for a large movement measure Γ i,j   n+1 . This is performed within the prescribed limiting values α 0   min  and α 0   max . An advantageous implementation of this adaptation can be performed in this defined subdivision (also see  FIG. 2 ): 
     
       
         
           
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             ⁢ 
             
                 
             
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               0 
               
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                 + 
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               { 
               
                 
                   
                     
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                       ] 
                     
                   
                 
                 
                   
                     
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                         n 
                       
                     
                   
                   
                     
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                           ⁢ 
                           
                               
                           
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                       ] 
                     
                   
                 
                 
                   
                     
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                           α0 
                           
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                           n 
                         
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                             i 
                             , 
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                             n 
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                         &lt; 
                         1 
                       
                       ] 
                     
                   
                 
                 
                   
                     
                       = 
                       
                         α0 
                         min 
                       
                     
                   
                   
                     
                       [ 
                       
                         
                           for 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             Γ 
                             
                               i 
                               , 
                               j 
                             
                             
                               n 
                               + 
                               1 
                             
                           
                         
                         = 
                         1.0 
                       
                       ] 
                     
                   
                 
               
             
           
         
       
     
     The following adaptation resulting from threshold value overshooting: 
     
       
         
           
             
               α0 
               
                 i 
                 , 
                 j 
               
               
                 n 
                 + 
                 1 
               
             
             ⁢ 
             
               { 
               
                 
                   
                     
                       = 
                       
                         α0 
                         min 
                       
                     
                   
                   
                     
                       [ 
                       
                         
                           for 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             α 
                             
                               i 
                               , 
                               j 
                             
                             
                               n 
                               + 
                               1 
                             
                           
                         
                         &lt; 
                         
                           α0 
                           min 
                         
                       
                       ] 
                     
                   
                 
                 
                   
                     
                       = 
                       
                         α0 
                         max 
                       
                     
                   
                   
                     
                       [ 
                       
                         
                           for 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             α 
                             
                               i 
                               , 
                               j 
                             
                             
                               n 
                               + 
                               1 
                             
                           
                         
                         &gt; 
                         
                           α0 
                           max 
                         
                       
                       ] 
                     
                   
                 
                 
                   
                     
                       = 
                       
                         α0 
                         
                           i 
                           , 
                           j 
                         
                         
                           n 
                           + 
                           1 
                         
                       
                     
                   
                   
                     
                       
                         [ 
                         
                           for 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           otherwise 
                         
                         ] 
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
     If a pixel is classified as not moved or as noisy, the attenuation factor from the preceding filter pass is increased by the absolute value Δ α , and so the attenuation is increased. In the case of a pixel classified only as moderately moved, the attenuation factor α 0   i,j   n+1  is adopted without change. If the pixel is classified as moved, the damping factor α 0   i,j   n  from the previous filter pass is attenuated by the absolute value Δ α . A pixel classified as strong in movement resets the filter strength of the attenuation factor α 0   i,j   n+1  to the lower limiting value α 0   min , as a result of which only very slight attenuation, or no further attenuation takes place at this pixel. 
     If the adaptation of the attenuation factor α 0   i,j   n+1  is to be done yet more variably and exactly, it is advantageous to store a more comprehensive differentiating subdivision of ranges in a look-up table. 
     Symbols 
     
         
         α 0 —sequence of attenuation factors 
         α 0   max /α 0   min —upper/lower limiting value of the attenuation factor 
         α 0   i,j   n+1 —attenuation factor adapted to pixel 
         α 0   i,j   n —attenuation factor of the predecessor 
         α i,j   n+1 —attenuation function adapted to the pixel 
         α i,j   n —attenuation function that was applied to the predecessor 
         Δ α —absolute value of the change in the attenuation factor 
         Q0—reference grayscale value 
         GW max —maximum grayscale value 
         Q i,j   n+1 —grayscale value of the current pixel 
         dQ i,j   n+1 —grayscale value difference from P i,j   n  and Q i,j   n+1    
         P i,j   n+1 —grayscale value of the current pixel after fiItering 
         P i,j   n —grayscale value of the preceding pixel after filtering 
         dif_swl—threshold value for movement measure 
         γ i,j   n+1 —weighting factor (grayscale value) 
         Γ i,j   n+1 —movement measure