Patent Publication Number: US-2018048910-A1

Title: Method and associated device for filtering digital video images

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/766,035 filed Jan. 19, 2001, which claims priority to FI Application No. 20000120 filed Jan. 20, 2000, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected from a predetermined set of coding types, in which filtering is performed to reduce visual artefacts due to a block boundary. The present invention also relates to a device for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, the device comprising a filter for reducing visual artefacts due to a block boundary. Furthermore, the present invention relates to a video encoder comprising means for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which encoder comprises a filter for reducing visual artefacts due to a block boundary. The present invention also relates to a video decoder comprising means for reducing visual artefacts in a frame of a digital video signal, which is coded by blocks and then decoded, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video decoder comprises a filter for reducing visual artefacts due to a block boundary. The present invention also relates to a video codec comprising means for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video codec comprises a filter for reducing visual artefacts due to a block boundary. The present invention also relates to a mobile terminal comprising a video codec, which comprises means for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, which video codec comprises a filter for reducing visual artefacts due to a block boundary. The present invention further relates to a storage medium for storing a software program comprising machine executable steps for coding and decoding a digital video signal by blocks, a block type being defined according to the coding method for a block selected according to a predetermined set of coding types, for reducing visual artefacts due to a block boundary by filtering. 
     BRIEF DESCRIPTION OF RELATED DEVELOPMENTS 
     A transmission system like that shown in  FIG. 1  is generally used for transferring a digital video image in compressed form. The video image is formed of sequential frames. In some prior art video transmission systems, for example those according to the ITU-T H.261/H.263 recommendations, at least three frame types are defined: an I-frame (Intra), a P-frame (Predicted or Inter), and a B-frame (Bi-directional). The I-frame is generated solely on the basis of information contained in the image itself, wherein at the receiving end, this I-frame can be used to form the entire image. P-frames are formed on the basis of a preceding I-frame or P-frame, wherein at the receiving stage a preceding I-frame or P-frame is correspondingly used together with the received P-frame in order to reconstruct the image. In the composition of P-frames, for instance, motion compensation is used to compress the quantity of information. B-frames are formed on the basis of one or more preceding P-frames or I-frames and/or one or more following P- or I-frames. 
     The frames are further divided into blocks. One or more such blocks forms a block region. There can generally be four different region types: Intra region, copy region, coded region, and not-coded region. An intra region is a block region in which the blocks are coded independently without reference to any other frame. A copy region consists of blocks which are obtained by copying the content of the reference frame into exactly the same location without any motion compensated prediction. A coded region consists of blocks which are obtained using motion compensated prediction and prediction error coding. The prediction error is a difference between the pixel values of the actual frame and a reconstructed frame which is formed in the coding/decoding system using motion compensated prediction, as will be described in further detail later in the text. The prediction error is coded and sent to a receiver. A not-coded region is obtained using motion compensated prediction only. In fact the not-coded region is equivalent to a copy region if the motion information equals 0. All the block regions of one frame are not necessarily similar types, but one frame can comprise block regions which are of different types. 
     Referring to  FIG. 1 , which illustrates a typical encoding and decoding system (codec) used, for example, in the transmission of digital video, a current video frame to be coded comes to the transmission system  10  as input data l n (x,y). The input data l n (x,y) typically takes the form of pixel value information. In the differential summer  11  it is transformed into a prediction error frame E n (x,y) by subtracting from it a prediction frame P n (x,y) formed on the basis of a previous image. The prediction error frame is coded in block  12  in a manner described hereinafter, and the coded prediction error frame is directed to a multiplexer  13 . To form a new prediction frame, the coded prediction error frame is also directed to a decoder  14 , which produces a decoded prediction error frame Ê n (x,y) which is summed in a summer  15  with the prediction frame P n (x,y), resulting in a decoded frame Î n (x,y). The decoded frame is saved in a frame memory  16 . To code the next frame, the frame saved in the frame memory  16  is read as a reference frame R n (x,y) and is transformed into a new prediction frame P n (x,y) in a motion compensation and prediction block  17 , according to the formula: 
         P   n ( x,y )= R   n   [x+Dx ( x,y ), y+Dy ( x,y )]  (1)
 
     The pair of numbers [Dx(x,y), Dy(x,y)] is called the motion vector of the pixel at location (x,y) and the numbers Dx(x,y) and Dy(x,y) are the horizontal and vertical shifts of the pixel. They are calculated in a motion estimation block  18 . The set of motion vectors [Dx(•), Dy(•)] consisting of all motion vectors related to the pixels of the frame to be compressed is also coded using a motion model comprising basis functions and coefficients. The basis functions are known to both the encoder and the decoder. The coefficient values are coded and directed to the multiplexer  13 , which multiplexes them into the same data stream with the coded prediction error frame for sending to a receiver. In this way the amount of information to be transmitted is dramatically reduced. 
     Some frames can be partly, or entirely, so difficult to predict that it is not practical to use motion compensated prediction when coding them. These frames or parts of frames are coded using intra-coding without prediction, and therefore it is not necessary to send motion vector information relating to them to the receiver. 
     In the receiver system  20 , a demultiplexer  21  separates the coded prediction error frames and the motion information transmitted by the motion vectors and directs the coded prediction error frames to a decoder  22 . The decoder  22  produces a decoded prediction error frame Ê n (x,y), which is summed in a summer  23  with the prediction frame P n (x,y) formed on the basis of a previous frame, resulting in a decoded frame Î n (x,y). The decoded frame is directed to an output  24  of the decoder and at the same time saved in a frame memory  25 . When decoding the next frame, the frame saved in the frame memory is read as a reference frame and transformed into a new prediction frame in the motion compensation and prediction block  26 , according to formula (1) presented above. 
     The coding method applied in block  12  to the coding of the prediction error frame or to the intra-coding of a frame or part of a P-frame to be sent without prediction, is generally based on a transformation, the most common of which is the Discrete Cosine Transformation, DCT. The frame is divided into adjacent blocks having a size of e.g. 8×8 pixels. In coding and decoding, the blocks are processed independent of one another. The transformation is calculated for the block to be coded, resulting in a series of terms. The coefficients of these terms are quantized on a discrete scale in order that they can be processed digitally. Quantization causes rounding errors, which can become visible in an image reconstructed from blocks, so that there is a discontinuity of pixel values at the boundary between two adjacent blocks. Because a certain decoded frame is used for calculating the prediction frame for subsequent predicted frames, these errors can be propagated in sequential frames, thus causing visible edges in the image reproduced by the receiver. Image errors of this type are called blocking artefacts. 
     Some prior art methods are known for removing blocking artefacts. These methods are characterized by the following features:
         determining which pixel/pixels require value correction in order to remove the blocking artefact,   determining a suitable low-pass filtering for each pixel to be corrected, based on the values of other pixels contained by a filtering window placed around the pixel,   calculating a new value for the pixel to be corrected, and   rounding the new value to the closest digitized pixel value.       

     Factors that influence the selection of a filter and the decision whether to use filtering can be, for example, the difference between the values of pixels across the block boundary, the size of the quantization step of the coefficients received as the transformation result, and the difference of the pixel values on different sides of the pixel being processed. 
     It has been found that prior art methods tend to remove lines that belong to real features of the image. On the other hand, prior art methods are not always capable of removing all blocking artefacts. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to present a new kind of filtering arrangement for reducing blocking artefacts. The invention also has the objective that the method and associated device operate more reliably and efficiently than prior art solutions. 
     The method according to the invention adjusts filtering parameters according to the type of blocks whose boundary is to be filtered. Different filtering parameters are chosen according to the type of block on either side of the boundary in order to yield an improved filtering result. 
     The objectives of the invention are achieved by adapting the selection of pixels for filtering and the filtering process more flexibly than before to the features of the frame and the environment of the filtering point and by taking into account the nature/type of the blocks to be filtered. 
     According to a first aspect of the invention, there is provided a method for reducing visual artefacts in a frame that is coded by blocks, characterized in that the filtering performed on the block boundary depends on block types of the frame in the environment of the block boundary. 
     According to a second aspect of the invention, there is provided a device for implementing the method according to the invention. The device according to the invention is characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary. 
     According to a third aspect of the invention, there is provided an encoder characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary. 
     According to a fourth aspect of the invention, there is provided a decoder characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary. 
     According to a fifth aspect of the invention, there is provided a codec characterized in that the filter is arranged to operate adaptively according to the block types of the frame in the environment of the block boundary. 
     According to a sixth aspect of the invention, there is provided a mobile terminal characterized in that the filter is arranged to operate adaptively according to the the block types of the frame in the environment of the block boundary. 
     According to a seventh aspect of the invention, there is provided a storage medium characterized in that the software program further comprises machine executable steps for filtering adaptively according to the block types of the frame in the environment of the block boundary. 
     Because blocking artefacts only occur at block boundaries, according to the invention, filtering is advantageously only applied to pixels at block boundaries and the vicinity thereof. Edges that are part of the image can reside anywhere in the image area. In order that only pixels containing blocking artefacts are selected for corrective filtering and that the quality of edges that are part of the image itself is not affected during filtering, the following assumptions are made: 
     Changes in pixel values associated with edges that are part of the image are generally larger than those associated with blocking artefacts, and those edges within the image, where the pixel value change is small, do not suffer considerably from the rounding of the pixel value difference caused by filtering. 
     Because the image to be coded is generally divided into blocks both vertically and horizontally, the image contains both vertical and horizontal block boundaries. With regard to vertical block boundaries, there are pixels to the right and left of the boundary, and with regard to horizontal block boundaries, there are pixels above and below the boundary. In general, the location of the pixels can be described as being on a first or a second side of the block boundary. In an exemplary embodiment of the filtering method according to the invention, the number of pixels to be corrected, the characteristic features of the filter being used and the size of the filtering window depend on the following factors:
     a) The type of block on either side of the boundary (e.g. inter, copy, coded, not-coded),   b) the difference in pixel values Δ across the block boundary. The difference can be defined in many ways. One definition is Δ=|r 1 −l 1 |, where r 1  is the value of the pixel on the first side of the block boundary closest to the boundary, and II is the value of the pixel on the second side of the block boundary closest to the boundary,   c) the size of the quantization step QP of the coefficients received as the result of the transformation used in coding, and   d) differences in pixel values between the pixels on the first side of the block boundary, and correspondingly between the pixels on the second side of the block boundary.   

     In an advantageous embodiment of the method according to the invention, the number of the pixels selected for filtering can vary, and it is not necessarily the same on different sides of a block boundary. The number of pixels also depends on the type of block on either side of the boundary. Because the number of pixels is adapted to the general features of the image information contained by the frame in a particular region according to the factors mentioned above, the method produces a better filtering result than that provided by prior art methods. A “better” result in this context is one in which blocking artefacts are reduced to a greater extent while real edges in the image are affected to a lesser degree. This means that a larger amount of blocking artefacts can be removed without weakening the real image edges unreasonably. 
     It should be noted that in other embodiments of the invention, the factors affecting the filtering performed at a block boundary may differ from those presented above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be described in more detail with reference to the preferred embodiments and the accompanying drawings, in which 
         FIG. 1  represents a digital video encoding and decoding system (codec) according to prior art, 
         FIG. 2  represents the location of pixels in relation to a block boundary in an exemplary embodiment of the method according to the invention, 
         FIG. 3  represents alternatives for locating the filtering method according to the invention in a digital video encoding and decoding system, 
         FIG. 4  is a schematic representation of a device for implementing a method according to the invention, 
         FIG. 5  represents a device according to  FIG. 4  in operation, and 
         FIG. 6  is a schematic representation of a portable video telecommunications device implementing a method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     In the above, in connection with the description of the prior art, reference was made to  FIG. 1 . In the following description of the invention and its preferred embodiments, reference will be made mostly to  FIGS. 2 to 5 . The same reference numbers are used for corresponding parts in the figures. 
       FIG. 2  shows the location of the pixels r 1 -r 6  and l 1 -l 6  in relation to a vertical block boundary  30 . To implement the method according to the invention, certain parameters are specified. Parameter n is the largest number of pixels to be examined from the block boundary in one direction, and in the case of  FIG. 2  its value is 6. It is advantageous to select the value of the parameter n so that it has a certain relation to both the difference of the pixel values Δ across the block boundary and to the size of the quantization step QP of the coefficients received as the result of image coding. Furthermore, the value of parameter n is advantageously smaller than or equal to the number of pixels in the block in the direction of examination, to avoid possible blocking artefacts associated with previous block boundaries spreading to the block boundary under examination. The following definition is recommended for use in a preferred embodiment of the invention applied to image blocks comprising 8×8 pixels: 
     
       
         
           
             
               
                 
                   n 
                   = 
                   
                     { 
                     
                       
                         
                           
                             0 
                           
                           
                             
                                 
                             
                           
                           
                             Δ 
                           
                           
                             
                               ≥ 
                               
                                 2.00 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             1 
                           
                           
                             
                               
                                 1.50 
                                  
                                 α 
                               
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 2.00 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             2 
                           
                           
                             
                               
                                 1.00 
                                  
                                 α 
                               
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 1.50 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             3 
                           
                           
                             
                               
                                 0.66 
                                  
                                 α 
                               
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 1.00 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             4 
                           
                           
                             
                               
                                 0.40 
                                  
                                 α 
                               
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 0.66 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             5 
                           
                           
                             
                               
                                 0.25 
                                  
                                 α 
                               
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 0.40 
                                  
                                 α 
                               
                             
                           
                         
                         
                           
                             6 
                           
                           
                             
                               0 
                               ≤ 
                             
                           
                           
                             Δ 
                           
                           
                             
                               &lt; 
                               
                                 0.25 
                                  
                                 α 
                               
                             
                           
                         
                       
                       , 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     wherein α=QP·log(QP). If QP has a different value in blocks on different sides of the block boundary, the smaller value of QP is used in the calculations, as well as in all cases presented hereinafter, in which a definition includes reference to one QP value only. The invention does not place any limitations on the determination of the value of parameter n, but according to the guidelines of equation (2), it is advantageous that its value is generally higher when the difference of pixel values Δ across the block boundary is small in comparison with the size of the quantization step QP of the coefficients received as the result of the coding transformation. If the difference between the pixel values Δ is large, there is a high probability that there is a real image edge at the block boundary, and in this case the pixels are preferably not examined for filtering at all (n=0). 
     In the next step of the filtering method according to the invention, region type information concerning the two neighbouring blocks is examined, i.e. the type of the blocks on both sides of the block boundary in question is considered. According to the region type information, the value of the parameter n may further be limited (truncated) to provide even better results for reducing blocking artefacts. The region type information is included e.g. in the coded information relating to the pixels of a particular block, wherein that information is maintained, or temporarily stored, during decoding of the block until a truncated value n tr  for parameter n is determined. 
     Table 1 shows truncation values according to an advantageous embodiment of the present invention. Table 1 applies in a situation where the maximum value of n is 6, and, of course, different truncated values would be appropriate in situations where the maximum value is other than 6. The truncation values are used for the first and second sides of the block boundary depending on the region type of the block on the first side of the block boundary and on the region type of the block on the second side of the block boundary. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 type of the Block 
                 type of the Block on the 
               
               
                 on the First side 
                 Second side of a boundary 
               
            
           
           
               
               
               
               
               
            
               
                 of a boundary 
                 INTRA 
                 COPY 
                 CODED 
                 NOT_CODED 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 INTRA 
                 n 
                 n 
                 2 
                 2 
                 n 
                 4 
                 n 
                 2 
               
               
                 COPY 
                 2 
                 2 
                 2 
                 2 
                 2 
                 4 
                 2 
                 2 
               
               
                 CODED 
                 4 
                 n 
                 4 
                 2 
                 4 
                 4 
                 4 
                 2 
               
               
                 NOT_CODED 
                 2 
                 n 
                 2 
                 2 
                 2 
                 4 
                 2 
                 2 
               
               
                   
               
            
           
         
       
     
     Each cell of Table 1, corresponding to a particular region type combination, is split into two parts. The value on the left gives the truncation value trval for the first side of the block boundary and the value on the right gives the truncation value trval for the second side of the boundary. If the value of parameter n exceeds the value given in Table 1, n is truncated to the truncation value trval in Table 1. If, however, the value of the overall activity parameter n does not exceed the value given in Table 1, the value of the parameter n (originally determined from equation (2)) is retained. In Table 1 the symbol “n” indicates that further truncation is not performed and the parameter value is retained. The truncated value n tr  for the parameter n can also be represented by the formula: 
         n   tr =min( trval,n ),  (3)
 
     The same table can be used both for filtering across vertical block boundaries (horizontal filtering) by putting “Left”/“Right” in place of “First”/“Second” and for filtering across horizontal block boundaries (vertical filtering) by putting “Bottom”/“Up” in place of “First”/“Second”, respectively. Now, the value on the left gives the truncation value for the pixels on the left/below the block boundary and the value on the right gives the truncation value for the pixels on the right/above the boundary. 
     To further clarify the use of Table 1, an example situation is presented in the following. In this illustrative example situation “horizontal filtering” is performed across a vertical block boundary  30 . Assuming that the value for parameter n, calculated from equation 2, is e.g. 4, the block on the left-hand side of the block boundary  30  in question is of the Intra type, and the block on the right-hand side of the block boundary  30  in question is of the Not-coded type, Table 1 indicates that the truncation value for the left-hand side is n and the truncation value for the right-hand side is 2. This means that the 4 pixels closest to the block boundary (=the calculated value of n) are selected for examination from the left-hand side of the boundary, and 2 pixels closest to the block boundary (=the truncated value of n) are selected for examination from the right-hand side. 
     Another example situation is presented in the following. In this illustrative example situation “horizontal filtering” is performed across a vertical block boundary  30 . Assuming that the value of parameter n calculated from equation (2) is e.g. 4, and the blocks on both sides of the block boundary  30  in question are of the Copy type, Table 1 indicates that the truncation value for the left-hand side is 2 and the truncation value for the right-hand side is 2. This means that the 2 pixels closest to the block boundary (=the truncated value of n) are selected for examination from the left-hand side of the boundary, and the 2 pixels closest to the block boundary (=the truncated value of n) are selected for examination from the right-hand side. 
     For bi-directionally predicted frames (B-frames), truncation of the parameter n is advantageously not applied because there is no unique block type information. 
     The next step in the filtering method according to the invention is to determine the values of the parameters d l  and d r , which represent activity, or the differences of pixel values between pixels on one side of the block boundary. A preferred definition for d r  is the following: 
         d   r =6, if | r   1   −η|≦β/j  with all  jε[ 1,6], 
     otherwise: d r =i, where i fulfils the conditions 
         iε[ 1, n   tr ],  (4)
 
       | r   1   −r   i+1   |&gt;β/i , and 
       | r   1   −r   j   |≦β/j  with all  jε[ 1, i].    
     Here, the auxiliary parameter β=4·log(QP). The value of parameter d l  is determined similarly, except that all r&#39;s are replaced by l&#39;s and the corresponding truncated value n tr  for the parameter n must be used. The number 6 appears in definition (4) because, according to equation (2), the highest possible value of n is 6 in this case. If n is defined differently, but the parameters d r  and d l  are defined according to definition (4), the number 6 must be replaced by the highest possible value of n according to the new definition. 
     It is advantageous that the values of the parameters d r  and d l  are calculated independent of one another, because the image information contained by the frame can be different on different sides of the block boundary. The invention does not limit the definition of parameters d r  and d l , but according to the guidelines of definition (4), it is advantageous that these parameters are used to limit the blocking artefact processing relatively close to the block boundary, if there is a real image edge near the block boundary. The essential features of definition (4) can be summarised as follows: the value of parameter d r  (and correspondingly the value of parameter d l ) provides an indication of how many pixels counted from the block boundary have approximately the same value as the pixel at the block boundary. 
     A high value of parameter n (e.g. 6) indicates that the difference between the pixel values at the block boundary is relatively small compared with the general variation of the pixel values within the block. In this case, it is possible that there is a real image edge near the block boundary. By selecting a sufficiently small value of parameter d r  (or d l ), it is possible to restrict the filtering aimed at correcting blocking artefacts so that it does not have a deteriorating effect on a real image edge close to the block boundary. In some situations, a large number of pixels counted from the block boundary have approximately the same value as the pixel at the block boundary. In that case, definition (4) would give the parameter d r  (or d l ) a relatively high value. However, if there is a clear discontinuity in pixel values between the blocks, the parameter n has a small value and the truncated value n tr  is used in the definition (4) which make sure that an unreasonably high value is not selected as the value of the parameter d r  (or d l ). Otherwise, a relatively high value of the parameter d r  (or d l ) would result in unnecessary filtering. 
     If blocks on both sides of the block boundary are Intra-type blocks, the truncation has no effect on the selection of the parameter values n, d r  and d l . On the other hand, if at least one of the blocks has a type other than Intra, the truncation of the value n according to the formula (3) may limit the number of pixels filtered. This has the advantage that the block boundaries are not smoothed too much. 
     In addition, the largest possible number of pixels to be filtered must be decided. This does not have a notation of its own in  FIG. 2 , but it can be e.g. 3, which means that filtering can only be used to correct the value of the pixels r 1 , r 2 , r 3 , l 1 , l 2  and 13. 
     When the values of the parameters n, n tr , d r  and d l  have been determined, filtering is carried out using a suitable filter. The invention does not limit the kind of filter that can be used, but a filtering arrangement that has been found preferable, will be described in the following. Filtering is used to determine a new value for the pixels selected for filtering. In a preferred embodiment of the invention, a new pixel value is determined for a given pixel by calculating the mean of the pixel values that appear in a filtering window. In the preferred embodiment, the filtering window is symmetrical with regard to the pixel to be filtered and contains, in addition to the pixel to be filtered, one, two or three pixels from both sides, depending on the values of the parameters d r  and d l  as described hereinafter. Of course, these are only examples and other values could be chosen in situations where n, n tr , d r  and d l  are defined differently. The calculated mean value is rounded to the closest digitized pixel value, whereby it becomes the new value of the filtered pixel. 
     Table 2 shows the determination of the width of the filtering window for the pixels r 1 , r 2  and r 3  according to the value of parameter d r  in a preferred embodiment of the invention. The values of the pixels l 1 , l 2  and l 3  are determined in the same manner according to the value of the parameter d l . In the table, “X” means that the pixel in question is not filtered at all, and the number means that the filtering window includes a number of pixels shown by the number from each side of the pixel being examined. Among other things, Table 2 shows that for filtering to be applied to any pixel, parameters d r  and d l  must both have a value greater than 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 d r  (d l  &gt; 1) 
                 r 1   
                 r 2   
                 r 3   
               
               
                   
                   
               
             
            
               
                   
                 1 
                 X 
                 X 
                 X 
               
               
                   
                 2 
                 1 
                 X 
                 X 
               
               
                   
                 3 
                 1 
                  1* 
                 X 
               
               
                   
                 4 
                 2 
                 2 
                 X 
               
               
                   
                 5 
                 2 
                 2 
                  2** 
               
               
                   
                 6 
                 3 or 2*** 
                 3 
                 3 
               
               
                   
                   
               
               
                   
                 *the filtered value of pixel r 1  is used for filtering of pixel r 2   
               
               
                   
                 **the filtered values of pixels r 1  and r 2  are used for filtering pixel r 3   
               
               
                   
                 ***3 if d l  &gt; 2, otherwise 2. 
               
            
           
         
       
     
     The above description relates to implementing the filtering on one horizontal part of a pixel row, which is 12 pixels long and located symmetrically on both sides of a vertical block boundary. The description can be easily generalized to concern vertical parts of pixel columns, which are located symmetrically on both sides of a horizontal block boundary:  FIG. 2  can be turned 90 degrees counter-clockwise, whereby block boundary  30  becomes horizontal, and the pixels shown in the figure form part of the vertical pixel column so that pixels r 1 -r 6  are the pixels above and pixels l 1 -l 6  are the pixels below the boundary. To filter block boundaries throughout the whole frame, applying the method according to the invention, all vertical block boundaries of the frame are examined row by row and all horizontal block boundaries are examined column by column. The order has no significance as such, and thus all the horizontal block boundaries of the frame could be examined first column by column, and then all the vertical block boundaries row by row. In an advantageous embodiment of the invention, filtering is repeated line by line, i.e. the first line of the pixels in the blocks (besides the boundary) is filtered first, then the second line, etc. 
       FIG. 3  shows at which points a digital image encoding and decoding system (codec) according to prior art can be improved by applying filtering according to the invention. The first alternative is to place a block implementing the filtering method according to the invention in the output of the decoder of the receiver, as illustrated by reference number  31 . In this case, block boundaries in the video frame are filtered after all blocks within the frame have been decoded. This requires block type information to be stored for all the blocks in one frame. Another alternative is to place the filtering block in the receiver before the point at which the decoded frame is directed to the frame memory  25  for forming a prediction frame, as illustrated by reference number  32 . In this case, block type information for all blocks within the frame must also be stored, as block boundary filtering is still performed after decoding and reconstructing the entire frame. However, this alternative has the advantage that the removal of blocking artefacts also has an effect on the formation of a prediction frame, whereby blocking artefacts in one frame are not propagated via the prediction frame to subsequent frames. In order to achieve the effect just described, the block that performs filtering according to the invention can be placed either before or after the frame memory  25 . However, the location shown by reference number  32  is preferred, because when applied at this stage, the filtering influences simultaneously the frame to be output by the receiving decoder and the frame to be saved in the memory. In the transmitter, a block implementing the filtering method according to the invention can be placed as shown by reference numbers  33  and  34 , either before or after the frame memory  16 . In this way, the invention can also be used to produce a corrected prediction frame at the transmission end. 
     In a particularly advantageous embodiment of the invention, the block carrying out the filtering according to the invention is implemented in a digital signal processor or a corresponding device suited for processing a digital signal, which can be programmed to apply predetermined processing functions to a signal received as input data.  FIG. 4  presents the functional elements of a block carrying out the filtering method according to the invention, implemented as a digital signal processor. Correspondingly, operation of the filtering block is illustrated by  FIG. 5 . Referring first to  FIG. 4 , the functional elements are programmed with functional definitions ( 35 - 39 ) for calculating the parameters that control the filtering method according to the invention. Advantageously, the functional definitions are loaded into the signal processor in connection with its manufacture or programming. During operation according to  FIG. 5 , the frame is saved temporarily in register  41 , so that it can be processed by the signal processor. Processing of the frame proceeds block-by-block. At a given instant, a number of pixels indicated by the parameter n, n tr , according to the definitions provided in Table 1, are selected to be examined  42  from each side of a certain point on a certain block boundary, the d-parameters  43  are calculated, and filtering  44  is performed. These operations are repeated until all boundaries of all blocks have been filtered/processed, after which the frame can be output from register  41  and a new frame saved for processing. The measures according to  FIGS. 4 and 5  can be carried out in a separate signal processor or they can be implemented in a general processor which also contains other arrangements for signal processing. 
     A storage medium can be used for storing a software program comprising machine executable steps for performing the method according to the invention. In an advantageous embodiment of the invention, the software program is read from the storage medium to a device comprising programmable means, e.g. a processor, for performing the method of the invention. 
     The invention can be modified without departing from the scope defined by the claims presented hereinafter, using the capabilities of a person skilled in the art without actual inventive steps. For example, the parameter Δ can be calculated using the formula Δ=|(r 1 +r 2 )−(l 1 +l 2 )| or some other formula considered suitable. The definitions of the other parameters presented above should also be considered as examples only. A particularly advantageous use of the invention is in mobile video telecommunication applications, digital television receivers and other devices that at least receive and decode digital video image. It should further be noted that in general the method according to the invention may be applied to any image coding method in which an image is encoded/decoded on a block-by-block basis, including individual (i.e. still) digital images. 
       FIG. 6  shows a mobile terminal  46  intended for use as a portable video telecommunications device and applying the method according to the invention. Advantageously, the mobile terminal  46  comprises at least display means  47  for displaying images, audio means  48  for capturing and reproducing audio information, a keyboard  49  for inputting e.g. user commands, a radio part  50  for communicating with a mobile communications network (not shown), processing means  51  for controlling the operation of the device, memory means  52  for storing information, and preferably a camera  53  for taking images. 
     The present invention is not solely restricted to the above presented embodiments, but it can be modified within the scope of the appended claims.