Abstract:
A method of filtering to remove coding artifacts introduced at block edges in a block-based video coder, the method having the steps of: checking the content activity on every line of samples belonging to a boundary to be filtered and where content activity is based on a set of adaptively selected thresholds determined using Variable-Shift Table Indexing (VSTI); determining whether the filtering process will modify the sample values on that particular line based on said content activity; and selecting a filtering mode between at least two filtering modes to apply on a block boundary basis, implying that there would be no switching between the two primary modes on a line by line basis along a given block boundary. The two filtering modes include a default mode based on a non-recursive filter, and a strong filtering mode which features two strong filtering sub-modes and a new selection criterion that is one-sided with respect to the block boundary to determine which of the two strong filtering sub-modes to use. The two strong filtering sub-modes include a new 3-tap filter sub-mode and a 5-tap filter sub-mode that permits a more efficient implementation of the filter.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/300,849 filed Nov. 21, 2002, and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of video coding, more particularly it relates to a method of reducing blocking artifacts inherent in hybrid block-based video coding. 
     BACKGROUND OF THE INVENTION 
     Video compression is used in many current and emerging products. It has found applications in video-conferencing, video streaming, serial storage media, high definition television (HDTV), and broadcast television. These applications benefit from video compression in the fact that they may require less storage space for archived video information, less bandwidth for the transmission of the video information from one point to another, or a combination of both. 
     Over the years, several standards for video compression have emerged, such as the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T) recommended video-coding standards: H.261, H.262, H.263 and the emerging H.264 standard and the International Standardization Organization and International Electrotechnical Commission (ISO/IEC) recommended standards MPEG-1, MPEG-2 and MPEG-4. These standards allow interoperability between systems designed by different manufacturers. 
     Video is composed of a stream of individual pictures (or frames) made up of discrete areas known as picture elements or pixels. The pixels are organised into lines for display on a CRT or the like. Each pixel is represented as a set of values corresponding to the intensity levels of the luminance and chrominance components of a particular area of the picture. Compression is based mainly on the recognition that much of the information in one frame is present in the next frame and, therefore, by providing a signal based on the changes from frame to frame a much reduced bandwidth is required. For the purpose of efficient coding of video, the pictures or frames can be partitioned into individual blocks of 16 by 16 luminance pixels called “macroblocks”. This practice simplifies the processing which needs to be done at each stage of the algorithm by an encoder or decoded. To encode a macroblock (or sub-macroblock partition) using motion-compensated prediction, an estimation is made of the amount of motion that is present in the block relative to the decoded pixel data in one or more reference frames, usually recently decoded frames, and the appropriate manner in which to convey the information from which the current frame may be reconstructed. The residual signal, which is the difference between the original pixel data for the macroblock and its prediction, is spatially transformed and the resulting transform coefficients are quantized before being entropy coded. The basic processing blocks of an encoder are a motion estimator/compensator/predictor, a transform, a quantizer and an entropy coder. Due to the quantation of the transformed coefficients of the residual signal, the reconstructed pixel values are generally not identical to those of the original frame. Since the coding is block-based, the errors that are introduced by the quantization process tend to produce artifacts in the form of sharp transitions in image intensity across transform block boundaries in the reconstructed frame. Such artifacts are referred to as “blocking artifacts”. The appearance of blocking significantly affects the natural smoothness seen in video images and leads to a degradation of the overall video image quality. 
     Blocking artifacts are inherent in hybrid block-based video coders, especially in low bit rate video applications. A number of solutions have been presented to alleviate the degradation in visual quality due to the presence of blocking artifacts. Two general approaches have been proposed to deal with blocking artifacts. The first approach is based on using a deblocking filter in the decoder only as a post-processing stage, and applying the deblocking filter on the decoded and reconstructed video frames before they are displayed. The purpose of the filter is to modify the sample values around the block boundaries in order to smooth unnatural sharp transitions that have been introduced by the block-based coding process. Having, a deblocking filter applied outside of the motion-compensation loop can be viewed as an optional process for the decoder, placing no requirements on the video encoder. However, this scheme has a disadvantage in that the reference frames that are used for generating predictions for the coding of subsequent frames will contain blocking artifacts. This can lead to reduced coding efficiency and degraded visual quality. The second approach to reduce the visibility of blocking artifacts is to apply a deblocking filter inside the motion-compensation loop. In this case, the reference frames that are used for generating predictions for subsequent encoded frames represent filtered reconstructed frames, generally providing improved predictions and improved compression and visual quality. In order to create identical predictions at both the encoder and decoder, the deblocking filter (sometimes referred to as a “boop filter” if it is inside the motion-compensation loop) must be applied in both the encoder and the decoder. 
     In order to reduce the appearance of blocking artifacts, a number of video coding standards, including H.263 version 2, and most recently the emerging H.264 video coding standard specify the use of a deblocking filter inside the motion-compensation loop. In particular, the H.264 video coding standard fully specifies a deblocking filter that is to be used inside the motion-compensation loop in both the encoder and decoder. 
     One of the known prior art methods is described in a document “Working Draft Number 2, Revision 2 (WD-2)” by the Joint Video Team (JVT) of ISO/IEC MPEG and ITU-T VCEG. In this prior art method, filtering occurs on the edges of 4×4 blocks in both the luminance and chrominance components of each reconstructed video frame. The filtering takes place on one 16×16 macroblock at a time, with macroblocks processed in raster-scan order throughout the frame. Within each macroblock, vertical edges are filtered first from left to right, followed by filtering of the horizontal edges, from top to bottom. The filtering of samples for one line-based filtering operation occurs along the boundary separating unfiltered samples p 0 , p 1 , p 2  and p 3  on one side of the boundary, and unfiltered samples q 0 , q 1 , q 2  and q 3  on the other side, as illustrated in  FIG. 3   a . The block boundary lies between samples p 0  and q 0 . In some cases p 1 , p 2  may indicate samples that have been modified by filtering of a previous block edge. For each line-based filtering operation, unfiltered samples will be referred to with lower-case letters, and filtered samples with upper-case letters. For each block boundary segment (consisting of 4 rows or columns of samples), a “Boundary strength” parameter, referred to as “Bs”, is computed before filtering. The calculation of Bs is based on parameters that are used in encoding the bounding blocks of each segment. Each segment is assigned a Bs value from zero to four, with a value of zero indicating that no filtering will take place, and a value of 4 indicating that the strongest filtering mode will be used. 
     The process for determining Bs is as follows. For each boundary, a determination is made as to whether either one of the two blocks that neighbour the boundary is intra-coded. If either block is intra-coded, then a further determination is made as to whether the block boundary is also a macroblock boundary. If the block boundary is also a macroblock boundary, then Bs=4, else Bs=3. 
     Otherwise, if neither block is intra-coded then a further determination is made as to whether either block contains non-zero transform coefficients. If either block contains non-zero coefficients then Bs=2; otherwise if a prediction of the two blocks is formed using different reference frames or a different number of frames and if a pair of motion vectors from the two blocks reference the same frame and either component of this pair has a difference of more than one sample, then Bs=1; else Bs=0, in which case no filtering is performed on this boundary. The value of boundary strength, Bs, for a specific block boundary is determined by the encoding characteristics of the two 4×4 blocks along the boundary. Therefore, the control of the filtering process for each individual block boundary is well localized. The block boundary is filtered only when it is necessary, based on whether the coding modes used for the neighbouring blocks are likely to produce a visible blocking artifact. 
     The known filtering process starts with the step of filtering each 4×4 block edge in a reconstructed macroblock. The filtering “Boundary strength” parameter, Bs, is computed and assigned based on the coding parameters used for luma. Block boundaries of chroma blocks correspond to block boundaries of luma blocks, therefore, the corresponding Bs for luma is also used for chroma boundaries. 
     Filtering takes place in the order described above on all boundary segments with non-zero value for Bs. The following describes the process that takes place for each line-based filtering operation. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 QP av  dependent activity threshold parameters α and β 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 QP av   
               
             
          
           
               
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 19 
                 20 
                 21 
                 22 
                 23 
                 24 
                 25 
                 26 
                 27 
                 28 
                 29 
               
               
                   
               
               
                 α 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 4 
                 4 
                 5 
                 6 
                 7 
                 9 
                 10 
                 12 
                 14 
                 17 
               
               
                 β 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
                 6 
                 6 
                 7 
                 7 
               
               
                   
               
             
          
           
               
                   
                 QP av   
               
             
          
           
               
                   
                   
                 30 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
                 37 
                 38 
                 39 
                 40 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
               
               
                   
                   
               
               
                   
                 α 
                 20 
                 24 
                 28 
                 33 
                 39 
                 46 
                 55 
                 65 
                 76 
                 90 
                 106  
                 126  
                 148  
                 175  
                 207  
                 245  
                 255  
                 255  
                 255  
                 255  
                 255  
                 255  
               
               
                   
                 β 
                 8 
                 8 
                 9 
                 9 
                 10 
                 10 
                 11 
                 11 
                 12 
                 12 
                 13 
                 13 
                 14 
                 14 
                 15 
                 15 
                 16 
                 16 
                 17 
                 17 
                 18 
                 18 
               
               
                   
                   
               
             
          
         
       
     
     A content activity check is performed. If the check is passed, filtering continues; otherwise, the sample values are not modified on this line of the boundary segment. The activity check makes use of a pair of activity threshold parameters, ALPHA (α) and BETA (β), whose particular values are selected from the above Table 1, based on the average quantization parameter (QP av ) used in coding each boundary segment. It is noted that QP av  represents the average value of the quantization parameter values used in encoding the two blocks that neighbour the boundary, with rounding of the average by truncation of any fractional part. Accordingly, the content activity check is passed if
 
| p   0   −q   0 &lt;ALPHA(α) AND | p   1   −p   0 &lt;BETA(β) AND | q   1   −q   0 &lt;BETA(β).
 
     Further, if this first content activity check is passed, and Bs is not equal to 4, default mode filtering is performed. Otherwise, if the check is passed and Bs is equal to 4, a second, stricter activity check is performed. This activity check involves the evaluation of the condition
 
1&lt;| p   0   −q   0 |&lt;( QP   av &gt;&gt;2) AND | p   2   −p   0 &lt;BETA(β) AND | q   2   −q   0 &lt;BETA(β).
 
If this second condition is true on a particular line of samples, a strong mode filtering is used on this line of samples. Otherwise, a default mode filtering is used on this line of samples. It should be noted the symbol “&gt;&gt;” is used to represent the operation of bit-wise shifting to the right.
 
     Among the disadvantages of the above described known method is that it permits switching between two filtering modes with very different characterstics at the level of each line of samples within a boundary segment. This switching adds complexity to the filtering process and can significantly increase the worst-case critical path for processing on many architectures. 
     Further disadvantages include the particular values in the tables of filtering parameters, ALPHA (α) and BETA (β) which are not optimized to produce the best subjective viewing quality of reconstructed and filtered video. Further, the characteristics of the deblocking filter in terms of the threshold parameters used in the activity checks and equations used for generating filtered sample values are fixed in the known method, providing the encoder with little or no flexibility to control the properties of the deblocking filter. This hinders optimization of the subjective quality of the decoded video for different types of video content and displays. 
     In the default mode of the above identified filtering method, the value Δ, which represents the change from the unfiltered values of p 0  and q 0  to their respective filtered values is computed using:
 
Δ=Clip( −C,C ,((( q   0   −p   0 )&lt;&lt;2+( p   1   −q   1 )+4)&gt;&gt;3)),
 
where C is determined as specified below and the function “Clip” is defined as:
         Clip(a, b, c)=IF (c&lt;a) THEN a
           ELSE IF (c&gt;b) THEN b   ELSE C
 
Further, the filtered values P 0  and Q 0  are computed where
 
 P   0 =Clip(0,255, p   0 +Δ) and  Q   0 =Clip(0,255, q   0 −Δ).
   
               

     In order to compute the clipping value, C, that is used to determine Δ, and also determine whether the values of p 1  and q 1  will be modified on this set of samples, two intermediate variables, a p  and a q  are computed, where:
 
α p   =|p   2   −p   0 | and α q   =|q   2   −q   0 |.
 
     If α p &lt;β for a luminance edge, a filtered sample P 1  is produced as specified by:
 
 P   1   =p   1 +Clip(− C 0, C 0,( p   2   +P   0 −( p   1 &lt;&lt;1))&gt;&gt;1).
 
     If Δ q &lt;β for a luminance edge, a filtered sample Q 1  is produced as specified by Q 1 =q 1 +Clip(−C0, C0, (q 2 +q 0 −( q   1 &lt;&lt;1))&gt;&gt;1) where C0 is specified in Table 2 (see below), based on Bs and QP av  for the block boundary. For both luma and chroma, C is determined by setting it equal to C0 and then incrementing it by one if α p &lt;β, and again by one if α q &lt;β. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Value of filter clipping parameter C0 as a function of QP av  and Bs 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 QP av   
               
             
          
           
               
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 19 
                 20 
                 21 
                 22 
                 23 
                 24 
                 25 
               
               
                   
               
               
                 Bs = 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                 Bs = 2 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Bs = 3 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
           
               
                   
                 QP av   
               
             
          
           
               
                   
                 26 
                 27 
                 28 
                 29 
                 30 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
                 37 
                 38 
                 39 
                 40 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
               
               
                   
               
               
                 Bs = 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
                 5 
                 6 
                 6 
                  7 
                  8 
                  9 
                 10 
                 11 
                 13 
               
               
                 Bs = 2 
                 1 
                 1 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 5 
                 5 
                 6 
                 7 
                 8 
                 8 
                 10 
                 11 
                 12 
                 13 
                 15 
                 17 
               
               
                 Bs = 3 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
                 5 
                 6 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 13 
                 14 
                 16 
                 18 
                 20 
                 23 
                 25 
               
               
                   
               
             
          
         
       
     
     It is important to note that the computation of the filtered values P 1  and Q 1  require as an input to the filtering equation the filtered values of P 0  and Q 0  from the current line of samples. This recursive filtering method presents a disadvantage as the values of P 0  and Q 0  must be computed before the computation of P 1  and Q 1  can begin. This design can impede parallel processing of the different samples and thereby increases the critical path for the default mode filtering on most hardware architectures. 
     An additional disadvantage in the default mode filtering process of the known method is that the calculation of the clipping parameter, C, for chroma samples is unnecessarily complex. The chroma samples p 1  and q 1  are never filtered in the default mode and, therefore, the computation of the variables a p  and a q  is only necessary to determine the C parameter that is used to clip the value of Δ. These computations could be avoided by specifying a simpler method to compute C for chroma filtering. 
     For strong mode filtering in the known method, the following equations are applied to calculate the filtered sample values:
 
 P   0 =( p   2 +2* p   1 +2* p   0 +2* q   0   +q   1 +4)&gt;&gt;3,
 
 P   1 =( p   3 +2* p   2 +2* p   1 +2* p   0   +q   0 +4)&gt;&gt;3,
 
 Q   0 =( p   1 +2* p   0 +2* q   0 +2* q   1   +q   2 +4)&gt;&gt;3 and
 
 Q   1 =( p   0 +2* q   0 +2* q   1 +2* q   2   +q   3 +4)&gt;&gt;3.
 
For the luminance component only, p 2  and q 2  are also filtered as specified by:
 
 P   2 =(2* p   3 +3* p   2   +p   1   +p   0   +q   0 +4)&gt;&gt;3 and
 
 Q   2 =(2* q   3 +3* q   2   +q+q   0   +p   0 +4)&gt;&gt;3.
 
     Filtering with this set of equations can lead to insufficient reduction in the visibility of blocking artifacts. It is therefore an object of the present invention to obviate or mitigate the above-mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention there is provided a method of filtering samples to minimise coding artifacts introduced at a block boundary in a block-based video encoder, the method having the steps of:
         (a) checking content activity on every line of samples across said block boundary,   (b) determining whether the filtering process will modify the sample values on the line of samples based on content activity thresholds, said content activity thesholds being dependent on an average quantization parameter and a selected filter offset parameter,   (c) determining whether a block on either side of the block boundary was coded using either intra or inter prediction;   (d) providing a pair of filter modes,
           i) the first filter mode; a strong filtering mode consisting of a selection of a 3-tap filter sub-mode and an alternative 5-tap filter sub-mode with the selection between these two sub-modes made separately on each side of the block boundary based on the sample values on each line of samples across said block boundary,   ii) the second filter mode, a non-recursive default filtering mode; and   
           (e) selecting one of the pair of filter modes for filtering the samples.       

     The determination of whether the filtering process will modify the sample values on each particular line is based on a content activity check which makes use of a set of adaptively selected thresholds whose values are determined using Variable-Shift Table Indexig (VSTI. 
     In another aspect of the invention there is provided a method of controlling a filter&#39;s properties to adjust the properties of said filter at a block boundary, said method having the steps of:
         (a) computing average quantization parameter value (QP av ) at said block boundary   (b) adding offset values Filter_Offset_A and Filter_Offset_B to the average quantization parameter value QP av  and clipping these values within a given range to determine the tables indices Index A  and Index B ; and   (c) accessing said ALPHA (α) table, said BETA (β) table and said Clipping (C0) table using these indices computed based on the filter offsets and the average quantization parameter value such that:
 
ALPHA=ALPHA_TABLE[Index A ]
 
BETA=BETA_TABLE[Index B ]
 
 C 0=CLIP_TABLE( Bs )[Index A.] 
       

     In a still further aspect of the invention there is provided a method of filtering samples to minimise coding artifacts introduced at a block boundary in a block-based video encoder, the method having the steps of checking content activity on every line of samples belonging to the boundary to be filtered and determining whether the filtering process will modify the sample values on said line of samples based on content activity thresholds that are dependent on a quantization parameter and determined using a filter offset parameter. 
     In a still further aspect of the present invention there is provided a method of filtering samples to minimise coding artifacts introduced at a block boundary in a block-based video encoder, the method having the steps of: determining whether line samples adjacent to a boundary to be filtered are intra-coded; and employing a non-recursive default filtering mode for samples in the proximity of said block boundary that is within a macroblock; a series of filtered output samples being calculated solely from unfiltered input sample values. 
     In a still further aspect of the present invention there is provided a method of filtering samples to mininise coding artifacts introduced at a block boundary in a block-based video encoder for a line of samples to be filtered across a block boundary that is also a macroblock boundary, where at least one side of the boundary is intra coded, the method having the step of: selecting a strong filtering sub-mode for either side of said block boundary between at least two strong filtering sub-modes, a 3-tap filter sub-mode and an alternative 5-tap filter sub-mode, with the selection between these two sub-modes based on the sample values on each line of samples across the boundary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a schematic representation of a data transmission system; 
         FIG. 2  is a schematic representation of hierarchy of levels of an H.264 conformant bitstream; 
         FIG. 3   a  is schematic representation of a macroblock and a block; 
         FIG. 3   b  is a diagram showing relationship between unfiltered samples and activity thresholds; 
         FIG. 4  is a block diagram of a hybrid block-based video decoder including a deblocking filter inside the motion compensation loop of the system of  FIG. 1 ; 
         FIG. 5  is a flowchart of the operation of the deblocking filter process for the decoder of  FIG. 4 ; 
         FIG. 6  is the dependency graph for default mode filter for the decoder of  FIG. 4 ; and 
         FIG. 7  is flowchart for the process of calculating the boundary strength for the decoder of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a video conferencing system  10  used as an example of a video transmission system has participants A and B that exchange video data  12  between monitors  13 , formatted as a compressed bit stream  15  over a network  14  (such as but not limited to the Internet). Each participant A, B has a video processor  16  having an encoder  18  for encoding transmitted video data  12  and a decoder  20  for decoding the received bit stream  15 . Each image frame  22  displayed on the monitors  13  is made of a series of macroblocks  24 , such as but not limited to a block of 16×16 pixels, representing (for example) an object  26  which moves over a background  28  (for example a person giving a presentation while standing in front of a backdrop). Accordingly, the processors  16  coordinate the display of successive frames  22  on the monitors  13 , as the video data  12  is communicated between the participants A, B, which can include applications such as video conferencing. It will be appreciated recognised that the system  10  may also involve the exchange of video data  12  in the compressed bit stream  15  in either one direction or both and on peer-to-peer basis or broadcast. 
     The video data  12  is a temporal sequence of pictures, each referred to as a frame or field  22 . Each picture  22  is organized as a matrix of macroblocks  24 . Each macroblock  24  has a size of 16 by 16 pixels and the macroblocks  24  are stored from left to right and from top to bottom and groups of macroblocks  24  are combined in a slice  32  (see  FIG. 2 ). Generally, a slice  32  contains macroblocks  24  and each macroblock  24  consists of blocks  25  (see  FIG. 3 ). Generally, each macroblock  24  is composed of three images; one red (R), one green (G), and one blue (B). However, for compatibility with non-coloured media, the RGB model is represented as an equivalent YCbCr model, where Y is a luminance (luma) component, and Cb and Cr are chrominance (chroma) components, such that typically Y=0.299R+0.587G+0.114B, Cb=B−Y, and Cr=R−Y. Therefore, each frame  22  of the video data  12  is generically referred to as containing one luma image, one Cb chroma image, and one Cr chroma image. Standard formats have 8 bits per pixel to digitally represent each of the three components, where Cb and Cr images are typically downsampled by 2 in each dimension due to the sensitivity of human vision. Generally, each block  25  consists of four pixels for the luma components and one pixel for each chroma component of the 4:2:0 color data. The blocks  25  are processed and compressed for transmission as the bit stream  15  over the network  14  (see  FIG. 1 ). 
     Generally, one of three fundamental coding modes can be selected for each macroblock  24 , with the choice of coding mode determining how the prediction of a macroblock  24  is formed. Intra-coded (I) macroblocks  24  make use of intra-prediction, in which the prediction is formed using only the current picture. In predictive (P), or inter-coded, macroblocks  24  the prediction of each sample is formed by referring to one block  25  in the set of previously decoded and stored reference pictures  22 . In bi-predictive (B) macroblocks  24 , predictions can be formed in this way, but can also be formed by computing a weighted average of two different blocks  25  in the set of previously decoded reference pictures  22 . It will be noted that some of the previously decoded pictures  22  are typically temporally subsequent to the current picture in terms of their intended display order when bi-predictive coding is used. Depending on the mode of each slice  32 , which is indicated in the slice header  27 , P- and B-macroblocks  24  may not be permitted within certain slices  32 . 
     Referring again to  FIG. 2 , the bitstream  15  is organized into a hierarchy of syntax levels, with the 3 main levels being a sequence level  17 , a picture (or frame) level  19 , and slice level  21 . A concept known as “parameter sets” allows efficient transmission of infrequently changing data at the sequence  17  and picture level  19  in the H.264 standard. A sequence parameter set  29  in the first level  17  includes values of parameters that will remain unchanged for an entire video sequence, or from one instantaneous decoder refresh (IDR) picture to the next. (IDR pictures are used to provide points of random access into the bitstream). Examples of parameters in a sequence parameter set  29  include frame dimensions and the maximum number of reference frames. A unique ID number “N” identifies each sequence parameter set  29 . 
     A picture parameter set  31  in the second level  21  includes values of parameters that will remain unchanged within a coded representation of a picture (frame or field)  22 . Examples of parameters in the picture parameter set  31  include the entropy coding mode and a flag that specifies whether deblocking filter parameters will be transmitted in the slice headers  27  of the picture  22  (see  FIG. 1 ). Each picture parameter set  31 , labeled as “M”, refers to the unique ID of a valid sequence parameter set  29 , which selects the active sequence parameters that are used when decoding coded pictures  22  that use the particular picture parameter set  31 . The unique ID number “M” identifies each picture parameter set  31 . 
     A slice  32  in the bit stream  15  contains a picture data  35  representing a subset of the macroblocks  24  of the complete picture  22 . The macroblocks  24  in a slice  32  are ordered contiguously in raster scan order. The coded slice  32  includes the slice header  27  and the slice data  35  (coded macroblocks  24 ). The slice header  27  contains a coded representation of data elements  35  that pertain to the decoding of the slice data that follow the slice header  27 . One of these data elements contains a reference to a valid picture parameter set  31 , which specifies the picture parameter values (and indirectly the sequence parameter values) to be used when decoding the slice data  35 . Each slice header  27  within the same picture  22  must refer to the same picture parameter set  31 . Other data elements in the slice header  27  include the initial quantization parameter for the first macroblock  24  in the slice  32  and deblocking filter offset parameters  39  (as further explained below), if the trnsmission of such offst parameters  39  is specified in the active picture parameter set. 
     Thus, the filter offsets  39  are transmitted in the slice header  27 , and therefore the offsets  39  can be different for each slice  32  within the picture  22 . However, depending on the value of a flag in the picture parameter set  31  (“filter_parameters_flag”), the transmission of these offsets  39  in the slice header  27  might be disabled. In the case that offsets  39  are not transmitted, a default value of zero is used for both filter offsets  39  for example. Further, each picture parameter set  31  contains parameter values that pertain to the decoding of the pictures  22  for which the particular parameter set  31  is active (i.e. selected in the slice headers  27  of the picture  22 ). The parameter sets  31  also contain a reference to the sequence parameter sets  29 , which are active for decoding of the pictures  22 . The choice of sequence parameter sets  29  and picture parameter sets  31  can be chosen by the encoder  18  (see  FIG. 1 ), or set at the time of system  10  setup for sequential operation of the encoder  18 , decoder  20  pair. 
     Referring either to  FIG. 2 , each of the pictures  22  can select individual picture parameter sets that specify the picture structure and the picture coding type. For exemplary purposes only,  FIG. 3   a  contains the macroblock  24  each consisting of a grouping of pixels, such as a 16×16 luma block  25  with the two associated 8×8 chroma blocks  25 . However, it is recognized that other sizes of blocks  24  could be used to represent the frames  22 , if desired. Each slice  32  of the frame  22  is encoded by the encoder  18  (see  FIG. 1 ), independently from the other slices  32  in the frame  22 . Each of the slices  32  has the slice header  27  that provides information, such as but not limited to the position of the respective slice  32  in the frame  22  as well as the initial quantization parameter, and the slice data which provides information for reconstructing the macroblocks  24  of a slice  32 , such as but not limited to the prediction modes and quantised coefficients for each of the respective macroblocks  24 . 
     Referring to  FIG. 4 , the decoder  20  processes the received bit stream  15  and then reconstructs the predicted frame  46 , using a stored copy of the reference frame(s)  48 , the transmitted motion vectors  23 , and the decompressed or reassembled prediction error  54  contained in the bit stream  1 . 
     The bit stream  15  generated by the encoder  18  is processed by the decoder  20  to produce the reconstructed video images  55 . Referring to  FIG. 4 , the video decoder  20  is based on functional units or components similar to those found in other hybrid block-based video decoders. The functional units include a buffering unit  33  that receives the compressed bitstream  15 , an entropy decoder  34  which decodes the received bit stream  15  to produce syntax elements used in subsequent processing by the other decoder  20  components, a motion compensated prediction  36  to produce the predicted frame, an inverse scanning and quantization unit  38 , and inverse transform units  40  to reproduce the coded prediction error  54 . A reconstruction unit  42  adds the prediction error  54  to the predicted pixels  57  to produce the reconstructed frame  56 , and a deblocking filter  44  that smoothes the edges of sub-blocks within the reconstructed frame  56  to produce the filtered reconstructed frame  56 . Each of the above mentioned components is discussed in more detail in the following. 
     The incoming video bitstream  15  is stored in a buffer  33  at the input to the decoder  20 . The first stage in the decoding process includes the parsing and decoding of the entropy coded bitstream symbols that are stored in a buffer  46  to produce the syntax elements used by the other decoder  20  components. 
     The various syntax elements in the bitstream  15  are de-multiplexed for use in different processes within the decoder  20 . High-level syntax elements include temporal information for each frame, frame coding types and frame dimensions. The coding can be based primarily on macroblocks  24  consisting of 16×16 luminance-pixel blocks  25  and  2  8×8 chrominance pixel blocks  25 . On the macroblock  24  level, syntax elements include the coding mode of the macroblock  24 , information required for forming the prediction, such as motion vectors  23  and spatial prediction modes, and the coded information of the residual (difference) blocks, such as the coded block pattern (CBP) for each macroblock  24  and quantized transform coefficients for each of the underlying blocks  24 . 
     Depending on the coding mode of each macroblock  24 , the predicted macroblock  24  can be generated either temporally (inter prediction) or spatially (intra prediction). The prediction for an inter-coded macroblock  24  is specified by the motion vectors  23  that are associated with that macroblock  24 . The motion vectors  23  indicate the position within the set of previously decoded frames from which each block of pixels will be predicted. Each inter-coded macroblock  24  can be partitioned in a number of different ways, using blocks of seven different sizes, with luminance block sizes ranging from 16×16 pixels to 4×4 pixels. Also, a special SKIP mode exists in which no motion vector difference values  23  (or coded residual blocks) are transmitted and the prediction is taken from a location in the previous picture that is predicted by the values of previously decoded motion vectors  23  of macroblocks  24  neighbouring the current macroblock  24 . Thus, 0 to 16 motion vectors  23  can be transmitted for each inter-coded macroblock  24 . Additional predictive modes in which two different motion vectors  23  correspond to each pixel and the sample values are computed using a weighted average are supported when bi-predictive macroblock types are employed. 
     For each motion vector  23 , a predicted block  25  must be computed by the decoder  20  and then arranged with other blocks  24  to form the predicted macroblock  24 . Motion vectors  23  in H.264 are specified generally with quarter-pixel accuracy. Interpolation of the reference video frames is necessary to determine the predicted macroblock  24  using sub-pixel accurate motion vectors  23 . 
     Multiple (previous for P-pictures) reference pictures  22  can also be used for motion-compensated prediction. Selection of a particular reference pictures  22  is made on an 8×8 sub-macroblock  24  basis, or larger if a larger sub-macroblock partition size is used for generating the motion-compensated prediction. This feature can improve coding efficiency by providing a larger set of options from which to generate a prediction signal. 
     Two different modes are supported in intra prediction and coding of macroblocks  24 . In the 4×4 Intra mode, each 4×4 block within a macroblock  24  can use a different prediction mode. In the 16×16 Intra mode, a single prediction mode is used for the entire macroblock  24 . The prediction of intra-coded blocks  25  is always based on neighboring pixel values that have already been decoded and reconstructed. 
     The decoding of a residual (difference) macroblock  24  requires that a number of transforms be performed on any blocks for which non-zero transform coefficients were transmitted in the bitstream, along with associated scanning and coefficient scaling operations. The transforms that are required for each macroblock  24  are determined based on the coding mode and the coded block pattern (CBP) of the macroblock  24 . The decoding of a difference macroblock  24  is based primarily on the transformation of 4×4 blocks  25  of both the luminance and chrominance pixels, although in some circumstances, a second-level transform must be performed on the DC coefficients of a group of 4×4 blocks  25  for macroblocks  24  that are coded in the 16×16 Intra prediction mode. Additionally, a special 2×2 transform is applied to the 4 DC coefficients of the chrominance residual blocks  25  of a macroblock  24 . 
     The values of the quantized coefficients are parsed and decoded by the entropy decoder  34 . These are put into their correct order based on the run values through the scanning process and then the levels, which represent quantized transform coefficients, are scaled via multiplication by a scaling factor. Finally, the necessary transform to reconstruct the coded residual signal for a block is performed on the scaled coefficients. The result of the transforms for each macroblock  24  is added to the predicted macroblock  24  and stored in the reconstructed frame buffer  48 . 
     In the final stage of the decoding process, the decoder  20  applies the normative de-blocking filtering process, which reduces blocking artifacts that are introduced by the coding process. The filter  44  is applied within the motion compensation loop, so both the encoder  18  and decoder  20  must perform this filtering. The filtering is performed on the 4×4 block edges of both luminance and chrominance components. The type of filter  44  used, the length of the filter and its strength are dependent on several coding parameters as well as picture content on both sides of each edge. A stronger filtering mode is used if the edge lies on a macroblock boundary  49  where the block on one or both sides of the edge is coded using intra prediction. The length of the filtering is also determined by the sample values over the edge, which determine the so-called “activity measures”. These activity measures determine whether 0, 1, or 2 samples on either side of the edge are modified by the filter. 
     Filtering is applied across the 4×4 block edges of both luminance and chrominance components. Looking at  FIG. 3   a , the blocks  25  are separated by boundaries or block edges  47 , with unfiltered samples p 0 , p 1 , p 2  and p 3  on one side of the boundary  47  and unfiltered samples q 0 , q 1 , q 2  and q 3  on the other side, such that the boundary  47  lies between p 0  and q 0 . In some cases p 1 , p 2  may indicate samples that have been modified by filtering of the previous block edge  47 . The deblocking filter  44  (see  FIG. 4 ) is applied on the block boundaries  47  of each reconstructed frame  56 , which helps to reduce the visibility of coding artifacts that can be introduced at those block-boundaries  49 . The filter  44  includes a control function that determines the appropriate filtering to apply. The control algorithm is illustrated by  FIG. 5 . 
     One of the parameters used to control the filtering process of all the block boundaries  47  is the boundary strength, Bs. The procedure for determining the boundary strength, Bs, for the block boundary  47  between two neighbouring blocks j and k is illustrated in  FIG. 7 . For each edge  47 , a determination is made as to whether either one of the two blocks j and k across the boundary  47  is intra-coded, in step  140 . If either block j or k is intra-coded then a further determination is made as to whether the block boundary  47  is also a macroblock boundary  49 , in step  152 . If the block boundary  47  is also a macroblock boundary  49 , then Bs=4 (step  154 ), else Bs=3 (step  156 ). 
     Otherwise, if neither block j or k is incoded then a further determination is made as to whether either block  25  contains non-zero coefficients, in step  142 . If either block  25  contains non-zero coefficients then
         Bs=2 (step  144 ), otherwise the following condition is applied:
 
R(j)≠R(k) or |V(j, x)−V(k, x)|≧1 pixel or |V(j, y)−V(k, y)|≧1 pixel, where R (j) is the reference picture  22  used for predicting block j, and V(j) is the motion vector  23  used for predicting block j, consisting of x and y (horizontal and vertical) components. Therefore, if a prediction of the two blocks  25  is formed using different reference frames  22  or a different number of frames  22  or if a pair of motion vectors  23  from the two blocks  25  reference the same frame and either component of this pair has a difference if more than one sample distance, then this condition holds true and
   Bs=1(step  148 );
 
else,
   Bs=0 (step  150 ), in which case no filtering is performed.       

     The value of boundary strength, Bs, for a specific block boundary  47  is determined solely by characteristics of the two 4×4 blocks  24  across the boundary  47 . Therefore, the control of the filtering process for each individual block boundary  47  is well localized. A block boundary  47  is filtered only when it is necessary, so that unneeded computation and blurring can be effectively avoided. 
     The flowchart of  FIG. 5  describes the filtering process starting with step  100  for the purposes of filtering each 4×4 block edge  47  in a reconstructed macroblock  24 . The filtering “Boundary strength” parameter, Bs, is computed ( 102 ) and assigned for luma. Block boundaries  47  of chroma blocks  25  always correspond to block boundaries  47  of luma blocks  25 , therefore, the corresponding Bs for luma is also used for chroma boundaries  47 . The boundary strength is based on the parameters that are used in encoding the bounding blocks  25  of each segment ( 104 ). Each segment is assigned a Bs value from 0 to 4, with a value of zero indicating that no filtering will take place ( 108 ), and a value of 4 indicating that the strongest filtering mode will be used. 
     In step  110 , the filtering process takes place for each line of samples on the block boundary  47 . The set of filtering operations that take place on one line of a block boundary is referred to as a line-based filtering operation. A content activity check at the boundary  47  between the two blocks  25  is performed in step  112 . The content activity measure is derived from the absolute value of the separation between sample values of p 0 , p 1 , q 0 , q 1 , on either side of the boundary  47 . The activity check is based on two activity threshold parameters ALPHA (α) and BETA (β), whose particular values are selected based on the average quantization parameter (QP av ) used in coding each boundary segment, as well as upon a pair of encoder  18  selected parameter values, referred to as Filter_Offset_A and Filter_Offset_B (referred to as  39  in  FIG. 2 ). QP av  represents the average of the quantization parameter values used in coding the two blocks  25  that neighbour the boundary  47 , with rounding of the average by truncation of any fractional part. Thus, the content activity check is done by comparing difference in the unfiltered sample values p 0  and q 0  across the boundary  47  against the activity threshold ALPHA (α), and the difference in the unfiltered sample values p 0  and p 1  on one side of the boundary  47  and unfiltered sample values q 0  and q 1  on the other side of the boundary  47  against the activity threshold and BETA (β), as shown in  FIG. 3   b . A determination is made to discover whether the activity on the line is above or below the activity threshold. If the activity is above the threshold, the sample values are not modified, otherwise filtering continues. The ALPHA (α) and BETA (β) values are considered as activity thresholds for the difference in magnitude between sample values along the line of samples being filtered. 
     Referring to  FIG. 3 , the ALPHA (α) and BETA (β) parameters represent the activity thresholds for the difference in the values of unfiltered samples p 0 , p 1 , q 0 , q 1  across the boundary  47 . The content activity check is passed if:
 
| p   0   −q   0 |&lt;ALPHA(α) AND | p   1   −p   0 ↑&lt;BETA(β) AND | q   1   −q   0 |&lt;BETA(β)
 
     The sets of samples p 0 , p 1 , q 0 , q 1  across this edge  46  are only filtered if Bs is not equal to zero and the content activity check expressed in the above condition is passed. 
     The values in the ALPHA (α) and BETA (β)-tables used in the loop filter are optimal in terms of the resulting video visual quality and allow some flexibility in the encoder  18  in terms of adjusting the filter parameters, such as the activity threshold parameters and maximum change in a sample value produced by the default filter, through control of the indexing of these tables. The strength of the deblocking filter  44  refers to the magnitude of the change in sample intensities that is caused by the filtering process. Generally, the strength of the filter  44  varies with the coding mode, as well as the step-size used for quantization of the transform coefficients. Stronger filtering is applied when the quantization step-size (and its corresponding “quantization parameter”, QP) are larger, since it is more likely that large block artifacts are created when the quantization is coarse. Thus, flexibility in the properties of the loop filter  44  is provided by allowing the encoder  18  to select offsets  39  to the QP-based indices used to address these tables. This adds flexibility to the filter  44 , help making it more robust to different content, resolutions, display types, and other encoder  18  decision characteristics. 
     The α- and β-tables of the loop filter  44  are QP-dependent thresholds that define the maximum amount of activity at an edge for which the edge will still be filtered. The modified α-table of the preferred embodiment is based on the subjective evaluation of a number of sequences over the entire QP scale. In the preferred embodiment, the value of α doubles every 6 QP as it is related directly to the quantization step size, which also doubles every 6 QP in the H.264 standard. 
     A determination is made to find the QP value below which α should be zero, such that the filter is no longer used for values of α which equal zero. Looking at Table 1, in sequences with smooth areas, blocking artifacts are clearly visible using QP=19, which is the largest QP for which α is equal to zero. Based on Table 3, filtering will take place for QP values as low as 16, since blocking artifacts are still visible in smooth areas. The β-table is also extended at the low QP end in order to permit filtering at these lower QP values. 
     The content activity check ( 112 ) determines whether each sample line is to be filtered and uses the following specific values for α and β ( 114 ) as shown in Table 3 below, where the index used to access the tables is clipped to be within the range of valid QP values (0 to 51). 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Index A  (for α) or Index B  (for B) 
               
             
          
           
               
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 19 
                 20 
                 21 
                 22 
                 23 
                 24 
                 25 
                 26 
                 27 
                 28 
                 29 
               
               
                   
               
               
                 α 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 4 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 12 
                 13 
                 15 
                 17 
                 20 
                 22 
               
               
                 β 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 3 
                  4 
                  4 
                  4 
                  6 
                  6 
                  7 
                  7 
               
               
                   
               
             
          
           
               
                   
                 Index A  (for α) or Index B  (for β) 
               
             
          
           
               
                   
                   
                 30 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
                 37 
                 38 
                 39 
                 40 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
               
               
                   
                   
               
               
                   
                 α 
                 25 
                 28 
                 32 
                 36 
                 40 
                 45 
                 50 
                 56 
                 63 
                 71 
                 80 
                 90 
                 101  
                 113  
                 127  
                 144  
                 162  
                 182  
                 203  
                 226  
                 255  
                 255  
               
               
                   
                 β 
                  8 
                  8 
                  9 
                  9 
                 10 
                 10 
                 11 
                 11 
                 12 
                 12 
                 13 
                 13 
                 14 
                 14 
                 15 
                 15 
                 16 
                 16 
                 17 
                 17 
                 18 
                 18 
               
               
                   
                   
               
             
          
         
       
     
     Further, the particular values for α and β to be used on each block boundary  47  do not only depend on QP, but additionally upon a pair of parameter values, referred to as Filter_Offset_A and Filter_Offset_B, (referenced  39  in  FIG. 2 ) that are transmitted in the higher-level syntax (sequence  17 , picture  19 , or preferably the slice level  21 ) within the video bitstream  15 . These offsets  39  are added to the average QP value between the block  24  in order to calculate the indices that are used to access the tables of ALPHA (α) and BETA (β) values ( 114 ), as well as the C0 table:
 
Index A =Clip( QP   min   ,QP   max   ,QP   av +Filter_Offset —   A )
 
Index B =Clip( QP   min   ,QP   max   ,QP   av +Filter_Offset —   B )
 
     The variables QP min  and QP max  in the above equations represent the minimum and maximum permitted values, respectively, of the quantization parameter QP, and for example can be such that but not limited to the values 0 and 51, respectively. 
     However, because the values Index B  and Index A  are limited to lie in a predetermined interval, if any of the computed coefficients lie outside the interval, those values are limited to the permitted range by the “clip” function. The function “clip” is defined as:
         clip (a, b, c)=IF (c&lt;a) THEN a
           ELSE IF (c&gt;b) THEN b   ELSE c   
               

     By default, Filter_Offset_A and Filter_Offset_B values  39  are both assumed to have a value of zero. Further, within the default filtering, Index A  is also used to access the table of C0 values. Transmission of the Filter_Offset_A and Filter_Offset_B values  39  in the slice header  27  (see  FIG. 2 ) provides a means of adapting the properties of the deblocking filter  44  in terms of the magnitude of the thresholds used in the activity checks and the maximum change in sample values that can be produced by the default filter  44 . This flexibility helps to allow the encoder to achieve the optimal visual quality of the decoded and filtered video. Typically, the semantic in the slice header  27  slice_alpha_c0_offset_div2 specifies the offset  39  used in accessing the ALPHA (α) and C0 deblocking filter tables for filtering operations controlled by the macroblocks  24  within the slice  32 . The decoded value of this parameter is in the range from +6 to −6, inclusive. From this value, the offset  39  that shall be applied when addressing these tables is computed as:
 
Filter_Offset —   A= slice_alpha —   c 0_offset_div2&lt;&lt;1
 
     If this value is not present in the slice header  27 , then the value of this field shall be inferred to be zero. 
     Correspondingly, the semantic in the slice header  27  slice_beta_offset_div2 specifies the offset  39  used in accessing the BETA (β) deblocking filter tables for filtering operations controlled by the macroblocks  24  within the slice  32 . The decoded value of this parameter is in the range from +6 to −6, inclusive. From this value, the offset  39  that shall be applied when addressing these tables is computed as:
 
Filter_Offset —   B= slice_beta_offset_div2&lt;&lt;1.
 
     If is value  39  is not present in the slice header  27 , then the value of this field shall be inferred to be zero. The resulting Variable-Shift Table Indexing (VSTI) method (using the offsets  39  to shift selection of the α-, β-, and clipping (C0) values) allows the decoder  20  to make use of the offset  39  that is specified on the individual slice  32  basis and that will be added to the QP value used in indexing the α-, β-, and clipping (C0) tables. Thus,
 
Alpha(α)=ALPHA_TABLE[Index A ]
 
Beta(β)=BETA_TABLE[Index B ]
 
 C 0=CLIP_TABLE[ Bs ][Index A ]
 
     The offset  39  for indexing the clipping table is always the same as for the α-table. In general, it is desired have α and the clipping values remain in sync, although a different offset  39  for β can be beneficial. The implementation of this method can be simplified even further by applying the offset  39  to the base pointers that are used to access the tables. This way, the extra addition only occurs as often as the offset  39  can be changed (on a per-slice basis), not every time the table is accessed. Clipping of the index can be avoided by extending the tables with the last value in the valid range of indices at each end of the table. 
     A positive offset  39  results in more filtering by shifting a curve (of α, β, or C0 values) to the left on a horizontal QP scale, while a negative offset  39  results in less filtering by shifting a curve to the right. The range of permitted offsets  39  is −12 to +12, in increments of 2. This range is large enough to allow properties of the filter  44  to vary as widely, but is limited to limit additional memory requirements and/or added complexity. This variable-shift method provides both stronger and weaker filtering, and there is sufficient flexibility in the range of values, with reasonable constraints on the amount of variation permitted in the filtering, while maintaining the doubling rate of 6 QP&#39;s for α, consistent with the quantization step size. Also, the clipping (C0) and α values remain in sync with each other. 
     The specific decision on the choice of offsets  39  is varied, and dependent upon the content, resolution, and opinion of the viewer. Generally, less filtering is needed for slowly changing, detailed areas and for high-resolution pictures  22 , while more filtering (using positive offsets  39 ) is preferable for lower resolution pictures  22 , especially with smooth areas and human faces. More filtering can provide the viewer with a feeling of smoother motion. 
     Referring again to  FIG. 5 , if the check is not passed in step  116 , the sample values are not modified on this line ( 118 ), otherwise filtering continues. The selection of the filtering mode occurs at the block boundary  47  level. More specifically, switching between the default-mode filtering and the strong-mode filtering does not occur on a line-to-line basis, and default-mode filtering is not used for intra-coded macroblock boundaries  47 . In step  120 , a further determination is made as to whether the macroblocks  24  are intra-coded. If the macroblocks  24  are not intracoded, then a default filter is applied in step  122 , in which the edges  47  with Bs&lt;4 are filtered by computing the filtered samples P 0  and Q 0  based on the DELTA (Δ). The variable Δ represents the difference the between the unfiltered samples p 0  and q 0  and their respective filtered samples, P 0  and Q 0 , according to the following relation:
 
Δ=Clip( −C,C ,((( q   0   −p   0 )&lt;&lt;2+( p   1   −q   1 )+4)&gt;&gt;3))
 
 P   0 =Clip(0,255, p   0 +Δ)
 
 Q   0 Clip(0,255, q   0 −Δ)
 
     The two intermediate threshold variables α p  and α q  are used to determine the clipping value for the default filtering of luminance samples, as well as the choice of one of the two sub-modes of the strong mode filter, where
 
 a   p   =|p   2   −p   0 | and  a   q   =|q   2   −q   0 |.
 
     Thus, for default-mode filtering ( 122 ), the calculations of filtered samples P 1  and Q 1  are modified from the prior art to increase the parallelism of the filtering process. If a p &lt;β for a luma edge, a filtered P 1  sample generated as specified by:
 
 P   1   =p   1 +Clip( −C 0, C 0( p   2 +( p   0   +q   0 )&gt;&gt;1−( p   1 &lt;&lt;1))&gt;&gt;1).
 
     While if a q &lt;β for a luma edge, a filtered Q 1  sample generated as specified by:
 
 Q   1   =q   1 +Clip(− C 0, C 0,( q   2 +( p   0   +q   0 )&gt;&gt;1−( q   1 &lt;&lt;1))&gt;&gt;1)
 
where C0 is specified in Table 4. However, the adaptable parameter Index A  is used to address the table, rather than QP av .
 
     A dependency graph for the default mode filter with reduced critical path as shown in  FIG. 6  shows that the complexity can be reduced significantly. By shortening the critical path, a reduced cost of default filtering can be achieved and opportunities for parallel processing can be substantially increased, leading to reduced computational requirements. Also, from this figure, the complexity of Bs=4 filtering is potentially reduced by not permitting the filter  44  to switch between default and strong filter modes on a line-by-line basis to help minimise branching stalls and control logic. 
     For luminance only, C, which represents the maximum change in the level of intensity that the default filter can apply to the p 0  and q 0  samples, is determined by setting it equal to C0 and then incrementing it by one if α p &lt;β, and again by one if α q &lt;β. In the default luma filtering, P 1  and Q 1  are filtered only if α p &lt;β and α q &lt;β, respectively, evaluate to true, while P 1  and Q 1  are never filtered for chroma. Therefore, for chrominance filtering, instead of doing these calculations, C can be defined with the basic relationship:
 
 C=C 0+1
 
     Thus, there is a no need to perform the calculations of α p  and α q  for chrominance and therefore no need to load the sample values p 2  and q 2 . This can reduce the complexity of the default chroma filtering by approximately 20%. There is no reduction in quality, either objective or subjective, introduced by this simplification. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 Index A   
               
             
          
           
               
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 19 
                 20 
                 21 
                 22 
                 23 
                 24 
                 25 
               
               
                   
               
               
                 Bs = 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
               
               
                 Bs = 2 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Bs = 3 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
           
               
                   
                 Index A   
               
             
          
           
               
                   
                 26 
                 27 
                 28 
                 29 
                 30 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
                 37 
                 38 
                 39 
                 40 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
               
               
                   
               
               
                 Bs = 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
                 5 
                 6 
                 6 
                  7 
                  8 
                  9 
                 10 
                 11 
                 13 
               
               
                 Bs = 2 
                 1 
                 1 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 5 
                 5 
                 6 
                 7 
                 8 
                 8 
                 10 
                 11 
                 12 
                 13 
                 15 
                 17 
               
               
                 Bs = 3 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
                 5 
                 6 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 13 
                 14 
                 16 
                 18 
                 20 
                 23 
                 25 
               
               
                   
               
             
          
         
       
     
     For strong mode filtering where Bs=4 and the initial activity threshold check  112  has been passed, a further determination to check whether each side of the boundary  47  meet an additional smoothness criteria is performed in steps  124  and  126 . The smoothness criteria for the left/upper side of the boundary  47  is checked in step  124 , while the smoothness criteria for the right/lower side is checked in step  126 . Thus, a choice between a 3-tap filter or a 5-tap filter for the left/upper (P) or the right/lower (Q) side of the boundary  47  is made. If the smoothness criterion is not met on a particular side, a 3-tap filter is used to filter only a single pixel on that side of the boundary  47 . 
     Specifically, for strong-mode filtering:
 
α p   =|p   2   −p   0 |
 
α q   =|q   2   −q   0 |
 
     Therefore, in step  124 , for filtering of edges with Bs=4 if the following condition holds true
 
α p &lt;BETA(β) AND | p   0   −q   0 |((ALPHA(α)&gt;&gt;2)+2),
 
then filtering of the left/upper side of the block edge is specified by the equations ( 130 )
 
 P   0 =( p   2 +2* p   1 +2* p   0 +2* q   0   +q   1 +4)&gt;&gt;3
 
 P   1 =( p   2   +p   1   +p   0   +q   0 +2)&gt;&gt;2
 
     In the case of luminance filtering, then ( 130 )
 
 P   2 =(2* p   3 +3* p   2   +p   1   +p   0   +q   0 +4)&gt;&gt;3
 
     Otherwise, if the above condition does not hold, then filter only P0 using the 3-tap filter ( 128 )
 
 P   0 =(2* p   1   +p   0   +q   1 +2)&gt;&gt;2
 
     Identical but mirrored filters are applied to the right/lower side of the boundary  47 , substituting q and Q for p and P, respectively, in the above description (and vice-versa) ( 132 ,  134 ). 
     Therefore, if the following condition holds true ( 126 ):
 
α p &lt;BETA(β) AND | p   0   −q   0 |&lt;((ALPHA(α)&gt;&gt;2)+2)
 
filtering of the right/lower side of the block edge ( 134 ) is specified by the equations
 
 Q   0 =( p   1 +2* p   0 +2* q   0 +2* q   1   +q   2 +4)&gt;&gt;3
 
 Q   1 =( p   0   +q   0   +q   1   +q   2 +2)&gt;&gt;2
 
     In the case of luminance filtering, then ( 134 )
 
 Q   2 =(2* q   3 +3* q   2   +q   1   +q   0   +p   0 +4)&gt;&gt;3
 
     Otherwise, if the above condition does not hold, then only P0 is filtered with the 3-tap filter ( 132 )
 
 Q   0 =(2* q   1   +q   0   +p   1 +2)&gt;&gt;2
 
     The system  10  thus includes a set of equations for the strong mode filtering to generate samples P 1  and Q 1  that can provide a greater reduction in the visibility of blocking artifacts than alternative equations that were used in the prior known method. Typically, the filters for samples P 1  and Q 1  consist of only 4 taps, as opposed to the 5 taps used for the other filtered samples in this strongest filtering mode. However, this is referred to as a 5-tap filter, since 5 taps is the maximum used for any sample. In addition to providing an improved reduction in blocking artifacts, these equations for filtering P 1  and Q 1  are simpler than those used in the prior art method, potentially reducing the complexity of the filter by a small amount. 
     The system  10  includes tables for ALPHA (α) and BETA (β) that can improve the subjective quality of the filtered video and can also specify an efficient method to allow the encoder  18  to control the characteristics of the deblocking filter  44  by transmitting variable offsets  39  that affect the QP-based indexing of these tables. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.