Abstract:
Current deblocking filters are using the same filters with the same filtering strength irrespective of the block size and the size of the transform used. However, in the new video coding standards such as emerging HEVC the PU sizes can vary from 4 to 64 and the TU sizes can vary from 4 to 32. Therefore, filtering the same amount of pixels (e.g. two or three) from the block boundary for the block of size 4 can be excessive, while for the block size 32 it may not be enough, with the result that the boundary between two blocks is still visible. Hence, there is a need for an efficient deblocking filter control that can be used to reduce blocking artifacts at block boundaries and that does not have the above mentioned drawbacks. It is a general objective to provide an efficient deblocking filter control. Thus, the objective is solved by applying different filters for different block sizes such as CU, PU or/and TU sizes. Accordingly, the deblocking filtering strength is adjusted based on the block size, which implies that the amount of modification applied to pixels by the deblocking filter is varied depending on the block size. The amount of modification that is being varied is in one embodiment the number of pixels to be modified.

Description:
TECHNICAL FIELD 
       [0001]    The present embodiments generally relate to filtering control and in particular to controlling deblocking filtering over block boundaries in a video frame. 
       BACKGROUND 
       [0002]    Deblocking filters are used in the video coding standards in order to combat blocking artifacts. The blocking artifacts arise because the original video frames are split into blocks which are processed relatively independently. The blocking artifacts can, for instance, arise due to different intra predictions of the blocks, quantization effects and motion compensation. Two particular variants of deblocking are described below. 
         [0003]    In state of the art video coding, such as H.264, there is a deblocking filter, also denoted loop filter, after prediction and residual reconstruction, but before storage of the reconstruction for later reference when encoding or decoding the subsequent frames. The deblocking filtering consists of several steps such as filter decisions, filtering operations, a clipping function and change of pixel values. The decision to filter the boundary or not is made based on evaluation of several conditions. Filter decisions depend on macroblock (MB) type, motion vector (MV) difference between neighboring blocks, whether neighboring blocks have coded residuals and on the local structure of the current and/or neighboring blocks. 
         [0004]    Then the amount of filtering for a pixel depends, among others, on the position of that pixel relative to the block border or boundary and on the quantization parameter (QP) value used for residual coding. 
         [0005]    The filter decision is based on comparing three pixel differences with three thresholds. The thresholds are adapted to the quantization parameter (QP). For instance, assume a vertical block boundary of 
         [0000]    
       
      
       abcd|efgh  
      
     
         [0000]    where a, b c and d denote the pixel values of the pixels of a row of pixels in the current block with e, f, g and h denoting the corresponding pixel values of the pixels of a corresponding row of pixels in the neighboring block. If the following conditions are fulfilled the filter decision is positive, e.g. abs(d−e)&lt;thr1, abs(c−d)&lt;thr2, and abs(e−f)&lt;thr2, where thr1 and thr2 are adapted based on QP. 
         [0006]    There are two filtering modes in H.264. In the first filtering mode, referred to as normal filtering, filtering can be described with a delta value with which filtering changes the current value with. The filtering for the pixels closest to the block boundary is d′=d+delta and e′=e−delta, where delta has been clipped off to a threshold±thr3 to a value that is constrained by the QP. More filtering is thereby allowed for high QP than for low QP. Clipping can be described as delta_clipped=max(−thr3, min(thr3, delta)), where thr3 is controlling the filter strength. A larger value of thr3 means that the filtering is stronger which means that a stronger low-pass filtering effect will happen. 
         [0007]    The filter strength can be increased if any of the following two conditions also holds, e.g. abs(b−d)&lt;thr2 and abs(e−g)&lt;thr2. The filter strength is adapted by clipping the delta less, e.g. allow for more variation. 
         [0008]    The second filtering mode, referred to as strong filtering, is applied for intra macroblock boundaries only, when the following condition is fulfilled abs(d−e)&lt;thr1/4. 
         [0009]    For more information of deblocking filtering in H.264 reference is made to List et al., Adaptive Deblocking Filter,  IEEE Transactions on Circuits and Systems for Video Technology , vol. 13, no. 7, July 2003. 
         [0010]    In the draft HEVC (High Efficiency Video Coding) specification “Test Model under Consideration”, ITU-T SG16 WP3 document, JCTVC-B205, Chapter 6.5 In-loop filter process, the deblocking filter works differently from H.264. The filtering is performed if at least one of the blocks on the side of the boundary is intra, or has non-zero coefficients, or the difference between the motion vector components of the blocks is greater or equal to one integer pixel. For example, when filtering the border between the blocks with a vertical block boundary of 
         [0000]        p 3 i   p 2 i   p 1 i   p 0 i   |q 0 i   q 1 i   q 2 i   q 3 i    
         [0000]    with pj i  denoting the pixel value of pixel number j of row number i in the current block and qj i  denoting the pixel value of pixel number j of row number i in the neighboring block, i=0 . . . 7, j=0 . . . 3, then the following condition should also be satisfied: 
         [0000]        d=|p 2 2 −2× p 1 2   +p 0 2   |+|q 2 2 −2 ×q 1 2   +q 0 2   |+|p 2 5 −2 ×p 1 5   +p 0 5   |+|q 2 5 −2 ×q 1 5   +q 0 5 |β
 
         [0000]    where β depends on QP. In the above mentioned HEVC specification, there is a table of β, where β increases with QP. 
         [0011]    If the conditions are fulfilled and filtering is done between the current block and the neighboring block, one of two types of filtering is performed, referred to as weak and strong filtering, respectively. The choice between the strong and the weak filtering is done separately for each line depending on the following conditions. For each line i=0 . . . 7, the strong filtering is performed if all the following conditions are true, otherwise, weak filtering is performed: 
         [0000]        d &lt;(β&gt;&gt;2)
 
         [0000]      (| p 3 i   −p 0 i   |+|q 0 i   −q 3 i |)&lt;(β&gt;&gt;3)
 
         [0000]      | p 0 i   −q 0 i |&lt;((5 ×t   C +1)&gt;&gt;1) 
         [0000]    where t C  and β depend on QP and &gt;&gt; denotes a right shift operator. 
         [0012]    Weak filtering is performed based on the above conditions. The actual filtering works by computing an offset (Δ), adding it to the original pixel value and clip the sum to a filtered output pixel value in the range of 0-255: 
         [0000]      Δ=Clip(− t   C   ,t   C ,(13×( q 0 i   −p 0 i )+4×( q 1 i   −p 1 i )−5×( q 2 i   −p 2 i )+16)&gt;&gt;5))
 
         [0000]        p 0 i =Clip 0-255 ( p 0 i +Δ)
 
         [0000]        q 0 i =Clip 0-255 ( q 0 i −Δ)
 
         [0000]        p 1 i =Clip 0-255 ( p 1 i +Δ/2)
 
         [0000]        q 1 i =Clip 0-255 ( q 1 i −Δ/2)
 
         [0000]    where the clip function Clip(A, B, x) is defined as Clip(A, B, x)=A if x&lt;A, Clip(A, B, x)=B if x&gt;B and Clip(A, B, x)=x if A≦x≦B and Clip 0-255 (x) is defined as Clip(0, 255, x). 
         [0013]    Strong filtering mode is performed by the following set of operations: 
         [0000]        p 0 i =Clip 0-255 (( p 2 i +2 ×p 1 i +2 ×p 0 i +2 ×q 0 i   +q 1 i +4)&gt;&gt;3) 
         [0000]        q 0 i =Clip 0-255 (( p 1 i +2 ×p 0 i +2 ×q 0 i +2 ×q 1 i   +q 2 i +4)&gt;&gt;3) 
         [0000]        p 1 i =Clip 0-255 (( p 2 i   +p 1 i   +p 0 i   +q 0 i +2&gt;&gt;2) 
         [0000]        q 1 i =Clip 0-255 (( p 0 i   +q 0 i   +q 1 i   +q 2 i +2&gt;&gt;2) 
         [0000]        p 2 i =Clip 0-255 ((2 ×p 3 i +3 ×p 2 i   +p 1 i   +p 0 i   +q 0 i +4)&gt;&gt;3) 
         [0000]        q 2 i =Clip 0-255 (( p 0 i   +q 0 i   +q 1 i +3 ×q 2 i +2× q 3 i 30 4)&gt;&gt;3)
 
         [0014]    As video resolutions have increased, it has been noticed that large blocks can provide good video coding benefits. Traditional block sizes are in the order of 16×16 pixels or smaller (e.g. macroblocks in H.264), but it has been shown that block sizes of up to 128×128 pixels can provide improved coding efficiency. 
         [0015]    To enable large blocks while keeping the coding performance of small detailed areas in the same image, hierarchical coding is used. This is the case for HEVC. 
         [0016]    Large blocks, referred to as Largest Coding Units (LCU) in HEVC, are scanned left to right in the same way as normal macroblocks in H.264. Each LCU may be split into four smaller coding units (CU), and the may be split again hierarchically in a quad-tree fashion. There is also a smallest size for the Coding Unit defined, these blocks are called Smallest Coding Unit (SCU). 
         [0017]      FIG. 1  shows how a picture is divided in to a number of LCU blocks and how an LCU can be split in a quad-tree fashion into smaller CUs according to prior art. 
         [0018]      FIG. 2  shows how an LCU may be split into smaller CUs. It also shows the order in which the CUs are decoded. The current HEVC operates with an LCU of 64×64 and a SCU of 4×4, but this can be configured. 
         [0019]    Each CU has a coding type assigned, e.g. Intra, P, skip. Each CU may also be quad-tree split further in three independent quad-tree structures. 
         [0020]    The CU has its prediction type (e.g. intra prediction or inter-prediction). The CU is also a root of two structures called prediction units and transform units. Each prediction unit inside the CU can have its own prediction that is different from the predictions of the other PU (for example, a separate motion vector or intra prediction direction). A CU can contain one PU (which has then the same size as the CU) or can be split further into up to four PUs. Those PUs can have either square or rectangular form (in this case, the vertical and horizontal PU dimensions differ). As an example, there might be a CU of size 16×16 that is split once, creating 4 8×8 prediction unit blocks (PUs). If the coding type of the CU is Intra, the PUs may have different Intra prediction modes. If the coding type of the CU is Inter, the PUs may have different motion vectors. 
         [0021]    Then there is a transform quad-tree that also has the CU as its root. The resulting blocks are called Transform Units (TU). As an example, there might be a CU of size 16×16 that is split into 8×8 TUs. Then, one of the 8×8 TU can be split into 4×4 TUs. Then each TU is transformed with an 8×8 or a 4×4 transform. If the root TU was not split, then a 16×16 transform would have been used. Transforms can also have a non-square (rectangular) shape. 
       SUMMARY 
       [0022]    Current deblocking filters are using the same filters with the same filtering strength irrespective of the block size and the size of the transform used. However, in the new video coding standards such as emerging HEVC the PU sizes can vary from 4 to 64 and the TU sizes can vary from 4 to 32. 
         [0023]    Therefore, filtering the same amount of pixels (e.g. two or three) from the block boundary for the block of size 4 can be excessive, while for the block size 32 it may not be enough, with the result that the boundary between two blocks is still visible. 
         [0024]    Hence, there is a need for an efficient deblocking filter control that can be used to reduce blocking artifacts at block boundaries and that does not have the above mentioned drawbacks. 
         [0025]    It is a general objective to provide an efficient deblocking filter control. 
         [0026]    Thus, the object is solved by applying different filters for different block sizes such as CU, PU or/and TU sizes. Accordingly, the deblocking filtering strength is adjusted based on the block size, which implies that the amount of modification applied to pixels by the deblocking filter is varied depending on the block size. The amount of modification that is being varied is in one embodiment the number of pixels to be modified. 
         [0027]    According to a first aspect of embodiments of the present invention, a method for filter control applicable to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. In the method, a block size is determined, and a deblocking filtering strength is adjusted based on the determined block size. 
         [0028]    According to a second aspect of embodiments of the present invention, a filtering control device for applying a filter control to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. The filtering control device comprises a unit configured to determine block size and a unit configured to adjust a deblocking filtering strength based on the determined block size. 
         [0029]    According to a third aspect of embodiments of the present invention, an encoder comprising a filtering control device for applying a filter control to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. The filtering control device comprises a unit configured to determine block size and a unit configured to adjust a deblocking filtering strength based on the determined block size. 
         [0030]    According to a fourth aspect of embodiments of the present invention a user equipment is provided. The user equipment comprises a memory configured to store video frames; and an encoder according to the third aspect of embodiments of the present invention configured to encode said video frames into encoded video frames, wherein said memory is further configured to store said encoded video frames. 
         [0031]    According to a fifth aspect of embodiments of the present invention, a decoder comprising a filtering control device for applying a filter control to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. The filtering control device comprises a unit configured to determine block size and a unit configured to adjust a deblocking filtering strength based on the determined block size. 
         [0032]    According to a sixth aspect of embodiments of the present invention, a user equipment is provided. The user equipment comprises a memory configured to store video frames; and a decoder according to the fifth aspect of embodiments of the present invention configured to encode said video frames into encoded video frames, wherein said memory is further configured to store said encoded video frames. 
         [0033]    According to a seventh aspect of embodiments of the present invention, a post-filter comprising a filtering control device for applying a filter control to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. The filtering control device comprises a unit configured to determine block size and a unit configured to adjust a deblocking filtering strength based on the determined block size. 
         [0034]    According to an eighth aspect of embodiments of the present invention, a user equipment is provided. The user equipment comprises a memory configured to store video frames; and a decoder according to the fifth aspect of embodiments of the present invention configured to encode said video frames into encoded video frames, wherein said memory is further configured to store said encoded video frames that can be modified by deblocking filtering. 
         [0035]    According to a ninth aspect of embodiments of the present invention, a method for filter control applicable to a block of multiple pixels of a video frame is provided. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. In the method, a block size is determined, and a deblocking filtering strength is adjusted based on the determined block size, wherein the adjustment of the deblocking filtering strength comprises modifying a number of pixels at a block border that can be modified by deblocking filtering. 
         [0036]    According to a tenth aspect of embodiments of the present invention, a filtering control device for applying a filter control to a block of multiple pixels of a video frame. The video frame comprises a plurality of blocks and each pixel has a respective pixel value. The filtering control comprises a unit configured to determine block size and a unit configured to adjust a deblocking filtering strength based on the determined block size, wherein the unit ( 120 ) configured to adjust a deblocking filtering strength is further configured to adjust the deblocking filtering strength by modifying a number of pixels at a block border. 
         [0037]    An advantage with embodiments of the present invention is that the deblocking can improve the subjective quality of the reconstructed picture by applying different amount of filtering to blocks of different size. Another advantage is that some embodiments can also decrease the computation complexity as they decrease the number of filtered pixels for smaller blocks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  shows how a picture is divided in to a number of LCU blocks and how an LCU can be split in a quad-tree fashion into smaller blocks according to prior art. 
           [0039]      FIG. 2  shows how an LCU may be split into smaller CUs according to prior art, 
           [0040]      FIG. 3A  shows a vertical block boundary according to prior art. 
           [0041]      FIG. 3B  shows a horizontal block boundary according to prior art. 
           [0042]      FIG. 4  is a flowchart illustrating a method according to an embodiment of the present invention. 
           [0043]      FIGS. 5 and 6  are flowcharts showing an embodiment of the present invention. 
           [0044]      FIG. 7  is a schematic block diagram of a filtering control device according to embodiments of the present invention. 
           [0045]      FIG. 8  is a schematic block diagram of a software implementation of a filtering control device in a computer according to an embodiment. 
           [0046]      FIG. 9  shows an encoder embodying the filtering control device according to embodiments of the present invention. 
           [0047]      FIG. 10  shows a decoder embodying the filtering control device according to embodiments of the present invention. 
           [0048]      FIG. 11  a schematic block diagram of a user equipment according to an embodiment. 
           [0049]      FIG. 12  a schematic block diagram of a user equipment according to an embodiment. 
           [0050]      FIG. 13  is a schematic overview of a portion of a communication network comprising a network device according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0051]    Throughout the drawings, the same reference numbers are used for similar or corresponding elements. 
         [0052]    The embodiments of the present invention relate to a filter control mechanism to improve deblocking over block boundaries in a video frame. 
         [0053]    In the following, a block can denote either a PU or a TU. Since blocking artifacts can appear at both PU and TU boundaries due to different prediction and quantization effects, respectively, the deblocking filter can be turned on at both the PU and TU boundary. The block boundary can be, therefore a PU boundary, a TU boundary or both at the same time. Since different CU have different PUs and TUs, the CU boundary is always a PU and TU boundary and is always covered by the above definition. 
         [0054]    Blocks of different size can have blocking artifacts of different strength. However, current HEVC deblocking treats blocks of all sizes differently. Therefore, the deblocking is improved by applying deblocking of different strength to blocks of different size in accordance with the present invention. 
         [0055]    In H.264, a video frame is divided into non-overlapping blocks of pixels that are encoded and decoded according to the various available intra and inter coding modes. Generally, a video frame is divided into non-overlapping macroblocks of 16×16 pixels. Such a macroblock can in turn be divided into smaller blocks of different sizes, such as 4×4 or 8×8 pixels. However, also rectangular blocks (partitions) could be possible according to the embodiments, such as, 4×8, 8×4, 8×16 or 16×8. The embodiments can be applied to any such block of pixels, including macroblocks or even larger blocks of pixels. 
         [0056]    In the emerging High Efficiency Video Coding (HEVC) standard, coding units (CU), prediction units (PU) and transform units (TU) are used. The largest coding units (LCU) are coded in the raster scan order. Each CU can be split further into smaller CUs in the quad-tree fashion until the smallest CU size is reached. The prediction units are defined inside a coding unit. Each PU has its own prediction (motion vectors or an inter-prediction mode) depending on the CU prediction type. The transform coding tree is also defined inside a coding unit. The largest transform size is 32×32 pixels and the smallest size is 4×4 pixels. The CU size is currently varying from 64×64 pixels (largest) to 8×8 pixels (smallest). In this way, the largest CU can be split into smaller CUs with the “level of granularity” depending on the local characteristics of the frame. That means that the largest CU may be split into smaller CUs of different sizes. The embodiments can also be used in connection with prediction units and transform units, which are regarded as being encompassed by the expression “block of pixels” as used herein. In the following, the word “partition” means prediction unit (PU) and those words can be used interchangeably. The reason is that in HEVC, partitioning of CU into prediction partitions (in H.264 terminology) results in Prediction Units corresponding to each prediction partition and having the corresponding spatial shape. 
         [0057]    Each pixel in the block has a respective pixel value. Video frames generally have color values assigned to the pixels, where the color values are represented in a defined color formats. One of the common color formats uses one luminance component and two chrominance components for each pixel, although other formats exist, such as using red, green and blue components for each pixel. 
         [0058]    Traditionally, luminance component filtering and chrominance component filtering are done separately, possibly employing different filtering decisions and different deblocking filters. It is, though, possible that the luminance filtering decisions are used in chroma filtering, like in HEVC. The embodiments can be applied to filtering control for the luminance component, the chrominance component or both the luminance component and the chrominance component. In a particular embodiment, the embodiments are applied to control luminance or luma filtering. Filtering decisions, or parts of filtering decisions for one component, such as luma, can be then used when making the filtering decisions for other components, such as chroma. 
         [0059]    Deblocking filtering is conducted over a boundary, edge or border between neighboring blocks. As a consequence, such boundaries can be vertical boundaries  1 , which can be seen in  FIG. 3A , between two neighboring blocks  10 ,  20  present side by side in the video frame. Alternatively, the boundaries are horizontal boundaries  1 , which can be seen in  FIG. 3B , between two neighboring block  10 ,  20 , where one block  10  is positioned above the other block  20  in the video frame. 
         [0060]    Accordingly, the embodiments of the present invention relate to a filter control mechanism to improve deblocking over block boundaries in a video frame. This filter control mechanism adjusts the deblocking according to a block size, wherein the block size can be any PU or/and TU sizes. Accordingly, a filter control is applicable to a block of multiple pixels of a video frame, wherein the video frame comprises a plurality of blocks and each pixel has a respective pixel value. In a method according to one embodiment shown in a flowchart of  FIG. 4 , a block size is determined  401 , and a deblocking filtering strength is adjusted  402  based on the determined block size. According to one embodiment, the adjustment of the deblocking filtering strength comprises modifying a number of pixels at a block boundary that can be modified by deblocking filtering. According to another embodiment 
         [0061]    The method according to any of claims  1 - 8 , wherein the deblocking filtering strength is further adjusted by changing a clipping value tc based on the determined block size or by changing a threshold value, a beta parameter. 
         [0062]    The number of pixels to be modified may be equal on both sides of the block boundary, or the number of pixels to be modified can also differ between the blocks lying on different sides of the block boundary, also referred to as block border. 
         [0063]    The block size can be determined  401  by determining  401   a  a first block size wherein the first block size is a size associated with the block, and determining  401   b  a second block size wherein the second block size is a size associated with the neighboring block, wherein the determined block  401   c  size is a minimum of the first block size and the second block size. Thus the block sizes on both sides of the block boundary are calculated. The variable BlockSize is set to the value min(GetBlockSize(A), GetBlockSize(B)) e.g. with reference to  FIG. 3A . 
         [0064]    Furthermore according to embodiments, the size of the first block and the second block, respectively, is determined as a minimum of partition size (Prediction Unit (PU) size) and the transform size to which the pixels on the side of the block boundary belong and the partition size (PU size) is the smallest of the vertical partition size (PU size) and the horizontal partition size (PU size). Thus, the function GetBlockSize(Block) is defined as a minimum of GetPartitionSize(Block) and GetTransformSize(Block), i.e. min(GetPartitionSize(Block), GetTransformSize(Block)), to which the pixels on the side of the block boundary belong. GetPartitionSize(Block) returns the smaller of the two partition dimensions (vertical and horizontal). In an alternative implementation, for filtering of a horizontal line of pixels across the vertical boundary between two blocks the horizontal size of the partition (and/or respectively transform) is returned. Likewise, in the same embodiment, for filtering of a vertical line (column) of pixels across the horizontal boundary between two blocks, the vertical size of the partition (and/or respectively transform) is returned. 
         [0065]    In one particular embodiment, if the block size is 4, one pixel in the block at the block boundary is modified, if the block size is 8 at most two pixels in the block at the block boundary are modified, and if the block size is equal to or larger than 16 up to three pixels in the block at the block boundary are modified. 
         [0066]    This is also illustrated by the following sequence and in the flowchart of  FIG. 5 : 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 if ( BlockSize ==4 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 One pixel from at block border is modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if ( BlockSize ==8 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 At most two pixels from the block border are modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else // if (BlockSize &gt;= 16) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Up to three pixels from the block border are modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0067]    In another particular embodiment, if the block size is 4, one pixel in the block at the block border is modified, if the block size is 8 at most two pixels in the block at the block border are modified, if the block size is equal to than 16 up to three pixels in the block at the block border are modified, and if the block size is equal to 32 up to four pixels in the block at the block border are modified. This is also illustrated by the following sequence: 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 if ( BlockSize ==4 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 One pixel from the block border is modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if ( BlockSize ==8 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 At most two pixels from the block border are modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if (BlockSize == 16) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Up to three pixels from the block border are modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else // if (BlockSize == 32) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Up to four pixels from the block border are modified 
               
             
          
           
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0068]    Moreover, the filtering strength e.g. defined by a clipping value, tc, or a threshold value beta, the type of filtering and the decision for filtering can also be varied dependent on the block size such as CU, TU and PU. 
         [0069]    Thus, the amount of filtering for the block size can also be changed by changing the clipping value tc for a delta (offset) or/and the threshold value beta β depending on the BlockSize. In one of the embodiments, tc values can be chosen from a table tc(QP, BlockSize). The values can also be chosen as tc(QP+offset(BlockSize)) from a one-dimensional tc table. Here the offset depends on the block size and is chosen from the table. One example of adjusting the deblocking filtering strength by changing a clipping value tc based on the determined block size is illustrated below. 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 if (BlockSize ==4 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 tc = tc(QP, 4); 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if (BlockSize ==8) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 tc = tc(QP, 8); 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if (BlockSize == 16) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 tc = tc(QP, 16); 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else { 
               
             
          
           
               
                   
                 tc = tc(QP, 32); 
               
               
                   
                 } 
               
             
          
           
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0070]    The values of beta β can be chosen from the table β (QP, BlockSize). The values can also be chosen as β(QP+offset(BlockSize)) from a one-dimensional β table. Here the offset depends on the block size and is chosen from the table. 
         [0071]    The tc parameter is a clipping value that defines how much the pixel can be changed. The beta parameter, however, mostly determines if the block boundary is filtered and (in the current HEVC deblocking) how many pixels from the block boundary are filtered. The table for derivation of tc and beta parameters depending on the QP (quantization parameter) is shown below. One can see that the parameter tc or beta can be derived from the table. However, the default value of tc and beta for the current QP can be changed by adding an offset to QP and deriving parameter beta and tc from the table. 
         [0072]    Clipping can be described as delta_clipped=max(−thr, min(thr, delta)), where thr is controlling the filter strength. Therefore, the clipping parameter tc is applied to the delta value Δ as in the following. Δclipped=max(−tc, min(tc, delta)). The clipped value of delta is the modification of the pixel value. Therefore, smaller values of tc result in less filtering whereas a larger value of tc allows stronger filtering. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 
               
               
                   
               
               
                 Derivation of threshold variables β and tc from input QP 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Q 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
               
               
                   
               
               
                 β 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 6 
                 7 
                 8 
               
               
                 t C   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
               
               
                   
               
               
                 Q 
                 19 
                 20 
                 21 
                 22 
                 23 
                 24 
                 25 
                 26 
                 27 
                 28 
                 29 
                 30 
                 31 
                 32 
                 33 
                 34 
                 35 
                 36 
                 37 
               
               
                   
               
               
                 β 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
                 20 
                 22 
                 24 
                 26 
                 28 
                 30 
                 32 
                 34 
                 36 
               
               
                 t C   
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 2 
                 2 
                 2 
                 2 
                 3 
                 3 
                 3 
                 3 
                 4 
                 4 
                 4 
               
               
                   
               
             
          
           
               
                 Q 
                 38 
                 39 
                 40 
                 41 
                 42 
                 43 
                 44 
                 45 
                 46 
                 47 
                 48 
                 49 
                 50 
                 51 
                 52 
                 53 
                 54 
                 55 
               
               
                   
               
               
                 β 
                 38 
                 40 
                 42 
                 44 
                 46 
                 48 
                 50 
                 52 
                 54 
                 56 
                 58 
                 60 
                 62 
                 64 
                 64 
                 64 
                 64 
                 64 
               
               
                 t C   
                 5 
                 5 
                 6 
                 6 
                 7 
                 8 
                 9 
                 9 
                 10 
                 10 
                 11 
                 11 
                 12 
                 12 
                 13 
                 13 
                 14 
                 14 
               
               
                   
               
             
          
         
       
     
         [0073]    In a further embodiment, a first pixel is modified as a neighboring pixel away from the border with addition of a delta value Δ. 
         [0074]    This can be implemented as below (the implementation is shown without considering the clipping of delta values): 
         [0000]    
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 if (BlockSize ==4 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Δ = (9*(q0−p0) −3*(q1−p1))/16 
               
               
                   
                 p0’ = p1 + Δ 
               
               
                   
                 q0’ = q1 + Δ 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else if (BlockSize ==8) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Δ = (9*(q0−p0) −3*(q1−p1))/16 
               
               
                   
                 p0’ = p1 + Δ 
               
               
                   
                 q0’ = q1 + Δ 
               
               
                   
                 Δp1 = (p0+p2−2*p1+2*Δ)/4; 
               
               
                   
                 p1’ = p1 + Δp1 
               
               
                   
                 Δq1 = (q0+q2−2*q1−2*Δ)/4; 
               
               
                   
                 q1’ = q1 + Δq1 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else // if (BlockSize &gt;= 16) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 if (Criterion 1 ) 
               
               
                   
                 { 
               
             
          
           
               
                   
                 HEVC Strong filter (3pixels from the border) 
               
             
          
           
               
                   
                 } 
               
               
                   
                 else 
               
               
                   
                 { 
               
             
          
           
               
                   
                 Δ = (9*(q0−p0) −3*(q1−p1))/16 
               
               
                   
                 p0’ = p1 + Δ 
               
               
                   
                 q0’ = q1 + Δ 
               
               
                   
                 Δp1 = (p0+p2−2*p1+2*Δ)/4; 
               
               
                   
                 p1’ = p1 + Δp1 
               
               
                   
                 Δq1 = (q0−q2−2*q1−2*Δ)/4; 
               
               
                   
                 q1’ = q1 + Δq1 
               
             
          
           
               
                   
                 } 
               
             
          
           
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0075]    The implementation with clipping using a programming language may look like below. 
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 if (BlockSize ==4 ) 
               
               
                 { 
               
             
          
           
               
                   
                 delta = (9*(m4−m3) −3*(m5−m2) + 8)&gt;&gt;4 ; 
               
               
                   
                 delta = Clip3(−tc, tc, delta); 
               
               
                   
                 piSrc[−iOffset] = Clip((m3+delta)); 
               
               
                   
                 piSrc[0] = Clip((m4−delta)); 
               
             
          
           
               
                 } 
               
               
                 else if (BlockSize ==8) 
               
               
                 { 
               
             
          
           
               
                   
                 delta = (9*(m4−m3) −3*(m5−m2) + 8)&gt;&gt;4 ; 
               
               
                   
                 delta = Clip3(−tc, tc, delta); 
               
               
                   
                 piSrc[−iOffset] = Clip((m3+delta)); 
               
               
                   
                 piSrc[0] = Clip((m4−delta)); 
               
               
                   
                 Int delta1 = Clip3(−tc2, tc2, (( ((m1+m3+1)&gt;&gt;1)− m2+delta)&gt;&gt;1)); 
               
               
                   
                 piSrc[−iOffset*2] = Clip((m2+delta1)); 
               
               
                   
                 Int delta2 = Clip3(−tc2, tc2, (( ((m6+m4+1)&gt;&gt;1)− m5−delta)&gt;&gt;1)); 
               
               
                   
                 piSrc[ iOffset] = Clip((m5+delta2)); 
               
             
          
           
               
                 } 
               
               
                 else // if (BlockSize &gt;= 16) 
               
               
                 { 
               
             
          
           
               
                   
                  if (Criterion 1 ) 
               
               
                   
                  { 
               
             
          
           
               
                   
                 HEVC Strong filter (3pixels from the border) 
               
             
          
           
               
                   
                  } 
               
               
                   
                  else 
               
               
                   
                  { 
               
             
          
           
               
                   
                 delta = (9*(m4−m3) −3*(m5−m2) + 8)&gt;&gt;4 ; 
               
               
                   
                 delta = Clip3(−tc, tc, delta); 
               
               
                   
                 piSrc[−iOffset] = Clip((m3+delta)); 
               
               
                   
                 piSrc[0] = Clip((m4−delta)); 
               
               
                   
                 Int delta1 = Clip3(−tc2, tc2, (( ((m1+m3+1 )&gt;&gt;1)− 
               
               
                   
                 m2+delta)&gt;&gt;1)); 
               
               
                   
                 piSrc[−iOffset*2] = Clip((m2+delta1)); 
               
               
                   
                 Int delta2 = Clip3(−tc2, tc2, (( ((m6+m4+1)&gt;&gt;1)− 
               
               
                   
                 m5−delta)&gt;&gt;1)); 
               
               
                   
                 piSrc[ iOffset] = Clip((m5+delta2)); 
               
             
          
           
               
                   
                  } 
               
             
          
           
               
                 } 
               
               
                   
               
             
          
         
       
     
         [0076]    Here the Criterion1 is the criterion used in HEVC for strong filtering as described above. 
         [0077]    In a yet further embodiment, the number of pixels to be modified is being calculated for each block separately. An exemplary implementation is disclosed below and in the flowchart of  FIG. 6 : 
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
             
             
               
                 if ( BlockSizeA ==4 ) 
               
               
                 { 
               
             
          
           
               
                   
                 One pixel from the block border is modified for block A. 
               
             
          
           
               
                 } 
               
               
                 else if ( BlockSizeA ==8 ) 
               
               
                 { 
               
               
                 At most two pixels from the block border are modified for block A. 
               
               
                 } 
               
               
                 else // if (BlockSizeA &gt;= 16) 
               
               
                 { 
               
             
          
           
               
                   
                 Up to three pixels from the block border are modified for block A. 
               
             
          
           
               
                 } 
               
               
                 if ( BlockSizeB ==4 ) 
               
               
                 { 
               
             
          
           
               
                   
                 One pixel from the block border is modified for block B. 
               
             
          
           
               
                 } 
               
               
                 else if ( BlockSizeB ==8 ) 
               
               
                 { 
               
               
                 At most two pixels from the block border are modified for block B. 
               
               
                 } 
               
               
                 else // if (BlockSizeB &gt;= 16) 
               
               
                 { 
               
             
          
           
               
                   
                 Up to three pixels from the block border are modified for block B. 
               
             
          
           
               
                 } 
               
               
                   
               
             
          
         
       
     
         [0078]    In particular cases, embodiments allow to avoid overlapping of the deblocking filtering from two sides of the block border. For example, in the HEVC scenario, if the deblocking modifies only one pixel from the block border in a 4×4 block, then the deblocking operation in the block on the left block boundary would not influence the results of the deblocking operation on the block boundary at the right side of the block, even in case of filtering on the 4×4-block grid. Therefore, the deblocking operation can be parallelized. 
         [0079]      FIG. 7  is a schematic block diagram of an embodiment of a filtering control device  100 . The filtering control device  100  comprises, in this embodiment the filtering control device  100  comprises a unit  110  configured to determine block size and a unit  120  configured to adjust a deblocking filtering strength based on the determined block size. 
         [0080]    According to an embodiment, the unit  120  configured to adjust a deblocking filtering strength is further configured to adjust the deblocking filtering strength by modifying a number of pixels at a block boundary. 
         [0081]    According to a further embodiment, the unit  120  configured to adjust a deblocking filtering strength is further configured to adjust the deblocking filtering strength by changing a clipping value tc based on the determined block size. 
         [0082]    In another embodiment, the unit  120  configured to adjust a deblocking filtering strength is further configured to adjust the deblocking filtering strength by changing a threshold value, a beta parameter based on the determined block size. 
         [0083]    The unit  120  configured to adjust a deblocking filtering strength may also be configured to modify a first pixel away from the boundary with addition (or subtraction) of a delta value. 
         [0084]    Although the respective units  110 - 120  disclosed in conjunction with  FIG. 7  have been disclosed as physically separate units  110 - 196  in the filtering control device  100 , and all units may be special purpose circuits, such as ASICs (Application Specific Integrated Circuits), alternative embodiments of the filtering control device  100  are possible where some or all of the units  110 - 120  are implemented as computer program modules running on a general purpose processor. Such an embodiment is disclosed in  FIG. 8 . 
         [0085]      FIG. 8  schematically illustrates an embodiment of a computer  70  having a processing unit  72 , such as a DSP (Digital Signal Processor) or CPU (Central Processing Unit). The processing unit  72  can be a single unit or a plurality of units for performing different steps of the method described herein. The computer  70  also comprises an input/output (I/O) unit  71  for receiving recorded or generated video frames or encoded video frames and outputting encoded video frame or decoded video data. The I/O unit  71  has been illustrated as a single unit in  FIG. 8  but can likewise be in the form of a separate input unit and a separate output unit. 
         [0086]    Furthermore, the computer  70  comprises at least one computer program product  73  in the form of a non-volatile memory, for instance an EEPROM (Electrically Erasable Programmable Read-Only Memory), a flash memory or a disk drive. The computer program product  73  comprises a computer program  74 , which comprises code means which when run on or executed by the computer  70 , such as by the processing unit  72 , causes the computer  70  to perform the steps of the method described in the foregoing in connection with  FIG. 4 . Hence, in an embodiment the code means in the computer program  74  comprises a block size determination module  310  for determining block sizes according to the embodiments and a module  320  for adjusting deblocking filtering strength based on the determined block sizes according to the embodiments. These modules  310 - 320  essentially perform the steps of the flow diagram in  FIG. 4  when run on the processing unit  72 . Thus, when the different modules  310 - 320  are run on the processing unit  72  they correspond to the corresponding units  110 - 120  of  FIG. 7 . 
         [0087]    The computer  70  of  FIG. 8  can be a user equipment or be present in a user equipment. In such a case, the user equipment  80  may additionally comprise or be connected to a display to display video data. 
         [0088]    The filtering control device of  FIG. 7  is preferably used in video coding. It functions and is therefore preferably implemented both in a video encoder and in a video decoder. The filtering control device can also be implemented in a post-filter. The video decoder can be implemented preferably in hardware but also in software. The same holds for the video encoder. 
         [0089]      FIG. 9  is a schematic block diagram of an encoder  40  for encoding a block of pixels in a video frame of a video sequence according to an embodiment. 
         [0090]    A current block of pixels is predicted by performing a motion estimation by a motion estimator  50  from an already provided block of pixels in the same frame or in a previous frame. The result of the motion estimation is a motion or displacement vector associated with the reference block, in the case of inter prediction. The motion vector is utilized by a motion compensator  50  for outputting an inter prediction of the block of pixels. 
         [0091]    An intra predictor  49  computes an intra prediction of the current block of pixels. The outputs from the motion estimator/compensator  50  and the intra predictor  49  are input in a selector  51  that either selects intra prediction or inter prediction for the current block of pixels. The output from the selector  51  is input to an error calculator in the form of an adder  41  that also receives the pixel values of the current block of pixels. The adder  41  calculates and outputs a residual error as the difference in pixel values between the block of pixels and its prediction. 
         [0092]    The error is transformed in a transformer  42 , such as by a discrete cosine transform, and quantized by a quantizer  43  followed by coding in an encoder  44 , such as by entropy encoder. In inter coding, also the estimated motion vector is brought to the encoder  44  for generating the coded representation of the current block of pixels. 
         [0093]    The transformed and quantized residual error for the current block of pixels is also provided to a inverse quantizer  45  and inverse transformer  46  to retrieve the original residual error. This error is added by an adder  47  to the block prediction output from the motion compensator  50  or the intra predictor  49  to create a reference block of pixels that can be used in the prediction and coding of a next block of pixels. This new reference block is first processed by a filtering control device  100  according to the embodiments in order to control any deblocking filtering that is applied to the reference block to combat any blocking artifact. The processed new reference block is then temporarily stored in a frame buffer  48 , where it is available to the intra predictor  49  and the motion estimator/compensator  50 . 
         [0094]      FIG. 10  is a corresponding schematic block diagram of a decoder  60  comprising a filtering control device  100  according to the embodiments. The decoder  60  comprises a decoder  61 , such as entropy decoder, for decoding an encoded representation of a block of pixels to get a set of quantized and transformed residual errors. These residual errors are dequantized in an inverse quantizer  62  and inverse transformed by an inverse transformer  63  to get a set of residual errors. 
         [0095]    These residual errors are added in an adder  64  to the pixel values of a reference block of pixels. The reference block is determined by a motion estimator/compensator  67  or intra predictor  66 , depending on whether inter or intra prediction is performed. A selector  68  is thereby interconnected to the adder  64  and the motion estimator/compensator  67  and the intra predictor  66 . The resulting decoded block of pixels output from the adder  64  is input to a filtering control device  100  according to the embodiments in order to control any deblocking filter that is applied to combat any blocking artifacts. The filtered block of pixels is output form the decoder  60  and is furthermore preferably temporarily provided to a frame buffer  65  and can be used as a reference block of pixels for a subsequent block of pixels to be decoded. The frame buffer  65  is thereby connected to the motion estimator/compensator  67  to make the stored blocks of pixels available to the motion estimator/compensator  67 . 
         [0096]    The output from the adder  64  is preferably also input to the intra predictor  66  to be used as an unfiltered reference block of pixels. 
         [0097]    In the embodiments disclosed in  FIGS. 9 and 10  the filtering control device  100  controls deblocking filtering in the form of so called in-loop filtering. In an alternative implementation at the decoder  60  the filtering control device  100  is arranged to perform so called post-processing filtering. In such a case, the filtering control device  100  operates on the output frames outside of the loop formed by the adder  64 , the frame buffer  65 , the intra predictor  66 , the motion estimator/compensator  67  and the selector  68 . No deblocking filtering and filter control is then typically done at the encoder. 
         [0098]      FIG. 11  is a schematic block diagram of a user equipment or media terminal  80  housing a decoder  60  with a filtering control device. The user equipment  80  can be any device having media decoding functions that operates on an encoded video stream of encoded video frames to thereby decode the video frames and make the video data available. Non-limiting examples of such devices include mobile telephones and other portable media players, tablets, desktops, notebooks, personal video recorders, multimedia players, video streaming servers, set-top boxes, TVs, computers, decoders, game consoles, etc. The user equipment  80  comprises a memory  84  configured to store encoded video frames. These encoded video frames can have been generated by the user equipment  80  itself. Alternatively, the encoded video frames are generated by some other device and wirelessly transmitted or transmitted by wire to the user equipment  80 . The user equipment  80  then comprises a transceiver (transmitter and receiver) or input and output port  82  to achieve the data transfer. 
         [0099]    The encoded video frames are brought from the memory  84  to a decoder  60 , such as the decoder illustrated in  FIG. 10 . The decoder  60  comprises a filtering control device  100  according to embodiments. The decoder  60  then decodes the encoded video frames into decoded video frames. The decoded video frames are provided to a media player  86  that is configured to render the decoded video frames into video data that is displayable on a display or screen  88  of or connected to the user equipment  80 . 
         [0100]    In  FIG. 12 , the user equipment  80  has been illustrated as comprising both the decoder  60  and the media player  86 , with the decoder  60  implemented as a part of the media player  86 . This should, however, merely be seen as an illustrative but non-limiting example of an implementation embodiment for the user equipment  80 . Also distributed implementations are possible where the decoder  60  and the media player  86  are provided in two physically separated devices are possible and within the scope of user equipment  80  as used herein. The display  88  could also be provided as a separate device connected to the user equipment  80 , where the actual data processing is taking place. 
         [0101]      FIG. 11  illustrates another embodiment of a user equipment  80  that comprises en encoder, such as the encoder of  FIG. 9 , comprising a filtering control device according to the embodiments. The encoder  40  is then configured to encode video frames received by the I/O unit  82  and/or generated by the user equipment  80  itself. In the latter case, the user equipment  80  preferably comprises a media engine or recorder, such as in the form of or connected to a (video) camera. The user equipment  80  may optionally also comprise a media player  86 , such as a media player  86  with a decoder and filtering control device according to the embodiments, and a display  88 . 
         [0102]    As illustrated in  FIG. 13 , the encoder  40  and/or decoder  60 , such as illustrated in  FIGS. 9 and 10 , may be implemented in a network device  30  being or belonging to a network node in a communication network  32  between a sending unit  34  and a receiving user equipment  36 . Such a network device  30  may be a device for converting video according to one video coding standard to another video coding standard, for example, if it has been established that the receiving user equipment  36  is only capable of or prefers another video coding standard than the one sent from the sending unit  34 . The network device  30  can be in the form of or comprised in a radio base station, a Node-B or any other network node in a communication network  32 , such as a radio-based network. 
         [0103]    The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.