Patent Publication Number: US-10778970-B2

Title: Method and apparatus of decoding video data

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Continuation of U.S. patent application Ser. No. 15/724,934 filed on Oct. 4, 2017, which is a Continuation of U.S. patent application Ser. No. 14/349,436 filed on Apr. 3, 2014 (now U.S. Pat. No. 9,838,681 issued on Dec. 5, 2017), which was filed as the National Phase of PCT International Application No. PCT/CN2012/084177 filed on Nov. 7, 2012, which claims the priority benefit under 35 U.S.C. § 119(a) to Korean Patent Application No. 10-2011-0115217 filed in Republic of Korea on Nov. 7, 2011, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an apparatus of decoding video data, and more particularly, to an apparatus of generating a prediction block and residual block to reconstruct a reconstructed block using motion information and quantization information. 
     Discussion of the Related Art 
     Methods for compressing video data include MPEG-2, MPEG-4 and H.264/MPEG-4 AVC. According to these methods, one picture is divided into macroblocks and each macroblock is encoded by generating a prediction block using inter prediction or intra prediction. The difference between an original block and the prediction block is transformed to generate a transformed block, and the transformed block is quantized using a quantization parameter and a predetermined quantization matrix. The quantized coefficient of the quantized block are scanned by a predetermined scan pattern and then entropy-coded. The quantization parameter is adjusted per macroblock and encoded using a previous quantization parameter. 
     In H.264/MPEG-4 AVC, motion estimation is used to eliminate temporal redundancy between consecutive pictures. To detect the temporal redundancy, one or more reference pictures are used to estimate motion of a current block, and motion compensation is performed to generate a prediction block using motion information. The motion information includes one or more reference picture indexes and one or more motion vectors. 
     According to the H.264/MPEG-4 AVC, only the motion vectors are predicted and encoded using neighboring motion vectors, and the reference picture indexes are encoded without neighboring reference picture indexes. 
     However, if various sizes are used for inter prediction, the correlation between motion information of a current block and motion information of one or more neighboring block increases. Also, the correlation between motion vector of a current block and motion vector of neighboring block within a reference picture becomes higher as the picture size becomes larger if motion of image is almost constant or slow. Also, the various sizes of coding unit and transform unit results in increase of coding bits of residual block when scanning a large quantized block. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus of decoding video data by generating a prediction block and a residual block using motion information and scanning information. 
     One aspect of the present invention provides an apparatus of decoding video data, comprising: an entropy decoding unit configured to extract quantized coefficient components and inter prediction information from a received bit stream; an inverse scanning unit configured to an inverse scan pattern to the quantized coefficient components to generate a quantized block having a size of a transform unit; an inverse quantization unit configured to generate a quantization parameter and to inverse-quantize the quantized block to generate a transformed block; an inverse transform unit configured to generate a residual block by inverse-transforming the transformed block; an inter prediction unit configured to derive motion information and to generate a prediction block; and an adder configured to generate a reconstructed block using the residual block and the prediction block. The transform unit is larger than a 4×4 block, each of the quantized coefficient components are inversely scanned in the unit of subset to generate multiple subsets, and the multiple subsets are inversely scanned to generate the quantized block. 
     An apparatus according to the present invention includes an entropy decoding unit configured to extract quantized coefficient components and inter prediction information from a received bit stream, an inverse scanning unit configured to an inverse scan pattern to the quantized coefficient components to generate a quantized block having a size of a transform unit, an inverse quantization unit configured to generate a quantization parameter and to inverse-quantize the quantized block to generate a transformed block, an inverse transform unit configured to generate a residual block by inverse-transforming the transformed block, an inter prediction unit configured to derive motion information and to generate a prediction block, and an adder configured to generate a reconstructed block using the residual block and the prediction block. The transform unit is larger than a 4×4 block, each of the quantized coefficient components are inversely scanned in the unit of subset to generate multiple subsets, and the multiple subsets are inversely scanned to generate the quantized block. Accordingly, the coding efficiency of the motion information is improved by including various motion vector candidates. Also, the computational complexity of an encoder and a decoder is reduced maintaining improvement of coding efficiency by adaptively storing motion information of reference picture and adaptively generating a temporal motion vector candidate. Also, the amount of coding bits of the residual block is reduced by applying diagonal scan pattern to each subset. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an image coding apparatus according to the present invention. 
         FIG. 2  is a flow chart illustrating a method of encoding video data in inter prediction mode according to the present invention. 
         FIG. 3  is a flow chart illustrating a method of encoding motion information in an AMVP mode according to the present invention. 
         FIG. 4  is a conceptual diagram illustrating positions of spatial motion vector candidate blocks according to the present invention. 
         FIG. 5  is a conceptual diagram illustrating position of temporal candidate block according to the present invention. 
         FIG. 6  is a block diagram of an image decoding apparatus according to the present invention. 
         FIG. 7  is a conceptual diagram illustrating an inverse scan pattern according to the present invention. 
         FIG. 8  is a conceptual diagram illustrating a method of generating a quantized block according to the present invention. 
         FIG. 9  is a flow chart illustrating a method of decoding an image in inter prediction mode according to the present invention. 
         FIG. 10  is a flow chart illustrating a method of deriving motion information in an AMVP mode according to the present invention. 
         FIG. 11  is a flow chart illustrating a method of generating a residual block in inter prediction mode according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the exemplary embodiments disclosed below, but can be implemented in various types. Therefore, many other modifications and variations of the present invention are possible, and it is to be understood that within the scope of the disclosed concept, the present invention may be practiced otherwise than as has been specifically described. 
     An image encoding apparatus and an image decoding apparatus according to the present invention may be a user terminal such as a personal computer, a personal mobile terminal, a mobile multimedia player, a smartphone or a wireless communication terminal. The image encoding device and the image decoding device may be include a communication unit for communicating with various devices, a memory for storing various programs and data used to encode or decode images. 
       FIG. 1  is a block diagram of an image coding apparatus  100  according to the present invention. 
     Referring to  FIG. 1 , the image coding apparatus  100  according to the present invention includes a picture division unit  110 , an intra prediction unit  120 , an inter prediction unit  130 , a transform unit  140 , a quantization unit  150 , a scanning unit  160 , an entropy coding unit  170 , an inverse quantization unit  155 , an inverse transform unit  145 , a post-processing unit  180 , a picture storing unit  190 , a subtracter  192  and an adder  194 . 
     The picture division unit  110  divides a picture or a slice into a plurality of largest coding units (LCUs), and divides each LCU into one or more coding units. The size of LCU may be 32×32, 64×64 or 128×128. The picture division unit  110  determines prediction mode of each coding unit and a size of prediction unit. 
     An LCU includes one or more coding units. The LCU has a recursive quad tree structure to specify a division structure of the LCU. Parameters for specifying the maximum size and the minimum size of the coding unit are included in a sequence parameter set. The division structure is specified by one or more split coding unit flags (split_cu_flags). The size of a coding unit is 2N×2N. If the LCU is a 64×64 block and a smallest coding unit (SCU) is a 8×8 block, the size of the coding unit may be 64×64, 32×32, 16×16 or 8×8. 
     A coding unit includes one or more prediction units. In intra prediction, the size of the prediction unit is 2N×2N or N×N. In inter prediction, the size of the prediction unit is specified by a partitioning mode. The partitioning mode is 2N×2N, 2N×N, N×2N or N×N if the coding unit is partitioned symmetrically. The partitioning mode is hN×2N, (2−h)N×2N, 2N×hN and 2N×(2−h)N if the coding unit is partitioned asymmetrically. The value of h is ½. If the size of the coding unit is smaller than 16×16, the asymmetric partitioning modes are not allowed. 
     A coding unit includes one or more transform units. The transform unit has a recursive quad tree structure to specify a division structure of the coding unit. The division structure is specified by one or more split transform unit flags (split_transform_flags). Parameters for specifying the maximum size and the minimum size of the transform unit are included in a sequence parameter set. The chroma transform unit has half the height and half the width of the transform unit and the minimum size of the chroma transform unit is 4×4. 
     The intra prediction unit  120  determines an intra prediction mode of a current prediction unit and generates a prediction block using the intra prediction mode. 
     The inter prediction unit  130  determines motion information of the current prediction unit using one or more reference pictures stored in the picture storing unit  190 , and generates a prediction block of the prediction unit. The motion information includes one or more reference picture indexes and one or more motion vectors. 
     The transform unit  140  transforms residual signals generated using a current block and a prediction block to generate a transformed block. The residual signals are transformed in the unit of transform unit. A transform matrix is determined by the prediction mode and the size of the transform unit. The transform matrix is a DCT-based integer transform matrix or a DST-based integer transform matrix. The DCT-based integer transform matrix is used in inter prediction. 
     The quantization unit  150  determines a quantization parameter for quantizing the transformed block. The quantization parameter is a quantization step size. The quantization parameter is determined per quantization unit. The size of the quantization unit may vary and be one of allowable sizes of coding unit. If the size of a coding unit is equal to or larger than the minimum size of the quantization unit, the coding unit becomes the quantization unit. A plurality of coding units may be included in a quantization unit of minimum size. The minimum size of the quantization unit is determined per picture and a parameter for specifying the minimum size is included in a picture parameter set. 
     The quantization unit  150  generates a quantization parameter predictor and generates a differential quantization parameter by subtracting the quantization parameter predictor from the quantization parameter. The differential quantization parameter is entropy-coded. 
     The quantization parameter predictor is generated by using quantization parameters of neighboring coding units and quantization parameter of previous coding unit as follows. 
     A left quantization parameter, an above quantization parameter and a previous quantization parameter are sequentially retrieved in this order. An average of the first two available quantization parameters retrieved in that order is set as the quantization parameter predictor when two or more quantization parameters are available, and when only one quantization parameter is available, the available quantization parameter is set as the quantization parameter predictor. That is, if the left and above quantization parameters are available, an average of the left and above quantization parameter is set as the quantization parameter predictor. If only one of the left and above quantization parameters is available, an average of the available quantization parameter and the previous quantization parameter is set as the quantization parameter predictor. If both of the left and above quantization parameters are unavailable, the previous quantization parameter is set as the quantization parameter predictor. The average is rounded off. 
     The differential quantization parameter is converted into bins for the absolute value of the differential quantization parameter and a bin for indicating sign of the differential quantization parameter through a binarization process, and the bins are arithmetically coded. If the absolute value of the differential quantization parameter is 0, the bin for indicating sign may be omitted. Truncated unary is used for binarization of the absolute. 
     The quantization unit  150  quantizes the transformed block using a quantization matrix and the quantization parameter to generate a quantized block. The quantized block is provided to the inverse quantization/transform unit  180  and the scanning unit  160 . 
     The scanning unit  160  determines applies a scan pattern to the quantized block. 
     In inter prediction mode, a diagonal scan is used as the scan pattern if CABAC is used for entropy coding. The quantized coefficients of the quantized block are split into three coefficient components. The coefficient components are significant coefficients, coefficient flags and coefficient levels. The diagonal scan is applied to each of the coefficient components. The significant flag indicates whether the corresponding quantized coefficient is zero or not. The coefficient sign indicates a sign of non-zero quantized coefficient, and the coefficients level indicates an absolute value of non-zero quantized coefficient. 
     When the size of the transform unit is larger than a predetermined size, the quantized block is divided into multiple subsets and the diagonal scan is applied to each subset. Significant flags, coefficient signs and coefficients levels of each subset are scanned respectively according to the diagonal scan. The predetermined size is 4×4. The subset is a 4×4 block containing 16 transform coefficients. 
     The scan pattern for scanning the subsets is the same as the scan pattern for scanning the coefficient components. The significant flags, the coefficient signs and the coefficients levels of each subset are scanned in the reverse direction. The subsets are also scanned in the reverse direction. 
     A parameter indicating last non-zero coefficient position is encoded and transmitted to the decoder. The parameter specifies a position of last non-zero quantized transform coefficient within the transform unit. A non-zero subset flag may be set for each subset other than the first subset and the last subset. The first subset covers a DC coefficient. The last subset covers the last non-zero coefficient. The non-zero subset flag indicates whether the subset contains non-zero coefficients or not. 
     The inverse quantization/transform unit  180  inversely quantizes the quantized coefficients of the quantized block, and inversely transforms the inverse-quantized block to generate residual block. 
     The post-processing unit  190  performs a deblocking filtering process for removing blocking artifact generated in a reconstructed picture. 
     The picture storing unit  195  receives post-processed image from the post-processing unit  190 , and stores the image in picture units. A picture may be a frame or a field. 
     The entropy coding unit  170  entropy-codes the scanned coefficient component received from the scanning unit  160 , intra prediction information received from the intra prediction unit  120 , motion information received from the inter prediction unit  130 , and so on. 
       FIG. 2  is a flow chart illustrating a method of encoding video data in inter prediction mode according to the present invention. 
     Motion information of a current block is determined (S 110 ). The current block is a prediction unit. A size of the current block is determined by a size of a coding unit and a partitioning mode. 
     The motion information varies according to a prediction type. If the prediction type is a uni-directional prediction, the motion information includes a reference index specifying a picture of a reference list 0, and a motion vector. If the prediction type is a bi-directional prediction, the motion information includes a reference index specifying a picture of a reference list 0, a reference index specifying a picture of a reference list 1, and a list 0 motion vector and a list 1 motion vector. 
     A prediction block of the current block is generated using the motion information (S 120 ). If the motion vector indicates a pixel position, the prediction block is generated by copying a block of the reference picture specified by the motion vector. If the motion vector indicates a sub-pixel position, the prediction block is generated by interpolating the pixels of the reference picture. 
     A residual block is generated using the current block and the prediction block (S 130 ). 
     The residual block is encoded (S 140 ). The residual block has the same size of the transform unit. If the prediction unit is larger than the transform unit, the residual signals between the current block and the prediction block are divided into multiple residual blocks. The one or more residual blocks are encoded by the transform unit  140 , the quantization unit  150 , the scanning unit  160  and the entropy coding unit  170  of  FIG. 1 . 
     The motion information is encoded (S 150 ). The motion information may be encoded predictively using spatial candidates and a temporal candidate of the current block. The motion information is encoded in a skip mode, a merge mode or an AMVP mode. In the skip mode, the prediction unit has the size of coding unit and the motion information is encoded using the same method as that of the merge mode. 
       FIG. 3  is a flow chart illustrating a method of encoding motion information in the AMVP mode according to the present invention.  FIG. 4  is a conceptual diagram illustrating positions of spatial motion vector candidate blocks according to the present invention. 
     A left motion vector candidate is a motion vector of a left block (block A) or a left-below block (block D) of a current block. An above motion vector candidate is a motion vector of an above block (block B), an above-right block (block C) or an above-left block (block E) of the current block. The neighboring block belongs to one of four types with respect to the current block. If the neighboring block has same reference picture and same reference picture list with the current block, the neighboring block belongs to the first type. If the neighboring block has same reference picture and different reference picture list with the current block, the neighboring block belongs to the second type. If the neighboring block has different reference picture and same reference picture list with the current block, the neighboring block belongs to the third type. If the neighboring block has different reference picture and different reference picture list with the current block, the neighboring block belongs to the fourth type. 
     The left motion vector candidate is derived (S 210 ). It is determined whether the block D belongs to the first type or the second type. If the block D belongs to the first type or the second type, the motion vector of the bock D is set as the left motion vector candidate. If the block D does not belong to the first type or the second type, it is determined whether the block A belongs to the first type or the second type. If the block A belongs to the first type or the second type, the motion vector of the bock A is set as the left motion vector candidate. If the block A does not belong to the first type or the second type, it is determined whether the block D belongs to the third type or the fourth type. If the block D belongs to the third type or the fourth type, the scaled motion vector of the bock D is set as the left motion vector candidate. If the block D does not belong to the third type or the fourth type, it is determined whether the block A belongs to the third type or the fourth type. If the block A belongs to the third type or the fourth type, the scaled motion vector of the bock A is set as the left motion vector candidate. If the block A does not belong to the third type or the fourth type, the left motion vector candidate is set as unavailable. 
     The above motion vector candidate is derived (S 220 ). The above motion vector candidate is determined depending on the left motion vector candidate. If any one of the blocks C, B and E belongs to the first type or the second type, the motion vector of the first block encountered when the blocks C, B and E are retrieved in this order is set as the above motion vector candidate. But, if any one of the blocks C, B and E does not belong to the first type or the second type, the above motion vector is determined as follows. 
     If the left motion vector candidate belongs to the first type or the second type, the motion vector of the first block belonging to the third type or the fourth type is set as the above motion vector candidate. If any one of the blocks C, B and E does not belong to the third type or the fourth type, the above motion vector is set as unavailable. The first block is a block encountered first when the blocks C, B and E are retrieved in this order. 
     If the left motion vector candidate belongs to the third type or the fourth type, the above motion vector candidate is set as unavailable. 
     The temporal motion vector candidate is derived (S 230 ). The temporal motion vector candidate is derived as follows. If the left motion vector candidate and the above motion vector candidate are available and different each other, the temporal motion vector candidate may not be derived. 
     First, a temporal candidate picture is determined. The temporal candidate picture includes a temporal candidate block. One temporal candidate picture is used within a slice. A reference picture index of the temporal candidate picture may be set to zero. 
     If the current slice is a P slice, one of the reference pictures of the reference picture list 0 is set to the temporal candidate picture. If the current slice is a B slice, one of the reference pictures of the reference picture lists 0 and 1 is set to the temporal candidate picture. A list indicator specifying whether the temporal candidate picture belongs to the reference picture lists 0 or 1 is included in a slice header if the current slice is a B slice. The reference picture index specifying the temporal candidate picture may be included in the slice header. 
     Next, the temporal candidate block is determined. The temporal candidate block may be a first candidate block or a second candidate block. If the first candidate block is available, the first candidate block is set to the temporal candidate block. If the first candidate block is unavailable, the second candidate block is set to the temporal candidate block. If the second candidate block is unavailable, the temporal candidate block is set as unavailable. 
       FIG. 5  is a conceptual diagram illustrating position of temporal candidate block according to the present invention. As shown in  FIG. 5 , the first candidate block may be a right-below corner block (block H) of the block C. The block C has same size and same location of the current block and is located within the temporal candidate picture. The second candidate block is a block covering an upper-left pixel of the center of the block C. 
     If the temporal candidate block is determined, the motion vector of the temporal candidate block is set to the temporal motion vector candidate. 
     A motion vector candidate list is constructed (S 240 ). The motion vector candidates are listed in the order of the left motion vector candidate, the above motion vector candidate and the temporal motion vector candidate. If the left motion vector candidate and the above motion vector candidate are same, the above motion vector candidate is removed from the motion vector candidate list. 
     If the number of the motion vector candidates is smaller than a predetermined number, one or more zero motion vectors are added to the motion vector list (S 250 ). The predetermined number may be 2. 
     A motion vector candidate is selected as a motion vector predictor and the differential motion vector is generated by subtracting the motion vector candidate from the motion vector of the current block (S 260 ). 
     The reference picture index, the differential motion vector and an amvp index are encoded (S 270 ). The amvp index specifies the motion vector predictor. 
       FIG. 6  is a block diagram of an image decoding apparatus  200  according to the present invention. 
     The image decoding apparatus  200  according to the present invention includes an entropy decoding unit  210 , an inverse scanning unit  220 , an inverse quantization unit  230 , an inverse transform unit  240 , an intra prediction unit  250 , an inter prediction unit  260 , a post-processing unit  270 , a picture storing unit  280  and an adder  290 . 
     The entropy decoding unit  210  extracts the intra prediction information, the inter prediction information and the quantized coefficient components from a received bit stream using a context-adaptive binary arithmetic decoding method. 
     The inverse scanning unit  220  applies an inverse scan pattern to the quantized coefficient components to generate a quantized block. In inter prediction, the inverse scan pattern is a diagonal scan. The quantized coefficient components include the significant flags, the coefficient signs and the coefficients levels. 
     When the size of the transform unit is larger than a predetermined size, the significant flags, the coefficient signs and the coefficients levels are inversely scanned in the unit of subset using the diagonal scan to generate subsets, and the subsets are inversely scanned using the diagonal scan to generate the quantized block. The predetermined size is equal to the size of the subset. The subset is a 4×4 block including 16 transform coefficients. The significant flags, the coefficient signs and the coefficient levels are inversely scanned in the reverse direction. The subsets are also inversely scanned in the reverse direction. 
       FIG. 7  is a conceptual diagram illustrating an inverse scan pattern according to the present invention. 
     As shown in  FIG. 7 , if the size of the transform unit is 16×16, sixteen 4×4 subsets are generated. The subsets are generated in reverse direction. The quantized coefficient components are inversely scanned in the reverse direction to generate a subset. 
       FIG. 8  is a conceptual diagram illustrating a method of generating a quantized block according to the present invention. 
     In  FIG. 8 , the first subset contains DC coefficient and the last subset contains a last non-zero coefficient. The subset flag is defined for each subset other than the first subset and the last subset. The subset flag indicates whether the subset contains non-zero coefficient. 
     The inverse scanning unit  220  determines the number of subsets from the last subset to the first subset to be generated using the location of the last non-zero coefficient of the transform unit. Then, the inverse scanning unit  220  generates each subset from the last subset to the first subset. If the subset flag indicates that the subset contains non-zero coefficient, the subset is generated by inversely scanning the significant coefficients, coefficient signs and coefficient levels. If the subset flag indicates that the subset does not contain non-zero coefficient, all of the coefficient of the subset are set to 0. The last subset and the first subset are generated by inversely scanning the significant coefficients, coefficient signs and coefficient levels. 
     The inverse quantization unit  230  receives the differential quantization parameter from the entropy decoding unit  210  and generates the quantization parameter predictor to generate the quantization parameter of the coding unit. The operation of generating the quantization parameter is the same as the operation of the quantization unit  150  of  FIG. 1 . Then, the quantization parameter of the current coding unit is generated by adding the differential quantization parameter and the quantization parameter predictor. If the differential quantization parameter for the current coding unit is not received from the encoder, the differential quantization parameter is set to 0. 
     The inverse quantization unit  230  inversely quantizes the quantized block. 
     The inverse transform unit  240  inversely transforms the inverse-quantized block to restore a residual block. The inverse transform type is adaptively determined according to the prediction mode and the size of the transform unit. The inverse transform type is the DCT-based integer transform or the DST-based integer transform. In inter prediction, DCT-based integer transforms are used. 
     The intra prediction unit  250  restores the intra prediction mode of the current prediction unit using the received intra prediction information, and generates a prediction block according to the restored intra prediction mode. 
     The inter prediction unit  260  derives the motion information of the current prediction unit using the received inter prediction information, and generates a prediction block using the motion information. 
     The post-processing unit  270  operates the same as the post-processing unit  180  of  FIG. 1 . 
     The picture storing unit  280  receives post-processed image from the post-processing unit  270 , and stores the image in picture units. A picture may be a frame or a field. 
     The adder  290  adds the restored residual block and a prediction block to generate a reconstructed block. 
       FIG. 9  is a flow chart illustrating a method of decoding an image in inter prediction mode according to the present invention. 
     Motion information of a current block is derived (S 310 ). The current block is a prediction unit. A size of the current block is determined by a size of a coding unit and a partitioning mode. 
     The motion information varies according to a prediction type. If the prediction type is a uni-directional prediction, the motion information includes a reference index specifying a picture of a reference list 0, and a motion vector. If the prediction type is a bi-directional prediction, the motion information includes a reference index specifying a picture of a reference list 0, a reference index specifying a picture of a reference list 1, and a list 0 motion vector and a list 1 motion vector. 
     The motion information is adaptively decoded according the coding mode of the motion information. The coding mode of the motion information is determined by a skip flag and a merge flag. If the skip flag is equal to 1, the merge flag does not exist and the coding mode is a skip mode. If the skip flag is equal to 0 and the merge flag is equal to 1, the coding mode is a merge mode. If the skip flag and the merge flag are equal to 0, the coding mode is an AMVP mode. 
     A prediction block of the current block is generated using the motion information (S 320 ). 
     If the motion vector indicates a pixel position, the prediction block is generated by copying a block of the reference picture specified by the motion vector. If the motion vector indicates a sub-pixel position, the prediction block is generated by interpolating the pixels of the reference picture. 
     A residual block is generated (S 330 ). The residual block is generated by the entropy decoding unit  210 , the inverse scanning unit  220 , the inverse quantization unit  230  and the inverse transform unit  240  of  FIG. 6 . 
     A reconstructed block is generated using the prediction block and the residual block (S 340 ). 
     The prediction block has the size of the prediction unit, and the residual block has the size of the transform unit. Therefore, the residual signals and the prediction signals of same size are added to generate reconstructed signals. 
       FIG. 10  is a flow chart illustrating a method of deriving motion information in the AMVP mode according to the present invention. 
     A reference picture index, a differential motion vector and an amvp index is extracted from a bit stream (S 410 ). The amvp index specifies a motion vector predictor. 
     A left motion vector candidate is derived (S 420 ). The left motion vector candidate is derived as described in S 210  of  FIG. 3 . 
     An above motion vector candidate is derived (S 430 ). The above motion vector candidate is derived as described in S 220  of  FIG. 3 . 
     A temporal motion vector candidate is derived (S 440 ). The temporal motion vector candidate is derived as describe in S 230  of  FIG. 3 . If the left motion vector candidate and the above motion vector candidate are available and different each other, the temporal motion vector may not be derived. 
     A motion vector candidate list is constructed (S 450 ). The motion vector candidates are listed in the order of the left motion vector candidate, the above motion vector candidate and the temporal motion vector candidate. If the left motion vector candidate and the above motion vector candidate are same, the above motion vector candidate is removed from the motion vector candidate list. 
     If the number of the motion vector candidates is smaller than 2, one or two zero motion vectors are added to the motion vector list (S 460 ). 
     A motion vector candidate specified by the amvp index is set as the motion vector predictor (S 470 ). 
     The motion vector of the current block is generated by adding the differential motion vector and the motion vector predictor (S 480 ). 
       FIG. 11  is a flow chart illustrating a method of generating a residual block in inter prediction mode according to the present invention. 
     Quantized coefficient components are generated by the entropy decoding unit (S 510 ). 
     A quantized block is generated by inversely scanning the quantized coefficient components according to the diagonal scan (S 520 ). The quantized coefficient components include the significant flags, the coefficient signs and the coefficients levels. 
     When the size of the transform unit is larger than the a predetermined size, the significant flags, the coefficient signs and the coefficients levels are inversely scanned in the unit of subset using the diagonal scan to generate subsets, and the subsets are inversely scanned using the diagonal scan to generate the quantized block. The predetermined size is equal to the size of the subset. The subset is a 4×4 block including 16 transform coefficients. The significant flags, the coefficient signs and the coefficient levels are inversely scanned in the reverse direction. The subsets are also inversely scanned in the reverse direction. 
     The last non-zero coefficient position and the non-zero subset flags are extracted from the bit stream. The number of encoded subsets is determined according to the last non-zero coefficient position. The non-zero subset flags are used to determine whether the subset has at least one non-zero coefficient. If the non-zero subset flag is equal to 0, the subset is generated using the diagonal scan. The first subset and the last subset are generated using the inverse scan pattern. 
     The quantized block is inversely quantized using an inverse quantization matrix and a quantization parameter (S 530 ). 
     The quantization parameter is derived as follows. 
     A size of minimum quantization unit is determined. A parameter cu_qp_delta_enabled_info is extracted from a picture parameter set of a bit stream, and the size of the minimum quantization unit is determined using the parameter. 
     A differential quantization parameter of the current coding unit is derived. The coded differential quantization parameter is arithmetically decoded to generate bin string indicating the absolute value of the differential quantization parameter and a bin indicating the sign of the differential quantization parameter. The bin string may be a truncated unary code. If the absolute value of the differential quantization parameter is zero, the bin indicating the sign does not exist. The differential quantization parameter is derived using the bin string indicating the absolute value and the bin indicating the sign. 
     A quantization parameter predictor of the current coding unit is derived. The quantization parameter predictor is generated by using quantization parameters of neighboring coding units and quantization parameter of previous coding unit as follows. 
     A left quantization parameter, an above quantization parameter and a previous quantization parameter are sequentially retrieved in this order. An average of the first two available quantization parameters retrieved in that order is set as the quantization parameter predictor when two or more quantization parameters are available, and when only one quantization parameter is available, the available quantization parameter is set as the quantization parameter predictor. That is, if the left and above quantization parameter are available, the average of the left and above quantization parameter is set as the quantization parameter predictor. If only one of the left and above quantization parameter is available, the average of the available quantization parameter and the previous quantization parameter is set as the quantization parameter predictor. If both of the left and above quantization parameter are unavailable, the previous quantization parameter is set as the quantization parameter predictor. 
     If multiple coding units belong to the minimum quantization unit, the quantization parameter predictor for the first coding unit in decoding order is derived and used for the other coding units. 
     The quantization parameter of the current coding unit is generated using the differential quantization parameter and the quantization parameter predictor. 
     A residual block is generated by inverse-transforming the inverse-quantized block (S 540 ). One dimensional horizontal and vertical inverse DCT based-transforms are used. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.