Patent Publication Number: US-11647208-B2

Title: Picture coding method, picture coding apparatus, picture decoding method, and picture decoding apparatus

Description:
FIELD 
     One or more exemplary embodiments disclosed herein relate to a picture coding method and a picture decoding method. 
     BACKGROUND 
     Generally, in coding processing of a moving picture, the amount of information is reduced by compression for which temporal redundancy and spatial redundancy in a moving picture is utilized. Generally, transform into frequency domain is performed as a method in which spatial redundancy is utilized, and coding using prediction between pictures (the prediction is hereinafter referred to as inter prediction) is performed as a method of compression for which temporal redundancy is utilized. In the inter prediction coding, a current picture is coded using, as a reference picture, a coded picture which precedes or follows the current picture in order of display time. A motion vector is derived by estimating motion between the current picture and the reference picture. Then, difference between picture data of the current picture and prediction picture data obtained by motion compensation based on the derived motion vector is calculated to reduce temporal redundancy (see Non-patent Literature 1, for example). In the motion estimation, difference values between current blocks in the current picture and blocks in the reference picture are calculated, and a block having the smallest difference value in the reference picture is determined as a reference block. Then, a motion vector is estimated for the current block and the reference block. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [Non-patent Literature 1] ITU-T Recommendation H.264 “Advanced video coding for generic audiovisual services”, March 2010 
         [Non-patent Literature 2] JCT-VC, “WD3: Working Draft 3 of High-Efficiency Video Coding”, JCTVC-E603, March 2011 
       
    
     SUMMARY 
     Technical Problem 
     It is still desirable to increase coding efficiency in coding and decoding of pictures using inter prediction with the above-described conventional technique. 
     Non-limiting and exemplary embodiments provide picture coding methods and picture decoding methods with which coding efficiency in coding and decoding of pictures using inter prediction is increased. 
     Solution to Problem 
     In one general aspect, the techniques disclosed here feature a picture coding method which is a method for coding a picture on a block-by-block basis to generate a bitstream and includes: performing a first derivation process for deriving a first merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in coding of a current block; performing a second derivation process for deriving a second merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the second derivation process being different from the first derivation process; selecting a merging candidate to be used in the coding of the current block from among the first merging candidate and the second merging candidate; and attaching an index for identifying the selected merging candidate to the bitstream, wherein in the performing of a first derivation process, the first derivation process is performed so that a total number of the first merging candidates does not exceed a predetermined number, and the second derivation process is performed when the total number of the first merging candidates is less than a predetermined maximum number of merging candidates. 
     These general and specific aspects can be implemented as a system, a method, an integrated circuit, a computer program, a computer-readable recording medium such as a CD-ROM (compact disc read-only memory), or as any combination of a system, a method, an integrated circuit, a computer program, and a computer-readable recording medium. 
     Additional benefits and advantages of the disclosed embodiments will be apparent from the Specification and Drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the Specification and Drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
     Advantageous Effects 
     A picture coding method according to one or more exemplary embodiments or features disclosed herein provide increased coding efficiency in coding and decoding of pictures using inter prediction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG.  1 A  is a diagram for illustrating an exemplary reference picture list for a B-picture. 
         FIG.  1 B  shows an example of a reference picture list 0 (L0) for a prediction direction 0 in bi-prediction of a B-picture. 
         FIG.  1 C  shows an example of a reference picture list 1 (L1) for a prediction direction 1 in bi-prediction of a B-picture. 
         FIG.  2    is a diagram for illustrating motion vectors for use in a temporal motion vector prediction mode. 
         FIG.  3    shows an exemplary motion vector of a neighboring block used in the merging mode. 
         FIG.  4    is a diagram for illustrating an example of a merging candidate list. 
         FIG.  5    shows a relationship between the size of a merging candidate list and bit sequences assigned to merging candidate indices. 
         FIG.  6    is a flowchart showing an example of a process for coding of a current block when the merging mode is used. 
         FIG.  7    is a flowchart showing a process for decoding using the merging mode. 
         FIG.  8    shows syntax for attachment of merging candidate indices to a bitstream. 
         FIG.  9    is a block diagram showing a configuration of a picture coding apparatus according to Embodiment 1. 
         FIG.  10 A  is a flowchart showing processing operations of a picture coding apparatus according to Embodiment 1. 
         FIG.  10 B  is a flowchart showing derivation of merging candidates according to Embodiment 1. 
         FIG.  11    shows an example of a merging candidate list generated by the picture coding apparatus according to Embodiment 1. 
         FIG.  12    is a block diagram showing a configuration of a picture decoding apparatus according to Embodiment 2. 
         FIG.  13 A  is a flowchart showing processing operations of the picture decoding apparatus according to Embodiment 2. 
         FIG.  13 B  is a flowchart showing derivation of merging candidates according to Embodiment 2. 
         FIG.  14    is a block diagram showing a configuration of a picture coding apparatus according to Embodiment 3. 
         FIG.  15    is a flowchart showing processing operations of the picture coding apparatus according to Embodiment 3. 
         FIG.  16    is a flowchart showing the process for selecting a merging candidate according to Embodiment 3. 
         FIG.  17    is a block diagram showing a configuration of a picture decoding apparatus according to Embodiment 4. 
         FIG.  18    is a flowchart showing processing operations of the picture decoding apparatus according to Embodiment 4. 
         FIG.  19    is a flowchart showing derivation of a zero merging candidate according to Embodiment 5. 
         FIG.  20    shows an example of a derived zero merging candidate in Embodiment 5. 
         FIG.  21    is a flowchart showing derivation of a combined merging candidate according to Embodiment 6. 
         FIG.  22    is a flowchart showing derivation of a scaling merging candidate according to Embodiment 7. 
         FIG.  23    shows an example of a motion vector and a reference picture index calculated in Embodiment 7. 
         FIG.  24    shows an overall configuration of a content providing system for implementing content distribution services. 
         FIG.  25    shows an overall configuration of a digital broadcasting system. 
         FIG.  26    shows a block diagram illustrating an example of a configuration of a television. 
         FIG.  27    shows a block diagram illustrating an example of a configuration of an information reproducing/recording unit that reads and writes information from and on a recording medium that is an optical disk. 
         FIG.  28    shows an example of a configuration of a recording medium that is an optical disk. 
         FIG.  29 A  shows an example of a cellular phone. 
         FIG.  29 B  is a block diagram showing an example of a configuration of a cellular phone. 
         FIG.  30    illustrates a structure of multiplexed data. 
         FIG.  31    schematically shows how each stream is multiplexed in multiplexed data. 
         FIG.  32    shows how a video stream is stored in a stream of PES packets in more detail. 
         FIG.  33    shows a structure of TS packets and source packets in the multiplexed data. 
         FIG.  34    shows a data structure of a PMT. 
         FIG.  35    shows an internal structure of multiplexed data information. 
         FIG.  36    shows an internal structure of stream attribute information. 
         FIG.  37    shows steps for identifying video data. 
         FIG.  38    shows an example of a configuration of an integrated circuit for implementing the moving picture coding method and the moving picture decoding method according to each of embodiments. 
         FIG.  39    shows a configuration for switching between driving frequencies. 
         FIG.  40    shows steps for identifying video data and switching between driving frequencies. 
         FIG.  41    shows an example of a look-up table in which video data standards are associated with driving frequencies. 
         FIG.  42 A  is a diagram showing an example of a configuration for sharing a module of a signal processing unit. 
         FIG.  42 B  is a diagram showing another example of a configuration for sharing a module of the signal processing unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Underlying Knowledge Forming Basis of the Present Disclosure 
     In a moving picture coding scheme already standardized, which is referred to as H.264, the amount of information is reduced by compression using three types of pictures: I-picture, P-picture, and B-picture. 
     The I picture is not coded using inter prediction. Specifically, the I-picture is coded by prediction within the picture (the prediction is hereinafter referred to as intra prediction). The P-picture is coded using inter prediction with reference to one previously coded picture preceding or following the current picture in order of display time. The B-picture is coded using inter prediction with reference to two previously coded pictures preceding and following the current picture in order of display time. 
     In coding using inter prediction, a reference picture list for identifying a reference picture is generated. In the reference picture list, reference picture indices are assigned to coded reference pictures to be referenced in inter prediction. For example, two reference picture lists (L0 and L1) are generated for a B-picture because it is coded with reference to two pictures. 
       FIG.  1 A  is a diagram for illustrating an exemplary reference picture list for a B-picture.  FIG.  1 B  shows an example of a reference picture list 0 (L0) for a prediction direction 0 in bi-prediction. In the reference picture list 0, the reference picture index 0 having a value of 0 is assigned to a reference picture  0  with a display order of 2. The reference picture index 0 having a value of 1 is assigned to a reference picture  1  with a display order of 1. The reference picture index 0 having a value of 2 is assigned to a reference picture  2  with a display order of 0. In other words, a reference picture temporally closer to the current picture in display order is assigned with a reference picture index having a smaller value. 
       FIG.  1 C  shows an example of a reference picture list 1 (L1) for a prediction direction 1 in bi-prediction. In the reference picture list 1, the reference picture index 1 having a value of 0 is assigned to a reference picture  1  with a display order of 1. The reference picture index 1 having a value of 1 is assigned to a reference picture  0  with a display order of 2. The reference picture index 1 having a value of 2 is assigned to a reference picture  2  with a display order of 0. 
     In this manner, reference picture indices assigned to a reference picture may have values different between prediction directions (the reference pictures  0  and  1  in  FIG.  1 A ), and may have the same value for both directions (the reference picture  2  in  FIG.  1 A ). 
     In a moving picture coding method referred to as H.264 (see Non-patent Literature 1), a motion vector estimation mode is available as a coding mode for inter prediction of each current block in a B-picture. In the motion vector estimation mode, a difference value between picture data of a current block and prediction picture data and a motion vector used for generating the prediction picture data are coded. In addition, in the motion vector estimation mode, bi-prediction and uni-prediction can be selectively performed. In bi-prediction, a prediction picture is generated with reference to two coded pictures one of which precedes a current picture to be coded and the other of which follows the current picture. In uni-prediction, a prediction picture is generated with reference to one coded picture preceding or following a current picture to be coded. 
     Furthermore, in the moving picture coding method referred to as H.264, a coding mode referred to as a temporal motion vector prediction mode can be selected for derivation of a motion vector in coding of a B-picture. The inter prediction coding method performed in temporal motion vector prediction mode will be described below using  FIG.  2   . 
       FIG.  2    is a diagram for illustrating motion vectors for use in the temporal motion vector prediction mode. Specifically,  FIG.  2    shows a case where a block a in a picture B 2  is coded in temporal motion vector prediction mode. 
     In the coding, a motion vector vb is used which has been used for coding of a block b in a picture P 3 , which is a reference picture following the picture B 2 . The position of the motion vector vb in the picture P 3  is the same as the position of the block a in the picture B 2  (the block b is hereinafter referred to as a “co-located block” of the block a). The motion vector vb has been used for coding the block b with reference to the picture P 1 . 
     Motion vectors parallel to the motion vector vb are used for obtaining two reference blocks for the block a from a preceding reference picture and a following reference picture, that is, a picture P 1  and a picture P 3 . Then, the block a is coded using bi-prediction based on the two obtained reference blocks. Specifically, the block a is coded with reference the picture P 1  using a motion vector va 1  and with reference to the picture P 3  using a motion vector vat. 
     In addition, a merging mode has been discussed which is an inter prediction mode for coding of each current block in a B-picture or a P-picture (see Non-patent Literature 2). In the merging mode, a current block is coded using a set of a prediction direction, a motion vector, and a reference picture index which is a copy of a set thereof used for coding a neighboring block of the current block. In the coding of a current block, an index and others indicating the set of a prediction direction, a motion vector, and a reference picture index which is used as a set for the coding of the neighboring block is attached to a bitstream. This makes it possible to select, in decoding of the current block, the set of a prediction direction, a motion vector, and a reference picture index used as a set for the coding of the neighboring block. A concrete example is given below with reference to  FIG.  3   . 
       FIG.  3    shows an exemplary motion vector of a neighboring block used in the merging mode. In  FIG.  3   , a neighboring block A is a coded block located on the immediate left of a current block. A neighboring block B is a coded block located immediately above the current block. A neighboring block C is a coded block located immediately above to the right of the current block. A neighboring block D is a coded block located immediately below to the left of the current block. 
     The neighboring block A is a block coded using uni-prediction in the prediction direction 0. The neighboring block A has a motion vector MvL0_A having the prediction direction 0, which is a motion vector to a reference picture indicated by a reference picture index RefL0_A for the prediction direction 0. Here, MvL0 represents a motion vector which references a reference picture specified in a reference picture list 0 (L0). MvL1 represents a motion vector which references a reference picture specified in a reference picture list 1 (L1). 
     The neighboring block B is a block coded using uni-prediction in the prediction direction 1. The neighboring block B has a motion vector MvL1_B having the prediction direction 1, which is a motion vector to a reference picture indicated by a reference picture index RefL1_B for the prediction direction 1. 
     The neighboring block C is a block coded using intra prediction. 
     The neighboring block D is a block coded using uni-prediction in the prediction direction 0. The neighboring block D has a motion vector MvL0_D having the prediction direction 0, which is a motion vector to a reference picture indicated by a reference picture index RefL0_D for the prediction direction 0. 
     In the case illustrated in  FIG.  3   , for example, a set of a prediction direction, a motion vector, and a reference picture index with which the current block can be coded with the highest coding efficiency is selected as a set of a prediction direction, a motion vector, and a reference picture index of the current block from among such sets of the neighboring blocks A to D and a set of a prediction direction, a motion vector, and a reference picture index which are calculated using a co-located block in temporal motion vector prediction mode. One or more candidate sets of a prediction direction, a motion vector, and a reference picture index compose a merging candidate. A merging candidate index indicating the selected merging candidate is attached to a bitstream. 
     For example, when the merging candidate of the neighboring block A is selected, the current block is coded using the reference picture index RefL0_A and the motion vector MvL0_A having the prediction direction 0. Then, only a merging candidate index having a value of 0 is attached to a bitstream, indicating that the merging candidate of the neighboring block A as shown in  FIG.  4    is used for the coding of the current block. The amount of information on a prediction direction, a motion vector, and a reference picture index is thereby reduced. 
     Furthermore, in the merging mode, a candidate which cannot be used for coding of a current block (hereinafter referred to as an “unusable-for-merging candidate”), and a candidate having a set of a prediction direction, a motion vector, and a reference picture index identical to a set of a prediction direction, a motion vector, and a reference picture index of any other merging block (hereinafter referred to as an “identical candidate”) are removed from the merging candidate list as shown in  FIG.  4   . 
     The total number of merging candidates is thus reduced, and thereby the amount of codes assigned to merging candidate indices is saved. Examples of the merging candidate which cannot be used for coding of a current block includes: (1) a merging candidate of a block coded using intra prediction, (2) a merging candidate of a block outside the slice including the current block or outside the boundary of a picture including the current block, and (3) a merging candidate of a block yet to be coded. 
     In the example shown in  FIG.  4   , the neighboring block C is a block coded using intra prediction. The merging candidate of the neighboring block C (indicated by the merging candidate index having a value of 3) is an unusable-for-merging candidate and therefore removed from the merging candidate list. In addition, the neighboring block D is identical in prediction direction, motion vector, and reference picture index to the neighboring block A. The merging candidate of the neighboring block D (indicated by the merging candidate index having a value of 4) is therefore removed from the merging candidate list. As a result, the final total number of merging candidates is three, and the size of the merging candidate list is set at three. 
     Merging candidate indices are coded by variable-length coding by assigning bit sequences according to the size of each merging candidate list as shown in  FIG.  5   . In the merging mode, bit sequences assigned to merging candidate indices are thus changed depending on the size of each merging candidate list, and thereby the amount of code is reduced. 
       FIG.  6    is a flowchart showing an example of a process for coding of a current block when the merging mode is used. In Step S 1001 , sets each including a prediction direction, a motion vector, and a reference picture index of neighboring blocks and a co-located block are obtained as merging candidates. In Step S 1002 , identical candidates and unusable-for-merging candidates are removed from the merging candidates. In Step S 1003 , the total number of the merging candidates after the removing is set as the size of the merging candidate list. In Step S 1004 , a merging candidate index to be used for coding of the current block is determined. In Step S 1005 , the determined merging candidate index is coded by variable-length coding in bit sequence according to the size of the merging candidate list. 
       FIG.  7    is a flowchart showing an example of a process for decoding using the merging mode. In Step S 2001 , sets each including a prediction direction, a motion vector, and a reference picture index of neighboring blocks and a co-located block are obtained as merging candidates. In Step S 2002 , identical candidates and unusable-for-merging candidates are removed from the merging candidates. In Step S 2003 , the total number of the merging candidates after the removing is set as the size of the merging candidate list. In Step S 2004 , the merging candidate index to be used in decoding of a current block is decoded from a bitstream using the size of the merging candidate list. In Step S 2005 , the current block is decoded by generating a prediction picture using the merging candidate indicated by the decoded merging candidate index. 
       FIG.  8    shows syntax for attaching a merging candidate index to a bitstream. In  FIG.  8   , merge_idx represents a merging candidate index, and merge_flag represents a merging flag. NumMergeCand represents the size of a merging candidate list. NumMergeCand is set at the total number of merging candidates after unusable-for-merging candidates and identical candidates are removed from the merging candidates. 
     In the merging mode, when identical candidates are removed from merging candidates, a merging candidate index cannot be correctly decoded due to a discrepancy in bit sequence assigned to merging candidate indices between a picture coding apparatus and a picture decoding apparatus. Such a discrepancy may occur when, for example, there is a difference in the total number of merging candidates between the picture coding apparatus and the picture decoding apparatus. 
     Use of merging candidate lists having a fixed size has been discussed as a solution to the problem. 
     When the total number of merging candidates is equivalent to the size of a merging candidate list, it is more likely that the merging candidate list has a merging candidate including a motion vector for accurate prediction. It is therefore possible to achieve increased coding efficiency. 
     On the other hand, when the size of merging candidate lists is fixed, the total number of merging candidates after removing of identical candidates may be smaller than the size. In such a case, it is less likely that the merging candidate list has a merging candidate including a motion vector for accurate prediction. This may lead to decrease in coding efficiency. 
     In one general aspect, the techniques disclosed here feature a picture coding method which is a method for coding a picture on a block-by-block basis to generate a bitstream and includes: performing a first derivation process for deriving a first merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in coding of a current block; performing a second derivation process for deriving a second merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the second derivation process being different from the first derivation process; selecting a merging candidate to be used in the coding of the current block from among the first merging candidate and the second merging candidate; and attaching an index for identifying the selected merging candidate to the bitstream, wherein in the performing of a first derivation process, the first derivation process is performed so that a total number of the first merging candidates does not exceed a predetermined number, and the second derivation process is performed when the total number of the first merging candidates is less than a predetermined maximum number of merging candidates. 
     With this, it is possible to perform the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. The total number of first merging candidates is thus controlled, and the variety of merging candidates thereby increases. As a result, coding efficiency increases. 
     For example, the picture coding method may further include performing a third derivation process for deriving a third merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the third derivation process being different from the first derivation process and the second derivation process, wherein the second derivation process is performed when the total number of the first merging candidates and third merging candidates is less than the predetermined maximum number of merging candidates, and in the selecting, the merging candidate to be used in the coding of the current block is selected from among the first merging candidate, the second merging candidate, and the third merging candidate. 
     With this, it is possible to further perform the third derivation process in which a method different from methods used in the first derivation process and the second derivation process is used. The variety of merging candidates thus further increases, and coding efficiency thereby increases. 
     For example, in the performing of a third derivation process, a plurality of the third merging candidates may be derived by performing the third derivation process, and in the performing of a first derivation process, the first derivation process may be a process for deriving, as the first merging candidate, a bi-predictive merging candidate which is a combination of two sets each including a prediction direction, a motion vector, and a reference picture index and included in the third merging candidates. 
     With this, it is possible to derive a bi-predictive first merging candidate by making a combination from a plurality of third merging candidates. A new bi-predictive first merging candidate can be thus derived even when none of the plurality of third merging candidates is a bi-predictive merging candidate. As a result, the variety of merging candidates is increased, and coding efficiency thereby increases. 
     For example, in the performing of a third derivation process, the third derivation process may be a process for deriving the third merging candidate using a set of a prediction direction, a motion vector, and a reference picture index which are used as a set for coding a block spatially or temporally neighboring the current block. 
     With this, it is possible to derive a third merging candidate using a set of a prediction direction, a motion vector, and a reference picture index used for coding of a block spatially or temporally neighboring the current block. The third merging candidate derived in this manner is reliable, and coding efficiency therefore increases. 
     For example, the second derivation process may be repeatedly performed until a total number of the first merging candidates, second merging candidates, and third merging candidates reaches the predetermined maximum number of merging candidates. 
     With this, it is possible to repeat the second derivation process until the total number of second merging candidates and third merging candidates reaches the predetermined maximum number of merging candidates. Merging candidates are thus derived to the maximum number, and coding efficiency therefore increases. 
     For example, in the performing of a second derivation process, the second derivation process may be a process for deriving, as the second merging candidate, a merging candidate including a motion vector which is a zero vector. 
     With this, it is possible to derive a second merging candidate having a zero vector as a motion vector. The merging candidate derived in this manner is reliable when the current block is a stationary region, and coding efficiency therefore increases. 
     For example, the predetermined number may depend on a maximum number of the first merging candidates to be derived using the first derivation process. 
     With this, it is possible to derive a first merging candidate using, as a predetermined number, a number dependent on the total number of first merging candidates which can be derived by the first derivation process. A first merging candidate is thus derived using an appropriate predetermined number so that the variety of merging candidates may increase, and coding efficiency therefore increases. 
     For example, the picture coding method may further include switching a coding process between a first coding process conforming to a first standard and a second coding process conforming to a second standard; and attaching, to the bitstream, identification information indicating either the first standard or the second standard to which the coding process after the switching conforms, wherein when the coding process after the switching is the first coding process, the first coding process is performed by performing the first derivation process, the second derivation process, the selecting, and the attaching. 
     With this, it is possible to switchably perform the first coding process conforming to the first standard and the second coding process conforming to the second standard. 
     Furthermore, in one general aspect, the techniques disclosed here feature a picture decoding method which is a method for decoding, on a block-by-block basis, a coded image included in a bitstream, and includes: performing a first derivation process for deriving a first merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in decoding of a current block; performing a second derivation process for deriving a second merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the decoding of the current block, the second derivation process being different from the first derivation process; obtaining an index from the bitstream; and selecting, based on the obtained index, a merging candidate to be used in the decoding of the current block from among the first merging candidate and the second merging candidate, wherein in the performing of a first derivation process, the first derivation process is performed so that a total number of the first merging candidates does not exceed a predetermined number, and the second derivation process is performed when the total number of the first merging candidates is less than a predetermined maximum number of merging candidates. 
     With this, it is possible to perform the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. The total number of first merging candidates is thus controlled, and the variety of merging candidates thereby increases. As a result, a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, the picture decoding method may further include performing a third derivation process for deriving a third merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the third derivation process being different from the first derivation process and the second derivation process, performing a third derivation process for deriving a third merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the third derivation process being different from the first derivation process and the second derivation process, wherein the second derivation process is performed when the total number of the first merging candidates and third merging candidates is less than the predetermined maximum number of merging candidates, and in the selecting, the merging candidate to be used in the decoding of the current block is selected from among the first merging candidate, the second merging candidate, and the third merging candidate. 
     With this, it is possible to further possible to derive a third merging candidate by performing the third derivation process in which a method different from methods used in the first derivation process and the second derivation process is used. The variety of merging candidates thus further increases, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, in the performing of a third derivation process, a plurality of the third merging candidates may be derived by performing the third derivation process, and in the performing of a first derivation process, the first derivation process may be a process for deriving, as the first merging candidate, a bi-predictive merging candidate which is a combination of two sets each including a prediction direction, a motion vector, and a reference picture index and included in the third merging candidates. 
     With this, it is possible to derive a bi-predictive first merging candidate by making a combination from a plurality of third merging candidates. A new bi-predictive first merging candidate can be thus derived even when none of the plurality of third merging candidates is a bi-predictive merging candidate. As a result, the variety of merging candidates is thus increased, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, in the performing of a third derivation process, the third derivation process may be a process for deriving the third merging candidates using a set of a prediction direction, a motion vector, and a reference picture index which are used as a set in decoding a block spatially or temporally neighboring the current block. 
     With this, it is possible to derive a third merging candidate using a set of a prediction direction, a motion vector, and a reference picture index used for coding of a block spatially or temporally neighboring the current block. The third merging candidate derived in this manner is reliable, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, the second derivation process may be repeatedly performed until a total number of the first merging candidates, second merging candidates, and third merging candidates reaches the predetermined maximum number of merging candidates. 
     With this, it is possible to repeat the second derivation process until the total number of second merging candidates and the third merging candidates reaches the predetermined maximum number of merging candidates. Merging candidates are thus derived to the maximum number, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, in the performing of a second derivation process, the second derivation process may be a process for deriving, as the second merging candidate, a merging candidate including a motion vector which is a zero vector. 
     With this, it is possible to derive a second merging candidate having a zero vector as a motion vector. The merging candidate derived in this manner is reliable when the current block is a stationary region, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, the predetermined number may depend on a maximum number of the first merging candidates to be derived using the first derivation process. 
     With this, it is possible to derive a first merging candidate using, as a predetermined number, a number dependent on the total number of first merging candidates which can be derived by the first derivation process. A first merging candidate is thus derived using an appropriate predetermined number so that the variety of merging candidates may be increased, and therefore a bitstream coded with increased coding efficiency can be appropriately decoded. 
     For example, the picture decoding method may further include: switching a decoding process between a first decoding process conforming to a first standard and a second decoding process conforming to a second standard, according to identification information attached to the bitstream and indicating either the first standard or the second standard, wherein when the decoding process after the switching is the first decoding process, the first decoding process is performed by performing the first derivation process, the second derivation process, the obtaining, and the selecting. 
     With this, it is possible to switchably perform the first coding process conforming to the first standard and the second coding process conforming to the second standard. 
     These general and specific aspects can be implemented as a system, a method, an integrated circuit, a computer program, a computer-readable recording medium such as a CD-ROM, or as any combination of a system, a method, an integrated circuit, a computer program, and a computer-readable recording medium. 
     Exemplary embodiments will be described below with reference to the drawings. 
     Each of the exemplary embodiments described below shows a general or specific example. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the processing order of the steps, etc. shown in the following exemplary embodiments are mere examples, and therefore do not limit the scope of the appended Claims and their equivalents. Therefore, among the constituent elements in the following exemplary embodiments, constituent elements not recited in any one of the independent claims defining the most generic part of the inventive concept are described as structural elements included as appropriate. 
     Embodiment 1 
       FIG.  9    is a block diagram showing a configuration of a picture coding apparatus  100  according to Embodiment 1. The picture coding apparatus  100  codes a picture on a block-by-block basis to generate a bitstream. As shown in  FIG.  9   , the picture coding apparatus  100  includes a merging candidate derivation unit  110 , a prediction control unit  120 , and a coding unit  130 . 
     The merging candidate derivation unit  110  derives merging candidates. Then, the merging candidate derivation unit  110  generates a merging candidate list in which each of the derived merging candidates is associated with an index for identifying the merging candidate (hereinafter referred to as merging candidate index). Specifically, the merging candidate derivation unit  110  includes a third derivation unit  111 , a first derivation unit  112 , and a second derivation unit  113  as shown in  FIG.  9   . 
     The third derivation unit  111  performs a third derivation process in which a merging candidate is derived using a third derivation method. The merging candidate derived using the third derivation process is hereinafter referred to as a third merging candidate. Then, the third derivation unit  111  registers the third merging candidate in the merging candidate list in association with a merging candidate index. 
     Specifically, the third derivation unit  111  performs, as the third derivation process, a process for deriving a third merging candidate using, for example, a set of a prediction direction, a motion vector, and a reference picture index used for coding of a block spatially or temporally neighboring a current block. Third merging candidate derived from spatially neighboring blocks in this manner are referred to as spatial merging candidates, and third merging candidates derived from temporally neighboring blocks are referred to as temporal merging candidates. 
     The spatially neighboring block is a block within a picture including the current block and neighbors the current block. Specifically, the neighboring blocks A to D shown in  FIG.  3    are examples of the spatially neighboring block. 
     The spatially neighboring block is not limited to the neighboring blocks A to D shown in  FIG.  3   . Examples of the spatially neighboring block may further include blocks neighboring any of the neighboring blocks A to D. 
     The temporally neighboring block is a block which is within a picture different from a picture including the current block and corresponds to the current block. Specifically, a co-located block is an example of the temporally neighboring block. 
     The temporally neighboring block is not limited to a block located in a position which is the same as the position of the current block in the respective picture (co-located block). For example, the temporally neighboring block may be a block neighboring the co-located block. 
     The third derivation unit  111  may perform the third derivation process in which a merging candidate is derived using a method other than the third derivation method. In other words, the third derivation unit  111  need not perform the process for deriving a spatial merging candidate or a temporal merging candidate as the third derivation process. 
     The first derivation unit  112  performs a first derivation process for deriving a merging candidate, using a first derivation method which is different from the third derivation method. The merging candidate derived using the first derivation process is hereinafter referred to as a first merging candidate. The first derivation unit  112  performs the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. Then, the first derivation unit  112  registers the first merging candidate in the merging candidate list in association with a merging candidate index. 
     The predetermined number is a maximum number of first merging candidates. The predetermined number may be fixed or variable. For example, the predetermined number may be set depending on the total number of merging candidates which can be derived using the first derivation process. Specifically, the first derivation unit  112  may set the predetermined number depending on, for example, the total number of third merging candidates or the total number of referable pictures. Because of dependency of the predetermined number on the total number of merging candidates which can be derived using the first derivation process, the variety of merging candidates can be increased by deriving first merging candidates using an appropriate predetermined number, and coding efficiency thereby increases. 
     Specifically, the first derivation unit  112  performs, as the first derivation process, a process for deriving, for example, a bi-predictive merging candidate as a first merging candidate by making a combination of sets each including a prediction direction, a motion vector, and a reference picture index. The sets are included in the third merging candidates. Merging candidates derived in this manner are hereinafter referred to as combined merging candidates. The process for deriving a combined merging candidate will be described in detail in Embodiment 6. 
     The first derivation unit  112  may perform the first derivation process in which a merging candidate is derived using a method other than the first derivation method. In other words, the first derivation unit  112  may perform, as the first derivation process, a process other than the process for deriving a combined merging candidate. 
     The second derivation unit  113  performs a second derivation process for deriving a merging candidate, using a second derivation method when the total number of first merging candidates and third merging candidates is smaller than a predetermined maximum number of merging candidates. The second derivation method is different from the first derivation method and the third derivation method. The merging candidate derived using the second derivation process is hereinafter referred to as a second merging candidate. Then, the second derivation unit  113  registers the second merging candidate in the merging candidate list in association with a merging candidate index. 
     Specifically, the second derivation unit  113  performs, as the second derivation process, a process for deriving, for example, a merging candidate including a motion vector which is a zero vector. Merging candidates derived in this manner are hereinafter referred to as zero merging candidates. The process for deriving a zero merging candidate will be described in detail in Embodiment 5. 
     The second derivation unit  113  may perform the second derivation process in which a merging candidate is derived using a method other than the second derivation method. In other words, the second deriving unit  113  need not perform the process for deriving a zero merging candidate as the second derivation process. 
     The predetermined maximum number of merging candidates is a number provided in a standard, for example. Optionally, the predetermined maximum number of merging candidates may be determined according to, for example, features of a current picture. In this case, the determined maximum number may be attached to a bitstream. 
     The prediction control unit  120  selects a merging candidate to be used for coding a current block from the first to third merging candidates. In other words, the prediction control unit  120  selects a merging candidate to be used for coding a current block from the merging candidate list. 
     The coding unit  130  attaches an index for identifying the selected merging candidate (merging candidate index) to a bitstream. For example, the coding unit  130  codes an index using the total number of first to third merging candidates (total number of merging candidates), and attaches the coded index to a bitstream. Then, the coding unit  130  attaches the coded index to a bitstream. 
     Optionally, the coding unit  130  may code an index using not the total number of first to third merging candidates but, for example, a predetermined maximum number of merging candidates. Specifically, the coding unit  130  may determine a bit sequence assigned to the value of an index using a predetermined maximum number of merging candidates as shown in  FIG.  5    and code the determined bit sequence by variable-length coding. By doing this, the coding unit  130  can code an index independently of the total number of actually derived merging candidates. Therefore, even when information necessary for derivation of a merging candidate (for example, information on a co-located block) is lost, an index can be still decoded and error resistance is thereby enhanced. Furthermore, an index can be decoded independently of the total number of actually derived merging candidates. In other words, an index can be decoded without waiting for derivation of merging candidates. In other words, a bitstream can be generated with which deriving of merging candidates and decoding of indices can be performed in parallel. 
     Operations of the picture coding apparatus  100  in the above-described configuration will be described below. 
       FIG.  10 A  is a flowchart showing processing operations of the picture coding apparatus  100  according to Embodiment 1. 
     First, the merging candidate derivation unit  110  derives merging candidates (S 110 ), and registers the derived merging candidates in a merging candidate list. 
     Next, the prediction control unit  120  selects a merging candidate to be used for coding a current block from the first to third merging candidates (S 120 ). For example, the prediction control unit  120  selects, from the derived merging candidates, a merging candidate which minimizes cost indicating the amount of code for the current block and others. 
     Next, the coding unit  130  attaches an index for identifying the selected merging candidate to a bitstream (S 130 ). Furthermore, the coding unit  130  generates inter-prediction picture of the current block by performing inter prediction using the selected merging candidate. Input picture data is coded using inter-prediction picture generated in this manner. 
     Step S 110  in  FIG.  10 A  will be described in detail below with reference to  FIG.  10 B  and  FIG.  11   . 
       FIG.  10 B  is a flowchart of the deriving of merging candidates according to Embodiment 1.  FIG.  11    shows an example of the merging candidate list generated by the picture coding apparatus  100  according to Embodiment 1. For  FIG.  11   , it is assumed that a predetermined maximum number of merging candidates is five, and a predetermined number is two. 
     First, the third derivation unit  111  performs the third derivation process (S 111 ). Note that a third merging candidate is not always derived in Step S 111 . For example, the third derivation unit  111  derives no third merging candidate by performing the third derivation process when a third merging candidate to be derived as a result of the third derivation process presently performed is identical to a previously derived third merging candidate. Here, one merging candidate being identical to another merging candidate means that the sets each including a prediction direction, a motion vector, and a reference picture index and included in the respective merging candidates are identical to each other. In other examples, the third derivation unit  111  does not derive a third merging candidate from a block spatially or temporally neighboring a current block when the block is (1) a block coded by intra prediction, (2) a block outside a slice including the current block or outside the boundary of a picture including the current block, or (3) a block yet to be coded. 
     Next, the third derivation unit  111  determines whether or not to end the third derivation process (S 112 ). For example, to determine whether or not to end the third derivation process, the third derivation unit  111  determines whether the third derivation process has been performed for all predetermined neighboring blocks. 
     When the third derivation unit  111  determines not to end the third derivation process (S 112 , No), the third derivation unit  111  performs the third derivation process again (S 111 ). 
     Referring to  FIG.  11   , two third merging candidates (a spatial merging candidate and a temporal merging candidate) are derived from the neighboring blocks A to D and a co-located block. The third merging candidates are provided with merging candidate indices having values of “0” and “1”, respectively. 
     When the third derivation unit  111  determines to end the third derivation process (S 112 , Yes), the first derivation unit  112  performs the first derivation process (S 113 ). Next, the first derivation unit  112  determines whether or not the total number of first merging candidates derived using the first derivation process is below a predetermined number (S 114 ). 
     When the total number of first merging candidates is below the predetermined number (S 114 , Yes), the first derivation unit  112  performs the first derivation process again (S 113 ). In other words, the first derivation unit  112  performs the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. 
     Referring to  FIG.  11   , two first merging candidates (combined merging candidates) are derived by making combinations from the two third merging candidates. The first merging candidates are provided with merging candidate indices having values of “2” and “3”, which are larger than those of the third merging candidates. 
     When the total number of first merging candidates is not below the predetermined number (S 114 , No), the second derivation unit  113  performs the second derivation process (S 115 ). Next, the second derivation unit  113  determines whether or not the total number of first to third merging candidates is below a predetermined maximum number of merging candidates (S 116 ). 
     When the total number of first to third merging candidates is below the predetermined maximum number (S 116 , Yes), the second derivation unit  113  performs the second derivation process again (S 115 ). In other words, the second derivation unit  113  repeats the second derivation process until the total number of first to third merging candidates reaches the predetermined maximum number of merging candidates. 
     Referring to  FIG.  11   , the total number of first and third merging candidates is four, and the predetermined maximum number of merging candidates is five, and therefore one second merging candidate (zero merging candidate) is derived. The second merging candidate is provided with a merging candidate index having a value of “4”, which is larger than those of the first and third merging candidates. 
     When the total number of first to third merging candidates is not below the maximum number (S 116 , No), the process proceeds to Step S 120  shown in  FIG.  10 A . 
     In this manner, the picture coding apparatus  100  according to Embodiment 1 performs the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. The picture coding apparatus  100  thereby controls the total number of first merging candidates to increase the variety of merging candidates. As a result, the picture coding apparatus  100  can code pictures with increased efficiency. 
     Furthermore, the second derivation unit  113  can repeat the second derivation process until the total number of first to third merging candidates reaches a predetermined maximum number of merging candidates. The second derivation unit  113  thereby derives merging candidates to the maximum number of merging candidates, and coding efficiency therefore increases. 
     Furthermore, merging candidates can be derived in descending order of reliability by performing the deriving in an order of spatial or temporal merging candidates as third merging candidates, combined merging candidates as first merging candidates, and zero merging candidates as second merging candidates as shown in  FIG.  11   . It is therefore more likely that derived merging candidates are more reliable. 
     The merging candidate derivation unit may assign merging candidate indices to merging candidates in such a manner that the merging candidate indices of combined merging candidates (first merging candidates) are larger than those of the spatial or temporal merging candidates (third merging candidates) and the merging candidate indices of zero merging candidates (second merging candidates) are larger than those of the combined merging candidates (first merging candidates) as shown in  FIG.  11   . The merging candidate derivation unit  110  thereby assigns indices having smaller values to merging candidates which are more likely to be selected, and therefore the amount of codes assigned to merging candidate indices is saved. 
     Note that the first to third merging candidates are not limited to combined merging candidates, zero merging candidates, or spatial or temporal merging candidates. Note also that the values of the indices assigned to the first to third merging candidates are not limited to the values of the indices shown in  FIG.  11   . 
     Note that the picture coding apparatus  100  need not derive third merging candidates in Embodiment 1. In other words, the merging candidate derivation unit  110  may not include the third derivation unit  111  shown in  FIG.  9   . In this case, the picture coding apparatus  100  skips Step S 111  and Step S 112  in the process shown in  FIG.  10 B . The process is performed without using third merging candidates in Step S 113  to Step S 116 . For example, in Step S 115 , the second derivation unit  113  determines whether or not the total number of first merging candidates is below a predetermined maximum number of merging candidates. 
     For example, the picture coding apparatus  100  may further derive fourth merging candidates. For example, the merging candidate derivation unit  110  may derive a scaling merging candidate as a fourth merging candidate when it is impossible to derive as many second merging candidates as to make the total number of first to third merging candidates equal to a maximum number of merging candidates. The process for deriving a scaling merging candidate will be described in detail in Embodiment 7. Note also that in Embodiment 1, the second derivation unit need not repeat the second derivation process until the total number of first to third merging candidates reaches a predetermined maximum number of merging candidates. For example, the total number of first to third merging candidates is not equal to a predetermined maximum number of merging candidates when the difference between the predetermined maximum number of merging candidates and the total number of first to third merging candidates is larger than the total number of second merging candidates which can be derived using the second derivation process. 
     Embodiment 2 
     Embodiment 2 will be described below. 
       FIG.  12    is a block diagram showing a configuration of a picture decoding apparatus  200  according to Embodiment 2. The picture decoding apparatus  200  is an apparatus corresponding to the picture coding apparatus  100  according to Embodiment 1. Specifically, for example, the picture decoding apparatus  200  decodes, on a block-by-block basis, coded pictures included in a bitstream generated by the picture coding apparatus  100  according to Embodiment 1. As shown in  FIG.  12   , the picture decoding apparatus  200  includes a merging candidate derivation unit  210 , a decoding unit  220 , and a prediction control unit  230 . 
     As with the merging candidate derivation unit  110  in Embodiment 1, the merging candidate derivation unit  210  derives merging candidates. The merging candidate derivation unit  210  generates a merging candidate list in which each of the derived merging candidates is associated with a merging candidate index. Specifically, the merging candidate derivation unit  210  includes a third derivation unit  211 , a first derivation unit  212 , and a second derivation unit  213  as shown in  FIG.  12   . 
     The third derivation unit  211  performs the same process as the process performed by the third derivation unit  111  in Embodiment 1. In other words, the third derivation unit  211  performs the third derivation process for deriving a third merging candidate using the third derivation method. Then, the third derivation unit  111  registers the third merging candidate in the merging candidate list in association with a merging candidate index. 
     Specifically, the third derivation unit  211  performs, as the third derivation process, a process for deriving a third merging candidate using, for example, a set of a prediction direction, a motion vector, and a reference picture index used for decoding of a block spatially or temporally neighboring a current block. 
     The first derivation unit  212  performs the same process as the process performed by the first derivation unit  112  in Embodiment 1. In other words, the first derivation unit  212  performs the first derivation process for deriving a first merging candidate, using the first derivation method. The first derivation unit  212  performs the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. Then, the first derivation unit  212  registers the first merging candidate in the merging candidate list in association with a merging candidate index. 
     Specifically, the first derivation unit  212  performs, as the first derivation process, a process for deriving, for example, a bi-predictive merging candidate as a first merging candidate by making a combination of sets each including a prediction direction, a motion vector, and a reference picture index. The sets are included the third merging candidates. 
     The term “bi-predictive” means prediction with reference to the first reference picture list and the second reference picture list. Note that being “bi-predictive” does not always involve references both to a temporally preceding reference picture and to a temporally following reference picture. In other words, a bi-predictive merging candidate may be coded and decoded with reference to reference pictures in the same direction (preceding reference pictures or following reference pictures). 
     The second derivation unit  213  performs the same process as the process performed by the second derivation unit  113  in Embodiment 1. In other words, the second derivation unit  213  performs a second derivation process for deriving a second merging candidate, using the second derivation method when the total number of first merging candidates and third merging candidates is smaller than a predetermined maximum number of merging candidates. Then, the second derivation unit  213  registers the second merging candidate in the merging candidate list in association with a merging candidate index. 
     Specifically, the second derivation unit  213  performs, as the second derivation process, a process for deriving, for example, a merging candidate including a motion vector which is a zero vector (zero merging candidate). In this case, the second derivation unit  213  performs the second derivation process using indices of referable pictures sequentially as reference picture indices included in zero merging candidates. 
     The decoding unit  220  obtains an index for identifying a merging candidate (merging candidate index) from a bitstream. For example, the decoding unit  220  obtains a merging candidate index by decoding, using the total number of first to third merging candidates or a predetermined maximum number of merging candidates, a merging candidate index coded and attached to a bitstream. 
     The prediction control unit  230  selects, using the index obtained by the decoding unit  220 , a merging candidate to be used for decoding a current block from the first to third merging candidates. In other words, the prediction control unit  230  selects a merging candidate from the merging candidate list. The selected merging candidate is to be used for generating a prediction picture of a current block to be decoded. 
     Operations of the picture decoding apparatus  200  in the above-described configuration will be described below. 
       FIG.  13 A  is a flowchart showing processing operations of the picture decoding apparatus  200  according to Embodiment 2. 
     First, the merging candidate derivation unit  210  derives merging candidates in the same manner as in Step S 110  in  FIG.  10 A  (S 210 ). 
     Next, the decoding unit  220  obtains a merging candidate index from a bitstream (S 220 ). For example, the decoding unit  220  obtains a merging candidate index by decoding a coded merging candidate index using the total number of first to third merging candidates (the number of merging candidates). 
     Optionally, the decoding unit  220  may obtain a merging candidate index by decoding a coded merging candidate index using a predetermined maximum number of merging candidates. In this case, the decoding unit  220  may obtain a merging candidate index (S 220 ) before the deriving of merging candidates (S 210 ). Alternatively, the decoding unit  220  may obtain a merging candidate index (S 220 ) in parallel with the deriving of merging candidates (S 210 ). 
     Next, the prediction control unit  230  selects, using the obtained merging candidate index, a merging candidate to be used for decoding a current block from the first to third merging candidates (S 230 ). 
     Step S 210  in  FIG.  13 A  will be described in detail below with reference to  FIG.  13 B . 
       FIG.  13 B  is a flowchart showing the deriving of merging candidates according to Embodiment 2. 
     First, the third derivation unit  111  performs the third derivation process in the same manner as in Step S 111  in  FIG.  10 B  (S 211 ). Next, the third derivation unit  211  determines whether or not to end the third derivation process (S 212 ). When the third derivation unit  211  determines not to end the third derivation process (S 212 , No), the third derivation unit  211  performs the third derivation process again (S 211 ). 
     When the third derivation unit  211  determines to end the third derivation process (S 212 , Yes), the first derivation unit  212  performs the first derivation process in the same manner as in Step S 113  in  FIG.  10 B  (S 213 ). Next, the first derivation unit  212  determines whether or not the total number of first merging candidates derived using the first derivation process is below a predetermined number (S 214 ). 
     When the total number of first merging candidates is not below the predetermined number (S 214 , No), the second derivation unit  213  performs the second derivation process in the same manner as in Step S 115  in  FIG.  10 B  (S 215 ). Next, the second derivation unit  213  determines whether or not the total number of first to third merging candidates is below a predetermined maximum number of merging candidates (S 216 ). 
     When the total number of first to third merging candidates is below the predetermined maximum number (S 216 , Yes), the second derivation unit  213  performs the second derivation process again (S 215 ). In other words, the second derivation unit  213  repeats the second derivation process until the total number of first to third merging candidates reaches the predetermined maximum number of merging candidates. 
     When the total number of first to third merging candidates is not below the maximum number (S 216 , No), the process proceeds to Step S 220  shown in  FIG.  13 A . 
     In this manner, the picture decoding apparatus  200  according to Embodiment 2 performs the first derivation process so that the total number of first merging candidates does not exceed a predetermined number. The picture decoding apparatus  200  thereby controls the total number of first merging candidates, and the variety of merging candidates thereby increases. As a result, the picture decoding apparatus  200  can appropriately decode a bitstream coded with increased coding efficiency. 
     Furthermore, the second derivation unit  213  can repeat the second derivation process until the total number of first to third merging candidates reaches a predetermined maximum number of merging candidates. The second derivation unit  213  thereby derives merging candidates to the maximum number of merging candidates, and coding efficiency therefore increases. The increase allows the picture decoding apparatus  200  to appropriately decode a bitstream coded with increased coding efficiency. 
     Note that the picture decoding apparatus  200  need not derive third merging candidates in Embodiment 2. In other words, the merging candidate derivation unit  210  may not include the third derivation unit  211  shown in  FIG.  12   . In this case, the picture decoding apparatus  200  skips Step S 211  and Step S 212  in the process shown in  FIG.  10 B . The process is performed without using third merging candidates in Step S 213  to Step S 216 . For example, in Step S 215 , the second derivation unit  213  determines whether or not the total number of first merging candidates is below a predetermined maximum number of merging candidates. 
     Embodiment 3 
     A picture coding apparatus according to Embodiment 3 will be specifically described below with reference to drawings. The picture coding apparatus according to Embodiment 3 is an example of possible applications of the picture coding apparatus according to Embodiment 1. 
       FIG.  14    is a block diagram showing a configuration of a picture coding apparatus  300  according to Embodiment 3. The picture coding apparatus  300  codes a picture on a block-by-block basis to generate a bitstream. 
     As shown in  FIG.  14   , the picture coding apparatus  300  includes a subtractor  301 , an orthogonal transformation unit  302 , a quantization unit  303 , an inverse-quantization unit  304 , an inverse-orthogonal transformation unit  305 , an adder  306   a  block memory  307 , a frame memory  308 , an intra prediction unit  309 , an inter prediction unit  310 , an inter prediction control unit  311 , a picture-type determination unit  312 , a switch  313 , a merging candidate derivation unit  314 , a colPic memory  315 , and a variable-length-coding unit  316 . 
     The subtractor  301  subtracts, on a block-by-block basis, prediction picture data from input picture data included in an input image sequence to generate prediction error data. 
     The orthogonal transformation unit  302  transforms the generated prediction error data from picture domain into frequency domain. 
     The quantization unit  303  quantizes the prediction error data in a frequency domain as a result of the transform. 
     The inverse-quantization unit  304  inverse-quantizes the prediction error data quantized by the quantization unit  303 . 
     The inverse-orthogonal-transformation unit  305  transforms the inverse-quantized prediction error data from frequency domain into picture domain. 
     The adder  306  generates reconstructed picture data by adding, on a block-by-block basis, prediction picture data and the prediction error data inverse-quantized by the inverse-orthogonal-transformation unit  305 . 
     The block memory  307  stores the reconstructed picture data in units of a block. 
     The frame memory  308  stores the reconstructed picture data in units of a frame. 
     The picture-type determination unit  312  determines in which of the picture types of I-picture, B-picture, and P-picture the input picture data is to be coded. Then, the picture-type determination unit  312  generates picture-type information indicating the determined picture type. 
     The intra prediction unit  309  generates intra prediction picture data of a current block by performing intra prediction using reconstructed picture data stored in the block memory  307  in units of a block. 
     The inter prediction unit  310  generates inter prediction picture data of a current block by performing inter prediction using reconstructed picture data stored in the frame memory  308  in units of a frame and a motion vector derived by a process including motion estimation. For example, when the merging mode is selected as a prediction mode to be used, the inter prediction unit  310  generates prediction picture data of a current block by performing inter prediction using a merging candidate. 
     When a current block is coded using intra prediction, the switch  313  outputs intra prediction picture data generated by the intra prediction unit  309  as prediction picture data of the current block to the subtractor  301  and the adder  306 . When a current block is coded using inter prediction, the switch  313  outputs inter prediction picture data generated by the inter prediction unit  310  as prediction picture data of the current block to the subtractor  301  and the adder  306 . 
     As with the merging candidate derivation unit  110  in Embodiment 1, the merging candidate derivation unit  314  derives merging candidates. Specifically, the merging candidate derivation unit  314  performs processes for deriving merging candidates (the first derivation process and the second derivation process) using at least two different derivation methods (the first derivation method and the second derivation method). For example, the merging candidate derivation unit  314  derives merging candidates using neighboring blocks of a current block and colPic information stored in the colPic memory  315 . The colPic information indicates information on a co-located block of the current block, such as a motion vector. 
     The merging candidate derivation unit  314  limits the total number of first merging candidates derived using the first derivation method but does not limit the total number of second merging candidates derived using the second derivation method. In other words, the merging candidate derivation unit  314  derives first merging candidates so that the total number of first merging candidates does not exceed a predetermined number. When the total number of derived first merging candidates is less than the size of a merging candidate list, the merging candidate derivation unit  314  derives second merging candidates until the total number of the derived first and second merging candidates becomes equivalent to the size of the merging candidate list. 
     In this manner, the total number of first merging candidates is limited and the total number of second merging candidates is not limited. The merging candidate derivation unit  314  can therefore derive a variety of merging candidates. Furthermore, the merging candidate derivation unit  314  derives merging candidates until the total number of the derived merging candidates becomes equivalent to the size of the merging candidate list. The merging candidate list is therefore more likely to include a merging candidate having a motion vector for accurate prediction. The merging candidate derivation unit  314  thereby contributes to increase in coding efficiency. 
     Furthermore, the merging candidate derivation unit  314  assigns merging candidate indices to the derived merging candidates. Then, the merging candidate derivation unit  314  transmits the merging candidates and the merging candidate indices to the inter prediction control unit  311 . Furthermore, the merging candidate derivation unit  314  transmits the total number of the derived merging candidates (the number of merging candidates) to the variable-length-coding unit  316 . 
     The inter prediction control unit  311  selects, from a prediction mode in which a motion vector derived by motion estimation is used (motion estimation mode) and a prediction mode in which a merging candidate is used (merging mode), a prediction mode which provides the smaller prediction error. Furthermore, the inter prediction control unit  313  transmits a merging flag indicating whether or not the selected prediction mode is the merging mode to the variable-length-coding unit  316 . Furthermore, when the selected prediction mode is the merging mode, the inter prediction control unit  311  transmits a merging candidate index corresponding to the selected merging candidate to the variable-length-coding unit  316 . Furthermore, the inter prediction control unit  311  transmits colPic information including a motion vector of the current block to the colPic memory  315 . 
     The variable-length-coding unit  316  generates a bitstream by performing variable-length coding on the quantized prediction error data, the merging flag, and the picture-type information. Furthermore, the variable-length-coding unit  316  sets the total number of the derived merging candidates as the size of the merging candidate list. Then, the variable-length-coding unit  316  performs variable-length coding on a bit sequence by assigning, according to the size of the merging candidate list, a bit sequence to the merging candidate index to be used for coding of the current block. 
     Operations of the picture coding apparatus  300  in the above-described configuration will be described below. 
       FIG.  15    is a flowchart showing processing operations of the picture coding apparatus  300  according to Embodiment 3. 
     In Step S 310 , the merging candidate derivation unit  314  derives merging candidates in the manner described in Embodiment 1. 
     In Step S 320 , the inter prediction control unit  311  selects a prediction mode based on comparison, using a method described later, between prediction error of a prediction picture generated using a motion vector derived by motion estimation and prediction error of a prediction picture generated using a merging candidate. The inter prediction control unit  311  sets the merging flag to “1” when the selected prediction mode is the merging mode, and sets the merging flag to “0” when otherwise. In Step S 330 , a determination is made as to whether or not the value of the merging flag is “1” (that is, the selected prediction mode is the merging mode). 
     When the result of the determination in Step S 330  is true (Yes, S 330 ), the variable-length-coding unit  316  attaches the merging flag to a bitstream in Step S 340 . In Step S 350 , the variable-length-coding unit  316  assigns a bit sequence according to the size of the merging candidate list as shown in  FIG.  5    to the merging candidate index of merging candidates to be used for coding of the current picture. Then, the variable-length-coding unit  316  performs variable-length coding on the assigned bit sequence. 
     When the result of the determination in Step S 330  is false (S 333 , No), the variable-length-coding unit  316  attaches a merging flag and information for motion estimation vector mode to a bitstream in Step S 360 . 
     Note that in Step S 350 , the variable-length-coding unit  316  need not attach a merging candidate index to a bitstream when, for example, the size of the merging candidate list is “1”. The amount of information on the merging candidate index is thereby reduced. 
       FIG.  16    is a flowchart showing details of the process in Step S 320  in  FIG.  15   . Specifically,  FIG.  16    illustrates a process for selecting a merging candidate.  FIG.  16    will be described below. 
     In Step S 321 , the inter prediction control unit  311  initializes settings for the process. Specifically, the inter prediction control unit  311  sets a merging candidate index at “0”, the minimum prediction error at the prediction error (cost) in the motion vector estimation mode, and a merging flag at “0”. The cost is calculated using the following equation for an R-D optimization model, for example.
 
Cost= D+λR   (Equation 1)
 
     In Equation 1, D denotes coding distortion. For example, D is the sum of absolute differences between original pixel values of a current block to be coded and pixel values obtained by coding and decoding of the current block using a prediction picture generated using a motion vector. R denotes the amount of generated codes. For example, R is the amount of codes necessary for coding a motion vector used for generation of a prediction picture. λ denotes an undetermined Lagrange multiplier. 
     In Step S 322 , the inter prediction control unit  311  determines whether or not the value of a merging candidate index is smaller than the total number of merging candidates of a current block. In other words, the inter prediction control unit  311  determines whether or not there is still any merging candidate on which the process from Step S 323  to Step S 325  has not been performed yet. 
     When the result of the determination in Step S 322  is true (S 322 , Yes), in Step S 323 , the inter prediction control unit  311  calculates the cost for a merging candidate to which a merging candidate index is assigned. Then, in Step S 324 , the inter prediction control unit  311  determines whether or not the calculated cost for the merging candidate is smaller than the minimum prediction error. 
     When the result of the determination in Step S 324  is true, (S 324 , Yes), the inter prediction control unit  311  updates the minimum prediction error, the merging candidate index, and the value of the merging flag in Step S 325 . When the result of the determination in Step S 324  is false (S 324 , No), the inter prediction control unit  311  does not update the minimum prediction error, the merging candidate index, or the value of the merging flag. 
     In Step S 326 , the inter prediction control unit  311  increments the merging candidate index by one, and repeats the process from Step S 322  to Step S 326 . 
     When the result of the determination in Step S 322  is false (Step S 322 , No), that is, when there is no more merging candidate on which this process has not been performed, the inter prediction control unit  311  settles the values of the merging flag and the merging candidate index in Step S 327 . 
     Note that in Embodiment 3, it is not always necessary in the merging mode to attach a merging flag to a bitstream. For example, a merging flag need not be attached to a bitstream when the merging mode is forcibly selected for a current block which satisfies a predetermined condition. This reduces the amount of information, and coding efficiency thereby increases. 
     Note that the picture coding apparatus according to Embodiment 3 is not limited to the example described therein where the merging mode is used in which a current block is coded using a prediction direction, a motion vector, and a reference picture index copied from a neighboring block of the current block. For example, a current block may be coded in skip merging mode. In the skip merging mode, a current block is coded using a merging candidate as in the merging mode. When all items in prediction error data are “0” for the current block, a skip flag is set at “1” and the skip flag and a merging candidate index are attached to a bitstream. When prediction error includes an item which is not “0” for a current block, a skip flag is set at “0” and the skip flag, a merging flag, a merging candidate index, and the prediction error data are attached to a bitstream. 
     Note that the picture coding apparatus according to Embodiment 3 is not limited to the example described therein in which a current block is coded using a merging candidate. For example, a motion vector in the motion vector estimation mode may be coded using a merging candidate. Specifically, a difference may be calculated by subtracting a motion vector of a merging candidate indicated by a merging candidate index from a motion vector in the motion vector estimation mode. Then, the difference and the merging candidate index are attached to a bitstream. Optionally, a difference may be calculated by scaling a motion vector MV_Merge of a merging candidate using a reference picture index RefIdx_ME in the motion vector estimation mode and a reference picture index RefIdx_Merge of the merging candidate as represented by Equation 2, and subtracting a motion vector scaledMV_Merge of the merging candidate after the scaling from the motion vector in the motion vector estimation mode. Then, the calculated difference and the merging candidate index are attached to a bitstream.
 
scaledMV_Merge=MV_Merge×(POC(RefIdx_ME)−curPOC)/(POC (RefIdx_Merge)−curPOC)  (Equation 2)
 
     Here, POC (RefIdx_ME) denotes the display order of reference picture indicated by a reference picture index RefIdx_ME. POC (RefIdx_Merge) denotes the display order of a reference picture indicated by a reference picture index RefIdx_Merge. curPOC denotes the display order of a current picture to be coded. 
     Embodiment 4 
     Embodiment 4 will be described below. 
       FIG.  17    is a block diagram showing a configuration of a picture decoding apparatus  400  according to Embodiment 4. The picture decoding apparatus  400  is an apparatus corresponding to the picture coding apparatus  300  according to Embodiment 3. Specifically, for example, the picture decoding apparatus  400  decodes, on a block-by-block basis, coded pictures included in a bitstream generated by the picture coding apparatus  300  according to Embodiment 3. 
     As shown in  FIG.  17   , the picture decoding apparatus  400  includes a variable-length decoding unit  401 , an inverse-quantization unit  402 , an inverse-orthogonal-transformation unit  403 , an adder  404 , a block memory  405 , a frame memory  406 , an intra prediction unit  407 , an inter prediction unit  408 , an inter prediction control unit  409 , a switch  410 , a merging candidate derivation unit  411 , and a colPic memory  412 . 
     The variable-length-decoding unit  401  generates picture-type information, a merging flag, and a quantized coefficient by performing variable-length decoding on an input bitstream. Furthermore, the variable-length-decoding unit  401  variable-length decodes a merging candidate index using the size of a merging candidate list. 
     The inverse-quantization unit  402  inverse-quantizes the quantized coefficient obtained by the variable-length decoding. 
     The inverse-orthogonal-transformation unit  403  generates prediction error data by transforming an orthogonal transform coefficient obtained by the inverse quantization from frequency domain into picture domain. 
     The block memory  405  stores, in units of a block, decoded picture data generated by adding prediction error data and prediction picture data. 
     The frame memory  406  stores decoded picture data in units of a frame. 
     The intra prediction unit  407  generates prediction picture data of a current block by performing intra prediction using the decoded picture data stored in the block memory  405  in units of a block. 
     The inter prediction unit  408  generates prediction picture data of a current block by performing inter prediction using the decoded picture data stored in the frame memory  406  in units of a frame. For example, when a merging flag is set to 1, the inter prediction unit  408  generates prediction picture data of a current block by performing inter prediction using a merging candidate. 
     The switch  410  outputs, as prediction picture data of a current block, intra prediction picture data generated by the intra prediction unit  407  or inter prediction picture data generated by the inter prediction unit  408  to the adder  404 . 
     The merging candidate derivation unit  411  performs processes for deriving merging candidates (the first derivation process and the second derivation process) using at least two different derivation methods (the first derivation method and the second derivation method) as in Embodiment 3. For example, the merging candidate derivation unit  411  derives merging candidates using neighboring blocks of a current block and colPic information stored in the colPic memory  412 . The colPic information indicates information on a co-located block of the current block, such as a motion vector. 
     The merging candidate derivation unit  411  limits the total number of first merging candidates derived using the first derivation method but does not limit the total number of second merging candidate derived using the second derivation method. In other words, the merging candidate derivation unit  411  derives first merging candidates so that the total number of first merging candidates does not exceed a predetermined number. When the total number of derived first merging candidates is less than the size of a merging candidate list, the merging candidate derivation unit  411  derives second merging candidates until the total number of the derived first and second merging candidates becomes equivalent to the size of the merging candidate list. 
     In this manner, the total number of first merging candidates is limited and the total number of second merging candidates is not limited. The merging candidate derivation unit  411  can therefore derive a variety of merging candidates. Furthermore, the merging candidate derivation unit  411  derives merging candidates until the total number of derived merging candidates becomes equivalent to the size of the merging candidate list. The merging candidate list is therefore more likely to include a merging candidate having a motion vector for accurate prediction. 
     Furthermore, the merging candidate derivation unit  411  assigns merging candidate indices to the derived merging candidates. Then, the merging candidate derivation unit  411  transmits the merging candidates and the merging candidate indices to the inter prediction control unit  409 . Furthermore, the merging candidate derivation unit  411  transmits the total number of the derived merging candidates (the number of merging candidates) to the variable-length-decoding unit  401 . 
     The inter prediction control unit  409  causes the inter prediction unit  408  to generate an inter prediction picture using information for motion vector estimation mode, when a decoded merging flag has a value of “0”. When a decoded merging flag has a value of “1”, the inter prediction control unit  409  selects, based on a decoded merging candidate index, a merging candidate for inter prediction from the derived merging candidates. Then, the inter prediction control unit  409  causes the inter prediction unit  408  to generate an inter prediction picture using the selected merging candidate. Furthermore, the inter prediction control unit  409  transfers colPic information including the motion vector of the current block to the colPic memory  412 . 
     Finally, the adder  404  generates decoded picture data by adding prediction picture data and prediction error data. 
       FIG.  18    is a flowchart showing processing operations of the picture decoding apparatus  400  according to Embodiment 4. 
     In Step S 414 , the variable-length-decoding unit  401  decodes a merging flag. 
     When it is determined in Step S 420  that the merging flag has a value of “1” (S 420 , Yes), a merging candidate is derived in Step S 430  using the same method as the method used in Step S 310  in  FIG.  15   . 
     In Step S 440 , the variable-length-decoding unit  401  performs variable-length decoding on a merging candidate index from a bitstream using the size of a merging candidate list. 
     In Step S 450 , the inter prediction control unit  409  generates inter prediction picture using a prediction direction, a motion vector, and a reference picture index which are included in the merging candidate indicated the decoded merging index. 
     When it is determined in Step S 420  that the merging flag has a value of “0” (S 420 , No), in Step S 460 , the inter prediction unit  408  generates an inter prediction picture using information for motion vector estimation mode decoded by the variable-length-decoding unit  401 . 
     Optionally, when the total number of merging candidates (the size of merging candidate list) derived in Step S 430  is “1”, a merging candidate index may be assumed to be “0” instead of being decoded. 
     Embodiment 5 
     In Embodiment 5, a process for deriving a zero merging candidate will be described in detail using drawings. The process for deriving a zero merging candidate described herein is an example of the first derivation process or the second derivation process. 
       FIG.  19    is a flowchart showing the process for deriving a zero merging candidate according to Embodiment 5. Specifically,  FIG.  19    shows part of processing operations of the merging candidate derivation unit  110 ,  210 ,  314 , or  411  in Embodiments 1 to 4. In other words,  FIG.  19    shows processing operations of the first derivation unit or the second derivation unit. 
     In Step S 501 , the merging candidate derivation unit updates the value of reference picture index refIdxL0 for the prediction direction 0 and the value of reference picture index refIdxL1 for the prediction direction 1 which are to be used for deriving a zero merging candidate. The reference picture indices refIdxL0 and refIdxL1 each have an initial value of “−1”, and are incremented by “+1” each time the process in Step S 501  is performed. 
     Specifically, in the first cycle of the process for deriving a merging candidate, a zero merging candidate including a motion vector having a value of zero (zero vector) and a reference picture index having a value of 0 is added to a merging candidate list as a zero merging candidate for stationary region. Next, in the second cycle of the process for deriving a merging candidate, a zero merging candidate including a motion vector having a value of zero (zero vector) and a reference picture index having a value of 1 is added to a merging candidate list. 
     In Step S 502 , the merging candidate derivation unit determines whether it is true or false that (i) the updated value of the reference picture index refIdxL0 for the prediction direction 0 is smaller than a maximum number of reference pictures in the reference picture list 0 for the prediction direction 0 and (ii) the updated value of the reference picture index refIdxL1 for the prediction direction 1 is smaller than a maximum number of reference pictures in the reference picture list 1 for the prediction direction 1. 
     When the result of the determination in Step S 502  is true, (S 502 , Yes), the merging candidate derivation unit assigns a motion vector (0, 0) and the reference picture index refIdxL0 to the motion vector and reference picture index for the prediction direction 0 of the zero merging candidate in Step S 503 . Moreover, in Step S 504 , the merging candidate derivation unit assigns a motion vector (0, 0) and the reference picture index refIdxL1 to the motion vector and reference picture index for the prediction direction 1 of the zero merging candidate. 
     The merging candidate derivation unit thereby derives a bi-predictive zero merging candidate by the processes in Step S 503  and Step S 504 .  FIG.  20    shows an example of a derived zero merging candidate. 
     In Step S 505 , the merging candidate derivation unit determines whether or not the merging candidate list already includes a merging candidate which is identical in prediction direction, motion vector, and reference picture index to the derived zero merging candidate. In other words, the merging candidate derivation unit determines whether or not the derived zero merging candidate is an identical candidate. 
     When the result of Step S 505  is false (S 505 , No), the merging candidate derivation unit registers the derived zero merging candidate in the merging candidate list in Step S 506 . 
     When the result of the determination in Step S 502  is false (S 502 , No) or the result of the determination in Step S 505  is true (S 505 , Yes), the merging candidate derivation unit does not register the derived zero merging candidate in the merging candidate list in Step S 506 . 
     The merging candidate derivation unit thereby derives a zero merging candidate which has a motion vector having zero values to referable reference pictures. Next, the merging candidate derivation unit adds the derived zero merging candidate to the merging candidate list. The picture coding apparatus thus can increase efficiency of coding in merging mode especially when a current block to be coded is a stationary region. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 5, in which a bi-predictive zero merging candidate is derived using a motion vector having zero values, a reference picture index for the prediction direction 0, and a reference picture index for the prediction direction 1. For example, the merging candidate derivation unit may derive a zero merging candidate for the prediction direction 0 using a motion vector having zero values and a reference picture index for the prediction direction 0. Similarly, the merging candidate derivation unit may derive a zero merging candidate for the prediction direction 1 using a motion vector having zero values and a reference picture index for the prediction direction 1. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 5, in which zero merging candidates are derived using reference picture indices starting from the value of 0 and incremented by +1. For example, the merging candidate derivation unit may derive zero merging candidates using reference picture indices in ascending order of distance from a current picture to reference pictures in display order. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 5, in which the merging candidate derivation unit determines in Step S 505  in  FIG.  19    whether or not a zero merging candidate is an identical candidate. For example, the merging candidate derivation unit may skip the determination in Step S 505 . This reduces computational complexity in deriving a merging candidate for the merging candidate derivation unit. 
     The merging candidate derivation unit according to Embodiment 5 thereby derives, as a first merging candidate or a second merging candidate, a merging candidate including zero vectors which are motion vectors for a stationary region, and coding efficiency therefore increases. More specifically, the merging candidate derivation unit derives a merging candidate including a motion vector which is a zero vector to a referable reference picture, and newly registers the derived merging candidate in a merging candidate list. The merging candidate derived in this manner is reliable when the current block is a stationary region, and coding efficiency therefore increases. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 5, in which a derived merging candidate includes a motion vector for a stationary region which is a zero vector. For example, a derived merging candidate may include a motion vector having a value slightly larger or smaller than a zero vector (0, 0) (for example, a motion vector (0, 1)) with consideration for small camera shake during video shooting. Optionally, a derived merging candidate may have a motion vector (OffsetX, OffsetY) which is provided by adding an offset parameter (OffsetX, OffsetY) to a header or the like of a sequence, a picture, or a slice. 
     Embodiment 6 
     In Embodiment 6, a process for deriving a combined merging candidate will be described in detail using a drawing. The process for deriving a combined merging candidate described herein is an example of the first derivation process or the second derivation process. 
       FIG.  21    is a flowchart showing the process for deriving a combined merging candidate according to Embodiment 6. Specifically,  FIG.  21    shows part of processing operations of the merging candidate derivation unit  110 ,  210 ,  314 , or  411  in Embodiments 1 to 4. In other words,  FIG.  21    shows processing operations of the first derivation unit or the second derivation unit. 
     In Step S 601 , the merging candidate derivation unit updates merging candidate indices idx1 and idx2. The merging candidate indices idx1 and idx2 are indices for determining two merging candidates to be used for deriving a combined merging candidate. 
     For example, the merging candidate derivation unit updates merging candidate indices idx1 and idx2 to “0” and “1”, respectively. In this case, the merging candidate derivation unit performs Steps S 602  to S 610  described below to derive a combined merging candidate by combining a set of a prediction direction, a motion vector, and a reference picture index included in a merging candidate [0] and a set of a prediction direction, a motion vector, and a reference picture index included in a merging candidate [1]. The merging candidate [0] is a merging candidate provided with a merging candidate index having a value of 0 in a merging candidate list, and the merging candidate [1] is a merging candidate provided with a merging candidate index having a value of 1 in the merging candidate list. The merging candidate derivation unit updates merging candidate indices idx1 and idx2 in Step S 601  for each cycle of derivation of a combined merging candidate. Note that details of the process for updating the merging candidate indices idx1 and idx2 is not limited to a specific procedure. Any procedure is applicable through which a combined merging candidate is derived using any combination of merging candidates derived before the derivation of the combined merging candidate. 
     In Step S 602 , the merging candidate derivation unit determines whether it is true or false that (1) the values of the merging candidate indices idx1 and idx2 are not identical, (2) a merging candidate [idx1] is not a combined merging candidate, and (3) a merging candidate [idx2] is not a combined merging candidate. 
     When the result of the determination in Step S 602  is true (S 142 , Yes), the merging candidate derivation unit determines in Step S 603  whether at least one of the following is true: (1) the prediction directions of the merging candidate [idx1] and the merging candidate [idx2] are different; and (2) both the merging candidate [idx1] and the merging candidate [idx2] are bi-predictive. When the result of the determination in Step S 603  is true, (S 603 , Yes), the merging candidate derivation unit determines in Step S 604  whether both of the following are true: (1) the merging candidate [idx1] is a merging candidate for the prediction direction 0 or bi-predictive; and (2) the merging candidate [idx2] is a merging candidate for the prediction direction 1 or bi-predictive. In other words, the merging candidate derivation unit determines whether it is true or false that the merging candidate [idx1] includes at least a motion vector having the prediction direction 0, and the merging candidate [idx2] includes at least a motion vector having the prediction direction 1. 
     When the result of the determination in Step S 604  is true (S 604 , Yes), the merging candidate derivation unit in Step S 605  assigns the motion vector and reference picture index for the prediction direction 0 which are included in the merging candidate [idx1] to the motion vector and reference picture index for the prediction direction 0 of the combined merging candidate. Moreover, in Step S 606 , the merging candidate derivation unit assigns the motion vector and reference picture index for the prediction direction 1 which are included in the merging candidate [idx2] to the motion vector and reference picture index for the prediction direction 1 of the combined merging candidate. The merging candidate derivation unit thereby derives a bi-predictive combined merging candidate. 
     When the result of the determination in Step S 604  is false (S 604 , No), the merging candidate derivation unit in Step S 607  assigns the motion vector and reference picture index for the prediction direction 0 which are included in the merging candidate [idx2] to the motion vector and reference picture index for the prediction direction 0 of the combined merging candidate. Moreover, in Step S 608 , the merging candidate derivation unit assigns the motion vector and reference picture index for the prediction direction 1 which are included in the merging candidate [idx1] to the motion vector and reference picture index for the prediction direction 1 of the combined merging candidate. The merging candidate derivation unit thereby derives a bi-predictive combined merging candidate. 
     In Step S 609 , the merging candidate derivation unit determines whether or not the merging candidate list already includes a merging candidate which is identical in prediction direction, motion vector, and reference picture index to the derived combined merging candidate. In other words, the merging candidate derivation unit determines whether or not the derived combined merging candidate is an identical candidate. 
     When the result of Step S 609  is false (S 609 , No), the merging candidate derivation unit registers the derived combined merging candidate in the merging candidate list in Step S 610 . 
     When the result of the determination in Step S 602  or Step S 603  is false (S 602  or S 603 , No), the merging candidate derivation unit does not register the derived combined merging candidate in the merging candidate list. 
     In this manner, the merging candidate derivation unit derives a combined merging candidate and registers the derived combined merging candidate in a merging candidate list. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 6, in which the merging candidate derivation unit determines in Step S 609  whether or not a combined merging candidate is an identical candidate. For example, the merging candidate derivation unit may skip the determination in Step S 609 . This reduces computational complexity in deriving a merging candidate for the merging candidate derivation unit. 
     In this manner, the merging candidate derivation unit according to Embodiment 6 derives a bi-predictive merging candidate by making a combination from previously derived merging candidates. The merging candidate derivation unit is thus capable of deriving a new bi-predictive first merging candidate even when previously derived merging candidates include no bi-predictive merging candidate. As a result, the merging candidate derivation unit increases the variety of merging candidates, and coding efficiency thereby increases. 
     Embodiment 7 
     In Embodiment 7, a process for deriving a scaling merging candidate will be described in detail using drawings. The process for deriving a scaling merging candidate described herein is an example of the first derivation process or the second derivation process. 
       FIG.  22    is a flowchart showing the process for deriving a scaling merging candidate according to Embodiment 7. Specifically,  FIG.  22    shows part of processing operations of the merging candidate derivation unit  110 ,  210 ,  314 , or  411  in Embodiments 1 to 4. In other words,  FIG.  22    shows processing operations of the first derivation unit or the second derivation unit. 
     In Step S 701 , the merging candidate derivation unit updates a prediction direction index X. In Step S 702 , the merging candidate derivation unit updates a merging candidate index idx. The prediction direction index X and the merging candidate index idx are indices for determination of a prediction direction and a merging candidate which are used for deriving a scaling merging candidate. 
     For example, the merging candidate derivation unit updates the prediction direction index X to “0” and the merging candidate index idx to “0”. In this case, the merging candidate derivation unit performs Steps S 702  to S 711  described below to derive a scaling merging candidate using a motion vector and a reference picture index for a prediction direction 0 included in a merging candidate [0], which is provided with a merging candidate index of 0 in a merging candidate list. The merging candidate derivation unit updates the prediction direction X in Step S 701  and the merging candidate index idx in Step S 702  for each cycle of derivation of a scaling merging candidate. 
     In Step S 703 , the merging candidate derivation unit determines whether it is true or false that (i) the merging candidate [idx] is not a scaling merging candidate and (ii) the merging candidate [idx] includes a motion vector having a prediction direction X. When the result of the determination in Step S 703  is true (S 703 , Yes), the merging candidate derivation unit in Step S 704  calculates a motion vector mvL(1−X) and a reference picture index refIdxL(1−X) for a prediction direction (1−X) using the motion vector mvLX and reference picture index refIdxLX for the prediction direction X which are included in the merging candidate [idx]. For example, the merging candidate derivation unit calculates the mvL(1−X) and refIdxL(1−X) using Equations 2 and 3 shown below.
 
refIdx L (1 −X )=refIdx LX   (Equation 3)
 
mv L (1 −X )=mv LX ×(POC(refIdx L (1 −X ))−curPOC)/(POC (refIdx LX )−curPOC)  (Equation 4)
 
     POC (refIdxLX) denotes the display order of a reference picture indicated by a reference picture index refIdxLX. POC (refIdxLX (1−X)) denotes the display order of a reference picture indicated by a reference picture index refIdxLX (1−X). curPOC denotes the display order of a current picture to be coded. 
       FIG.  23    shows an example of a motion vector and a reference picture index calculated in Embodiment 7. As shown in  FIG.  23   , the merging candidate derivation unit performs scaling using a motion vector mvLX and a reference picture index refIdxLX, which are a motion vector and a reference picture index for one prediction direction (prediction direction X) and included in a merging candidate, to calculate a motion vector mvL(1−X) and a reference picture index refIdxL(1−X), which are a motion vector and a reference picture index for the other prediction direction (a prediction direction (1−X)). 
     In Step S 705 , the merging candidate derivation unit determines whether or not the value of the prediction direction index X is “0”. When the result of the determination in Step S 705  is true (S 705 , Yes), the merging candidate derivation unit in Step S 706  assigns the motion vector and reference picture index for the prediction direction 0 which are included in the merging candidate [idx1] to the motion vector and reference picture index for the prediction direction 0 of the scaling merging candidate. Moreover, in Step S 707 , the merging candidate derivation unit assigns the calculated motion vector mvL(1−X) and reference picture index refIdxL1(1−X) for the prediction direction (1−X) to the motion vector and reference picture index for the prediction direction 1 of the scaling merging candidate. The merging candidate derivation unit thereby derives a bi-predictive scaling merging candidate. 
     When the result of the determination in Step S 705  is false (that is, when the value of the prediction direction X is “1”) (S 705 , No), the merging candidate derivation unit in Step S 708  assigns the calculated motion vector mvL(1−X) and reference picture index refIdxL1(1−X) for the prediction direction (1−X) to the motion vector and reference picture index for the prediction direction 0 of the scaling merging candidate. Moreover, in Step S 709 , the merging candidate derivation unit assigns the motion vector and reference picture index for the prediction direction X which are included in the merging candidate [idx] to the motion vector and reference picture index for the prediction direction 1 of the scaling merging candidate. The merging candidate derivation unit thereby derives a bi-predictive scaling merging candidate. 
     In Step S 710 , the merging candidate derivation unit determines whether or not the merging candidate list already includes a merging candidate which is identical in prediction direction, motion vector, and reference picture index to the derived scaling merging candidate. In other words, the merging candidate derivation unit determines whether or not the derived scaling merging candidate is an identical candidate. 
     When the result of Step S 710  is false (S 710 , No), the merging candidate derivation unit registers the derived scaling merging candidate in the merging candidate list in Step S 711 . 
     When the result of the determination in Step S 703  is false (S 703 , No), or when the result of the determination in Step S 710  is true (S 710 , Yes), the merging candidate derivation unit does not register the derived scaling merging candidate in the merging candidate list. 
     In this manner, the merging candidate derivation unit derives a scaling merging candidate and registers the derived scaling merging candidate in a merging candidate list. 
     Note that the merging candidate derivation unit need not add a derived scaling merging candidate to a merging candidate list when POC (refIdxLX) and POC (refIdxL(1−X)) are identical (that is, refIdxLX and refIdxL(1−X) indicates the same picture), and thus providing mvL(1−X) and mvLX having the same values. Also note that when the value of a calculated refIdxL(1−X) is not included in a reference picture list L(1−X), the merging candidate derivation unit need not register a scaling merging candidate in a merging candidate list. 
     Optionally, the merging candidate derivation unit may calculate mvL(1−X) by directly assigning−mvLX to mvL(1−X) only when a condition that the values of POC (refIdxLX) and POC (refIdxL(1−X)) are different and a condition that the absolute values of (POC (refIdxL(1−X))−curPOC) and (POC (refIdxLX)−curPOC) are equal are both satisfied. The former condition is satisfied when the picture indicated by refIdxLX and the picture indicated by refIdxL(1−X) are different. The latter condition is satisfied when the picture indicated by refIdxLX and the picture indicated by refIdxL(1−X) are equidistant in display order from the current picture. When both are satisfied, mvL(1−X) is the inverse vector of mvLX. When this is the case, the merging candidate derivation unit can derive a scaling merging candidate without performing the scaling represented by Equation 4. Coding efficiency thereby increases with a small increase in computational complexity. 
     Note that the picture coding apparatus is not limited to the example described in Embodiment 7, in which the merging candidate derivation unit determines in Step S 710  whether or not a scaling merging candidate is an identical candidate. For example, the merging candidate derivation unit may skip the determination in Step S 710 . This reduces computational complexity in deriving a merging candidate for the merging candidate derivation unit. 
     Although the picture coding apparatus and picture decoding apparatus according to one or more aspects of the present disclosure have been described using exemplary embodiments, the present invention is not limited to the exemplary embodiments. Those skilled in the art will readily appreciate that many modifications of the exemplary embodiments or embodiments in which the constituent elements of the exemplary embodiments are combined are possible without materially departing from the novel teachings and advantages described in the present disclosure. All such modifications and embodiments are also within scopes of the one or more aspects. 
     In the exemplary embodiments, each of the constituent elements may be implemented as a piece of dedicated hardware or implemented by executing a software program appropriate for the constituent element. The constituent elements may be implemented by a program execution unit such as a CPU or a processor which reads and executes a software program recorded on a recording medium such as a hard disk or a semiconductor memory. Here, examples of the software program which implements the picture coding apparatus or the picture decoding apparatus in the embodiments include a program as follows. 
     One is a program which causes a computer to execute a picture coding method for coding a picture on a block-by-block basis to generate a bitstream, and the method includes: performing a first derivation process for deriving a first merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in coding of a current block; performing a second derivation process for deriving a second merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the coding of the current block, the second derivation process being different from the first derivation process; selecting a merging candidate to be used in the coding of the current block from among the first merging candidate and the second merging candidate; and attaching an index for identifying the selected merging candidate to the bitstream, wherein in the performing of a first derivation process, the first derivation process is performed so that a total number of the first merging candidates does not exceed a predetermined number, and the second derivation process is performed when the total number of the first merging candidates is less than a predetermined maximum number of merging candidates. 
     Another is a program which causes a computer to execute a picture decoding method for decoding, on a block-by-block basis, a coded image included in a bitstream, and the method includes: performing a first derivation process for deriving a first merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in decoding of a current block; performing a second derivation process for deriving a second merging candidate which includes a candidate set of a prediction direction, a motion vector, and a reference picture index for use in the decoding of the current block, the second derivation process being different from the first derivation process; obtaining an index from the bitstream; and selecting, based on the obtained index, a merging candidate to be used in the decoding of the current block from among the first merging candidate and the second merging candidate, wherein in the performing of a first derivation process, the first derivation process is performed so that a total number of the first merging candidates does not exceed a predetermined number, and the second derivation process is performed when the total number of the first merging candidates is less than a predetermined maximum number of merging candidates. 
     Embodiment 8 
     The processing described in each of embodiments can be simply implemented in an independent computer system, by recording, in a recording medium, a program for implementing the configurations of the moving picture coding method (image coding method) and the moving picture decoding method (image decoding method) described in each of embodiments. The recording media may be any recording media as long as the program can be recorded, such as a magnetic disk, an optical disk, a magnetic optical disk, an IC card, and a semiconductor memory. 
     Hereinafter, the applications to the moving picture coding method (image coding method) and the moving picture decoding method (image decoding method) described in each of embodiments and systems using thereof will be described. The system has a feature of having an image coding and decoding apparatus that includes an image coding apparatus using the image coding method and an image decoding apparatus using the image decoding method. Other configurations in the system can be changed as appropriate depending on the cases. 
       FIG.  24    illustrates an overall configuration of a content providing system ex 100  for implementing content distribution services. The area for providing communication services is divided into cells of desired size, and base stations ex 106 , ex 107 , ex 108 , ex 109 , and ex 110  which are fixed wireless stations are placed in each of the cells. 
     The content providing system ex 100  is connected to devices, such as a computer ex 111 , a personal digital assistant (PDA) ex 112 , a camera ex 113 , a cellular phone ex 114  and a game machine ex 115 , via the Internet ex 101 , an Internet service provider ex 102 , a telephone network ex 104 , as well as the base stations ex 106  to ex 110 , respectively. 
     However, the configuration of the content providing system ex 100  is not limited to the configuration shown in  FIG.  24   , and a combination in which any of the elements are connected is acceptable. In addition, each device may be directly connected to the telephone network ex 104 , rather than via the base stations ex 106  to ex 110  which are the fixed wireless stations. Furthermore, the devices may be interconnected to each other via a short distance wireless communication and others. 
     The camera ex 113 , such as a digital video camera, is capable of capturing video. A camera ex 116 , such as a digital camera, is capable of capturing both still images and video. Furthermore, the cellular phone ex 114  may be the one that meets any of the standards such as Global System for Mobile Communications (GSM) (registered trademark), Code Division Multiple Access (CDMA), Wideband-Code Division Multiple Access (W-CDMA), Long Term Evolution (LTE), and High Speed Packet Access (HSPA). Alternatively, the cellular phone ex 114  may be a Personal Handyphone System (PHS). 
     In the content providing system ex 100 , a streaming server ex 103  is connected to the camera ex 113  and others via the telephone network ex 104  and the base station ex 109 , which enables distribution of images of a live show and others. In such a distribution, a content (for example, video of a music live show) captured by the user using the camera ex 113  is coded as described above in each of embodiments (i.e., the camera functions as the image coding apparatus according to an aspect of the present disclosure), and the coded content is transmitted to the streaming server ex 103 . On the other hand, the streaming server ex 103  carries out stream distribution of the transmitted content data to the clients upon their requests. The clients include the computer ex 111 , the PDA ex 112 , the camera ex 113 , the cellular phone ex 114 , and the game machine ex 115  that are capable of decoding the above-mentioned coded data. Each of the devices that have received the distributed data decodes and reproduces the coded data (i.e., functions as the image decoding apparatus according to an aspect of the present disclosure). 
     The captured data may be coded by the camera ex 113  or the streaming server ex 103  that transmits the data, or the coding processes may be shared between the camera ex 113  and the streaming server ex 103 . Similarly, the distributed data may be decoded by the clients or the streaming server ex 103 , or the decoding processes may be shared between the clients and the streaming server ex 103 . Furthermore, the data of the still images and video captured by not only the camera ex 113  but also the camera ex 116  may be transmitted to the streaming server ex 103  through the computer ex 111 . The coding processes may be performed by the camera ex 116 , the computer ex 111 , or the streaming server ex 103 , or shared among them. 
     Furthermore, the coding and decoding processes may be performed by an LSI ex 500  generally included in each of the computer ex 111  and the devices. The LSI ex 500  may be configured of a single chip or a plurality of chips. Software for coding and decoding video may be integrated into some type of a recording medium (such as a CD-ROM, a flexible disk, and a hard disk) that is readable by the computer ex 111  and others, and the coding and decoding processes may be performed using the software. Furthermore, when the cellular phone ex 114  is equipped with a camera, the video data obtained by the camera may be transmitted. The video data is data coded by the LSI ex 500  included in the cellular phone ex 114 . 
     Furthermore, the streaming server ex 103  may be composed of servers and computers, and may decentralize data and process the decentralized data, record, or distribute data. 
     As described above, the clients may receive and reproduce the coded data in the content providing system ex 100 . In other words, the clients can receive and decode information transmitted by the user, and reproduce the decoded data in real time in the content providing system ex 100 , so that the user who does not have any particular right and equipment can implement personal broadcasting. 
     Aside from the example of the content providing system ex 100 , at least one of the moving picture coding apparatus (image coding apparatus) and the moving picture decoding apparatus (image decoding apparatus) described in each of embodiments may be implemented in a digital broadcasting system ex 200  illustrated in  FIG.  25   . More specifically, a broadcast station ex 201  communicates or transmits, via radio waves to a broadcast satellite ex 202 , multiplexed data obtained by multiplexing audio data and others onto video data. The video data is data coded by the moving picture coding method described in each of embodiments (i.e., data coded by the image coding apparatus according to an aspect of the present disclosure). Upon receipt of the multiplexed data, the broadcast satellite ex 202  transmits radio waves for broadcasting. Then, a home-use antenna ex 204  with a satellite broadcast reception function receives the radio waves. Next, a device such as a television (receiver) ex 300  and a set top box (STB) ex 217  decodes the received multiplexed data, and reproduces the decoded data (i.e., functions as the image decoding apparatus according to an aspect of the present disclosure). 
     Furthermore, a reader/recorder ex 218  ( i ) reads and decodes the multiplexed data recorded on a recording medium ex 215 , such as a DVD and a BD, or (i) codes video signals in the recording medium ex 215 , and in some cases, writes data obtained by multiplexing an audio signal on the coded data. The reader/recorder ex 218  can include the moving picture decoding apparatus or the moving picture coding apparatus as shown in each of embodiments. In this case, the reproduced video signals are displayed on the monitor ex 219 , and can be reproduced by another device or system using the recording medium ex 215  on which the multiplexed data is recorded. It is also possible to implement the moving picture decoding apparatus in the set top box ex 217  connected to the cable ex 203  for a cable television or to the antenna ex 204  for satellite and/or terrestrial broadcasting, so as to display the video signals on the monitor ex 219  of the television ex 300 . The moving picture decoding apparatus may be implemented not in the set top box but in the television ex 300 . 
       FIG.  26    illustrates the television (receiver) ex 300  that uses the moving picture coding method and the moving picture decoding method described in each of embodiments. The television ex 300  includes: a tuner ex 301  that obtains or provides multiplexed data obtained by multiplexing audio data onto video data, through the antenna ex 204  or the cable ex 203 , etc. that receives a broadcast; a modulation/demodulation unit ex 302  that demodulates the received multiplexed data or modulates data into multiplexed data to be supplied outside; and a multiplexing/demultiplexing unit ex 303  that demultiplexes the modulated multiplexed data into video data and audio data, or multiplexes video data and audio data coded by a signal processing unit ex 306  into data. 
     The television ex 300  further includes: a signal processing unit ex 306  including an audio signal processing unit ex 304  and a video signal processing unit ex 305  that decode audio data and video data and code audio data and video data, respectively (which function as the image coding apparatus and the image decoding apparatus according to the aspects of the present disclosure); and an output unit ex 309  including a speaker ex 307  that provides the decoded audio signal, and a display unit ex 308  that displays the decoded video signal, such as a display. Furthermore, the television ex 300  includes an interface unit ex 317  including an operation input unit ex 312  that receives an input of a user operation. Furthermore, the television ex 300  includes a control unit ex 310  that controls overall each constituent element of the television ex 300 , and a power supply circuit unit ex 311  that supplies power to each of the elements. Other than the operation input unit ex 312 , the interface unit ex 317  may include: a bridge ex 313  that is connected to an external device, such as the reader/recorder ex 218 ; a slot unit ex 314  for enabling attachment of the recording medium ex 216 , such as an SD card; a driver ex 315  to be connected to an external recording medium, such as a hard disk; and a modem ex 316  to be connected to a telephone network. Here, the recording medium ex 216  can electrically record information using a non-volatile/volatile semiconductor memory element for storage. The constituent elements of the television ex 300  are connected to each other through a synchronous bus. 
     First, the configuration in which the television ex 300  decodes multiplexed data obtained from outside through the antenna ex 204  and others and reproduces the decoded data will be described. In the television ex 300 , upon a user operation through a remote controller ex 220  and others, the multiplexing/demultiplexing unit ex 303  demultiplexes the multiplexed data demodulated by the modulation/demodulation unit ex 302 , under control of the control unit ex 310  including a CPU. Furthermore, the audio signal processing unit ex 304  decodes the demultiplexed audio data, and the video signal processing unit ex 305  decodes the demultiplexed video data, using the decoding method described in each of embodiments, in the television ex 300 . The output unit ex 309  provides the decoded video signal and audio signal outside, respectively. When the output unit ex 309  provides the video signal and the audio signal, the signals may be temporarily stored in buffers ex 318  and ex 319 , and others so that the signals are reproduced in synchronization with each other. Furthermore, the television ex 300  may read multiplexed data not through a broadcast and others but from the recording media ex 215  and ex 216 , such as a magnetic disk, an optical disk, and a SD card. Next, a configuration in which the television ex 300  codes an audio signal and a video signal, and transmits the data outside or writes the data on a recording medium will be described. In the television ex 300 , upon a user operation through the remote controller ex 220  and others, the audio signal processing unit ex 304  codes an audio signal, and the video signal processing unit ex 305  codes a video signal, under control of the control unit ex 310  using the coding method described in each of embodiments. The multiplexing/demultiplexing unit ex 303  multiplexes the coded video signal and audio signal, and provides the resulting signal outside. When the multiplexing/demultiplexing unit ex 303  multiplexes the video signal and the audio signal, the signals may be temporarily stored in the buffers ex 320  and ex 321 , and others so that the signals are reproduced in synchronization with each other. Here, the buffers ex 318 , ex 319 , ex 320 , and ex 321  may be plural as illustrated, or at least one buffer may be shared in the television ex 300 . Furthermore, data may be stored in a buffer so that the system overflow and underflow may be avoided between the modulation/demodulation unit ex 302  and the multiplexing/demultiplexing unit ex 303 , for example. 
     Furthermore, the television ex 300  may include a configuration for receiving an AV input from a microphone or a camera other than the configuration for obtaining audio and video data from a broadcast or a recording medium, and may code the obtained data. Although the television ex 300  can code, multiplex, and provide outside data in the description, it may be capable of only receiving, decoding, and providing outside data but not the coding, multiplexing, and providing outside data. 
     Furthermore, when the reader/recorder ex 218  reads or writes multiplexed data from or on a recording medium, one of the television ex 300  and the reader/recorder ex 218  may decode or code the multiplexed data, and the television ex 300  and the reader/recorder ex 218  may share the decoding or coding. 
     As an example,  FIG.  27    illustrates a configuration of an information reproducing/recording unit ex 400  when data is read or written from or on an optical disk. The information reproducing/recording unit ex 400  includes constituent elements ex 401 , ex 402 , ex 403 , ex 404 , ex 405 , ex 406 , and ex 407  to be described hereinafter. The optical head ex 401  irradiates a laser spot in a recording surface of the recording medium ex 215  that is an optical disk to write information, and detects reflected light from the recording surface of the recording medium ex 215  to read the information. The modulation recording unit ex 402  electrically drives a semiconductor laser included in the optical head ex 401 , and modulates the laser light according to recorded data. The reproduction demodulating unit ex 403  amplifies a reproduction signal obtained by electrically detecting the reflected light from the recording surface using a photo detector included in the optical head ex 401 , and demodulates the reproduction signal by separating a signal component recorded on the recording medium ex 215  to reproduce the necessary information. The buffer ex 404  temporarily holds the information to be recorded on the recording medium ex 215  and the information reproduced from the recording medium ex 215 . The disk motor ex 405  rotates the recording medium ex 215 . The servo control unit ex 406  moves the optical head ex 401  to a predetermined information track while controlling the rotation drive of the disk motor ex 405  so as to follow the laser spot. The system control unit ex 407  controls overall the information reproducing/recording unit ex 400 . The reading and writing processes can be implemented by the system control unit ex 407  using various information stored in the buffer ex 404  and generating and adding new information as necessary, and by the modulation recording unit ex 402 , the reproduction demodulating unit ex 403 , and the servo control unit ex 406  that record and reproduce information through the optical head ex 401  while being operated in a coordinated manner. The system control unit ex 407  includes, for example, a microprocessor, and executes processing by causing a computer to execute a program for read and write. 
     Although the optical head ex 401  irradiates a laser spot in the description, it may perform high-density recording using near field light. 
       FIG.  28    illustrates the recording medium ex 215  that is the optical disk. On the recording surface of the recording medium ex 215 , guide grooves are spirally formed, and an information track ex 230  records, in advance, address information indicating an absolute position on the disk according to change in a shape of the guide grooves. The address information includes information for determining positions of recording blocks ex 231  that are a unit for recording data. Reproducing the information track ex 230  and reading the address information in an apparatus that records and reproduces data can lead to determination of the positions of the recording blocks. Furthermore, the recording medium ex 215  includes a data recording area ex 233 , an inner circumference area ex 232 , and an outer circumference area ex 234 . The data recording area ex 233  is an area for use in recording the user data. The inner circumference area ex 232  and the outer circumference area ex 234  that are inside and outside of the data recording area ex 233 , respectively are for specific use except for recording the user data. The information reproducing/recording unit  400  reads and writes coded audio, coded video data, or multiplexed data obtained by multiplexing the coded audio and video data, from and on the data recording area ex 233  of the recording medium ex 215 . 
     Although an optical disk having a layer, such as a DVD and a BD is described as an example in the description, the optical disk is not limited to such, and may be an optical disk having a multilayer structure and capable of being recorded on a part other than the surface. Furthermore, the optical disk may have a structure for multidimensional recording/reproduction, such as recording of information using light of colors with different wavelengths in the same portion of the optical disk and for recording information having different layers from various angles. 
     Furthermore, a car ex 210  having an antenna ex 205  can receive data from the satellite ex 202  and others, and reproduce video on a display device such as a car navigation system ex 211  set in the car ex 210 , in the digital broadcasting system ex 200 . Here, a configuration of the car navigation system ex 211  will be a configuration, for example, including a GPS receiving unit from the configuration illustrated in  FIG.  26   . The same will be true for the configuration of the computer ex 111 , the cellular phone ex 114 , and others. 
       FIG.  29 A  illustrates the cellular phone ex 114  that uses the moving picture coding method and the moving picture decoding method described in embodiments. The cellular phone ex 114  includes: an antenna ex 350  for transmitting and receiving radio waves through the base station ex 110 ; a camera unit ex 365  capable of capturing moving and still images; and a display unit ex 358  such as a liquid crystal display for displaying the data such as decoded video captured by the camera unit ex 365  or received by the antenna ex 350 . The cellular phone ex 114  further includes: a main body unit including an operation key unit ex 366 ; an audio output unit ex 357  such as a speaker for output of audio; an audio input unit ex 356  such as a microphone for input of audio; a memory unit ex 367  for storing captured video or still pictures, recorded audio, coded or decoded data of the received video, the still pictures, e-mails, or others; and a slot unit ex 364  that is an interface unit for a recording medium that stores data in the same manner as the memory unit ex 367 . 
     Next, an example of a configuration of the cellular phone ex 114  will be described with reference to  FIG.  29 B . In the cellular phone ex 114 , a main control unit ex 360  designed to control overall each unit of the main body including the display unit ex 358  as well as the operation key unit ex 366  is connected mutually, via a synchronous bus ex 370 , to a power supply circuit unit ex 361 , an operation input control unit ex 362 , a video signal processing unit ex 355 , a camera interface unit ex 363 , a liquid crystal display (LCD) control unit ex 359 , a modulation/demodulation unit ex 352 , a multiplexing/demultiplexing unit ex 353 , an audio signal processing unit ex 354 , the slot unit ex 364 , and the memory unit ex 367 . 
     When a call-end key or a power key is turned ON by a user&#39;s operation, the power supply circuit unit ex 361  supplies the respective units with power from a battery pack so as to activate the cell phone ex 114 . 
     In the cellular phone ex 114 , the audio signal processing unit ex 354  converts the audio signals collected by the audio input unit ex 356  in voice conversation mode into digital audio signals under the control of the main control unit ex 360  including a CPU, ROM, and RAM. Then, the modulation/demodulation unit ex 352  performs spread spectrum processing on the digital audio signals, and the transmitting and receiving unit ex 351  performs digital-to-analog conversion and frequency conversion on the data, so as to transmit the resulting data via the antenna ex 350 . Also, in the cellular phone ex 114 , the transmitting and receiving unit ex 351  amplifies the data received by the antenna ex 350  in voice conversation mode and performs frequency conversion and the analog-to-digital conversion on the data. Then, the modulation/demodulation unit ex 352  performs inverse spread spectrum processing on the data, and the audio signal processing unit ex 354  converts it into analog audio signals, so as to output them via the audio output unit ex 357 . 
     Furthermore, when an e-mail in data communication mode is transmitted, text data of the e-mail inputted by operating the operation key unit ex 366  and others of the main body is sent out to the main control unit ex 360  via the operation input control unit ex 362 . The main control unit ex 360  causes the modulation/demodulation unit ex 352  to perform spread spectrum processing on the text data, and the transmitting and receiving unit ex 351  performs the digital-to-analog conversion and the frequency conversion on the resulting data to transmit the data to the base station ex 110  via the antenna ex 350 . When an e-mail is received, processing that is approximately inverse to the processing for transmitting an e-mail is performed on the received data, and the resulting data is provided to the display unit ex 358 . 
     When video, still images, or video and audio in data communication mode is or are transmitted, the video signal processing unit ex 355  compresses and codes video signals supplied from the camera unit ex 365  using the moving picture coding method shown in each of embodiments (i.e., functions as the image coding apparatus according to the aspect of the present disclosure), and transmits the coded video data to the multiplexing/demultiplexing unit ex 353 . In contrast, during when the camera unit ex 365  captures video, still images, and others, the audio signal processing unit ex 354  codes audio signals collected by the audio input unit ex 356 , and transmits the coded audio data to the multiplexing/demultiplexing unit ex 353 . 
     The multiplexing/demultiplexing unit ex 353  multiplexes the coded video data supplied from the video signal processing unit ex 355  and the coded audio data supplied from the audio signal processing unit ex 354 , using a predetermined method. Then, the modulation/demodulation unit (modulation/demodulation circuit unit) ex 352  performs spread spectrum processing on the multiplexed data, and the transmitting and receiving unit ex 351  performs digital-to-analog conversion and frequency conversion on the data so as to transmit the resulting data via the antenna ex 350 . 
     When receiving data of a video file which is linked to a Web page and others in data communication mode or when receiving an e-mail with video and/or audio attached, in order to decode the multiplexed data received via the antenna ex 350 , the multiplexing/demultiplexing unit ex 353  demultiplexes the multiplexed data into a video data bit stream and an audio data bit stream, and supplies the video signal processing unit ex 355  with the coded video data and the audio signal processing unit ex 354  with the coded audio data, through the synchronous bus ex 370 . The video signal processing unit ex 355  decodes the video signal using a moving picture decoding method corresponding to the moving picture coding method shown in each of embodiments (i.e., functions as the image decoding apparatus according to the aspect of the present disclosure), and then the display unit ex 358  displays, for instance, the video and still images included in the video file linked to the Web page via the LCD control unit ex 359 . Furthermore, the audio signal processing unit ex 354  decodes the audio signal, and the audio output unit ex 357  provides the audio. 
     Furthermore, similarly to the television ex 300 , a terminal such as the cellular phone ex 114  probably have 3 types of implementation configurations including not only (i) a transmitting and receiving terminal including both a coding apparatus and a decoding apparatus, but also (ii) a transmitting terminal including only a coding apparatus and (iii) a receiving terminal including only a decoding apparatus. Although the digital broadcasting system ex 200  receives and transmits the multiplexed data obtained by multiplexing audio data onto video data in the description, the multiplexed data may be data obtained by multiplexing not audio data but character data related to video onto video data, and may be not multiplexed data but video data itself. 
     As such, the moving picture coding method and the moving picture decoding method in each of embodiments can be used in any of the devices and systems described. Thus, the advantages described in each of embodiments can be obtained. 
     Furthermore, various modifications and revisions can be made in any of the embodiments in the present disclosure. 
     Embodiment 9 
     Video data can be generated by switching, as necessary, between (i) the moving picture coding method or the moving picture coding apparatus shown in each of embodiments and (ii) a moving picture coding method or a moving picture coding apparatus in conformity with a different standard, such as MPEG-2, MPEG-4 AVC, and VC-1. 
     Here, when a plurality of video data that conforms to the different standards is generated and is then decoded, the decoding methods need to be selected to conform to the different standards. However, since to which standard each of the plurality of the video data to be decoded conforms cannot be detected, there is a problem that an appropriate decoding method cannot be selected. 
     In order to solve the problem, multiplexed data obtained by multiplexing audio data and others onto video data has a structure including identification information indicating to which standard the video data conforms. The specific structure of the multiplexed data including the video data generated in the moving picture coding method and by the moving picture coding apparatus shown in each of embodiments will be hereinafter described. The multiplexed data is a digital stream in the MPEG-2 Transport Stream format. 
       FIG.  30    illustrates a structure of the multiplexed data. As illustrated in  FIG.  30   , the multiplexed data can be obtained by multiplexing at least one of a video stream, an audio stream, a presentation graphics stream (PG), and an interactive graphics stream. The video stream represents primary video and secondary video of a movie, the audio stream (IG) represents a primary audio part and a secondary audio part to be mixed with the primary audio part, and the presentation graphics stream represents subtitles of the movie. Here, the primary video is normal video to be displayed on a screen, and the secondary video is video to be displayed on a smaller window in the primary video. Furthermore, the interactive graphics stream represents an interactive screen to be generated by arranging the GUI components on a screen. The video stream is coded in the moving picture coding method or by the moving picture coding apparatus shown in each of embodiments, or in a moving picture coding method or by a moving picture coding apparatus in conformity with a conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1. The audio stream is coded in accordance with a standard, such as Dolby-AC-3, Dolby Digital Plus, MLP, DTS, DTS-HD, and linear PCM. 
     Each stream included in the multiplexed data is identified by PID. For example, 0x1011 is allocated to the video stream to be used for video of a movie, 0x1100 to 0x111F are allocated to the audio streams, 0x1200 to 0x121F are allocated to the presentation graphics streams, 0x1400 to 0x141F are allocated to the interactive graphics streams, 0x1B00 to 0x1B1F are allocated to the video streams to be used for secondary video of the movie, and 0x1A00 to 0x1A1F are allocated to the audio streams to be used for the secondary audio to be mixed with the primary audio. 
       FIG.  31    schematically illustrates how data is multiplexed. First, a video stream ex 235  composed of video frames and an audio stream ex 238  composed of audio frames are transformed into a stream of PES packets ex 236  and a stream of PES packets ex 239 , and further into TS packets ex 237  and TS packets ex 240 , respectively. Similarly, data of a presentation graphics stream ex 241  and data of an interactive graphics stream ex 244  are transformed into a stream of PES packets ex 242  and a stream of PES packets ex 245 , and further into TS packets ex 243  and TS packets ex 246 , respectively. These TS packets are multiplexed into a stream to obtain multiplexed data ex 247 . 
       FIG.  32    illustrates how a video stream is stored in a stream of PES packets in more detail. The first bar in  FIG.  32    shows a video frame stream in a video stream. The second bar shows the stream of PES packets. As indicated by arrows denoted as yy 1 , yy 2 , yy 3 , and yy 4  in  FIG.  32   , the video stream is divided into pictures as I pictures, B pictures, and P pictures each of which is a video presentation unit, and the pictures are stored in a payload of each of the PES packets. Each of the PES packets has a PES header, and the PES header stores a Presentation Time-Stamp (PTS) indicating a display time of the picture, and a Decoding Time-Stamp (DTS) indicating a decoding time of the picture. 
       FIG.  33    illustrates a format of TS packets to be finally written on the multiplexed data. Each of the TS packets is a 188-byte fixed length packet including a 4-byte TS header having information, such as a PID for identifying a stream and a 184-byte TS payload for storing data. The PES packets are divided, and stored in the TS payloads, respectively. When a BD ROM is used, each of the TS packets is given a 4-byte TP_Extra_Header, thus resulting in 192-byte source packets. The source packets are written on the multiplexed data. The TP_Extra_Header stores information such as an Arrival_Time_Stamp (ATS). The ATS shows a transfer start time at which each of the TS packets is to be transferred to a PID filter. The source packets are arranged in the multiplexed data as shown at the bottom of  FIG.  33   . The numbers incrementing from the head of the multiplexed data are called source packet numbers (SPNs). 
     Each of the TS packets included in the multiplexed data includes not only streams of audio, video, subtitles and others, but also a Program Association Table (PAT), a Program Map Table (PMT), and a Program Clock Reference (PCR). The PAT shows what a PID in a PMT used in the multiplexed data indicates, and a PID of the PAT itself is registered as zero. The PMT stores PIDs of the streams of video, audio, subtitles and others included in the multiplexed data, and attribute information of the streams corresponding to the PIDs. The PMT also has various descriptors relating to the multiplexed data. The descriptors have information such as copy control information showing whether copying of the multiplexed data is permitted or not. The PCR stores STC time information corresponding to an ATS showing when the PCR packet is transferred to a decoder, in order to achieve synchronization between an Arrival Time Clock (ATC) that is a time axis of ATSs, and an System Time Clock (STC) that is a time axis of PTSs and DTSs. 
       FIG.  34    illustrates the data structure of the PMT in detail. A PMT header is disposed at the top of the PMT. The PMT header describes the length of data included in the PMT and others. A plurality of descriptors relating to the multiplexed data is disposed after the PMT header. Information such as the copy control information is described in the descriptors. After the descriptors, a plurality of pieces of stream information relating to the streams included in the multiplexed data is disposed. Each piece of stream information includes stream descriptors each describing information, such as a stream type for identifying a compression codec of a stream, a stream PID, and stream attribute information (such as a frame rate or an aspect ratio). The stream descriptors are equal in number to the number of streams in the multiplexed data. 
     When the multiplexed data is recorded on a recording medium and others, it is recorded together with multiplexed data information files. 
     Each of the multiplexed data information files is management information of the multiplexed data as shown in  FIG.  35   . The multiplexed data information files are in one to one correspondence with the multiplexed data, and each of the files includes multiplexed data information, stream attribute information, and an entry map. 
     As illustrated in  FIG.  35   , the multiplexed data information includes a system rate, a reproduction start time, and a reproduction end time. The system rate indicates the maximum transfer rate at which a system target decoder to be described later transfers the multiplexed data to a PID filter. The intervals of the ATSs included in the multiplexed data are set to not higher than a system rate. The reproduction start time indicates a PTS in a video frame at the head of the multiplexed data. An interval of one frame is added to a PTS in a video frame at the end of the multiplexed data, and the PTS is set to the reproduction end time. 
     As shown in  FIG.  36   , a piece of attribute information is registered in the stream attribute information, for each PID of each stream included in the multiplexed data. Each piece of attribute information has different information depending on whether the corresponding stream is a video stream, an audio stream, a presentation graphics stream, or an interactive graphics stream. Each piece of video stream attribute information carries information including what kind of compression codec is used for compressing the video stream, and the resolution, aspect ratio and frame rate of the pieces of picture data that is included in the video stream. Each piece of audio stream attribute information carries information including what kind of compression codec is used for compressing the audio stream, how many channels are included in the audio stream, which language the audio stream supports, and how high the sampling frequency is. The video stream attribute information and the audio stream attribute information are used for initialization of a decoder before the player plays back the information. 
     In the present embodiment, the multiplexed data to be used is of a stream type included in the PMT. Furthermore, when the multiplexed data is recorded on a recording medium, the video stream attribute information included in the multiplexed data information is used. More specifically, the moving picture coding method or the moving picture coding apparatus described in each of embodiments includes a step or a unit for allocating unique information indicating video data generated by the moving picture coding method or the moving picture coding apparatus in each of embodiments, to the stream type included in the PMT or the video stream attribute information. With the configuration, the video data generated by the moving picture coding method or the moving picture coding apparatus described in each of embodiments can be distinguished from video data that conforms to another standard. 
     Furthermore,  FIG.  37    illustrates steps of the moving picture decoding method according to the present embodiment. In Step exS 100 , the stream type included in the PMT or the video stream attribute information included in the multiplexed data information is obtained from the multiplexed data. Next, in Step exS 101 , it is determined whether or not the stream type or the video stream attribute information indicates that the multiplexed data is generated by the moving picture coding method or the moving picture coding apparatus in each of embodiments. When it is determined that the stream type or the video stream attribute information indicates that the multiplexed data is generated by the moving picture coding method or the moving picture coding apparatus in each of embodiments, in Step exS 102 , decoding is performed by the moving picture decoding method in each of embodiments. Furthermore, when the stream type or the video stream attribute information indicates conformance to the conventional standards, such as MPEG-2, MPEG-4 AVC, and VC-1, in Step exS 103 , decoding is performed by a moving picture decoding method in conformity with the conventional standards. 
     As such, allocating a new unique value to the stream type or the video stream attribute information enables determination whether or not the moving picture decoding method or the moving picture decoding apparatus that is described in each of embodiments can perform decoding. Even when multiplexed data that conforms to a different standard is input, an appropriate decoding method or apparatus can be selected. Thus, it becomes possible to decode information without any error. Furthermore, the moving picture coding method or apparatus, or the moving picture decoding method or apparatus in the present embodiment can be used in the devices and systems described above. 
     Embodiment 10 
     Each of the moving picture coding method, the moving picture coding apparatus, the moving picture decoding method, and the moving picture decoding apparatus in each of embodiments is typically achieved in the form of an integrated circuit or a Large Scale Integrated (LSI) circuit. As an example of the LSI,  FIG.  38    illustrates a configuration of the LSI ex 500  that is made into one chip. The LSI ex 500  includes elements ex 501 , ex 502 , ex 503 , ex 504 , ex 505 , ex 506 , ex 507 , ex 508 , and ex 509  to be described below, and the elements are connected to each other through a bus ex 510 . The power supply circuit unit ex 505  is activated by supplying each of the elements with power when the power supply circuit unit ex 505  is turned on. 
     For example, when coding is performed, the LSI ex 500  receives an AV signal from a microphone ex 117 , a camera ex 113 , and others through an AV IO ex 509  under control of a control unit ex 501  including a CPU ex 502 , a memory controller ex 503 , a stream controller ex 504 , and a driving frequency control unit ex 512 . The received AV signal is temporarily stored in an external memory ex 511 , such as an SDRAM. Under control of the control unit ex 501 , the stored data is segmented into data portions according to the processing amount and speed to be transmitted to a signal processing unit ex 507 . Then, the signal processing unit ex 507  codes an audio signal and/or a video signal. Here, the coding of the video signal is the coding described in each of embodiments. Furthermore, the signal processing unit ex 507  sometimes multiplexes the coded audio data and the coded video data, and a stream IO ex 506  provides the multiplexed data outside. The provided multiplexed data is transmitted to the base station ex 107 , or written on the recording medium ex 215 . When data sets are multiplexed, the data should be temporarily stored in the buffer ex 508  so that the data sets are synchronized with each other. 
     Although the memory ex 511  is an element outside the LSI ex 500 , it may be included in the LSI ex 500 . The buffer ex 508  is not limited to one buffer, but may be composed of buffers. Furthermore, the LSI ex 500  may be made into one chip or a plurality of chips. 
     Furthermore, although the control unit ex 501  includes the CPU ex 502 , the memory controller ex 503 , the stream controller ex 504 , the driving frequency control unit ex 512 , the configuration of the control unit ex 501  is not limited to such. For example, the signal processing unit ex 507  may further include a CPU. Inclusion of another CPU in the signal processing unit ex 507  can improve the processing speed. Furthermore, as another example, the CPU ex 502  may serve as or be a part of the signal processing unit ex 507 , and, for example, may include an audio signal processing unit. In such a case, the control unit ex 501  includes the signal processing unit ex 507  or the CPU ex 502  including a part of the signal processing unit ex 507 . 
     The name used here is LSI, but it may also be called IC, system LSI, super LSI, or ultra LSI depending on the degree of integration. 
     Moreover, ways to achieve integration are not limited to the LSI, and a special circuit or a general purpose processor and so forth can also achieve the integration. Field Programmable Gate Array (FPGA) that can be programmed after manufacturing LSIs or a reconfigurable processor that allows re-configuration of the connection or configuration of an LSI can be used for the same purpose. 
     In the future, with advancement in semiconductor technology, a brand-new technology may replace LSI. The functional blocks can be integrated using such a technology. The possibility is that the present disclosure is applied to biotechnology. 
     Embodiment 11 
     When video data generated in the moving picture coding method or by the moving picture coding apparatus described in each of embodiments is decoded, compared to when video data that conforms to a conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1 is decoded, the processing amount probably increases. Thus, the LSI ex 500  needs to be set to a driving frequency higher than that of the CPU ex 502  to be used when video data in conformity with the conventional standard is decoded. However, when the driving frequency is set higher, there is a problem that the power consumption increases. 
     In order to solve the problem, the moving picture decoding apparatus, such as the television ex 300  and the LSI ex 500  is configured to determine to which standard the video data conforms, and switch between the driving frequencies according to the determined standard.  FIG.  39    illustrates a configuration ex 800  in the present embodiment. A driving frequency switching unit ex 803  sets a driving frequency to a higher driving frequency when video data is generated by the moving picture coding method or the moving picture coding apparatus described in each of embodiments. Then, the driving frequency switching unit ex 803  instructs a decoding processing unit ex 801  that executes the moving picture decoding method described in each of embodiments to decode the video data. When the video data conforms to the conventional standard, the driving frequency switching unit ex 803  sets a driving frequency to a lower driving frequency than that of the video data generated by the moving picture coding method or the moving picture coding apparatus described in each of embodiments. Then, the driving frequency switching unit ex 803  instructs the decoding processing unit ex 802  that conforms to the conventional standard to decode the video data. 
     More specifically, the driving frequency switching unit ex 803  includes the CPU ex 502  and the driving frequency control unit ex 512  in  FIG.  38   . Here, each of the decoding processing unit ex 801  that executes the moving picture decoding method described in each of embodiments and the decoding processing unit ex 802  that conforms to the conventional standard corresponds to the signal processing unit ex 507  in  FIG.  38   . The CPU ex 502  determines to which standard the video data conforms. Then, the driving frequency control unit ex 512  determines a driving frequency based on a signal from the CPU ex 502 . Furthermore, the signal processing unit ex 507  decodes the video data based on the signal from the CPU ex 502 . For example, the identification information described in Embodiment B is probably used for identifying the video data. The identification information is not limited to the one described in Embodiment B but may be any information as long as the information indicates to which standard the video data conforms. For example, when which standard video data conforms to can be determined based on an external signal for determining that the video data is used for a television or a disk, etc., the determination may be made based on such an external signal. Furthermore, the CPU ex 502  selects a driving frequency based on, for example, a look-up table in which the standards of the video data are associated with the driving frequencies as shown in  FIG.  41   . The driving frequency can be selected by storing the look-up table in the buffer ex 508  and in an internal memory of an LSI, and with reference to the look-up table by the CPU ex 502 . 
       FIG.  40    illustrates steps for executing a method in the present embodiment. First, in Step exS 200 , the signal processing unit ex 507  obtains identification information from the multiplexed data. Next, in Step exS 201 , the CPU ex 502  determines whether or not the video data is generated by the coding method and the coding apparatus described in each of embodiments, based on the identification information. When the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, in Step exS 202 , the CPU ex 502  transmits a signal for setting the driving frequency to a higher driving frequency to the driving frequency control unit ex 512 . Then, the driving frequency control unit ex 512  sets the driving frequency to the higher driving frequency. On the other hand, when the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1, in Step exS 203 , the CPU ex 502  transmits a signal for setting the driving frequency to a lower driving frequency to the driving frequency control unit ex 512 . Then, the driving frequency control unit ex 512  sets the driving frequency to the lower driving frequency than that in the case where the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiment. 
     Furthermore, along with the switching of the driving frequencies, the power conservation effect can be improved by changing the voltage to be applied to the LSI ex 500  or an apparatus including the LSI ex 500 . For example, when the driving frequency is set lower, the voltage to be applied to the LSI ex 500  or the apparatus including the LSI ex 500  is probably set to a voltage lower than that in the case where the driving frequency is set higher. 
     Furthermore, when the processing amount for decoding is larger, the driving frequency may be set higher, and when the processing amount for decoding is smaller, the driving frequency may be set lower as the method for setting the driving frequency. Thus, the setting method is not limited to the ones described above. For example, when the processing amount for decoding video data in conformity with MPEG-4 AVC is larger than the processing amount for decoding video data generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the driving frequency is probably set in reverse order to the setting described above. 
     Furthermore, the method for setting the driving frequency is not limited to the method for setting the driving frequency lower. For example, when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the voltage to be applied to the LSI ex 500  or the apparatus including the LSI ex 500  is probably set higher. When the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1, the voltage to be applied to the LSI ex 500  or the apparatus including the LSI ex 500  is probably set lower. As another example, when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the driving of the CPU ex 502  does not probably have to be suspended. When the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1, the driving of the CPU ex 502  is probably suspended at a given time because the CPU ex 502  has extra processing capacity. Even when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, in the case where the CPU ex 502  has extra processing capacity, the driving of the CPU ex 502  is probably suspended at a given time. In such a case, the suspending time is probably set shorter than that in the case where when the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1. 
     Accordingly, the power conservation effect can be improved by switching between the driving frequencies in accordance with the standard to which the video data conforms. Furthermore, when the LSI ex 500  or the apparatus including the LSI ex 500  is driven using a battery, the battery life can be extended with the power conservation effect. 
     Embodiment 12 
     There are cases where a plurality of video data that conforms to different standards, is provided to the devices and systems, such as a television and a cellular phone. In order to enable decoding the plurality of video data that conforms to the different standards, the signal processing unit ex 507  of the LSI ex 500  needs to conform to the different standards. However, the problems of increase in the scale of the circuit of the LSI ex 500  and increase in the cost arise with the individual use of the signal processing units ex 507  that conform to the respective standards. 
     In order to solve the problem, what is conceived is a configuration in which the decoding processing unit for implementing the moving picture decoding method described in each of embodiments and the decoding processing unit that conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1 are partly shared. Ex 900  in  FIG.  42 A  shows an example of the configuration. For example, the moving picture decoding method described in each of embodiments and the moving picture decoding method that conforms to MPEG-4 AVC have, partly in common, the details of processing, such as entropy coding, inverse quantization, deblocking filtering, and motion compensated prediction. The details of processing to be shared probably include use of a decoding processing unit ex 902  that conforms to MPEG-4 AVC. In contrast, a dedicated decoding processing unit ex 901  is probably used for other processing unique to an aspect of the present disclosure. Since the aspect of the present disclosure is characterized by inverse quantization in particular, for example, the dedicated decoding processing unit ex 901  is used for inverse quantization. Otherwise, the decoding processing unit is probably shared for one of the entropy decoding, deblocking filtering, and motion compensation, or all of the processing. The decoding processing unit for implementing the moving picture decoding method described in each of embodiments may be shared for the processing to be shared, and a dedicated decoding processing unit may be used for processing unique to that of MPEG-4 AVC. 
     Furthermore, ex 1000  in  FIG.  42 B  shows another example in that processing is partly shared. This example uses a configuration including a dedicated decoding processing unit ex 1001  that supports the processing unique to an aspect of the present disclosure, a dedicated decoding processing unit ex 1002  that supports the processing unique to another conventional standard, and a decoding processing unit ex 1003  that supports processing to be shared between the moving picture decoding method according to the aspect of the present disclosure and the conventional moving picture decoding method. Here, the dedicated decoding processing units ex 1001  and ex 1002  are not necessarily specialized for the processing according to the aspect of the present disclosure and the processing of the conventional standard, respectively, and may be the ones capable of implementing general processing. Furthermore, the configuration of the present embodiment can be implemented by the LSI ex 500 . 
     As such, reducing the scale of the circuit of an LSI and reducing the cost are possible by sharing the decoding processing unit for the processing to be shared between the moving picture decoding method according to the aspect of the present disclosure and the moving picture decoding method in conformity with the conventional standard. 
     The herein disclosed subject matter is to be considered descriptive and illustrative only, and the appended Claims are of a scope intended to cover and encompass not only the particular embodiment(s) disclosed, but also equivalent structures, methods, and/or uses. 
     INDUSTRIAL APPLICABILITY 
     The picture coding method and picture decoding method according to one or more exemplary embodiments disclosed herein are advantageously applicable to a method of coding moving pictures and a method of decoding moving pictures.