Image decoding method, image coding method, image decoding apparatus, image coding apparatus, and image coding and decoding apparatus   

Abstract: An image decoding method for decoding, on a block-by-block basis, image data included in a coded bitstream includes: obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block (S303); and obtaining, from the coded bitstream, an index for identifying a merging candidate for the current block (S304), wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed. The fixed number is greater than or equal to two.

The present application claims the benefit of U.S. Provisional Patent Application No. 61/503,074 filed Jun. 30, 2011. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a moving picture coding method and a moving picture decoding method.

BACKGROUND ART

Generally, in coding processing of a moving picture, the amount of information is reduced by compression for which redundancy of a moving picture in spatial direction and temporal direction is made use of. Generally, conversion to a frequency domain is performed as a method in which redundancy in spatial direction is made use of, and coding using prediction between pictures (the prediction is hereinafter referred to as inter prediction) is performed as a method of compression for which redundancy in temporal direction is made use of. 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. Subsequently, a motion vector is derived by performing motion estimation on the current picture with reference to the reference picture. Then, redundancy in temporal direction is removed using a calculated difference between picture data of the current picture and prediction picture data which is obtained by motion compensation based on the derived motion vector (see Non-patent Literature 1, for example). Here, 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 from the current block and the reference block.

[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

It is still desirable to enhance error resistance of image coding and decoding in which inter prediction is used, beyond the above-described conventional technique.

In view of this, the object of the present disclosure is to provide an image coding method and an image decoding method with which error resistance of image coding and image decoding using inter prediction is enhanced.

An image decoding method according to an aspect of the present disclosure is a method for decoding, on a block-by-block basis, a coded image included in a bitstream, and includes: obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block, the fixed number being greater than or equal to two; obtaining, from the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the decoding of the current block; and identifying the merging candidate using the obtained index, and decoding the current block using the identified merging candidate, wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

It should be noted that these general or specific aspects can be implemented as a system, a method, an integrated circuit, a computer program, a computer-readable recording medium such as a compact disc read-only memory (CD-ROM), or as any combination of a system, a method, an integrated circuit, a computer program, and a computer-readable recording medium.

According to an aspect of the present disclosure, error resistance of image coding and decoding using inter prediction can be enhanced.

These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the present invention. In the Drawings:

FIG. 1A is a diagram for illustrating an exemplary reference picture list for a B-picture;

FIG. 1B is a diagram for illustrating an exemplary reference picture list of a prediction direction 0 for a B-picture;

FIG. 1C is a diagram for illustrating an exemplary reference picture list of a prediction direction 1 for a B-picture;

FIG. 2 is a diagram for illustrating motion vectors for use in the temporal motion vector prediction mode;

FIG. 3 shows an exemplary motion vector of a neighboring block for use in the merging mode;

FIG. 4 is a diagram for illustrating an exemplary merging block candidate list;

FIG. 5 shows a relationship between the size of a merging block candidate list and bit sequences assigned to merging block candidate indexes;

FIG. 6 is a flowchart showing an example of a process for coding when the merging mode is used;

FIG. 7 is a block diagram showing a configuration of an image coding apparatus;

FIG. 8 is a flowchart showing a process for decoding using the merging mode;

FIG. 9 is a block diagram showing a configuration of an image decoding apparatus;

FIG. 10 shows syntax for attachment of a merging block candidate index to a coded bitstream;

FIG. 11 is a block diagram showing a configuration of an image coding apparatus according to Embodiment 1;

FIG. 12 is a flowchart showing processing operations of the image coding apparatus according to Embodiment 1;

FIG. 13A shows an exemplary merging block candidate list according to Embodiment 1;

FIG. 13B shows an exemplary merging block candidate list according to Embodiment 1;

FIG. 13C shows an exemplary merging block candidate list according to Embodiment 1;

FIG. 14A is a flowchart illustrating a process for calculating merging block candidates and the size of a merging block candidate list according to Embodiment 1;

FIG. 14B is a flowchart illustrating a process for calculating merging block candidates and the size of a merging block candidate list according to a modification of an embodiment;

FIG. 14C is a flowchart illustrating a process for calculating merging block candidates and the size of a merging block candidate list according to a modification of an embodiment;

FIG. 15A is a flowchart illustrating a process for determining whether or not a merging block candidate is a usable-for-merging candidate and updating the total number of usable-for-merging candidates according to Embodiment 1;

FIG. 15B is a flowchart illustrating a process for determining whether or not a merging block candidate is a usable-for-merging candidate and updating the total number of usable-for-merging candidates according to a modification of an embodiment;

FIG. 16 is a flowchart illustrating a process for adding a new candidate according to Embodiment 1;

FIG. 17 is a flowchart illustrating a process for adding a second candidate according to a modification of an embodiment;

FIG. 18 is a flowchart illustrating a process for selecting a merging block candidate according to Embodiment 1;

FIG. 19 is a block diagram showing a configuration of an image decoding apparatus according to Embodiment 2;

FIG. 20 is a flowchart showing processing operations of the image decoding apparatus according to Embodiment 2;

FIG. 21 is a flowchart illustrating a process for determining whether or not a merging block candidate is a usable-for-merging candidate and updating the total number of usable-for-merging candidates according to Embodiment 2;

FIG. 22 is a flowchart illustrating a process for generating a merging block candidate list according to Embodiment 2;

FIG. 23 shows exemplary syntax for attachment of a merging block candidate index to a coded bitstream;

FIG. 24 shows exemplary syntax in the case where the size of a merging block candidate list is fixed at the maximum value of the total number of merging block candidates;

FIG. 25 shows an overall configuration of a content providing system for implementing content distribution services;

FIG. 26 shows an overall configuration of a digital broadcasting system;

FIG. 27 shows a block diagram illustrating an example of a configuration of a television;

FIG. 28 is 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. 29 shows an example of a configuration of a recording medium that is an optical disk;

FIG. 30A shows an example of a cellular phone;

FIG. 30B is a block diagram showing an example of a configuration of a cellular phone;

FIG. 31 illustrates a structure of multiplexed data;

FIG. 32 schematically shows how each stream is multiplexed in multiplexed data;

FIG. 33 shows how a video stream is stored in a stream of PES packets in more detail;

FIG. 34 shows a structure of TS packets and source packets in the multiplexed data;

FIG. 35 shows a data structure of a PMT;

FIG. 36 shows an internal structure of multiplexed data information;

FIG. 37 shows an internal structure of stream attribute information;

FIG. 38 shows steps for identifying video data;

FIG. 39 is a block diagram showing 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. 40 shows a configuration for switching between driving frequencies;

FIG. 41 shows steps for identifying video data and switching between driving frequencies;

FIG. 42 shows an example of a look-up table in which video data standards are associated with driving frequencies;

FIG. 43A is a diagram showing an example of a configuration for sharing a module of a signal processing unit; and

FIG. 43B is a diagram showing another example of a configuration for sharing a module of the signal processing unit.

In a moving picture coding scheme already standardized, which is referred to as H.264, three picture types of I-picture, P-picture, and B-picture are used for reduction of the amount of information by compression.

The I-picture is not coded by inter prediction coding. 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 by inter prediction coding with reference to one coded picture preceding or following the current picture in order of display time. The B-picture is coded by inter prediction coding with reference to two coded pictures preceding and following the current picture in order of display time.

In inter prediction coding, a reference picture list for identifying a reference picture is generated. In a reference picture list, reference picture indexes are assigned to coded reference pictures to be referenced in inter prediction. For example, two reference picture lists (L0, L1) are generated for a B-picture because it can be coded with reference to two pictures.

FIG. 1A is a diagram for illustrating an exemplary reference picture list for a B-picture. FIG. 1B shows an exemplary 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 number 2. The reference picture index 0 having a value of 1 is assigned to a reference picture 1 with a display order number 1. The reference picture index 0 having a value of 2 is assigned to a reference picture 2 with a display order number 0. In other words, the shorter the temporal distance of a reference picture from the current picture, the smaller the reference picture index assigned to the reference picture.

On the other hand, FIG. 1C shows an exemplary 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 number 1. The reference picture index 1 having a value of 1 is assigned to a reference picture 0 with a display order number 2. The reference picture index 1 having a value of 2 is assigned to a reference picture 2 with a display order number 0.

In this manner, it is possible to assign reference picture indexes having values different between prediction directions to a reference picture (the reference pictures 0 and 1 in FIG. 1A) or to assign the reference picture index having the same value for both directions to a reference picture (the reference picture 2 in FIG. 1A).

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 the 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 B2 is coded in temporal motion vector prediction mode.

In the coding, a motion vector vb is used which has been used in coding of a block b located in the same position in a picture P3, which is a reference picture following the picture B2, as the position of the block a in the picture B2 (the block b is hereinafter referred to as a “co-located block” of the block a). The motion vector vb is a motion vector used in coding the block b with reference to the picture P1.

Motion vectors parallel to the motion vector vb are used for obtaining two reference blocks for the block a are obtained from a forward reference picture and a backward reference picture, that is, a picture P1 and a picture P3. Then, the block a is coded using bi-prediction based on the two obtained reference blocks. Specifically, in the coding of the block a, a motion vector va1 is used to reference the picture P1, and a motion vector va2 is used to reference the picture P3.

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 prediction direction, a motion vector, and a reference picture index which are copies of those used in coding a neighboring block of the current block. At this time, the copies of the index and others of the neighboring block are attached to a coded bitstream (hereinafter simply referred to as a “bitstream” as appropriate) so that the motion direction, motion vector, and reference picture index used for the coding can be selected in decoding.

FIG. 3 shows an exemplary motion vector of a neighboring block for use 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 on the immediate above right of the current block. A neighboring block D is a coded block located on the immediate below 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 as a motion vector to a reference picture indicated by a reference picture index RefL0_A of 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 as a motion vector to a reference picture indicated by a reference picture index RefL1_B of 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 as a motion vector to a reference picture indicated by a reference picture index RefL0_D of the prediction direction 0.

In this case, 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 the sets of prediction directions, motion vectors, and reference picture indexes of the neighboring blocks A to D and the 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. Then, a merging block candidate index indicating a block having the selected set of a prediction direction, a motion vector, and a reference picture index is attached to a bitstream.

For example, when the neighboring block A is selected, the current block is coded using the motion vector MvL0_A having the prediction direction 0 and the reference picture index RefL0_A. Then, only the merging block candidate index having a value of 0 which indicates use of the neighboring block A as shown in FIG. 4 is attached to a bitstream. 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 (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 merging block candidates as shown in FIG. 4.

In this manner, the total number of merging block candidates is reduced so that the amount of code assigned to merging block candidate indexes can be reduced. Here, “unusable for merging” means (1) that the merging block candidate has been coded using intra prediction, (2) that the merging block candidate is outside the boundary of a slice including the current block or the boundary of a picture including the current block, or (3) that the merging block candidate is yet to be coded.

In the example shown in FIG. 4, the neighboring block C is a block coded using intra prediction. The merging block candidate having the merging block candidate index 3 is therefore an unusable-for-merging candidate and removed from the merging block candidate list. The neighboring block D is identical in prediction direction, motion vector, and reference picture index to the neighboring block A. The merging block candidate having the merging block candidate index 4 is therefore removed from the merging block candidate list. As a result, the total number of the merging block candidates is finally three, and the size of the merging block candidate list is set at three.

Merging block candidate indexes are coded by variable-length coding by assigning bit sequences according to the size of each merging block candidate list as shown in FIG. 5. Thus, in the merging mode, bit sequences assigned to merging mode indexes are changed according to the size of each merging block candidate list so that the amount of code can be reduced.

FIG. 6 is a flowchart showing an example of a process for coding when the merging mode is used. In Step S1001, sets of a motion vector, a reference picture index, and a prediction direction of merging block candidates are obtained from neighboring blocks and a co-located block. In Step S1002, identical candidates and unusable-for-merging candidates are removed from the merging block candidates. In Step S1003, the total number of the merging block candidates after the removing is set as the size of the merging block candidate list. In Step S1004, the merging block candidate index to be used in coding of the current block is determined. In Step S1005, the determined merging block candidate index is coded by performing variable-length coding in bit sequence according to the size of the merging block candidate list.

FIG. 7 is a block diagram illustrating an exemplary configuration of an image coding apparatus in which the merging mode is used. In FIG. 7, the merging block candidate calculation unit derives a merging block candidate list (Steps S1001 and S1002) and transmits the total number of merging block candidates to the variable-length-coding unit. The variable-length-coding unit 116 sets the total number of merging block candidates as the size of the merging block candidate list (Step 1003). Furthermore, the variable-length-coding unit determines a merging block candidate index to be used in coding of a current block (Step 1004). Furthermore, the variable-length-coding unit performs variable-length coding on the determined merging block candidate index using a bit sequence according to the size of the merging block candidate list (Step S1005).

FIG. 8 is a flowchart showing an example of a process for decoding using the merging mode. In Step S2001, sets of a motion vector, a reference picture index, and a prediction direction of merging block candidate are obtained from neighboring blocks and a co-located block. In Step S2002, identical candidates and unusable-for-merging candidates are removed from the merging block candidates. In Step S2003, the total number of the merging block candidates after the removing is set as the size of the merging block candidate list. In Step S2004, the merging block candidate index to be used in decoding of a current block is decoded from a bitstream using the size of the merging block candidate list. In Step S2005, the current block is decoded by generating a prediction picture using the merging block candidate indicated by the decoded merging block candidate index.

FIG. 9 is a block diagram illustrating an exemplary configuration of an image decoding apparatus in which the merging mode is used. In FIG. 9, the merging block candidate calculation unit derives a merging block candidate list (Steps S2001 and S2002) and transmits the total number of merging block candidates to the variable-length-decoding unit. The variable-length-decoding unit sets the total number of merging block candidates as the size of the merging block candidate list (Step S2003). Furthermore, using the size of the merging block candidate list, the variable-length-decoding unit decodes, from a bitstream, a merging block candidate index to be used in decoding of a current block (Step S2004).

FIG. 10 shows syntax for attachment of a merging block candidate index to a bitstream. In FIG. 10, merge_idx represents a merging block candidate index, and merge_flag represents a merging flag. NumMergeCand represents the size of a merging block candidate list. NumMergeCand is set at the total number of merging block candidates after unusable-for-merging candidates and identical candidates are removed from the merging block candidates.

Coding or decoding of an image is performed using the merging mode in the above-described manner.

As described above, in the conventional merging mode, a merging block candidate list is derived by removing unusable-for-merging candidates and identical candidates based on information on reference pictures including a co-located block. Then, the total number of merging block candidates in the merging block candidate list after the removing is set as the size of the merging block candidate list. In the case where there is a difference in the total number of merging block candidates between an image coding apparatus and an image decoding apparatus, a discrepancy arises in bit sequence assigned to a merging block candidate index between the image coding apparatus and the image decoding apparatus, which causes a problem that a bitstream cannot be normally decoded.

For example, when information on a reference picture referenced as a co-located block is lost due to packet loss in a transmission path, the motion vector or reference picture index of the co-located block becomes unknown so that information on a merging block candidate to be generated from the co-located block is no longer unavailable. Then, it is impossible to correctly remove unusable-for-merging candidates and identical candidates from merging block candidates in decoding, and a correct size of a merging block candidate list is therefore no longer obtainable. As a result, it is impossible to normally decode a merging block candidate index.

This problem can be solved by using merging block candidate lists having a fixed size. When merging block candidate lists have a fixed size, it is no longer necessary to calculate the size of merging block candidate lists.

However, such a merging block candidate list having a fixed size includes an empty entry when the size of the merging block candidate list is larger than the total number of candidates derived from spatially neighboring blocks (usable-for-merging candidates except identical candidates) and a candidate which is derived from a co-located block, that is, a temporally neighboring block (first candidate). In this case, there is a problem that an unexpected operation may be performed when the empty entry is referenced in the image decoding apparatus due to an error.

Here, an image decoding method according to an aspect of the present disclosure is a method for decoding, on a block-by-block basis, image data included in a coded bitstream, and includes: obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block, the fixed number being greater than or equal to two; obtaining, from the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the decoding of the current block; and identifying the merging candidate using the obtained index, and decoding the current block using the identified merging candidate, wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

In the image decoding method, a fixed number (greater than or equal to two) of merging candidates are obtained, that is, a merging block candidate list has a fixed size (hereinafter simply referred to as “candidate list size” as appropriate), and any empty entry after deriving the first candidates is filled with a second candidate. This prevents an unexpected operation which may be performed when such an empty entry is referenced, so that error resistance can be enhanced.

It should be noted that the phrase “having a predetermined fixed value” means that second candidates in a merging block candidate list are identical in prediction direction, motion vector, and reference picture index. In other words, second candidates in different merging block candidate lists may be different in prediction direction, motion vector, or reference picture index.

It should be noted that a third candidate may be further added to increase coding efficiency in the image decoding method. Also in this case, when a merging block candidate list (hereinafter simply referred to as a “candidate list” as appropriate) has any empty entry after first candidates and third candidates are derived, the empty entry is filled with a second candidate so that error resistance can be enhanced. It should be noted that unlike the second candidates, the third candidates added in a single merging block candidate list are different in at least one of prediction direction, motion vector, and reference picture index from each other because the third candidates are added for the purpose of increasing coding efficiency (however, the third candidates may be identical to any of a first candidate and a second candidate as a result).

Furthermore, for example, the obtaining of a fixed number of merging candidates may include: deriving the one or more first candidates and including the one or more first candidates in the fixed number of merging candidates; deriving one or more third candidates and including the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block; and deriving the one or more second candidates and including the one or more second candidates in the fixed number of merging candidates so that a total number of the first candidates, the second candidates, and the third candidates equals the fixed number, when a total number of the first candidates and the third candidates is smaller than the fixed number.

Furthermore, for example, in the deriving of one or more third candidates, the one or more third candidates may be derived by selecting, according to a predetermined priority order, one or more candidates from among a plurality of prepared candidates different from each other.

Furthermore, for example, the obtaining of a fixed number of merging candidates may include: initializing the fixed number of merging candidates by setting all the fixed number of merging candidates to the second candidates; deriving the one or more first candidates and updating part of the fixed number of merging candidates so as to include the one or more first candidates in the fixed number of merging candidates; and deriving one or more third candidates and updating part of the fixed number of merging candidates so as to include the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block.

An image coding method according to an aspect of the present disclosure is a method for coding an image on a block-by-block basis to generate a coded bitstream, and includes: obtaining a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in coding of a current block, the fixed number being greater than or equal to two; and attaching, to the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the coding of the current block, wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for coding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

Furthermore, for example, the obtaining of a fixed number of merging candidates may include: deriving the one or more first candidates and including the one or more first candidates in the fixed number of merging candidates; deriving one or more third candidates and including the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block; and deriving the one or more second candidates and including the one or more second candidates in the fixed number of merging candidates so that a total number of the first candidates, the second candidates, and the third candidates equals the fixed number, when a total number of the first candidates and the third candidates is smaller than the fixed number.

Furthermore, for example, in the deriving of one or more third candidates, the one or more third candidates may be derived by selecting, according to a predetermined priority order, one or more candidates from among a plurality of prepared candidates different from each other.

Furthermore, for example, the obtaining of a fixed number of merging candidates may include: initializing the fixed number of merging candidates by setting all the fixed number of merging candidates to the second candidates; deriving the one or more first candidates and updating part of the fixed number of merging candidates so as to include the one or more first candidates in the fixed number of merging candidates; and deriving one or more third candidates and updating part of the fixed number of merging candidates so as to include the one or more third candidates in the fixed number of merging candidates, when a total number of the first candidates is smaller than the fixed number, the third candidates each having a picture index for a picture referable in the decoding of the current block.

An image decoding apparatus according to an aspect of the present disclosure is an image decoding apparatus which decodes, on a block-by-block basis, image data included in a coded bitstream, and includes: an merging candidate obtaining unit configured to obtain a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index which are to be referenced in decoding of a current block, the fixed number being greater than or equal to two; an index obtaining unit configured to obtain, from the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the decoding of the current block; and a decoding unit configured to identify the merging candidate using the obtained index and decode the current block using the identified merging candidate, wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for decoding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

An image coding apparatus according to an aspect of the present disclosure is an image coding apparatus which codes an image on a block-by-block basis to generate a coded bitstream, and includes: an merging candidate obtaining unit configured to obtain a fixed number of merging candidates each of which is a candidate set of a prediction direction, a motion vector, and a reference picture index to be referenced in decoding of a current block, the fixed number being greater than or equal to two; and a coding unit configured to attach, to the coded bitstream, an index for identifying a merging candidate among the fixed number of merging candidates, the identified merging candidate being a merging candidate to be referenced in the coding of the current block, wherein the fixed number of merging candidates include: one or more first candidates each derived based on a prediction direction, a motion vector, and a reference picture index which have been used for coding a neighboring block spatially or temporally neighboring the current block; and one or more second candidates having a predetermined fixed value.

An image coding and decoding apparatus according to an aspect of the present disclosure includes: the image decoding apparatus; and the image coding apparatus.

It should be noted that these general or specific aspects can be implemented as a system, a method, an integrated circuit, a computer program, a computer-readable recording medium such as a compact disc read-only memory (CD-ROM), or as any combination of a system, a method, an integrated circuit, a computer program, and a computer-readable recording medium.

An image coding apparatus and an image decoding apparatus according to an aspect of the present disclosure will be described specifically below with reference to the drawings.

Each of the exemplary embodiments described below shows a specific example for the present disclosure. The numerical values, shapes, materials, constituent elements, the arrangement and connection of the constituent elements, steps, the processing order of the steps etc. shown in the following exemplary embodiments are mere examples, and therefore do not limit the inventive concept in the present disclosure. Furthermore, 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 not necessarily required in order to overcome the disadvantages.

An image coding apparatus using an image coding method according to Embodiment 1 will be described with reference to FIG. 11 to FIG. 18. FIG. 11 is a block diagram showing a configuration of an image coding apparatus according to Embodiment 1. An image coding apparatus 100 codes an image on a block-by-block basis to generate a bitstream.

As shown in FIG. 11, the image coding apparatus 100 includes a subtractor 101, an orthogonal transformation unit 102, a quantization unit 103, an inverse-quantization unit 104, an inverse-orthogonal-transformation unit 105, an adder 106, block memory 107, frame memory 108, an intra prediction unit 109, an inter prediction unit 110, an inter prediction control unit 111, a picture-type determination unit 112, a switch 113, a merging block candidate calculation unit 114, colPic memory 115, and a variable-length-coding unit 116.

The subtractor 101 subtracts, on a block-by-block basis, prediction picture data from input image data included in an input image sequence to generate prediction error data.

The orthogonal transformation unit 102 transforms the generated prediction error data from a picture domain into a frequency domain.

The quantization unit 103 quantizes the prediction error data transformed into a frequency domain.

The inverse-quantization unit 104 inverse-quantizes the prediction error data quantized by the quantization unit 103. The inverse-orthogonal-transformation unit 105 transforms the inverse-quantized prediction error data from a frequency domain into a picture domain.

The adder 106 adds, on a block-by-block basis, prediction picture data and the prediction error data inverse-quantized by the inverse-orthogonal-transformation unit 105 to generate reconstructed image data.

The block memory 107 stores the reconstructed image data in units of a block.

The frame memory 108 stores the reconstructed image data in units of a frame.

The picture-type determination unit 112 determines in which of the picture types of I-picture, B-picture, and P-picture the input image data is to be coded. Then, the picture-type determination unit 112 generates picture-type information indicating the determined picture type.

The intra prediction unit 109 generates intra prediction picture data of a current block by performing intra prediction using reconstructed image data stored in the block memory 107 in units of a block.

The inter prediction unit 110 generates inter prediction picture data of a current block by performing inter prediction using reconstructed image data stored in the frame memory 108 in units of a frame and a motion vector derived by a process including motion estimation.

When a current block is coded by intra prediction coding, the switch 113 outputs intra prediction picture data generated by the intra prediction unit 109 as prediction picture data of the current block to the subtractor 101 and the adder 106. On the other hand, when a current block is coded by inter prediction coding, the switch 113 outputs inter prediction picture data generated by the inter prediction unit 110 as prediction picture data of the current block to the subtractor 101 and the adder 106.

The merging block candidate calculation unit 114 according to Embodiment 1 generates a merging block candidate list to include a fixed number of merging block candidates.

Specifically, the merging block candidate calculation unit 114 derives first candidates which are merging block candidates for merging mode using motion vectors and others of neighboring blocks of the current block and a motion vector and others of the co-located block (colPic information) stored in the colPic memory 115. Furthermore, the merging block candidate calculation unit 114 adds the derived merging block candidates to the merging block candidate list.

Furthermore, when the merging block candidate list has any empty entry, the merging block candidate calculation unit 114 selects a third candidate, which is a new candidate, from among predetermined merging block candidates to increase coding efficiency. Then, the merging block candidate calculation unit 114 adds the derived new candidate as a new merging block candidate to the merging block candidate list. Furthermore, the merging block candidate calculation unit 114 calculates the total number of the merging block candidates.

Furthermore, the merging block candidate calculation unit 114 assigns merging block candidate indexes each having a different value to the derived merging block candidates. Then, the merging block candidate calculation unit 114 transmits the merging block candidates and merging block candidate indexes to the inter prediction control unit 111. Furthermore, the merging block candidate calculation unit 114 transmits the calculated total number of the merging block candidates to the variable-length-coding unit 116.

The inter prediction control unit 111 selects a prediction mode using which prediction error is the smaller 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 motion vector derived from a merging block candidate is used (merging mode). The inter prediction control unit 111 also transmits a merging flag indicating whether or not the selected prediction mode is the merging mode to the variable-length-coding unit 116. Furthermore, the inter prediction control unit 111 transmits a merging block candidate index corresponding to the determined merging block candidates to the variable-length-coding unit 116 when the selected prediction mode is the merging mode. Furthermore, the inter prediction control unit 111 transfers the colPic information including the motion vector and others of the current block to the colPic memory 115.

The variable-length-coding unit 116 generates a bitstream by performing variable-length coding on the quantized prediction error data, the merging flag, and the picture-type information. The variable-length-coding unit 116 also sets the total number of merging block candidates as the size of the merging block candidate list. Furthermore, the variable-length-coding unit 116 performs variable-length coding on a merging block candidate index to be used in coding, by assigning, according to the size of the merging block candidate list, a bit sequence to the merging block candidate index.

FIG. 12 is a flowchart showing processing operations of the image coding apparatus 100 according to Embodiment 1.

In Step S101, the merging block candidate calculation unit 114 derives merging block candidates from neighboring blocks and a co-located block of a current block. Furthermore, the merging block candidate calculation unit 114 calculates the size of a merging block candidate list using a method described later when the size of the merging block candidate list is set variable.

For example, in the case shown in FIG. 3, the merging block candidate calculation unit 114 selects the neighboring blocks A to D as merging block candidates. Furthermore, the merging block candidate calculation unit 114 calculates, as a merging block candidate, a co-located merging block having a motion vector, a reference picture index, and a prediction direction which are calculated from the motion vector of a co-located block using the time prediction mode.

The merging block candidate calculation unit 114 assigns merging block candidate indexes to the respective merging block candidates. (a) in FIG. 13A is a table of a merging block candidate list in which merging block candidate indexes are assigned to neighboring blocks. The left column of the merging block candidate list in (a) in FIG. 13A lists merging block candidate indexes. The right column lists sets of a prediction directions, reference picture indexes, and motion vectors. Furthermore, using a method described later, the merging block candidate calculation unit 114 removes unusable-for-merging candidates and identical candidates and adds new candidates to update the merging block candidate list, and calculates the size of the merging block candidate list. (b) in FIG. 13A is a merging block candidate list after removing an unusable-for-merging candidate and an identical candidate and adding a new candidate. The neighboring block A and the neighboring block D are identical and the neighboring block D is removed in Embodiment 1, but the neighboring block A may be removed instead.

Shorter codes are assigned to merging block candidate indexes of smaller values. In other words, the smaller the value of a merging block candidate index, the smaller the amount of information necessary for indicating the merging block candidate index.

On the other hand, the larger the value of a merging block candidate index, the larger the amount of information necessary for the merging block candidate index. Therefore, coding efficiency will be increased when merging block candidate indexes of smaller values are assigned to merging block candidates which are more likely to have motion vectors of higher accuracy and reference picture indexes of higher accuracy.

Therefore, there may be case in which the merging block candidate calculation unit 114 counts the total number of times of selection of each merging block candidates as a merging block, and assigns merging block candidate indexes of smaller values to blocks with a larger total number of the times. Specifically, this can be achieved by specifying a merging block selected from neighboring blocks and assigning a merging block candidate index of a smaller value to the specified merging block when a current block is coded.

When a merging block candidate does not have information such as a motion vector (for example, when the merging block has been a block coded by intra prediction, it is located outside the boundary of a picture or the boundary of a slice, or it is yet to be coded), the merging block candidate is unusable for coding.

In Embodiment 1, such a merging block candidate unusable for coding is referred to as an unusable-for-merging candidate, and a merging block candidate usable for coding is referred to as a usable-for-merging candidate. In addition, among a plurality of merging block candidates, a merging block candidate identical in motion vector, reference picture index, and prediction direction to any other merging block is referred to as an identical candidate.

In the case shown in FIG. 3, the neighboring block C is an unusable-for-merging candidate because it is a block coded by intra prediction. The neighboring block D is an identical candidate because it is identical in motion vector, reference picture index, and prediction direction to the neighboring block A.

In Step S102, the inter prediction control unit 111 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 motion vector obtained from a merging block candidate. When the selected prediction mode is the merging mode, the inter prediction control unit 111 sets the merging flag to 1, and when not, the inter prediction control unit 111 sets the merging flag to 0.

In Step S103, whether or not the merging flag is 1 (that is, whether or not the selected prediction mode is the merging mode) is determined.

When the result of the determination in Step S103 is true (Yes, S103), the variable-length-coding unit 116 attaches the merging flag to a bitstream in Step S104. Subsequently, in Step S105, the variable-length-coding unit 116 assigns bit sequences according to the size of the merging block candidate list as shown in FIG. 5 to the merging block candidate indexes of merging block candidates to be used for coding. Then, the variable-length-coding unit 116 performs variable-length coding on the assigned bit sequence.

On the other hand, when the result of the determination in Step S103 is false (S103, No), the variable-length-coding unit 116 attaches information on a merging flag and a motion estimation vector mode to a bitstream in Step S106.

In Embodiment 1, a merging block candidate index having a value of “0” is assigned to the neighboring block A as shown in (a) in FIG. 13A. A merging block candidate index having a value of “1” is assigned to the neighboring block B. A merging block candidate index having a value of “2” is assigned to the co-located merging block. A merging block candidate index having a value of “3” is assigned to the neighboring block C. A merging block candidate index having a value of “4” is assigned to the neighboring block D.

It should be noted that the merging block candidate indexes having such a value may be assigned otherwise. For example, when a new candidate is added using a method described later, the variable-length-coding unit 116 may assign smaller values to preexistent merging block candidates and a larger value to the new candidate. In other words, the variable-length-coding unit 116 may assign a merging block candidate index of a smaller value to a preexistent merging block candidate in priority to a new candidate.

Furthermore, merging block candidates are, not limited to the blocks at the positions of the neighboring blocks A, B, C, and D. For example, a neighboring block located above the lower left neighboring block D can be used as a merging block candidate. Furthermore, it is not necessary to use all the neighboring blocks as merging block candidates. For example, it is also possible to use only the neighboring blocks A and B as merging block candidates.

Furthermore, although the variable-length-coding unit 116 attaches a merging block candidate index to a bitstream in Step S105 in FIG. 12 in Embodiment 1, attaching such a merging block candidate index to a bitstream is not always necessary. For example, the variable-length-coding unit 116 need not attach a merging block candidate index to a bitstream when the size of the merging block candidate list is “1”. The amount of information on the merging block candidate index is thereby reduced.

FIG. 14A is a flowchart showing details of the process in Step S101 in FIG. 12. Specifically, FIG. 14A illustrates a method of calculating merging block candidates and the size of a merging block candidate list. FIG. 14A will be described below.

Before the process shown in FIG. 14A, the merging block candidate calculation unit 114 assigns index values to the neighboring blocks (the neighboring blocks A to D and the co-located merging block) as shown in (a) in FIG. 13A.

Here, N denotes an index value for identifying a merging block candidate. In Embodiment 1, N takes values from 0 to 4. Specifically, the neighboring block A in FIG. 3 is assigned to a merging block candidate [0]. The neighboring block B in FIG. 3 is assigned to a merging block candidate [1]. The co-located merging block is assigned to a merging block candidate [2]. The neighboring block C in FIG. 3 is assigned to a merging block candidate [3]. The neighboring block D in FIG. 3 is assigned to a merging block candidate [4].

After assigning the index values to the neighboring blocks, the merging block candidate calculation unit 114 determines whether or not each of the merging block candidates [0] to [4] is usable for merging (Step S111), and obtains information on the merging block candidates [0] to [4] to enter in the right column of the merging block candidate list shown in FIG. 13A (Step S112).

In Step S111, the merging block candidate calculation unit 114 determines whether or not the merging block candidate [N] is a usable-for-merging candidate using a method described later, and derives the total number of merging block candidates.

In Step S112, the merging block candidate calculation unit 114 obtains a set of a motion vector, a reference picture index, and a prediction direction of the merging block candidate [N], and adds them to a merging block candidate list (the right column).

In Step S113, the merging block candidate calculation unit 114 searches the merging block candidate list for any unusable-for-merging candidate and identical candidate, and removes the unusable-for-merging candidate and identical candidate from the merging block candidate list as shown in (b) in FIG. 13A. Furthermore, the merging block candidate calculation unit 114 subtracts the total number of the removed identical candidates from the total number of the merging block candidates.

In Step S114, the merging block candidate calculation unit 114 adds a new candidate (third candidate) to the merging block candidate list using a method described later. Here, when the new candidate is added, merging block candidate indexes may be reassigned so that the merging block candidate indexes of smaller values are assigned to preexistent merging block candidates in priority to the new candidate. In other words, the merging block candidate calculation unit 114 may reassign the merging block candidate indexes so that a merging block candidate index of a larger value is assigned to the new candidate. The amount of code of merging block candidate indexes is thereby reduced.

In Step S115, the merging block candidate calculation unit 114 sets the total number of merging block candidates after the adding of the new candidate as the size of the merging block candidate list. In the example shown in (b) in FIG. 13A, the total number of merging block candidates is calculated to be “5”, and the size of the merging block candidate list is set at “5”. It should be noted that when the size of the merging block candidate list is set not variable but at a fixed number, for example, a number greater than or equal to two, the fixed number greater than or equal to two is set as the size of the merging block candidate list.

The new candidate in Step S114 is a candidate newly added to merging block candidates using a method described later when the total number of merging block candidates is smaller than a maximum number of merging block candidates. Examples of such a new candidate include a neighboring block located above the lower-left neighboring block D in FIG. 3, a block which is included in a reference picture including a co-located block and corresponds to one of the neighboring blocks A, B, C, and D, and a block having values statistically obtained from motion vectors, reference picture indexes, and prediction directions of the whole or a certain region of a reference picture. Examples of such a new candidate further include a zero candidate which has a motion vector having a value of zero for each referable reference picture. Examples of such a new candidate further include a bi-predictive merging block candidate which is a combination of a set of a motion vector and a reference picture index for a prediction direction 0 of one of derived merging block candidates and a set of a motion vector and a reference picture index for a prediction direction 1 of a different one of the derived merging block candidates. Such a bi-predictive merging block candidate is hereinafter referred to as a combined merging block. In this manner, when the total number of merging block candidates is smaller than a maximum number of merging block candidates, the image coding apparatus 100 adds a new candidate so that coding efficiency can be increased.

FIG. 15A is a flowchart showing details of the process in Step S111 in FIG. 14A. Specifically, FIG. 15A illustrates a method of determining whether or not a merging block candidate [N] is a usable-for-merging candidate and updating the total number of usable-for-merging candidates. FIG. 15A will be described below.

In Step S121, the merging block candidate calculation unit 114 determines whether it is true or false that (1) a merging block candidate [N] has been coded by intra prediction, (2) the merging block candidate [N] is a block outside the boundary of a slice including the current block or the boundary of a picture including the current block, or (3) the merging block candidate [N] is yet to be coded.

When the result of the determination in Step 121 is true (Step S121, Yes), the merging block candidate calculation unit 114 sets the merging block candidate [N] as an unusable-for-merging candidate in Step S122. On the other hand, when the result of the determination in Step S121 is false (Step S121, No), the merging block candidate calculation unit 114 sets the merging block candidate [N] as a usable-for-merging candidate in Step S123.

In Step S124, the merging block candidate calculation unit 114 determines whether it is true or false that the merging block candidate [N] is either a usable-for-merging candidate or a co-located merging block candidate.

Here, when the result of the determination in Step S124 is true (Step S124, Yes), the merging block candidate calculation unit 114 updates the total number of merging block candidates by incrementing it by one in Step S125. When the result of the determination in Step S124 is false (Step S124, No), the merging block candidate calculation unit 114 does not update the total number of merging block candidates.

In this manner, when a co-located merging block is calculated as a merging block candidate, the merging block candidate calculation unit 114 according to Embodiment 1 increments the total number of merging block candidates by one regardless of whether the co-located block is a usable-for-merging candidate or an unusable-for-merging candidate. This prevents discrepancy in the total number of merging block candidates between the image coding apparatus and the image decoding apparatus even when information on a co-located merging block is lost due to an incident such as packet loss. In Step S115 in FIG. 14A, the merging block candidate calculation unit 114 sets the total number of merging block candidates as the size of the merging block candidate list. Furthermore, in Step S105 in FIG. 12, the merging block candidate calculation unit 114 performs variable-length coding on a merging block candidate index by assigning a bit sequence according to the size of the merging block candidate list. This makes it possible to generate a bitstream which can be normally decoded so that a merging block candidate index can be obtained even when information on reference picture including a co-located block is lost.

FIG. 16 is a flowchart showing details of the process in Step S114 in FIG. 14A. Specifically, FIG. 16 illustrates a method of adding a new candidate (third candidate) to increase coding efficiency. FIG. 16 will be described below.

In Step S131, the merging block candidate calculation unit 114 determines whether or not the total number of merging block candidates is smaller than the size of the merging block candidate list. More specifically, when the size of the merging block candidate list is variable, the merging block candidate calculation unit 114 determines whether or not the total number of merging block candidates is smaller than a maximum value of the candidate list size (a maximum number of merging block candidates). On the other hand, when the size of the merging block candidate list is invariable (the size of the merging block candidate list is a fixed number greater than or equal to two), the merging block candidate calculation unit 114 determines whether or not the total number of merging block candidates is smaller than the fixed number greater than or equal to two.

Here, when the result of the determination in Step S131 is true (Step S131, Yes), in Step S132, the merging block candidate calculation unit 114 determines whether or not there is a new candidate which can be added as a merging block candidate to the merging block candidate list.

The new candidate is a prepared candidate, such as a zero candidate which has a motion vector having a value of zero for each referable reference picture. In this case, the total number of referable reference pictures is the total number of candidates which can be added as new candidates. The new candidate may be a candidate other than such a zero candidate, such as a combined candidate as described above.

When the result of the determination in Step S132 is true (Step S132, Yes), the merging block candidate calculation unit 114 assigns a merging block candidate index having a value to the new candidate and adds the new candidate to the merging block candidate list in Step S133.

Furthermore, in Step S134, the merging block candidate calculation unit 114 increments the total number of merging block candidates by one.

On the other hand, when the result of the determination in Step S131 or in Step S132 is false (Step S131 or Step S132, No), the process for adding a new candidate ends. In other words, when the total number of merging block candidates reaches the maximum number of merging block candidates or when there is no more new candidate (that is, all new candidates have been added as merging block candidates to the candidate list), the process for adding a new candidate ends.

FIG. 18 is a flowchart showing details of the process in Step S102 in FIG. 12. Specifically, FIG. 18 illustrates a process for selecting a merging block candidate. FIG. 18 will be described below.

In Step S151, the inter prediction control unit 111 sets a merging block 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. Here, 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 code necessary for coding a motion vector used for generation of a prediction picture. λ denotes an undetermined Lagrange multiplier.

In Step S152, the inter prediction control unit 111 determines whether or not the value of a merging block candidate index is smaller than the total number of merging block candidates of a current block. In other words, the inter prediction control unit 111 determines whether or not there is still a merging block candidate on which the process from Step S153 to Step S155 has not been performed yet.

When the result of the determination in Step S152 is true (S152, Yes), in Step S153, the inter prediction control unit 111 calculates the cost for a merging block candidate to which a merging block candidate index is assigned. Then, in Step S154, the inter prediction control unit 111 determines whether or not the calculated cost for a merging block candidate is smaller than the minimum prediction error.

Here, when the result of the determination in Step S154 is true, (S154, Yes), the inter prediction control unit 111 updates the minimum prediction error, the merging block candidate index, and the value of the merging flag in Step S155. On the other hand, when the result of the determination in Step S154 is false (S154, No), the inter prediction control unit 111 does not update the minimum prediction error, the merging block candidate index, or the value of the merging flag.

In Step S156, the inter prediction control unit 111 increments the merging block candidate index by one, and repeats from Step S152 to Step S156.

On the other hand, when the result of the determination in Step S152 is false (S152, No), that is, there is no more unprocessed merging block candidate, the inter prediction control unit 111 fixes the final values of the merging flag and merging block candidate index in Step S157.

Thus, the image coding apparatus 100 according to Embodiment 1 calculates the size of a merging block candidate list for use in coding or decoding of a merging block candidate index, using a method independent of information on reference pictures including a co-located block so that error resistance can be enhanced. More specifically, in the image coding apparatus 100 according to Embodiment 1, the total number of merging block candidates is incremented by one for each co-located merging block regardless of whether the co-located merging block is a usable-for-merging candidate or an unusable-for-merging candidate. Then, bit sequences to be assigned to merging block candidate indexes are determined according to the total number of merging block candidates. This allows the image coding apparatus 100 and the image decoding apparatus 300 to have the same total number of merging block candidates so that a bitstream can be normally decoded to obtain a merging block candidate index even when information on reference picture including a co-located block is lost. Thus, when the total number of merging block candidates is smaller than the total number of usable-for-merging candidates, a new candidate having a new set of a motion vector, a reference picture index, and a prediction direction is added so that coding efficiency can be increased.

It should be noted that Embodiment 1 in which the total number of merging block candidates is incremented by one only for each co-located merging block regardless of whether the co-located merging block is a usable-for-merging candidate or an unusable-for-merging candidate as shown in Step S125 and Step S126 in FIG. 15A, is not limiting. The total number of merging block candidates may be incremented by one for any other block regardless of whether the block is a usable-for-merging candidate or an unusable-for-merging candidate.

Optionally, in Embodiment 1, when the size of a merging block candidate list is a fixed number greater than or equal to two, the fixed number greater than or equal to two may be set as a maximum value Max of the total number of merging block candidates. In other words, merging block candidate indexes may be coded using the size of a merging block candidate list fixed at a maximum value Max of the total number of merging block candidates on the assumption that the merging block candidates which are neighboring blocks are all usable-for-merging candidates. For example, in Embodiment 1, the maximum value Max of the total number of merging block candidates is 5 (neighboring block A, neighboring block B, co-located merging block, neighboring block C, and neighboring block D). In this case, merging block candidate indexes may be coded using the size of a merging block candidate list fixedly set at “5”.

Optionally, for example, when the maximum value Max of the total number of merging block candidates is set at 4 (neighboring block A, neighboring block B, neighboring block C, and neighboring block D) for a current picture which is to be coded without referencing a co-located merging block (a B-picture or a P-picture to be coded with reference to an I-picture), merging block candidate indexes may be coded using the size of a merging block candidate list fixedly set at “4”.

In this manner, when the size of a merging block candidate list is a fixed number greater than or equal to two, a maximum value Max of the total number of merging block candidates may be set at the fixed number greater than or equal to two to determine the size of the merging block candidate list according to the fixed number greater than or equal to two. In this case, the image coding apparatus 100 performs variable-length coding using the fixed number greater than or equal to two in Step S105 in FIG. 12.

It is therefore possible to generate a bitstream from which a variable-length-decoding unit of an image decoding apparatus can decode a merging block candidate index without referencing information on a neighboring block or on a co-located block, so that computational complexity for the variable-length-decoding unit can be reduced. Furthermore, for example, a fixed number greater than or equal to two (for example, a maximum value Max of the total number of merging block candidates) may be embedded in a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, or the like. This makes it possible to switch between fixed numbers greater than or equal to two for each current picture so that computational complexity can be reduced and coding efficiency can be increased.

It should be noted that the example described in Embodiment 1 in which merging flag is always attached to a bitstream in merging mode is not limiting. For example, the merging mode may be forcibly selected based on the shape of a reference block for use in inter prediction of a current block. In this case, the amount of information can be reduced by attaching no merging flag to a bitstream.

It should be noted that the example described in Embodiment 1 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 is not limiting. For example, a skip merging mode may be used. In the skip merging mode, a current block is coded with reference to a merging block candidate list created as shown in (b) in FIG. 13A, using a prediction direction, a motion vector, and a reference picture index copied from a neighboring block of the current block in the same manner as in the merging mode. When all resultant prediction errors are zero for the current block, a skip flag set at 1 and the skip flag and a merging block candidate index are attached to a bitstream. When any of the resultant prediction errors is non-zero, a skip flag is set at 0 and the skip flag, a merging flag, a merging block candidate index, and data of the prediction errors are attached to a bitstream.

It should be noted that the example described in Embodiment 1 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 is not limiting. For example, a motion vector in the motion vector estimation mode may be coded using a merging block candidate list created as shown in (b) in FIG. 13A. Specifically, a difference is calculated by subtracting a motion vector of a merging block candidate indicated by a merging block candidate index from a motion vector in the motion vector estimation mode. Furthermore, the calculated difference and the merging block candidate index may be attached to a bitstream.

Optionally, a difference may be calculated by scaling a motion vector MV_Merge of a merging block candidate using a reference picture index RefIdx_ME in the motion estimation mode and a reference picture index RefIdx_Merge of the merging block candidate and subtracting a motion vector scaledMV_Merge of the merging block candidate after the scaling from the motion vector in the motion estimation mode. Furthermore, the calculated difference and the merging block candidate index may be attached to a bitstream. The following is an exemplary formula for the scaling.

(Equation 2)

scaledMV_Merge=MV_Merge×(POC(RefIdx_ME)−curPOC)/(POC(RefIdx_Merge)−curPOC)  (2)

Here, POC (RefIdx_ME) denotes the display order of a 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.

It should be noted that the variable-length coding (see FIG. 5) which is performed in Embodiment 1 according to the size of a merging block candidate list in Step S105 in FIG. 12 may be performed optionally according to another parameter such as the total number of merging block candidates calculated as the total number of usable-for-merging candidates which is the sum of the total number of first candidates and the total number of identical candidates calculated in Step S111 (detailed in FIG. 15A) in FIG. 14A.

An image decoding apparatus using an image decoding method according to Embodiment 2 will be described with reference to FIG. 19 to FIG. 22. FIG. 19 is a block diagram showing a configuration of an image decoding apparatus 300 according to Embodiment 2. The image decoding apparatus 300 is an apparatus corresponding to the image coding apparatus 100 according to Embodiment 1. Specifically, for example, the image decoding apparatus 300 decodes, on a block-by-block basis, coded images included in a bitstream generated by the image coding apparatus 100 according to Embodiment 1.

As shown in FIG. 19, the image decoding apparatus 300 includes a variable-length-decoding unit 301, an inverse-quantization unit 302, an inverse-orthogonal-transformation unit 303, an adder 304, block memory 305, frame memory 306, an intra prediction unit 307, an inter prediction unit 308, an inter prediction control unit 309, a switch 310, a merging block candidate calculation unit 311, and colPic memory 312.

The variable-length-decoding unit 301 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 301 performs variable-length decoding on a merging block candidate index using the total number of merging block candidates calculated by the merging block candidate calculation unit 311.

The inverse-quantization unit 302 inverse-quantizes the quantized coefficient obtained by the variable-length decoding.

The inverse-orthogonal-transformation unit 303 generates prediction error data by transforming an orthogonal transformation coefficient obtained by the inverse quantization from a frequency domain to a picture domain.

The block memory 305 stores, in units of a block, decoded image data generated by adding the prediction error data and prediction picture data.

The frame memory 306 stores decoded image data in units of a frame.

The intra prediction unit 307 generates prediction picture data of a current block to be decoded, by performing intra prediction using the decoded image data stored in the block memory 305 in units of a block.

The inter prediction unit 308 generates prediction picture data of a current block to be decoded, by performing inter prediction using the decoded image data stored in the frame memory 306 in units of a frame.

When a current block is decoded by intra prediction decoding, the switch 310 outputs intra prediction picture data generated by the intra prediction unit 307 as prediction picture data of the current block to the adder 304. On the other hand, when a current block is decoded by inter prediction decoding, the switch 310 outputs inter prediction picture data generated by the inter prediction unit 308 as prediction picture data of the current block to the adder 304.

The merging block candidate calculation unit 311 derives merging block candidates from motion vectors and others of neighboring blocks of the current block and a motion vector and others of a co-located block (colPic information) stored in the colPic memory 312. Furthermore, the merging block candidate calculation unit 311 adds the derived merging block candidate to a merging block candidate list.

Furthermore, using a method described later, the merging block candidate calculation unit 311 derives, for example, a merging block candidate having a prediction direction, a motion vector, and a reference picture index for a stationary region as a new candidate (third candidate) for increasing coding efficiency. Then, the merging block candidate calculation unit 311 adds the derived new candidate as a new merging block candidate to the merging block candidate list. Furthermore, the merging block candidate calculation unit 311 calculates the total number of merging block candidates when the size of the merging block candidate list is variable.

Furthermore, the merging block candidate calculation unit 311 assigns merging block candidate indexes each having a different value to the derived merging block candidates. Then, the merging block candidate calculation unit 311 transmits the merging block candidates to which the merging block candidate indexes have been assigned to the inter prediction control unit 309. Furthermore, the merging block candidate calculation unit 311 transmits the calculated total number of merging block candidates to the variable-length-decoding unit 301 when the size of the merging block candidate list is variable.

The inter prediction control unit 309 causes the inter prediction unit 308 to generate an inter prediction picture using information on motion vector estimation mode when the merging flag decoded is “0”. On the other hand, when the merging flag is “1”, the inter prediction control unit 309 determines, based on a decoded merging block candidate index, a motion vector, a reference picture index, and a prediction direction for use in inter prediction from a plurality of merging block candidates. Then, the inter prediction control unit 309 causes the inter prediction unit 308 to generate an inter prediction picture using the determined motion vector, reference picture index, and prediction direction. Furthermore, the inter prediction control unit 309 transfers colPic information including the motion vector of the current block to the colPic memory 312.

Finally, the adder 304 generates decoded image data by adding the prediction picture data and the prediction error data.

FIG. 20 is a flowchart showing processing operations of the image decoding apparatus 300 according to Embodiment 2.

In Step S301, the variable-length-decoding unit 301 decodes a merging flag.

When the merging flag is “1” in Step S302 (Step S302, Yes), in Step S303, the merging block candidate calculation unit 311 calculates the total number of merging block candidates as the size of a merging block candidate list.