Image decoding device, image decoding method, and image decoding program

1. An image decoding device comprising:

a merge candidate list generation unit that generates a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a decoded image located at the same position as the block subject to prediction;

a merge candidate selection unit that selects, as a selected merge candidate, a merge candidate from the merge candidate list;

a bitstream decoding unit that decodes a bitstream to derive a motion vector difference; and

a merge candidate correction unit that derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture.

2. An image decoding method comprising:

generating a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a decoded image located at the same position as the block subject to prediction;

selecting, as a selected merge candidate, a merge candidate from the merge candidate list;

decoding a bitstream to derive a motion vector difference; and

deriving a corrected merge candidate of bidirectional prediction by adding the motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and deriving a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture.

3. A recording medium encoded with an image decoding program, the program comprising computer-implemented modules including:

a module that generates a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a decoded image located at the same position as the block subject to prediction;

a module that selects, as a selected merge candidate, a merge candidate from the merge candidate list;

a module that decodes a bitstream to derive a motion vector difference; and

a module that derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture.

4. An image coding device comprising:

a merge candidate list generation unit that generates a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a coded image located at the same position as the block subject to prediction;

a merge candidate selection unit that selects, as a selected merge candidate, a merge candidate from the merge candidate list;

a merge candidate correction unit that derives a corrected merge candidate of bidirectional prediction by adding a motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture; and

a bitstream coding unit that codes the motion vector difference into a bitstream.

5. An image coding method comprising:

generating a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a coded image located at the same position as the block subject to prediction;

selecting, as a selected merge candidate, a merge candidate from the merge candidate list;

deriving a corrected merge candidate of bidirectional prediction by adding a motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture; and

coding the motion vector difference into a bitstream.

6. A recording medium encoded with an image coding program, the program comprising computer-implemented modules including:

a module that generates a merge candidate list including, as a merge candidate, motion information on a plurality of blocks neighboring a block subject to prediction and motion information having a motion vector derived by scaling a motion vector of a block on a coded image located at the same position as the block subject to prediction;

a module that selects, as a selected merge candidate, a merge candidate from the merge candidate list;

a module that derives a corrected merge candidate of bidirectional prediction by adding a motion vector difference to a motion vector of the selected merge candidate for a first prediction without scaling and subtracting the motion vector difference from a motion vector of the selected merge candidate for a second prediction without scaling, when a reference picture of the selected merge candidate for a first prediction or a reference picture of the selected merge candidate for a second prediction is a long-term reference picture and derives a corrected merge candidate of bidirectional prediction by adding the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, to a motion vector of the selected merge candidate for a first prediction and subtracting the motion vector difference, which is scaled in accordance with an interval between a picture subject to prediction and the reference picture, from a motion vector of the selected merge candidate for a second prediction, when the reference picture of the selected merge candidate for the first prediction is not a long-term reference picture and the reference picture of the selected merge candidate for the second prediction is not a long-term reference picture; and

a module that codes the motion vector difference into a bitstream.