Inter-prediction method and image decoding device

1. A video decoding apparatus comprising:

a memory; and

one or more processors configured to:

determine that a current block is decoded using inter-prediction;

derive a motion vector of the current block based on motion information decoded from a bitstream;

acquire reference samples of a first reference block by using the motion vector, wherein reference samples of an external region located outside a reference picture among the first reference block are acquired from reference samples of a corresponding region corresponding to the external region within the reference picture;

predict the current block based on the acquired reference samples;

derive a residual block of the current block from the bitstream; and

reconstruct the current block based on the residual block and the prediction block,

wherein the reference samples of the corresponding region within the reference picture are determined by shifting sample positions in the external region toward an inside of the reference picture by an offset determined based on a picture width.

2. The video decoding apparatus of claim 1 , wherein the one or more processors is further configured to adjust the motion vector to point a second reference block corresponding to the first reference block, and

wherein the corresponding region is a region included in the second reference block.

3. The video decoding apparatus of claim 2 , wherein the one or more processors is further configured to adjust the motion vector when the motion vector points an outside of the reference picture.

4. The video decoding apparatus of claim 1 , wherein the one or more processors is further configured to indicate a directional relationship between the first reference block and a second reference block after the deriving of the motion vector,

wherein the second reference block corresponds to the first reference block and includes the corresponding region, and

wherein the sample acquirer acquires reference samples from the corresponding region according to the indicated directional relationship.

5. The video decoding apparatus of claim 4 , wherein the reference picture is in an equirectangular projection format for a 360-degree image,

wherein, when the external region is located outside an upper or lower end of the reference picture, the relationship indicator indicates that the first reference block and the second reference block are opposite to each other in terms of the directional relationship with respect to a vertical direction of the reference picture,

wherein the sample acquirer acquires the reference samples of the corresponding region in a direction opposite to a direction of arrangement of the reference samples of the external region with respect to the vertical direction of the reference picture.

6. The video decoding apparatus of claim 1 , wherein the corresponding region is a region obtained by shifting the external region in a horizontal direction by the offset derived from offset information contained in the bitstream.

7. The video decoding apparatus of claim 6 , wherein, when the external region is located outside a left side of the reference picture, the corresponding region is obtained by shifting the external region in a rightward direction by the offset, and

wherein, when the external region is located outside a right side of the reference picture, the corresponding region is obtained by shifting the external region in a leftward direction by the offset.

8. A video encoding apparatus comprising:

a memory; and

one or more processors configured to:

determine that a current block is encoded using inter-prediction;

determine a motion vector of the current block;

acquire reference samples of a first reference block related to the motion vector, wherein reference samples of an external region located outside a reference picture among the first reference block are acquired from reference samples of a corresponding region corresponding to the external region within the reference picture;

predict the current block based on the acquired reference samples;

derive a residual block of the current block based on original samples within the current block and predicted samples within the prediction block; and

encode a residual block of the current block into a bitstream,

wherein the reference samples of the corresponding region within the reference picture are determined by shifting sample positions in the external region toward an inside of the reference picture by an offset determined based on a picture width.

9. A non-transitory computer readable medium storing a bitstream containing encoded data for a block of video data, the encoded data being encoded by an inter-prediction coding method, the method comprising:

determining that a current block is encoded using inter-prediction;

determining a motion vector of the current block;

acquiring reference samples of a first reference block related to the motion vector, wherein reference samples of an external region located outside a reference picture among the first reference block are acquired from reference samples of a corresponding region corresponding to the external region within the reference picture;

predicting the current block based on the acquired reference samples;

deriving a residual block of the current block based on original samples within the current block and predicted samples within the prediction block; and

encoding a residual block of the current block into a bitstream,

wherein the reference samples of the corresponding region within the reference picture are determined by shifting sample positions in the external region toward an inside of the reference picture by an offset determined based on a picture width.