Method and device for encoding or decoding image on basis of inter mode

1. A method of decoding an image, the method comprising:

deriving, based on an inter mode pre-defined in an image decoding apparatus, motion information for bi-directional prediction of a current block in the image; and

performing inter prediction on the current block based on the motion information,

wherein, according to the pre-defined inter mode, the motion information for the bi-directional prediction is adjusted to motion information for uni-directional prediction,

wherein, in response to a case where the pre-defined inter mode is a merge mode, the motion information for the bi-directional prediction is adjusted to the motion information for the uni-directional prediction, and

wherein, in response to a case where the pre-defined inter mode is an affine mode, the motion information for the bi-directional prediction is not adjusted to the motion information for the uni-directional prediction.

2. The method of claim 1 ,

wherein, in response to the case where the pre-defined inter mode is the merge mode, the motion information for the bi-directional prediction is adjusted to the motion information for the uni-directional prediction in consideration of a size of the current block.

3. The method of claim 2 ,

wherein, in response to the case where the pre-defined inter mode is the merge mode, deriving the motion information comprises:

constructing a merge candidate list of the current block; and

deriving the motion information of the current block from the merge candidate list,

wherein the merge candidate list includes at least one of a spatial merge candidate, a temporal merge candidate, or a combined merge candidate.

4. The method of claim 1 ,

wherein, in response to the case where the pre-defined inter mode is the affine mode, deriving the motion information comprises:

generating a candidate list consisting of affine candidates of the current block;

deriving a control point vector of the current block based on the candidate list and a candidate index; and

deriving a motion vector of the current block based on the control point vector of the current block.

5. The method of claim 4 ,

wherein the affine candidates include at least one of a spatial candidate, a temporal candidate, or a configured candidate.

6. The method of claim 4 ,

wherein, in consideration of a size of a predetermined reference region, the motion vector of the current block is derived, and

wherein the reference region includes a reference block specified by motion vectors of n sub-blocks and a pixel used to interpolate a fractional pel of the reference block.

7. The method of claim 6 ,

wherein, in response to a case where the size of the reference region is less than or equal to a threshold value, the motion vector of the current block is derived for each sub-block, and

wherein, in response to a case where the size of the reference region is greater than the threshold value, one motion vector is derived for the current block.

8. The method of claim 7 ,

wherein the motion vector of the current block is derived based on an affine motion parameter, and

wherein the affine motion parameter is calculated based on the control point vector of the current block.

9. The method of claim 8 ,

wherein the current block belongs to a predetermined sub-picture, and

wherein the sub-picture is defined as a group of grids having the same index among a plurality of grids constituting one picture.

10. A method of encoding an image, the method comprising:

determining, based on an inter mode pre-defined in an image encoding apparatus, motion information for bi-directional prediction of a current block in the image; and

performing inter prediction on the current block in response to the motion information,

wherein, according to the pre-defined inter mode, the motion information for the bi-directional prediction is adjusted to motion information for uni-directional prediction,

wherein, in response to a case where the pre-defined inter mode is a merge mode, the motion information for the bi-directional prediction is adjusted to the motion information for the uni-directional prediction, and

wherein, in response to a case where the pre-defined inter mode is an affine mode, the motion information for the bi-directional prediction is not adjusted to the motion information for the uni-directional prediction.

11. A non-transitory computer-readable medium for storing a bitstream generated by an image encoding method,

the image encoding method comprising:

determining, based on an inter mode pre-defined, motion information for bi-directional prediction of a current block; and

performing inter prediction on the current block in response to the motion information,

wherein, according to the pre-defined inter mode, the motion information for the bi-directional prediction is adjusted to motion information for uni-directional prediction,

wherein, in response to a case where the pre-defined inter mode is a merge mode, the motion information for the bi-directional prediction is adjusted to the motion information for the uni-directional prediction, and

wherein, in response to a case where the pre-defined inter mode is an affine mode, the motion information for the bi-directional prediction is not adjusted to the motion information for the uni-directional prediction.