Image encoding/decoding method and device, and recording medium in which bitstream is stored

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

obtaining L0 motion information, including an L0 initial motion vector and an L0 reference picture, and L1 motion information, including an L1 initial motion vector and an L1 reference picture, of a current block, the L0 and L1 motion information being obtained from a merge candidate included in a merge candidate list;

determining motion vector refinement information based on the L0 and L1 reference pictures, based on motion vector refinement information, indicating whether to refine the L0 and L1 initial motion vectors being determined;

when the motion vector refinement information indicates that the L0 and L1 initial motion vectors are to be refined, determining L0 and L1 refined motion vectors by refining the L0 and L1 initial motion vectors by searching for a pair of L0 reference block, in the L0 reference picture, and an L1 reference block, in the L1 reference picture, having a minimum SAD (Sum of Absolute Difference); and

obtaining prediction samples of the current block based on the L0 and L1 refined motion vectors,

wherein the motion vector refinement information is determined based on whether a first POC difference between a current picture and the L0 reference picture is the same as a second POC difference between the L1 reference picture and the current picture.

2. The method of claim 1 , wherein the L0 reference block is determined by the L0 initial motion vector and an L0 differential vector and the L1 reference block is determined by the L1 initial motion vector and an L1 differential vector, and

the L1 differential vector is derived by mirroring the L0 differential vector.

3. The method of claim 1 , wherein the current block is divided into sub-blocks when a size of the current block is greater than a predetermined size, and the L0 and L1 refined motion vectors are determined in units of sub-blocks.

4. The method of claim 3 ,

wherein initial motion vectors of the sub-block are determined based on the initial motion vectors of the current block for determining the refined motion vector per a sub-block basis.

5. The method of claim 1 , wherein the searching for the pair of L0 reference block and the L1 reference block is performed in a searching range, the searching range being configured from +2 to −2 integer pixels in horizontal and vertical directions from a pixel position indicated by an initial motion vector.

6. The method of claim 1 , wherein the motion vector refinement information is determined to indicate that the L0 and L1 initial motion vectors are not to be refined, when a merge mode is not applied to the current block.

7. The method of claim 1 , wherein the motion vector refinement information is determined to indicate that the L0 and L1 initial motion vectors are not to be refined, when a size of the current block is less than 8×8.

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

determining obtaining L0 motion information, including an L0 initial motion vector and an L0 reference picture, and L1 motion information, including an L1 initial motion vector and an L1 reference picture, of a current block, index information specifying a merge candidate, that has the same L0 and L1 motion information of the current block, among a plurality of merge candidates being encoded;

determining motion vector refinement information based on the L0 and L1 reference pictures, based on motion vector refinement information, whether to refine the L0 and L1 initial motion vectors being determined;

when the motion vector refinement information indicates that the L0 and L1 initial motion vectors are to be refined, determining L0 and L1 refined motion vectors by refining the L0 and L1 initial motion vectors by searching for a pair of L0 reference block, in the L0 reference picture, and an L1 reference block, in the L1 reference picture, having a minimum SAD (Sum of Absolute Difference); and

obtaining prediction samples of the current block based on the L0 and L1 refined motion vectors of the current block,

wherein the motion vector refinement information is determined based on whether a first POC difference between a current picture and the L0 reference picture is the same as a second POC difference between the L1 reference picture and the current picture.

9. The method of claim 8 , wherein the L0 reference block is determined by the L0 initial motion vector and an L0 differential vector and the L1 reference block is determined by the L1 initial motion vector and an L1 differential vector, and

the L1 differential vector is derived by mirroring the L0 differential vector.

10. The method of claim 8 , wherein the current block is divided into sub-blocks when a size of the current block is greater than a predetermined size, and the L0 and L1 refined motion vectors are determined in units of sub-blocks.

11. The method of claim 10 ,

wherein initial motion vectors of the sub-block are determined based on the initial motion vectors of the current block for determining the refined motion vector per a sub-block basis.

12. The method of claim 8 , wherein the searching for the pair of L0 reference block and L1 reference block is performed in a searching range, the searching range being configured from +2 to −2 integer pixels in horizontal and vertical directions from a pixel position indicated by an initial motion vector.

13. The method of claim 8 , wherein the motion vector refinement information is determined to indicate that the L0 and L1 initial motion vectors are not to be refined, when a merge mode is not applied to the current block.

14. The method of claim 8 , wherein the motion vector refinement information is determined to indicate that the L0 and L1 initial motion vectors are not to be refined, when a size of the current block is less than 8×8.

15. An apparatus for transmitting compressed video data, the apparatus comprising:

a processor configured to obtain the compressed video data which comprising information to determine L0 motion information, including an L0 initial motion vector and an L0 reference picture, and L1 motion information, including an L1 initial motion vector and an L1 reference picture, of a current block; and

a transmitting unit configured to transmit the compressed video data,

wherein motion vector refinement information is determined based on the L0 and L1 reference pictures,

wherein, based on motion vector refinement information, whether to refine the L0 and L1 initial motion vectors is determined,

wherein when the motion vector refinement information indicates that the L0 and L1 initial motion vectors are to be refined, L0 and L1 refined motion vectors are determined by refining the L0 and L1 initial motion vectors by searching for a pair of L0 reference block, in the L0 reference picture, and an L1 reference block, in the L1 reference picture, having a minimum SAD (Sum of Absolute Difference),

wherein prediction samples of the current block based on the L0 and L1 refined motion vectors, and

wherein the motion vector refinement information is determined based on whether a first POC difference between a current picture and the L0 reference picture is the same as a second POC difference between the L1 reference picture and the current picture.