Inter prediction method and apparatus based on history-based motion vector
An image decoding method according to the present disclosure includes deriving a history-based motion vector prediction (HMVP) buffer for a current block, deriving motion information of the current block based on an HMVP candidate included in the HMVP buffer, generating prediction samples for the current block based on the motion information, and generating reconstructed samples based on the prediction samples, and the HMVP buffer is initialized when processing a first-ordered CTU in a CTU row in which a current CTU is located, and the current block is included in the current CTU.
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining image information including merge index information from a bitstream;
deriving a history-based motion vector prediction (HMVP) buffer for a current block;
deriving a merge candidate list including merge candidates for the current block,
wherein the merge candidates includes spatial candidates derived based on spatial candidate blocks and a temporal candidate derived based on a temporal candidate block,
wherein an HMVP candidate comprised in the HMVP buffer is inserted as a merge candidate of the merge candidate list,
wherein the HMVP candidate is inserted after the temporal candidate in the merge candidate list;
deriving motion information of the current block based on the merge index information and the merge candidate list;
generating prediction samples for the current block based on the motion information; and
generating reconstructed samples based on the prediction samples,
wherein the current block corresponds to a coding unit (CU) which is split from a coding tree unit (CTU), and
wherein the HMVP buffer is initialized at a CTU at a specific location,
wherein the CTU at the specific location is a first-ordered CTU in a CTU row of a slice.
2. The image decoding method of claim 1 , wherein based on the HMVP buffer being initialized, the number of HMVP candidates comprised in the HMVP buffer is set to zero.
3. The image decoding method of claim 1 , further comprising determining whether a current CTU is the first-ordered CTU in the CTU row of the slice.
4. The image decoding method of claim 3 , wherein based on a determination that the current CTU is the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block decoded earlier than the current block in the current CTU, and
wherein based on a determination that the current CTU is not the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block decoded earlier than the current block in the current CTU and an HMVP candidate derived based on motion information of a block previously decoded in a previous CTU in the CTU row.
5. The image decoding method of claim 3 , wherein whether the current CTU is the first-ordered CTU in the CTU row of the slice is determined based on the following equation,
ctu_idx % Num
where the ctu_idx refers to an index of the current CTU, the % refers to modulus operator, and the Num refers to the number of CTUs in the CTU row.
6. An image encoding method performed by an encoding apparatus, the method comprising:
deriving a history-based motion vector prediction (HMVP) buffer for a current block;
deriving a merge candidate list including merge candidates for the current block,
wherein the merge candidates includes spatial candidates derived based on spatial candidate blocks and a temporal candidate derived based on a temporal candidate block,
wherein an HMVP candidate comprised in the HMVP buffer is inserted as a merge candidate of the merge candidate list,
wherein the HMVP candidate is inserted after the temporal candidate in the merge candidate list;
deriving merge index information of the current block based on the merge candidate list; and
encoding image information including the merge index information,
wherein the current block corresponds to a coding unit (CU) which is split from a coding tree unit (CTU), and
wherein the HMVP buffer is initialized at a CTU at a specific location,
wherein the CTU at the specific location is a first-ordered CTU in a CTU row of a slice.
7. The image encoding method of claim 6 , wherein based on the HMVP buffer being initialized, the number of HMVP candidates comprised in the HMVP buffer is set to zero.
8. The image encoding method of claim 6 , further comprising determining whether a current CTU is the first-ordered CTU in the CTU row of the slice.
9. The image encoding method of claim 8 , wherein based on a determined that the current CTU is the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block encoded earlier than the current block in the current CTU, and
wherein based on a determined that the current CTU is not the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block encoded earlier than the current block in the current CTU and an HMVP candidate derived based on motion information of a block previously encoded in a previous CTU in the CTU row.
10. The image encoding method of claim 8 , whether the current CTU is the first-ordered CTU in the CTU row of the slice is determined based on the following equation,
ctu_idx % Num
where the ctu_idx refers to an index of the current CTU, the % refers to modulus operator, and the Num refers to the number of CTUs in the CTU row.
11. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
deriving a history-based motion vector prediction (HMVP) buffer for a current block;
deriving a merge candidate list including merge candidates for the current block,
wherein the merge candidates includes spatial candidates derived based on spatial candidate blocks and a temporal candidate derived based on a temporal candidate block,
wherein an HMVP candidate comprised in the HMVP buffer is inserted as a merge candidate of the merge candidate list,
wherein the HMVP candidate is inserted after the temporal candidate in the merge candidate list;
deriving merge index information of the current block based on the merge candidate list; and
encoding image information including the merge index information,
wherein the current block corresponds to a coding unit (CU) which is split from a coding tree unit (CTU), and
wherein the HMVP buffer is initialized at a CTU at a specific location,
wherein the CTU at the specific location is a first-ordered CTU in a CTU row of a slice.
12. The non-transitory computer-readable storage medium of claim 11 , wherein based on the HMVP buffer being initialized, the number of HMVP candidates comprised in the HMVP buffer is set to zero.
13. The non-transitory computer-readable storage medium of claim 11 , further comprising determining whether a current CTU is the first-ordered CTU in the CTU row of the slice.
14. The non-transitory computer-readable storage medium of claim 13 , wherein based on a determination that the current CTU is the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block encoded earlier than the current block in the current CTU, and
wherein based on a determination that the current CTU is not the first-ordered CTU in the CTU row of the slice, the HMVP buffer comprises an HMVP candidate derived based on motion information of a block encoded earlier than the current block in the current CTU and an HMVP candidate derived based on motion information of a block previously encoded in a previous CTU in the CTU row.
15. The non-transitory computer-readable storage medium of claim 13 , wherein whether the current CTU is the first-ordered CTU in the CTU row of the slice is determined based on the following equation,
ctu_idx % Num
where the ctu_idx refers to an index of the current CTU, the % refers to modulus operator, and the Num refers to the number of CTUs in the CTU row.