Image encoding / decoding method and apparatus for performing bdoF and method for transmitting bitstream

Abstract

An image encoding / decoding method and apparatus for performing bi-directional optical flow (BDOF) and a method for transmitting a bitstream are provided. An image decoding method according to the disclosure is an image decoding method performed by an image decoding apparatus. The image decoding method can include the steps of deriving prediction samples of a current block based on motion information of the current block, determining whether BDOF is to be applied to the current block, if the BDOF is to be applied to the current block, deriving a gradient for a current sub-block in the current block, deriving a refined motion vector (v x ,v y ) for the current sub-block based on the gradient, deriving a BDOF offset based on the gradient and the refined motion vector, and deriving refined prediction samples of the current block based on the prediction samples of the current block and the BDOF offset.

Claims

1. An image decoding method performed by an image decoding device, the image decoding method comprising the following steps: The prediction sample of the current block is derived based on the motion information of the current block; Determine whether to apply bidirectional optical flow (BDOF) to the current block; Based on applying the BDOF to the current block, the gradient of the current sub-block in the current block is derived; Based on the gradient, the motion refinement (v) of the current sub-block is derived. x , v y ); The BDOF offset is derived based on the gradient and the motion refinement. as well as The refined prediction sample of the current block is derived based on the predicted sample of the current block and the BDOF offset.

2. The image decoding method according to claim 1, in, The step of deriving the gradient includes shifting the predicted sample of the current block to the right by a first shift, and Regardless of the bit depth of the current block, the first shift is set to a fixed value.

3. The image decoding method according to claim 2, wherein, The first shift is 6.

4. The image decoding method according to claim 1, wherein, Derivation of the motion refinement (v) x , v y The steps include the following: The first intermediate parameter diff is derived based on the predicted samples of the current block; and The second intermediate parameters tempH and tempV are derived based on the gradient.

5. The image decoding method according to claim 4, in, The steps for deriving the first intermediate parameter diff include shifting the predicted sample of the current block to the right by a second shift, and Regardless of the bit depth of the current block, the second shift is set to a fixed value.

6. The image decoding method according to claim 5, wherein, The second shift is 4.

7. The image decoding method according to claim 4, in, The steps for deriving the second intermediate parameters tempH and tempV include right-shifting the values ​​derived based on the gradient by a third shift, and Regardless of the bit depth of the current block, the third shift is set to a fixed value.

8. The image decoding method according to claim 7, wherein, The third shift is 1.

9. The image decoding method according to claim 1, wherein, The motion refinement (v) x , v y The amplitude is limited to the predetermined range.

10. The image decoding method according to claim 9, wherein, Regardless of the bit depth of the current block, the motion refinement (v) x , v y The predetermined range that is limited is set to a fixed range.

11. The image decoding method according to claim 1, in, The steps for deriving the BDOF offset include shifting the value derived based on the gradient and the motion refinement to the right by a predetermined shift, and Regardless of the bit depth of the current block, the predetermined shift is set to a fixed range.

12. The image decoding method according to claim 1, in, The step of deriving the refined prediction sample of the current block includes limiting the BDOF offset within a predetermined range, and The predetermined range is set based on the bit depth of the current block.

13. An image decoding device, the image decoding device comprising the following steps: Memory; as well as At least one processor, Wherein, the at least one processor is configured to: The prediction sample of the current block is derived based on the motion information of the current block; Determine whether to apply bidirectional optical flow (BDOF) to the current block; Based on applying the BDOF to the current block, the gradient of the current sub-block in the current block is derived; Based on the gradient, the motion refinement (v) of the current sub-block is derived. x , v y ); The BDOF offset is derived based on the gradient and the motion refinement; and The refined prediction sample of the current block is derived based on the predicted sample of the current block and the BDOF offset.

14. An image encoding method performed by an image encoding device, the image encoding method comprising the following steps: The prediction sample of the current block is derived based on the motion information of the current block; Determine whether to apply bidirectional optical flow (BDOF) to the current block; Based on applying the BDOF to the current block, the gradient of the current sub-block in the current block is derived; Based on the gradient, the motion refinement (v) of the current sub-block is derived. x , v y ); The BDOF offset is derived based on the gradient and the motion refinement. as well as The refined prediction sample of the current block is derived based on the predicted sample of the current block and the BDOF offset.

15. A method for transmitting a bitstream generated by the image encoding method according to claim 14.

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