Video decoding method and device therefor, and video encoding method and device therefor
Provided is a video decoding method including: obtaining a first motion vector indicating a first reference block of a current block in a first reference picture and a second motion vector indicating a second reference block of the current block in a second reference picture; obtaining a parameter related to pixel group unit motion compensation of the current block, based on at least one of information of the parameter related to the pixel group unit motion compensation and a parameter related to an image including the current picture; generating a prediction block by performing, with respect to the current block, block unit motion compensation based on the first motion vector and the second motion vector and performing the pixel group unit motion compensation based on the parameter related to the pixel group unit motion compensation; and reconstructing the current block. Here, a pixel group may include at least one pixel.
1. A video decoding method comprising:
obtaining, from a bitstream, information related to a first motion vector and information related to a second motion vector, wherein the first motion vector indicates a first reference block of a current block in a first reference picture, and the second motion vector indicates a second reference block of the current block in a second reference picture;
determining whether to perform optical flow based compensation by using at least one of a size of the current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
if it is determined to perform the optical flow based compensation, obtaining a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group, wherein the pixel group comprises at least one pixel;
obtaining a predicted pixel value of the current block based on a first value and a second value, wherein the first value is obtained by using a pixel value of the first reference block and a pixel value of the second reference block, and the second value is obtained by using the first displacement vector and the second displacement vector for the pixel group; and
reconstructing the current block based on the predicted pixel value,
wherein the flag information is obtained from the bitstream, and
the POC difference is a difference between a POC of a reference picture and a POC of a current picture.
2. A video decoding apparatus comprising:
an obtainer configured to obtain, from a bitstream, information related to a first motion vector and information related to a second motion vector, wherein the first motion vector indicates a first reference block of a current block in a first reference picture, and the second motion vector indicates a second reference block of the current block in a second reference picture;
an inter predictor configured to:
determine whether to perform optical flow based compensation by using at least one of a size of the current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
if it is determined to perform the optical flow based compensation, obtain a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group, wherein the pixel group comprises at least one pixel; and
obtain a predicted pixel value of the current block based on a first value and a second value, wherein the first value is obtained by using a pixel value of the first reference block and a pixel value of the second reference block, and the second value is obtained by using the first displacement vector and the second displacement vector for the pixel group; and
a decoder configured to reconstruct the current block based on the predicted pixel value,
wherein the flag information is obtained from the bitstream, and
the POC difference is a difference between a POC of a reference picture and a POC of a current picture.
3. A video encoding method comprising:
determining whether to perform optical flow based compensation by using at least one of a size of a current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
if it is determined to perform the optical flow based compensation, obtaining a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group, wherein the pixel group comprises at least one pixel;
obtaining a predicted pixel value of the current block based on a first value and a second value, wherein the first value is obtained by using a pixel value of a first reference block and a pixel value of a second reference block, and the second value is obtained by using the first displacement vector and the second displacement vector for the pixel group; and
generating a bitstream comprising information related to a first motion vector and information related to a second motion vector, wherein the first motion vector indicates the first reference block of the current block in a first reference picture, and the second motion vector indicates the second reference block of the current block in a second reference picture
wherein the POC difference is a difference between a POC of a reference picture and a POC of a current picture.
4. A non-transitory computer-readable recording medium having recorded thereon a program which performs the video decoding method of claim 1 .