Method and apparatus for inter-prediction of pictures with different resolutions

An apparatus for encoding a sequence of pictures, some of which have different resolutions, the apparatus comprising at least one processor configured to:

• encode, into a high-level syntax structure of a bitstream, a first syntax element for specifying a reference size of pictures referring to the high-level syntax structure and a second syntax element for specifying a picture size scaled from the reference size of the pictures referring to the high-level syntax structure,
• wherein the first syntax element and the second syntax element are encoded for use in deriving at least one offset factor and at least one scale factor representing a scale difference between a current picture including a current block to be decoded and a reference picture referred to by the current block, and
• wherein the at least one scale factor and the at least one offset factor are used to modify a sample position indicated by a motion vector of the current block within the reference picture;
• generate a prediction block of the current block from the reference picture by using the motion vector of the current block, the at least one scale factor, and the at least one offset factor;
• generate a residual block of the current block based on the prediction block;
• and
• encode information on the residual block,
• wherein the at least one scale factor includes a horizontal scale factor and a vertical scale factor that are respectively applied to horizontal and vertical components of the motion vector of the current block, and
• wherein the at least one offset factor includes a horizontal offset factor and a vertical offset factor that are respectively applied to horizontal and vertical components of the motion vector of the current block.

An apparatus for storing a bitstream containing encoded video data, the apparatus comprising at least one processor configured to:

• generate the bitstream by encoding a sequence of pictures some of which have different resolutions;
• and
• store the bitstream in a non-transitory recording medium,
• wherein generating the bitstream includes
• (i) encoding, into a high-level syntax structure of the bitstream, a first syntax element for specifying a reference size of pictures referring to the high-level syntax structure and a second syntax element for specifying a picture size scaled from the reference size of the pictures referring to the high-level syntax structure,
• wherein the first syntax element and the second syntax element are encoded for use in deriving at least one offset factor and at least one scale factor representing a scale difference between a current picture including a current block to be decoded and a reference picture referred to by the current block, and
• wherein the at least one scale factor and the at least one offset factor are used to modify a sample position indicated by a motion vector of the current block within the reference picture,
• (ii) generating a prediction block of the current block from the reference picture by using the motion vector of the current block, the at least one scale factor, and the at least one offset factor,
• (iii) generating a residual block of the current block based on the prediction block, and
• (iv) encoding information on the residual block,
• wherein the at least one scale factor includes a horizontal scale factor and a vertical scale factor that are respectively applied to horizontal and vertical components of the motion vector of the current block, and
• wherein the at least one offset factor includes a horizontal offset factor and a vertical offset factor that are respectively applied to horizontal and vertical components of the motion vector of the current block.