Encoder, decoder, encoding method, and decoding method
An encoder is disclosed which includes circuitry and memory. Using the memory, the circuitry, in a first operating mode, derives first motion vectors for a first block obtained by splitting a picture, and generates a prediction image corresponding to the first block, with a bi-directional optical flow flag settable to true, and by referring to spatial gradients of luminance generated based on the first motion vectors. Using the memory, the circuitry, in a second operating mode, derives second motion vectors for a sub-block obtained by splitting a second block, the second block being obtained by splitting the picture, and generates a prediction image corresponding to the sub-block, with the bi-directional optical flow flag set to false.
1. An encoder comprising:
circuitry; and
memory,
wherein, using the memory, the circuitry:
in a first operating mode,
derives first motion vectors for a first block obtained by splitting a picture, and
generates a prediction image corresponding to the first block, with a bi-directional optical flow flag set to true, by referring to spatial gradients of luminance generated based on the first motion vectors; and
in a second operating mode,
derives second motion vectors for a sub-block obtained by splitting a second block, the second block being obtained by splitting the picture, and
generates a prediction image corresponding to the sub-block, with the bi-directional optical flow flag set to false.
2. An encoding method, comprising:
in a first operating mode,
deriving first motion vectors for a first block obtained by splitting a picture, and
generating a prediction image corresponding to the first block, with a bi-directional optical flow flag set to true, by referring to spatial gradients of luminance generated based on the first motion vectors; and
in a second operating mode,
deriving second motion vectors for a sub-block obtained by splitting a second block, the second block being obtained by splitting the picture, and
generating a prediction image corresponding to the sub-block, with the bi-directional optical flow flag set to false.
3. A non-transitory computer readable medium storing a bitstream, the bitstream comprising:
information according to which a decoder derives first motion vectors or second motion vectors in a decoding process; and
a picture including a first block and a second block obtained by splitting the picture, wherein
in the decoding process:
in a first operating mode,
the first motion vectors are derived for the first block, and
a prediction image corresponding to the first block is generated, with a bi-directional optical flow flag set to true, by referring to spatial gradients of luminance generated based on the first motion vectors; and
in a second operating mode,
the second motion vectors are derived for a sub-block obtained by splitting the second block, and
a prediction image corresponding to the sub-block is generated, with the bi-directional optical flow flag set to false.