Encoder, decoder, encoding method, and decoding method

A decoder, comprising:

• circuitry;
• and
• memory connected to the circuitry,
• wherein the circuitry, in operation,
• determines a block from a coding tree unit (CTU),
• determines a mode to be applied to the block from a plurality of merge modes based on a width of the block and a height of the block,
• when the mode determined is a first mode, obtains from a bitstream an index indicating a distance and an angle that define two partitions in the block, and decodes the block using the first mode,
• disables the first mode and obtaining of the index from the bitstream when (i) the width is 64 pixels and the height is 8 pixels or (ii) the height is 64 pixels and the width is 8 pixels, and
• applies the first mode to the block when (i) a ratio of the width to the height is smaller than 8 and (ii) a ratio of the height to the width is smaller than 8,
• wherein the plurality of merge modes include a combined inter merge/intra prediction (CIIP) mode and a sub-block mode.

A non-transitory computer readable medium storing a bitstream, the bitstream including information according to which a decoder determines a mode to be applied to a block from a plurality of merge modes, the information indicating a size of the block, wherein

• the block is determined from a coding tree unit (CTU),
• in a case where the mode determined is a first mode, the decoder obtains, from the bitstream, an index indicating a distance and an angle that define two partitions in the block, and
• in a case where (i) a width of the block is 64 pixels and a height of the block is 8 pixels or (ii) the height is 64 pixels and the width is 8 pixels, the decoder disables the first mode and obtaining of the index from the bitstream, and
• in a case where (i) a ratio of the width to the height is smaller than 8 and (ii) a ratio of the height to the width is smaller than 8, the decoder applies the first mode to the block,
• wherein the plurality of merge modes include a combined inter merge/intra prediction (CIIP) mode and a sub-block mode.