Encoder, decoder, encoding method, and decoding method

1. An encoder that encodes a block of an image, the encoder comprising:

a processor; and

memory connected to the processor,

wherein, using the memory, the processor:

determines prediction mode information;

determines a set of partition modes in accordance with the prediction mode information;

selects a partition mode from the determined set of partition modes;

writes a parameter into a bitstream;

partitions a block into a plurality of sub blocks using the selected partition mode; and

encodes a sub block included in the plurality of sub blocks in an encoding process,

wherein the processor:

sets the parameter to a first value in response to partitioning the block into three sub blocks, each of a width and a height of each of the three sub blocks being a power of two; and

sets the parameter to a second value in response to partitioning the block into an even number of sub blocks, the second value being different from the first value, each of the even number of sub blocks having a same size.

2. The encoder according to claim 1 , wherein the processor, in operation:

determines a first set of partition modes in response to the prediction information indicating inter prediction mode; and

determines a second set of partition modes in response to the prediction information indicating intra prediction mode, a partition mode included in the second set of partition modes producing smaller sub blocks than all of sub blocks produced by the first set of partition modes.

3. A decoder that decodes a block of an image, the decoder comprising:

a processor; and

memory connected to the processor,

wherein, using the memory, the processor:

determines prediction mode information;

determines a set of partition modes in accordance with the prediction mode information;

parses a parameter from the bitstream;

selects a partition mode from the set of partition modes in accordance with the parameter;

partitions a block into a plurality of sub blocks using the selected partition mode; and

decodes a sub block included in the plurality of sub blocks in a decoding process,

wherein the processor:

partitions the block into three sub blocks in response to the parameter indicating a first value, each of a width and a height of each of the three sub blocks being a power of two; and

partitions the block into an even number of sub blocks in response to the parameter indicating a second value, the second value being different from the first value, each of the even number of sub blocks having a same size.

4. The decoder according to claim 3 , wherein the processor, in operation:

determines a first set of partition modes in response to the prediction information indicating inter prediction mode; and

determines a second set of partition modes in response to the prediction information indicating intra prediction mode, a partition mode included in the second set of partition modes producing smaller sub blocks than all of sub blocks produced by the first set of partition modes.

5. An encoding method of encoding a block of an image, the encoding method comprising:

determining prediction mode information;

determining a set of prediction modes in accordance with the prediction mode information;

selecting a partition mode from the determined set of partition modes;

writing a parameter into a bitstream;

partitioning a block into a plurality of sub blocks using the selected partition mode; and

encoding a sub block included in the plurality of sub blocks in an encoding process,

wherein the encoding method comprises:

setting the parameter to a first value in response to partitioning the block into three sub blocks, each of a width and a height of each of the three sub blocks being a power of two; and

setting the parameter to a second value in response to partitioning the block into an even number of sub blocks, the second value being different from the first value, each of the even number of sub blocks having a same size.

6. The encoding method according to claim 5 , wherein the encoding method comprises:

determining a first set of partition modes in response to the prediction information indicating inter prediction mode; and

determining a second set of partition modes in response to the prediction information indicating intra prediction mode, a partition mode included in the second set of partition modes producing smaller sub blocks than all of sub blocks produced by the first set of partition modes.

7. A decoding method of decoding a block of an image, the decoding method comprising:

determining prediction mode information;

determining a set of partition modes in accordance with the prediction mode information;

parsing a parameter from the bitstream;

selecting a partition mode from the set of partition modes in accordance with the parameter;

partitioning a block into a plurality of sub blocks using the selected partition mode; and

decoding a sub block included in the plurality of sub blocks in a decoding process,

wherein the decoding method comprises:

partitioning the block into three sub blocks in response to the parameter indicating a first value, each of a width and a height of each of the three sub blocks being a power of two; and

partitioning the block into an even number of sub blocks in response to the parameter indicating a second value, the second value being different from the first value, each of the even number of blocks having a same size.

8. The decoding method according to claim 7 , wherein the decoding method comprises:

determining a first set of partition modes in response to the prediction information indicating inter prediction mode; and

determining a second set of partition modes in response to the prediction information indicating intra prediction mode, a partition mode included in the second set of partition modes producing smaller sub blocks than all of sub blocks produced by the first set of partition modes.