Encoder, decoder, encoding method, and decoding method of generating power of 2 transform block sizes

1. An encoder, comprising:

circuitry; and

memory, wherein

using the memory, the circuitry:

performs first determination processing of determining whether a size of a current block to be subjected to transform processing is a power of 2;

performs the transform processing on the current block when the size is a power of 2; and

uses, for the current block, a transform skip mode for skipping the transform processing when the size is not a power of 2.

2. The encoder according to claim 1 , wherein

when the size is a power of 2,

second determination processing of determining, according to a determination criterion different from a determination criterion for the first determination processing, whether the transform skip mode is used for the current block is performed, and

the transform skip mode or a mode other than the transform skip mode is used for the current block, according to a result of the second determination processing, and

when the size is not a power of 2,

the second determination processing is not performed, and

the transform skip mode is used for the current block.

3. The encoder according to claim 2 , wherein

when the size is a power of 2, a signal indicating the result of the second determination processing, and indicating whether the transform skip mode is used for the current block is encoded, and

when the size is not a power of 2, the signal is not encoded.

4. The encoder according to claim 1 , wherein

the current block having a size that is not a power of 2 is generated by splitting a block into two in a ratio of 1:3.

5. The encoder according to claim 1 , wherein

the current block having a size that is not a power of 2 is generated by splitting a block into two in a ratio of 3:5.

6. The encoder according to claim 1 , wherein

the current block having a size that is not a power of 2 is generated by splitting a block into three in a ratio of 2:3:2.

7. The encoder according to claim 1 , wherein

when at least one of a vertical side and a horizontal side of the current block has a length that is not a power of 2, the transform skip mode is used for both a vertical direction and a horizontal direction of the current block.

8. The encoder according to claim 1 , wherein

when one side among a vertical side and a horizontal side of the current block has a length that is a power of 2 and the other side has a length that is not a power of 2, the transform skip mode is used for a direction in which the other side of the current block extends, and a mode other than the transform skip mode is used for a direction in which the one side of the current block extends.

9. The encoder according to claim 1 , wherein

when a size of a transform block which is the current block and a size of a coding block are different,

the first determination processing is performed,

the transform processing is performed on the current block when the size of the transform block is a power of 2, and

a transform skip mode for skipping the transform processing is used for the current block when the size of the transform block is not a power of 2, and

when the size of the transform block and the size of the coding block are identical, the first determination processing is not performed.

10. A decoder, comprising:

circuitry; and

memory, wherein

using the memory, the circuitry:

performs first determination processing of determining whether a size of a current block to be subjected to transform processing is a power of 2;

when the size is a power of 2,

decodes a signal indicating whether a transform skip mode for skipping the transform processing is used for the current block; and

uses, for the current block, the transform skip mode or a mode other than the transform skip mode, according to the signal; and

uses the transform skip mode for the current block without decoding the signal when the size is not a power of 2.

11. The decoder according to claim 10 , wherein

when a size of a transform block which is the current block and a size of a coding block are different,

the first determination processing is performed,

when the size of the transform block is a power of 2, the signal is decoded, and the transform skip mode or the mode other than the transform skip mode is used for the current block according to the signal, and

when the size of the transform block is not a power of 2, the transform skip mode is used for the current block, without decoding the signal, and

when the size of the transform block and the size of the coding block are identical,

the signal is decoded without performing the first determination processing, and

the transform skip mode or the mode other than the transform skip mode is used for the current block according to the signal.

12. An encoding method, comprising:

performing first determination processing of determining whether a size of a current block to be subjected to transform processing is a power of 2;

performing the transform processing on the current block when the size is a power of 2; and

using, for the current block, a transform skip mode for skipping the transform processing when the size is not a power of 2.

13. A decoding method, comprising:

performing first determination processing of determining whether a size of a current block to be subjected to transform processing is a power of 2;

when the size is a power of 2,

decoding a signal indicating whether a transform skip mode for skipping the transform processing is used for the current block; and

using, for the current block, the transform skip mode or a mode other than the transform skip mode, according to the signal; and

using the transform skip mode for the current block without decoding the signal when the size is not a power of 2.