Image coding method, image decoding method, image coding apparatus, image decoding apparatus, and image coding and decoding apparatus

An encoder for coding an image comprising:

• circuitry;
• and
• memory coupled to the circuitry, wherein
• in operation, the circuitry:
• obtains a current signal to be coded of a current processing unit of the image;
• selects a context for the current signal to be coded from among a plurality of contexts;
• and
• performs arithmetic coding of the current signal to be coded by using the selected context,
• wherein, in said selecting, a shared context is selected as the context for the current signal to be coded in the case where a size of the processing unit which includes the current signal to be coded is a block size of 16×16, and in the case where the size of the processing unit which includes the current signal to be coded is a block size of 32×32, and
• wherein, in said selecting, a dedicated context is selected as the context for the current signal to be coded in the case where the size of the processing unit which includes the current signal to be coded is smaller than the block size of 16×16.

A non-transitory computer-readable medium storing a bitstream,

• the bitstream comprising:
• a first signal which is encoded by arithmetic coding using a first context, the first signal being included in a first processing unit according to which a decoder performs a selecting process,
• wherein in the selecting process:
• a shared context is selected as the first context for the first signal to be decoded in the case where a size of the first processing unit which includes the first signal to be decoded is a block size of 16×16, and in the case where the size of the first processing unit which includes the first signal to be decoded is a block size of 32×32;
• and
• a dedicated context is selected as the first context for the first signal to be decoded in the case where the size of the first processing unit which includes the first signal to be decoded is smaller than the block size of 16×16.