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

A coding apparatus comprising:

• processing circuitry;
• and
• storage coupled to the processing circuitry,
• wherein the processing circuitry performs the following using the storage:
• determining a context for a current block in an image, from among a plurality of contexts;
• and
• performing arithmetic coding on a control parameter for the current block, using the determined context to generate a bitstream corresponding to the current block,
• wherein the determining of a context for a current block in the image further includes:
• determining a signal type under which the control parameter of the current block is classified;
• determining the context by using both of coded control parameters for a left block and an upper block, when the signal type is a first type, the left block being a neighboring block to the left of the current block, and the upper block being a neighboring block on top of the current block;
• determining the context by using a predetermined fixed value, without using any of the coded control parameters for the left block and the upper block, when the signal type is a second type different from the first type;
• and
• determining the context by using a hierarchical depth of a data unit to which the control parameter for the current block belongs, without using both of the coded control parameters for the left block and the upper block, when the signal type is a third type different from the first type and the second type,
• wherein one of a split flag and a skip flag is classified under the first type, the split flag indicating whether or not the current block is partitioned into a plurality of blocks, and the skip flag indicating whether or not the current block is to be skipped,
• wherein a residual flag is classified under the second type, the residual flag indicating whether or not residual data is included in the current block, the residual data includes at least one of luma coefficient data and chroma coefficient data,
• wherein a luma coefficient flag and a chrominance coefficient flag are classified under the third type, the luma coefficient flag indicating whether or not the current block includes a non-zero luma coefficient, and the chrominance coefficient flag indicating whether or not the current block includes a non-zero chrominance coefficient,
• wherein a first number of contexts for the luma coefficient flag is different from a second number of contexts for the chrominance coefficient flag, and
• wherein the determining of a context for a current block in the image further includes determining the context by using the predetermined fixed value when (i) the signal type is the first type and (ii) both coded control parameters of the left block and the upper block are not available.

A non-transitory computer-readable medium storing a bitstream, the bitstream comprising:

• a first block;
• a left block being a neighboring block to the left of the first block;
• and
• an upper block being a neighboring block on top of the first block,
• wherein the bitstream is coded by a coding method including:
• determining a context for the first block in an image, from among a plurality of contexts;
• and
• performing arithmetic coding on a control parameter for the first block, using the determined context to generate a bitstream corresponding to the first block,
• wherein the determining of a context for a first block in the image further includes:
• determining a signal type under which the control parameter of the first block is classified;
• determining the context by using both of coded control parameters for the left block and the upper block, when the signal type is a first type;
• determining the context by using a predetermined fixed value, without using any of the coded control parameters for the left block and the upper block, when the signal type is a second type different from the first type;
• and
• determining the context by using a hierarchical depth of a data unit to which the control parameter for the first block belongs, without using both of the coded control parameters for the left block and the upper block, when the signal type is a third type different from the first type and the second type,
• wherein one of a split flag and a skip flag is classified under the first type, the split flag indicating whether or not the first block is partitioned into a plurality of blocks, and the skip flag indicating whether or not the first block is to be skipped,
• wherein a residual flag is classified under the second type, the residual flag indicating whether or not residual data is included in the current block, the residual data includes at least one of luma coefficient data and chroma coefficient data,
• wherein a luma coefficient flag and a chrominance coefficient flag are classified under the third type, the luma coefficient flag indicating whether or not the first block includes a non-zero luma coefficient, and the chrominance coefficient flag indicating whether or not the first block includes a non-zero chrominance coefficient,
• wherein a first number of contexts for the luma coefficient flag is different from a second number of contexts for the chrominance coefficient flag, and
• wherein the determining of a context for a first block in the image further includes determining the context by using the predetermined fixed value when (i) the signal type is the first type and (ii) both coded control parameters of the left block and the upper block are not available.