Image coding method including selecting a context for performing arithmetic coding on a parameter indicating a coding-target coefficient included in a sub-block

An image coding and decoding apparatus comprising:

• an image coding apparatus which codes an image on a block-by-block basis;
• and
• an image decoding apparatus which decodes the image coded on the block-by-block basis,
• wherein the image coding apparatus includes:
• a first context selecting unit configured to select, for each of a plurality of first sub-blocks included in a coding-target block and each including a plurality of coefficients, a first context for performing arithmetic coding on a parameter indicating a coding-target coefficient included in the first sub-block from a first context set corresponding to the first sub-block, based on at least one reference coefficient located around the coding-target coefficient, the coding-target block being a transform unit;
• and
• an arithmetic encoder configured to perform arithmetic coding on the parameter indicating the coding-target coefficient using probability information about the selected first context,
• wherein, the first context selecting unit is configured to select, the first context from the first context set, the first context set corresponding to a sum of (i) a first horizontal value indicating a position in a horizontal direction of the first sub-block in the coding-target block and (ii) a first vertical value indicating a position in a vertical direction of the first sub-block in the coding-target block,
• wherein the first horizontal value is an integer part of (a horizontal position of the coding-target coefficient/4), where the horizontal position of the coding-target coefficient is a horizontal direction distance from a position of an upper left coefficient in the coding-target block,
• wherein the first vertical value is an integer part of (a vertical position of the coding-target coefficient/4), where the vertical position of the coding target coefficient is a vertical direction distance from the position of the upper left coefficient in the coding-target block,
• wherein the image decoding apparatus includes:
• a second context selecting unit configured to select, for each of a plurality of second sub-blocks included in a decoding-target block and each including a plurality of coefficients, a second context for performing arithmetic decoding on a parameter indicating a decoding-target coefficient included in the second sub-block from a second context set corresponding to the second sub-block, based on at least one reference coefficient located around the decoding-target coefficient, the decoding-target block being a transform unit;
• and
• an arithmetic decoder configured to perform arithmetic decoding on the parameter indicating the decoding-target coefficient using probability information about the selected second context,
• wherein, the second context selecting unit is configured to select the second context from the second context set, the second context set corresponding to a sum of (i) a second horizontal value indicating a position in a horizontal direction of the second sub-block in the decoding-target block and (ii) a second vertical value indicating a position in a vertical direction of the second sub-block in the decoding-target block,
• wherein the second horizontal value is an integer part of (a horizontal position of the decoding-target coefficient/4), where the horizontal position of the decoding-target coefficient is a horizontal direction distance from a position of an upper left coefficient in the decoding-target block, and
• wherein the second vertical value is an integer part of (a vertical position of the decoding-target coefficient/4), where the vertical position of the decoding-target coefficient is a vertical direction distance from the position of the upper left coefficient in the decoding-target block.

An image coding and decoding apparatus comprising:

• a processor;
• and
• a non-transitory memory having stored thereon executable instructions, which when executed, cause the processor to perform a coding process for coding an image on a block-by-block basis and a decoding process for decoding the image coded on the block-by-block basis,
• wherein the coding process includes:
• first selecting, for each of a plurality of first sub-blocks included in a coding-target block and each including a plurality of coefficients, a first context for performing arithmetic coding on a parameter indicating a coding-target coefficient included in the first sub-block from a first context set corresponding to the first sub-block, based on at least one reference coefficient located around the coding-target coefficient, the coding-target block being a transform unit;
• and
• performing arithmetic coding on the parameter indicating the coding-target coefficient using probability information about the selected first context,
• wherein, in the first selecting, the first context is selected from the first context set, the first context set corresponding to a sum of (i) a first horizontal value indicating a position in a horizontal direction of the first sub-block in the coding-target block and (ii) a second vertical value indicating a position in a vertical direction of the first sub-block in the coding-target block,
• wherein the first horizontal value is an integer part of (a horizontal position of the coding-target coefficient/4), where the horizontal position of the coding target coefficient is a horizontal direction distance from a position of an upper left coefficient in the coding-target block,
• wherein the first vertical value is an integer part of (a vertical position of the coding-target coefficient/4), where the vertical position of the coding target coefficient is a vertical direction distance from the position of the upper left coefficient in the coding-target block,
• wherein the decoding process includes:
• second selecting, for each of a plurality of second sub-blocks included in a decoding-target block and each including a plurality of coefficients, a second context for performing arithmetic decoding on a parameter indicating a decoding-target coefficient included in the second sub-block from a second context set corresponding to the second sub-block, based on at least one reference coefficient located around the decoding-target coefficient, the decoding-target block being a transform unit;
• and
• performing arithmetic decoding on the parameter indicating the decoding-target coefficient using probability information about the selected second context,
• wherein, in the second selecting, the second context is selected from the second context set, the second context set corresponding to a sum of (i) a second horizontal value indicating a position in a horizontal direction of second the sub-block in the decoding-target block and (ii) a second vertical value indicating a position in a vertical direction of the second sub-block in the decoding-target block,
• wherein the second horizontal value is an integer part of (a horizontal position of the decoding-target coefficient/4), where the horizontal position of the decoding-target coefficient is a horizontal direction distance from a position of an upper left coefficient in the decoding-target block, and
• wherein the second vertical value is an integer part of (a vertical position of the decoding-target coefficient/4), where the vertical position of the decoding-target coefficient is a vertical direction distance from the position of the upper left coefficient in the decoding-target block.