Decoding device, encoding device, and decoding method

An encoding device for deriving a context index for a flag, the encoding device comprising:

• a context derivation circuitry that derives the context index for a significant coefficient flag indicating whether a corresponding transform coefficient is non-zero, wherein the context index is derived using a colour component index and a significant context;
• and
• a syntax encoding circuitry that performs arithmetic encoding a bin by using a probability state index associated with the context index,
• wherein
• if a sum of a X coordinate xC of a current location and a Y coordinate yC of the current location is equal to 0, the significant context is set equal to a first value,
• if the sum of the X coordinate xC and the Y coordinate yC is not equal to 0, the context derivation circuitry sets a sub-block position (xCG, yCG) to coordinates (xC»2, yC»2), and then when a sum of a X coordinate xCG of the sub-block position and a Y coordinate yCG of the sub-block position is greater than a threshold value, the significant context is derived by adding an first offset value to a second value,
• if the colour component is equal to 0, the context index is set equal to the significant context, and
• if the colour component is greater than 0, the context index is derived by adding the significant context to a second offset value.

A decoding method for deriving a context index for a flag, the decoding method comprising:

• deriving the context index for a significant coefficient flag indicating whether a corresponding transform coefficient is non-zero, wherein the context index is derived using a colour component index and a significant context;
• setting the significant context to a first value, if a sum of a X coordinate xC of a current location and a Y coordinate yC of the current location is equal to 0;
• setting a sub-block position (xCG, yCG) to coordinates (xC»2, yC»2), if the sum of the X coordinate xC and the Y coordinate yC is not equal to 0, and then when a sum of a X coordinate xCG of the sub-block position and a Y coordinate yCG of the sub-block position is greater than a threshold value, deriving the significant context by adding an first offset value to a second value;
• setting the context index equal to the significant context, if the colour component is equal to 0;
• deriving the context index by adding the significant context to a second offset value, if the colour component is greater than 0;
• and
• performing arithmetic decoding a bin by using a probability state index associated with the context index.