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

An image decoding method for decoding a bitstream generated by coding an image,

• the bitstream including a plurality of coefficient subsets each of which includes a plurality of frequency coefficients as coefficients each of which is replaced with a plurality of parameters and coded,
• the image decoding method comprising:
• selecting a context to be used in arithmetic coding, by referring to a value related to a coefficient neighboring a current coefficient to be decoded which is one of the coefficients included in each of the coefficient subsets;
• performing arithmetic decoding on a parameter corresponding to the current coefficient, using a symbol occurrence probability that is determined according to the selected context;
• reconstructing the current coefficient as a coefficient value, using the parameter that is arithmetically decoded;
• generating a decoding block by combining the coefficient subsets each including the coefficients which are reconstructed;
• generating a differential image by performing inverse frequency transformation on the decoding block;
• generating a prediction image;
• and
• generating a decoded image from the differential image and the prediction image,
• wherein in the performing of arithmetic decoding,
• the arithmetic decoding is performed, for each of the coefficients, on a related parameter among a plurality of parameters corresponding to the coefficient, the related parameter being a parameter related to a value which is referred to for selecting the context, and
• the arithmetic decoding is performed, for each of the coefficient subsets, collectively on a plurality of unrelated parameters, among a plurality of parameters corresponding to the coefficients in the coefficient subset, the unrelated parameters being parameters not related to the value which is referred to for selecting the context.