Encoder, decoder, encoding method, and decoding method

A decoder which obtains a bitstream in which image information has been encoded and decodes the image information, the decoder comprising:

• memory;
• circuitry accessible to the memory,
• wherein the circuitry accessible to the memory:
• obtains the bitstream which includes a binary data string in which the image information has been binarized;
• switches whether or not to apply arithmetic decoding to the binary data string included in the bitstream;
• when arithmetic decoding is applied to the binary data string, applies arithmetic decoding to the binary data string, and inverse binarizes the binary data string to which arithmetic decoding has been applied, to decode the image information;
• when arithmetic decoding is not applied to the binary data string, skips applying arithmetic decoding to the binary data string, and inverse binarizes the binary data string to which arithmetic decoding has not been applied, to decode the image information;
• when inverse binarizing the binary data string, inverse binarizes a first portion which is included in the binary data string and in which frequency transform coefficient information related to frequency components of an image has been binarized, according to different inverse binarization formats between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string;
• and
• when inverse binarizing the binary data string, inverse binarizes a second portion which is included in the binary data string and in which a part or an entirety of prediction parameter information related to prediction methods of an image has been binarized, according to a common inverse binarization format between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string.

An encoding method for encoding image information and outputting a bitstream in which the image information has been encoded, the encoding method comprising:

• binarizing the image information;
• switching whether or not to apply arithmetic encoding to a binary data string in which the image information has been binarized;
• when arithmetic encoding is applied to the binary data string, applying arithmetic encoding to the binary data string, and outputting the bitstream which includes the binary data string to which arithmetic encoding has been applied;
• when arithmetic encoding is not applied to the binary data string, skipping applying arithmetic encoding to the binary data string, and outputting the bitstream which includes the binary data string to which arithmetic encoding has not been applied;
• in the binarizing of the image information, binarizing frequency transform coefficient information which is related to frequency components of an image and included in the image information, according to different binarization formats between when arithmetic encoding is applied to the binary data string and when arithmetic encoding is not applied to the binary data string;
• and
• in the binarizing of the image information, binarizing a part or an entirety of prediction parameter information which is related to prediction methods of the image and included in the image information, according to a common binarization format between when arithmetic encoding is applied to the binary data string and when arithmetic encoding is not applied to the binary data string.

A decoding method for obtaining a bitstream in which image information has been encoded and decoding the image information, the decoding method comprising:

• obtaining the bitstream which includes a binary data string in which the image information has been binarized;
• switching whether or not to apply arithmetic decoding to the binary data string included in the bitstream;
• when arithmetic decoding is applied to the binary data string, applying arithmetic decoding to the binary data string, and inverse binarizing the binary data string to which arithmetic decoding has been applied, to decode the image information;
• when arithmetic decoding is not applied to the binary data string, skipping applying arithmetic decoding to the binary data string, and inverse binarizing the binary data string to which arithmetic decoding has not been applied, to decode the image information;
• in the inverse binarizing of the binary data string, inverse binarizing a first portion which is included in the binary data string and in which frequency transform coefficient information related to frequency components of an image has been binarized, according to different inverse binarization formats between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string;
• and
• in the inverse binarizing of the binary data string, inverse binarizing a second portion which is included in the binary data string and in which a part or an entirety of prediction parameter information related to prediction methods of an image has been binarized, according to a common inverse binarization format between when arithmetic decoding is applied to the binary data string and when arithmetic decoding is not applied to the binary data string.