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

A method for decoding, from a bitstream, a difference motion vector which indicates a difference between a motion vector used to predict a picture and a predicted motion vector that is a prediction of the motion vector, the method comprising:

• performing context adaptive binary arithmetic decoding to decode, from the bitstream, a code string, wherein the code string includes a first flag that indicates whether or not a horizontal component of the difference motion vector is zero and a second flag that indicates whether or not a vertical component of the difference motion vector is zero;
• in response to determining that the code string indicates that the horizontal component of the difference motion vector is not zero and that the vertical component of the difference motion vector is not zero:
• performing context adaptive binary arithmetic decoding with variable probability to decode, from the bitstream, first grouped data that includes (i) first prefix data that is a prefix portion of the horizontal component of the difference motion vector, and (ii) second prefix data that is a prefix portion of the vertical component of the difference motion vector;
• and
• performing bypass decoding to decode, from the bitstream, subsequent to decoding the first grouped data, second grouped data that includes (i) first suffix data that is a suffix portion of the horizontal component of the difference motion vector, (ii) first sign data that indicates the horizontal component being positive or negative, (iii) second suffix data that is a suffix portion of the vertical component of the difference motion vector, and (iv) second sign data that indicates the vertical component being positive or negative,
• wherein the first flag, the second flag, the first prefix data, the second prefix data, the first suffix data, the first sign data, the second suffix data, and the second sign data are decoded in this order from the bitstream.