Image coding method for encoding a difference motion vector into and decoding a difference motion vector from a bitstream
An image coding method includes coding a motion vector difference indicating a difference between the motion vector and a predicted motion vector, wherein the coding includes: coding a first portion that is a part of a first component which is one of a horizontal component and a vertical component of the motion vector difference; coding a second portion that is a part of a second component which is different from the first component and is the other one of the horizontal component and the vertical component; coding a third portion that is a part of the first component and is different from the first portion; coding a fourth portion that is a part of the second component and is different from the second portion; and generating a code string which includes the first portion, the second portion, the third portion, and the fourth portion in the stated order.
1. 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:
decoding, from the bitstream, a code string, wherein the code string indicates whether or not a horizontal component of the difference motion vector and a vertical component of the difference motion vector are 0;
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:
decoding, 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;
decoding, 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, and (ii) second suffix data that is a suffix portion of the vertical component of the difference motion vector;
deriving the horizontal component of the difference motion vector from a combination of the first prefix data and the first suffix data; and
deriving the vertical component of the difference motion vector from a combination of the second prefix data and the second suffix data.
2. The method of claim 1 wherein decoding the code string comprises performing context adaptive binary arithmetic decoding with variable probability.
3. The method of claim 1 wherein decoding the first grouped data comprises performing context adaptive binary arithmetic decoding with variable probability.
4. The method of claim 1 wherein decoding the second grouped data comprises performing bypass decoding.
5. An apparatus which decodes, 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 apparatus comprising:
processing circuitry; and
storage accessible from the processing circuitry, the storage having instructions which when executed by the processing circuitry cause the apparatus to:
decode, from the bitstream, a code string, wherein the code string indicates whether or not a horizontal component of the difference motion vector and a vertical component of the difference motion vector are 0;
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:
decode, from the bitstream subsequent to decoding the code string, first grouped data that includes (i) a first prefix data that is a prefix portion of the horizontal component of the difference motion vector, (ii) a second prefix data that is a prefix portion of the vertical component of the difference motion vector;
decode, from the bitstream, subsequent to decoding the first grouped data, second grouped data that includes (i) the first suffix data that is a suffix portion of the horizontal component of the difference motion vector, and (ii) the second suffix data that is a suffix portion of the vertical component of the difference motion vector;
derive the horizontal component of the difference motion vector from a combination of the first prefix data and the first suffix data; and
derive the vertical component of the difference motion vector from a combination of the second prefix data and the second suffix data.
6. The apparatus of claim 5 wherein decoding the code string comprises performing context adaptive binary arithmetic decoding with variable probability.
7. The apparatus of claim 5 wherein decoding the first grouped data comprises performing context adaptive binary arithmetic decoding with variable probability.
8. The apparatus of claim 5 wherein decoding the second grouped data comprises performing bypass decoding.
9. A method for encoding, into 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:
encoding, into the bitstream, a code string, wherein the code string indicates whether or not a horizontal component of the difference motion vector and a vertical component of the motion vector difference are 0;
when the encoded 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:
encoding, into 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
encoding, into the bitstream, subsequent to encoding 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, and (ii) second suffix data that is a suffix portion of the vertical component of the difference motion vector.
10. The method of claim 9 wherein encoding the code string comprises performing context adaptive binary arithmetic coding with variable probability.
11. The method of claim 9 wherein encoding the first grouped data comprises performing context adaptive binary arithmetic coding with variable probability.
12. The method of claim 9 wherein encoding the second grouped data comprises performing bypass coding.
13. An apparatus which encodes, into 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 apparatus comprising:
processing circuitry; and
storage accessible from the processing circuitry, the storage having instructions which when executed by the processing circuitry cause the apparatus to:
encode, into the bitstream, a code string, wherein the code string indicates whether or not a horizontal component of the difference motion vector and a vertical component of the difference motion vector are 0;
when the encoded 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:
encode, into the bitstream, first grouped data that includes (i) a first prefix data that is a prefix portion of the horizontal component of the difference motion vector, and (ii) a second prefix data that is a prefix portion of the vertical component of the difference motion vector; and
encode, into the bitstream, subsequent to encoding the first grouped data, second grouped data that includes (i) the first suffix data that is a suffix portion of the horizontal component of the difference motion vector, and (ii) the second suffix data that is a suffix portion of the vertical component of the difference motion vector.
14. The apparatus of claim 13 wherein encoding the code string comprises performing context adaptive binary arithmetic coding with variable probability.
15. The apparatus of claim 13 wherein encoding the first grouped data comprises performing context adaptive binary arithmetic coding with variable probability.
16. The apparatus of claim 13 wherein encoding the second grouped data comprises performing bypass coding.