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

Abstract

A decoding method decodes last position information indicating horizontal and vertical positions of a last non-zero coefficient in a predetermined order within a current block to be decoded, the current block including plural coefficients. The decoding includes obtaining a bitstream including first, second, third and fourth partial signals, in this order, performing first arithmetic decoding on the first and the third partial signals respectively to obtain decoded first and decoded third partial signals, performing second arithmetic decoding on the second and the fourth partial signals respectively to obtain decoded second and decoded fourth partial signals, the second arithmetic decoding being different from the first arithmetic decoding, deriving a horizontal component of the last position information from the decoded first and decoded third partial signals, and deriving a vertical component of the last position information from the decoded second and decoded fourth partial signals.

The invention claimed is: 
     
       1. A decoding method for decoding last position information indicating a horizontal position and a vertical position of a last non-zero coefficient in a predetermined order within a current block to be decoded, the current block being included in a picture and including a plurality of coefficients, the decoding method comprising:
 obtaining a bitstream including a first partial signal, a second partial signal, a third partial signal, and a fourth partial signal, in this order; 
 performing first arithmetic decoding on the first partial signal and the third second partial signal respectively to obtain a decoded first partial signal and a decoded third second partial signal, the first arithmetic decoding being context adaptive arithmetic decoding using a variable probability that is updated based on decoded last position information of a decoded block different from the current block; 
 generating a first flag that indicates when the decoded first partial signal is greater than or equal to a predetermined value; 
 generating a second flag that indicates when the decoded second partial signal is greater than or equal to the predetermined value; 
 when the first flag is generated, performing second arithmetic decoding on the second third partial signal, and when the second flag is generated, performing second arithmetic decoding on the fourth partial signal, respectively, to obtain a decoded second third partial signal and a decoded fourth partial signal, the second arithmetic decoding being bypass decoding without using a fixed variable probability; 
 deriving a horizontal component of the last position information indicating the horizontal position of the non-zero coefficient from the decoded first partial signal and the decoded third partial signal; and 
 deriving a vertical component of the last position information indicating the vertical position of the non-zero coefficient from the decoded second partial signal and the decoded fourth partial signal; and 
 reconstructing the horizontal component of the last position information and the vertical component of the last position information to provide decoded last position information for the current block in the picture. 
 
     
     
       2. A decoding apparatus which decodes last position information indicating a horizontal position and a vertical position of a last non-zero coefficient in a predetermined order within a current block to be decoded, the current block being included in a picture and including a plurality of coefficients, the decoding apparatus comprising:
 electrical processing circuitry; and 
 storage coupled to the electrical processing circuitry; 
 wherein the electrical processing circuitry, which includes one or more processors and one or more integrated circuits, is configured to perform the following using the storage:
 obtaining a bitstream including a first partial signal, a second partial signal, a third partial signal, and a fourth partial signal, in this order; 
 performing first arithmetic decoding on the first partial signal and the third second partial signal respectively to obtain a decoded first partial signal and a decoded third second partial signal; 
 generating a first flag that indicates when the decoded first partial signal is greater than or equal to a predetermined value; 
 generating a second flag that indicates when the decoded second partial signal is greater than or equal to the predetermined value; 
 when the first flag is generated, performing second arithmetic decoding on the second third partial signal and when the second flag is generated, performing second arithmetic decoding on the fourth partial signal, respectively, to obtain a decoded second third partial signal and a decoded fourth partial signal, the second arithmetic decoding being different from the first arithmetic decoding; 
 deriving a horizontal component of the last position information indicating the horizontal position of the non-zero coefficient from the decoded first partial signal and the decoded third partial signal; and 
 deriving a vertical component of the last position information indicating the vertical position of the non-zero coefficient from the decoded second partial signal and the decoded fourth partial signal. 
 
 
     
     
       3. A decoding apparatus which decodes last position information indicating a horizontal position and a vertical position of a last non-zero coefficient in a predetermined order within a current block to be decoded, the current block being included in a picture and including a plurality of coefficients, the decoding apparatus comprising:
 an obtainer configured to obtain a bitstream including a first partial signal, a second partial signal, a third partial signal, and a fourth partial signal, in this order; 
 an arithmetic decoder configured to:
 perform a first arithmetic decoding on the first partial signal and the third second partial signal respectively to obtain a decoded first partial signal and a decoded third second partial signal; and 
 generate a first flag that indicates when the decoded first partial signal is greater than or equal to a predetermined value; 
 generate a second flag that indicates when the decoded second partial signal is greater than or equal to the predetermined value; 
 when the first flag is generated, perform a second arithmetic decoding on the second third partial signal and when the second flag is generated, perform second arithmetic decoding on the fourth partial signal respectively to obtain a decoded second third partial signal and a decoded fourth partial signal, the second arithmetic decoding being different from the first arithmetic decoding; and 
 
 a deriver configured to:
 derive a horizontal component of the last non-zero coefficient indicting the horizontal position of the non-zero coefficient from the decoded first partial signal and the decoded third partial signal; and 
 derive a vertical component of the last non-zero coefficient indicting the vertical position of the non-zero coefficient from the decoded second partial signal and the decoded fourth partial signal. 
 
 
     
     
       4. A non-transitory computer-readable recording medium, having a computer program recorded thereon, for causing the a computer to execute the a decoding method according to  claim 1 , the decoding method comprising:
 obtaining a bitstream including a first partial signal, a second partial signal, a third partial signal, and a fourth partial signal, in this order; 
 performing first arithmetic decoding on the first partial signal and the second partial signal respectively to obtain a decoded first partial signal and a decoded second partial signal, the first arithmetic decoding being context adaptive arithmetic decoding using a variable probability that is updated based on decoded last position information of a decoded block different from the current block; 
 generating a first flag that indicates when the decoded first partial signal is greater than or equal to a predetermined value; 
 generating a second flag that indicates when the decoded second partial signal is greater than or equal to the predetermined value; 
 when the first flag is generated, performing second arithmetic decoding on the third partial signal, and when the second flag is generated, performing second arithmetic decoding on the fourth partial signal, respectively, to obtain a decoded third partial signal and a decoded fourth partial signal, the second arithmetic decoding being bypass decoding without using a variable probability; 
 deriving a horizontal component of the last position information indicating the horizontal position of the non-zero coefficient from the decoded first partial signal and the decoded third partial signal; 
 deriving a vertical component of the last position information indicating the vertical position of the non-zero coefficient from the decoded second partial signal and the decoded fourth partial signal; and 
 reconstructing the horizontal component of the last position information and the vertical component of the last position information to provide decoded last position information for the current block in the picture.