Image encoding method and device, and image decoding method and device

1. An image decoding method comprising:

determining at least one coding unit by hierarchically splitting a current image;

determining at least one transform unit in the at least one coding unit;

determining whether to decode residual data of the at least one transform unit according to a transform skip mode;

based on determining that the residual data is to be decoded according to the transform skip mode, obtaining a significant subgroup flag about a subgroup in the at least one transform unit;

based on the significant subgroup flag indicating that the subgroup comprises at least one non-zero significant coefficient and scanning information about coefficients of the subgroup, obtaining at least one first bin about at least one of a significant coefficient flag, a sign flag, a first flag, and a parity flag of a coefficient of a current scan position, the number of the at least one first bin being equal to or smaller than a predetermined number of bins, and the at least one first bin being arithmetic encoded by using a context model;

obtaining the residual data comprising at least one coefficient in the subgroup, based on at least one of a base level of the coefficient of the current scan position obtained by arithmetic decoding the first bin by using the context model and a remainder of an absolute value excluding the base level; and

obtaining a reconstruction block of the coding unit, based on the residual data,

wherein the significant subgroup flag indicates whether the subgroup comprises the at least one non-zero significant coefficient,

the significant coefficient flag indicates whether the coefficient of the current scan position is a non-zero coefficient,

the first flag indicates whether an absolute value of the coefficient of the current scan position is greater than 1 , and

the parity flag of the coefficient of the current scan position indicates whether the coefficient of the current scan position is an odd number or an even number.

2. An image decoding apparatus comprising:

at least one processor configured to:

determine at least one coding unit by hierarchically splitting a current image;

determine at least one transform unit in the at least one coding unit;

determine whether to decode residual data of the at least one transform unit according to a transform skip mode;

based on determining that the residual data is to be decoded according to the transform skip mode, obtain a significant subgroup flag about a subgroup in the at least one transform unit;

based on the significant subgroup flag indicating that the subgroup comprises at least one non-zero significant coefficient and scan information about coefficients of the subgroup, obtain at least one first bin about at least one of a significant coefficient flag, a sign flag, a first flag, and a parity flag of a coefficient of a current scan position, the number of the at least one first bin being equal to or smaller than a predetermined number of bins and the at least one first bin being arithmetic encoded by using a context model,

obtain the residual data comprising at least one coefficient in the subgroup, based on at least one of a base level of the coefficient of the current scan position obtained by arithmetic decoding the first bin by using the context model and a remainder of an absolute value excluding the base level, and

obtain a reconstruction block of the coding unit, based on the residual data,

wherein the significant subgroup flag indicates whether the subgroup comprises the at least one non-zero significant coefficient,

the significant coefficient flag indicates whether the coefficient of the current scan position is a non-zero coefficient,

the first flag indicates whether an absolute value of the coefficient of the current scan position is greater than 1 , and

the parity flag of the coefficient of the current scan position indicates whether the coefficient of the current scan position is an odd number or an even number.

3. An image encoding method comprising:

determining at least one coding unit by hierarchically splitting a current image;

determining at least one transform unit comprised in the coding unit;

determining whether to encode residual data of the at least one transform unit according to a transform skip mode;

based on determining that the residual data is to be encoded according to the transform skip mode, generating a significant subgroup flag about a subgroup in the at least one transform unit;

based on the subgroup including at least one non-zero significant coefficient and scanning information about coefficients of the subgroup, generating at least one first bin by arithmetic encoding at least one of a significant coefficient flag, a sign flag, a first flag, and a parity flag of a coefficient of a current scan position by using a context model, the number of the at least one first bin being equal to or less than a predetermined number of bins; and

generating a bitstream including the residual data that includes the significant subgroup flag and the first bin,

wherein the significant subgroup flag indicates whether the subgroup comprises at least one non-zero significant coefficient,

the significant coefficient flag indicates whether the coefficient of the current scan position is a non-zero coefficient,

the first flag indicates whether an absolute value of the coefficient of the current scan position is greater than 1 , and

the parity flag of the coefficient of the current scan position indicates whether the coefficient of the current scan position is an odd number or an even number.

4. A non-transitory computer-readable storage medium storing a bitstream, the bitstream being encoded by an image encoding method comprising:

determining at least one coding unit by hierarchically splitting a current image;

determining at least one transform unit comprised in the coding unit;

determining whether to encode residual data of the at least one transform unit according to a transform skip mode;

based on determining that the residual data is to be encoded according to the transform skip mode, generating a significant subgroup flag about a subgroup in the at least one transform unit;

based on the subgroup including at least one non-zero significant coefficient and scanning information about coefficients of the subgroup, generating at least one first bin by arithmetic encoding at least one of a significant coefficient flag, a sign flag, a first flag, and a parity flag of a coefficient of a current scan position by using a context model, the number of the at least one first bin being equal to or less than a predetermined number of bins; and

generating a bitstream including the residual data that includes the significant subgroup flag and the first bin,

wherein the significant subgroup flag indicates whether the subgroup comprises at least one non-zero significant coefficient,

the significant coefficient flag indicates whether the coefficient of the current scan position is a non-zero coefficient,

the first flag indicates whether an absolute value of the coefficient of the current scan position is greater than 1 , and

the parity flag of the coefficient of the current scan position indicates whether the coefficient of the current scan position is an odd number or an even number.