Video decoding method using residual information in video coding system, and apparatus thereof

1. An image decoding method performed by a decoding apparatus, the image decoding method comprising:

receiving residual information for a current block;

deriving a context model for a sign flag of a current residual coefficient in a current sub-block of the current block;

decoding the sign flag based on the context model;

deriving the current residual coefficient based on the sign flag;

deriving a residual sample based on the current residual coefficient; and

generating a reconstructed picture based on the residual sample,

wherein the current block is a transform skip block, and

wherein the context model for the sign flag is derived based on a sign flag of a residual coefficient decoded prior to the current residual coefficient in the current sub-block.

2. The image decoding method of claim 1 ,

wherein, when a value of the sign flag of the residual coefficient decoded prior to the current residual coefficient is 0, the context model for the sign flag of the current residual coefficient is derived as a context model 0, and

wherein, when the value of the sign flag of the residual coefficient decoded prior to the current residual coefficient is 1, the context model for the sign flag of the current residual coefficient is derived as a context model 1.

3. The image decoding method of claim 1 , further comprising determining whether the sign flag is decoded based on the context model, by comparing a specific value and a number of non-zero residual coefficients in the current sub-block.

4. The image decoding method of claim 3 ,

wherein the sign flag is bypass-decoded when the number of non-zero residual coefficients is less than the specific value, and

wherein the sign flag is decoded based on the context model when the number of non-zero residual coefficients is greater than or equal to the specific value.

5. The image decoding method of claim 3 , wherein the specific value is derived based on a size of the current block.

6. The image decoding method of claim 5 ,

wherein the specific value is derived as 5 when the size of the current block is an 8×8 size, and

wherein the specific value is derived as 4 when the size of the current block is a 4×4 size.

7. The image decoding method of claim 3 , wherein the specific value is derived based on a size of the current block and a position of the current sub-block in the current block.

8. The image decoding method of claim 7 ,

wherein the specific value is derived as 5 when the size of the current block is an 8×8 size and the current sub-block is a bottom-right sub-block of the current block, and

wherein the specific value is derived as 4 when the size of the current block is the 8×8 size and the current sub-block is a top-left block of the current block.

9. The image decoding method of claim 3 , wherein the specific value is derived based on a size of the current block, a position of the current sub-block in the current block, and a prediction mode of the current block.

10. The image decoding method of claim 9 ,

wherein the specific value is derived as 5 when the size of the current block is the 8×8 size, the current sub-block is a bottom-right sub-block of the current block, and the prediction mode of the current block is an intra prediction mode, and

wherein the specific value is derived as 0 when the size of the current block is the 8×8 size, the current sub-block is a top-left sub-block of the current block, and the prediction mode of the current block is the intra prediction mode.

11. The image decoding method of claim 1 , wherein the context model for the sign flag is derived as one of a plurality of context models based on sign flags of a plurality of residual coefficients decoded prior to the current residual coefficient in the current sub-block.

12. An image encoding method performed by an encoding apparatus, the image encoding method comprising:

deriving a current residual coefficient in a current sub-block of a current block;

deriving a context model for a sign flag of the current residual coefficient;

encoding the sign flag based on the context model; and

generating a bitstream including the sign flag,

wherein the current block is a transform skip block, and

wherein the context model for the sign flag is derived based on a sign flag of a residual coefficient encoded prior to the current residual coefficient in the current sub-block.

13. The image encoding method of claim 12 ,

wherein, when a value of the sign flag of the residual coefficient encoded prior to the current residual coefficient is 0, the context model for the sign flag of the current residual coefficient is derived as a context model 0, and

wherein, when the value of the sign flag of the residual coefficient encoded prior to the current residual coefficient is 1, the context model for the sign flag of the current residual coefficient is derived as a context model 1.

14. The image encoding method of claim 12 , further comprising determining whether the sign flag is encoded based on the context model, by comparing a specific value and a number of non-zero residual coefficients in the current sub-block.

15. The image encoding method of claim 14 ,

wherein the sign flag is bypass-encoded when the number of non-zero residual coefficients is less than the specific value, and

wherein the sign flag is encoded based on the context model when the number of non-zero residual coefficients is greater than or equal to the specific value.

16. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:

deriving a current residual coefficient in a current sub-block of a current block;

deriving a context model for a sign flag of the current residual coefficient;

encoding the sign flag based on the context model; and

generating the bitstream including the sign flag,

wherein the current block is a transform skip block, and

wherein the context model for the sign flag is derived based on a sign flag of a residual coefficient encoded prior to the current residual coefficient in the current sub-block.