Residual coding method and device for same

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

obtaining prediction mode information and residual information from a bitstream;

deriving prediction samples for a current block based on the prediction mode information;

deriving quantized transform coefficients for the current block based on the residual information;

deriving transform coefficients based on the quantized transform coefficients;

deriving residual samples for the current block based on the transform coefficients; and

generating reconstructed samples based on the prediction samples and the residual samples for the current block,

wherein the residual information includes a significant coefficient flag representing whether a quantized transform coefficient is a non-zero significant coefficient, a parity level flag for a parity of a transform coefficient level for the quantized transform coefficient, a first transform coefficient level flag about whether the transform coefficient level is larger than a first reference value, a second transform coefficient level flag about whether the transform coefficient level is larger than a second reference value and remainder information of the transform coefficient level,

wherein deriving the quantized transform coefficients comprising:

deriving the quantized transform coefficient by decoding the significant coefficient flag, the parity level flag, the first transform coefficient level flag, the second transform coefficient level flag and the remainder information,

wherein the decoding of the first transform coefficient level flag is performed prior to the decoding of the parity level flag.

2. The method of claim 1 , wherein a binarization value for the remainder information is derived based on a rice parameter,

wherein a value of the reminder information is derived based on the binarization value which is derived based on the rice parameter.

3. The method of claim 2 , wherein an initialization process is performed to derive at least one rice parameter for a current sub-block comprised in the current block.

4. The method of claim 3 , wherein the rice parameter for a current transform coefficient within the current sub-block is derived based on a last rice parameter for a transform coefficient of a previous order of the current transform coefficient, and

wherein if the current transform coefficient is the first transform coefficient of the current sub-block, a value of the last rice parameter for the transform coefficient of the previous order is zero.

5. The method of claim 3 , wherein a size of the current sub-block is 2×2 or 4×4.

6. The method of claim 4 , wherein the rice parameter for the current transform coefficient is derived based on neighboring reference transform coefficients of the current transform coefficient, and a number of neighboring reference transform coefficients is 4 or less.

7. The method of claim 6 , wherein a temporary sum coefficient is derived based on the neighboring reference transform coefficients, and

wherein a value of the rice parameter is determined as zero if a value of the temporary sum coefficient is smaller than a first threshold, the value of the rice parameter is determined as 1 if the value of the temporary sum coefficient is the first threshold or more and smaller than a second threshold, the value of the rice parameter is determined as 2 if the value of the temporary sum coefficient is the second threshold or more and smaller than a third threshold, and the value of the rice parameter is determined as 3 if the value of the temporary sum coefficient is the third threshold or more.

8. The method of claim 7 , wherein the first threshold is 1, 2, or 3, the second threshold is 4, 5, or 6, and the third threshold is 10, 11, or 12.

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

deriving a prediction mode of a current block;

deriving prediction samples of the current block based on the prediction mode;

generating prediction mode information based on the derived prediction mode

deriving residual samples for the current block based on the prediction samples;

deriving transform coefficients for the current block based on the residual samples;

deriving quantized transform coefficients based on the transform coefficients;

generating residual information for the quantized transform coefficients; and

encoding image information including the prediction mode information and the residual information,

wherein the residual information includes a significant coefficient flag representing whether a quantized transform coefficient is a non-zero significant coefficient, a parity level flag for a parity of a transform coefficient level for the quantized transform coefficient, a first transform coefficient level flag about whether the transform coefficient level is larger than a first reference value, a second transform coefficient level flag about whether the transform coefficient level is larger than a second reference value and remainder information of the transform coefficient level,

wherein the encoding of the residual information comprises:

encoding the significant coefficient flag, the parity level flag, the first transform coefficient level flag, the second transform coefficient level flag and the reminder information,

wherein the encoding of the first transform coefficient level flag is performed prior to the encoding of the parity level flag.

10. The method of claim 9 , wherein a binarization value for the remainder information is derived based on a rice parameter,

wherein the reminder information is encoded based on the binarization value which is derived based on the rice parameter.

11. The method of claim 10 , wherein an initialization process is performed to derive at least one rice parameter for a current sub-block comprised in the current block.

12. The method of claim 11 , wherein the rice parameter for a current transform coefficient within the current sub-block is derived based on a last rice parameter for a transform coefficient of a previous order of the current transform coefficient, and

wherein if the current transform coefficient is the first transform coefficient of the current sub-block, a value of the last rice parameter for the transform coefficient of the previous order is zero.

13. The method of claim 11 , wherein a size of the current sub-block is 2×2 or 4×4.

14. The method of claim 13 , wherein the rice parameter for the current transform coefficient is derived based on neighboring reference transform coefficients of the current transform coefficient, and a number of neighboring reference transform coefficients is 4 or less.

15. The method of claim 14 , wherein a temporary sum coefficient is derived based on the neighboring reference transform coefficients, and

wherein a value of the rice parameter is determined as zero if a value of the temporary sum coefficient is smaller than a first threshold, the value of the rice parameter is determined as 1 if the value of the temporary sum coefficient is the first threshold or more and smaller than a second threshold, the value of the rice parameter is determined as 2 if the value of the temporary sum coefficient is the second threshold or more and smaller than a third threshold, and the value of the rice parameter is determined as 3 if the value of the temporary sum coefficient is the third threshold or more.

16. The method of claim 15 , wherein the first threshold is 1, 2, or 3, the second threshold is 4, 5, or 6, and the third threshold is 10, 11, or 12.