Method for encoding/decoding video signal, and apparatus therefor

1. A method for decoding an image performed by a decoding apparatus, comprising:

determining a secondary transform set among a pre-determined number of secondary transform sets based on an intra prediction mode of a current block;

obtaining a first syntax element related to a secondary transform matrix to be applied to the current block among secondary transform matrices included in the secondary transform set;

performing a secondary inverse-transform on secondary transformed coefficients of the current block based on the secondary transform matrix specified by the first syntax element to obtain primary transformed coefficients, the secondary inverse-transform being a non-separable transform;

performing a primary inverse-transform on the primary transformed coefficients based on a primary transform matrix of the current block to obtain residual samples of the current block; and

generating reconstructed samples of the current block based on the residual samples,

wherein a number of input coefficients of the secondary inverse-transform and a number of output coefficients of the secondary inverse-transform are respectively determined as 8 and 16 based on the current block having a 4×4 size, and

wherein the number of the input coefficients of the secondary inverse-transform for the current block having the 4×4 size is equal to the number of the input coefficients of the secondary inverse-transform for the current block having an 8×8 size, while the number of the output coefficients of the secondary inverse-transform for the current block having the 4×4 size is not equal to the number of the output coefficients of the secondary inverse-transform for the current block having the 8×8 size.

2. The method for decoding the image of claim 1 ,

wherein each of the secondary transform sets comprises two secondary transform matrices.

3. The method for decoding the image of claim 1 , further comprising:

parsing a second syntax element related to the primary transform matrix; and

determining whether to perform the secondary inverse-transform based on the second syntax element.

4. The method for decoding the image of claim 3 ,

wherein the determining whether to perform the secondary inverse-transform comprises determining to perform the secondary inverse-transform based on the second syntax element related to information for a pre-determined transform type.

5. The method for decoding the image of claim 4 , wherein the pre-determined transform type is defined as DCT-2.

6. A method for encoding an image performed by an encoding apparatus, comprising:

generating prediction samples of a current block based on an intra prediction mode;

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

performing a primary transform on the residual samples to obtain primary-transformed coefficients of the current block;

determining a secondary transform set among a pre-determined number of secondary transform sets based on the intra prediction mode;

generating a first syntax element related to a secondary transform matrix to be applied to the current block among secondary transform matrices included in the determined secondary transform set; and

performing a secondary transform on the primary-transformed coefficients based on the secondary transform matrix, the secondary transform being a non-separable transform,

wherein a number of input coefficients of the secondary transform and a number of output coefficients of the secondary transform are respectively determined as 16 and 8 based on the current block having a 4×4 size, and

wherein the number of the input coefficients of the secondary transform for the current block having the 4×4 size is not equal to the number of the input coefficients of the secondary transform for the current block having an 8×8 size, while the number of the output coefficients of the secondary transform for the current block having the 4×4 size is equal to the number of the output coefficients of the secondary transform for the current block having the 8×8 size.

7. The method for encoding the image of claim 6 , wherein each of the secondary transform sets comprises two secondary transform matrices.

8. A non-transitory decoder-readable storage medium storing the bitstream generated by the method for encoding the image of claim 6 .

9. A transmission method of data for an image performed by a transmission apparatus, the transmission method comprising:

obtaining a bitstream for the image, wherein the bitstream is generated based on generating prediction samples of a current block based on an intra prediction mode, deriving residual samples of the current block based on the prediction samples, performing a primary transform on the residual samples to obtain primary-transformed coefficients of the current block, determining a secondary transform set among a pre-determined number of secondary transform sets based on the intra prediction mode, generating a first syntax element related to a secondary transform matrix to be applied to the current block among secondary transform matrices included in the determined secondary transform set, and performing a secondary transform on the primary-transformed coefficients based on the secondary transform matrix, the secondary transform being a non-separable transform; and

transmitting the data comprising the bitstream,

wherein a number of input coefficients of the secondary transform and a number of output coefficients of the secondary transform are respectively determined as 16 and 8 based on the current block having a 4×4 size, and

wherein the number of the input coefficients of the secondary transform for the current block having the 4×4 size is not equal to the number of the input coefficients of the secondary transform for the current block having an 8×8 size, while the number of the output coefficients of the secondary transform for the current block having the 4×4 size is equal to the number of the output coefficients of the secondary transform for the current block having the 8×8 size.