Method and apparatus for processing video signal by applying secondary transform to partitioned block

1. A method for reconstructing a video signal, the method comprising:

obtaining a secondary transform index for a current block;

deriving a secondary transform matrix related to a specific area in a transform block based on the secondary transform index, wherein the specific area represents an area including a top-left area of the transform block, and wherein the secondary transform matrix is denoted based on an input length and an output length of a secondary transform;

performing an inverse secondary transform for the specific area based on the secondary transform matrix;

performing an inverse primary transform for a block which the inverse secondary transform is applied to;

obtaining residual samples based on a block which the primary inverse transform is applied to; and

reconstructing the current block based on the residual samples and prediction samples,

wherein the input length of the inverse secondary transform is determined as 16, and the output length of the inverse secondary transform is determined as 48, based on that each of the height and the width of the transform block is equal to 8 or 16.

2. The method of claim 1 , wherein the specific area is divided into 4×4 subblocks, and

wherein the inverse secondary transform is performed for each of the 4×4 subblocks.

3. The method of claim 2 , wherein the same 4×4 secondary transform is applied or different 4×4 secondary transforms are applied to the 4×4 subblocks based on at least one of locations or prediction modes of the subblocks.

4. The method of claim 2 , wherein whether the specific area is divided into 4×4 subblocks is determined based on a size of the transform block.

5. The method of claim 2 , further comprising

checking whether the number of non-zero transform coefficients within in the 4×4 subblocks is equal to or more than a specific threshold,

wherein whether the 4×4 secondary transform is applied to the 4×4 subblocks is determined according to the checking result.

6. The method of claim 5 , wherein when the number of non-zero transform coefficients within the 4×4 subblocks is equal to or more than the specific threshold, the 4×4 secondary transform is applied to the 4×4 subblocks, and

otherwise, the 4×4 secondary transform is not applied to the 4×4 subblocks.

7. A method for encoding a video signal, the method comprising:

generating prediction data for a current block;

generating residual data based on the prediction data;

performing a primary transform for a transform block related to the residual data;

deriving a secondary transform matrix for a specific area in the transform block, wherein the specific area represents an area including a top-left area of the transform block, and wherein a secondary transform matrix is denoted based on an input length and an output length of a secondary transform;

performing the secondary transform for the specific area based on the secondary transform matrix;

generating a secondary transform index related to the secondary transform; and

performing a quantization and an entropy encoding for the transform block,

wherein the input length of the secondary transform is determined as 48, and the output length of the secondary transform is determined as 16, based on that each of the height and the width of the transform block is equal to 8 or 16.

8. A transmission method for data comprising a bitstream for an image, comprising:

obtaining the bitstream for the image, wherein the bitstream is generated by generating prediction data for a current block, generating residual data based on the prediction data, performing a primary transform for a transform block related to the residual data, deriving a secondary transform matrix for a specific area in the transform block, wherein the specific area represents an area including a top-left area of the transform block, and wherein a secondary transform matrix is denoted based on an input length and an output length of a secondary transform, performing the secondary transform for the specific area based on the secondary transform matrix, generating a secondary transform index related to the secondary transform, and performing a quantization and an entropy encoding for the transform block, wherein the input length of the secondary transform is determined as 48, and the output length of the secondary transform is determined as 16, based on that each of the height and the width of the transform block is equal to 8 or 16; and

transmitting the data comprising the bitstream.

9. A non-transitory computer-readable medium storing encoded video information generated by performing the steps of:

generating prediction data for a current block;

generating residual data based on the prediction data;

performing a primary transform for a transform block related to the residual data;

deriving a secondary transform matrix for a specific area in the transform block, wherein the specific area represents an area including a top-left area of the transform block, and wherein a secondary transform matrix is denoted based on an input length and an output length of a secondary transform;

performing the secondary transform for the specific area based on the secondary transform matrix;

generating a secondary transform index related to the secondary transform; and

performing a quantization and an entropy encoding for the transform block,

wherein the input length of the secondary transform is determined as 48, and the output length of the secondary transform is determined as 16, based on that each of the height and the width of the transform block is equal to 8 or 16.