Method and apparatus for processing image signal

A method for decoding an image signal by an apparatus, comprising:

• determining whether a non-separable transform is applied for a current block, based on a non-separable transform index obtained from a bitstream;
• based on the non-separable transform being determined to be applied for the current block, determining a non-separable transform set index indicating a non-separable transform set for the current block from among four predetermined non-separable transform sets based on an intra prediction mode of the current block;
• determining a transform kernel included in the non-separable transform set indicated by the non-separable transform set index as a non-separable transform matrix for the current block;
• and
• performing secondary inverse transform for the current block based on the non-separable transform matrix,
• wherein the non-separable transform matrix is applied for R coefficients included in the current block to generate N transformed coefficients, R being smaller than N, N being equal to a total number of coefficients for which the non-separable transform matrix is applied in the current block.

A method for encoding an image signal by an apparatus, comprising:

• determining whether a non-separable transform is applied for a current block, a non-separable transform index indicating a result of the determination is encoded into a bitstream;
• based on the non-separable transform being determined to be applied for the current block, determining a non-separable transform set for the current block from among four predetermined non-separable transform sets based on an intra prediction mode of the current block, each of the four predetermined non-separable transform sets consists of two transform kernels;
• determining a transform kernel included in the non-separable transform set as a non-separable transform matrix for the current block;
• and
• performing secondary transform for the current block based on the non-separable transform matrix,
• wherein a non-separable transform set index indicating the non-separable transform set from among predetermined non-separable transform sets is encoded into the bitstream, and
• wherein the non-separable transform matrix is applied for N coefficients included in the current block to generate R transformed coefficients, R being smaller than N, N being equal to a total number of coefficients for which the non-separable transform matrix is applied in the current block.

A method of transmitting a bitstream generated by a method for encoding an image signal, the method for encoding the image signal comprising:

• determining whether a non-separable transform is applied for a current block, a non-separable transform index indicating a result of the determination is encoded into the bitstream;
• based on the non-separable transform being determined to be applied for the current block, determining a non-separable transform set for the current block from among four predetermined non-separable transform sets based on an intra prediction mode of the current block, each of the four predetermined non-separable transform sets consists of two transform kernels;
• determining a transform kernel included in the non-separable transform set as a non-separable transform matrix for the current block;
• and
• performing secondary transform for the current block based on the non-separable transform matrix,
• wherein a non-separable transform set index indicating the non-separable transform set from among predetermined non-separable transform sets is encoded into the bitstream, and
• wherein the non-separable transform matrix is applied for N coefficients included in the current block to generate R transformed coefficients, R being smaller than N, N being equal to a total number of coefficients for which the non-separable transform matrix is applied in the current block.