Intra prediction method and device

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

• deriving a residual signal for a current block;
• deriving transform coefficients by performing a primary transform based on the residual signal;
• determining a transform set for a secondary transform based on an intra prediction mode of the current block;
• determining a transform kernel for the secondary transform based on the transform set and a size of the current block;
• deriving modified transform coefficients by performing the secondary transform on the transform coefficients based on the transform kernel for the secondary transform;
• deriving quantized transform coefficient levels based on the modified transform coefficients;
• and
• encoding image information including information on the quantized transform coefficient levels,
• wherein the secondary transform is a non-separable transform,
• wherein the transform set including plural transform kernel candidates is determined based on the intra prediction mode of the current block,
• wherein transform kernel candidates in a first transform set is different from transform kernel candidates in a second transform set,
• wherein one of the plural transform kernel candidates in the transform set is determined as the transform kernel for the secondary inverse-transform based on the size of the current block,
• wherein a first transform kernel determined from the transform set based on a first block size of the current block is different from a second transform kernel determined from the transform set based on a second block size of the current block.

A transmission method for image data, the method comprising:

• obtaining a bitstream of encoded image information, wherein the encoded image information is generated based on deriving a residual signal for a current block, deriving transform coefficients by performing a primary transform based on the residual signal, determining a transform set for a secondary transform based on an intra prediction mode of the current block, determining a transform kernel for the secondary transform based on the transform set and a size of the current block, deriving modified transform coefficients by performing the secondary transform based on the transform coefficients and the transform kernel for the secondary transform, deriving quantized transform coefficient levels based on the modified transform coefficients, and encoding image information including information on the quantized transform coefficient levels;
• and
• transmitting the image data comprising the bitstream,
• wherein the secondary transform is a non-separable transform,
• wherein the transform set including plural transform kernel candidates is determined based on the intra prediction mode of the current block,
• wherein transform kernel candidates in a first transform set is different from transform kernel candidates in a second transform set,
• wherein one of the plural transform kernel candidates in the transform set is determined as the transform kernel for the secondary inverse-transform based on the size of the current block,
• wherein a first transform kernel determined from the transform set based on a first block size of the current block is different from a second transform kernel determined from the transform set based on a second block size of the current block.