Method and device for decoding image according to intra prediction in image coding system

A video decoding method performed by a decoding apparatus, the method comprising:

• deriving a first intra-prediction mode for a current block:
• deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block:
• and
• determining whether to derive the second intra-prediction mode for the target sample based on a size of the current block,
• when the size of the current block is greater than a specific size, the second intra-prediction mode for the target sample is derived based on the position of the target sample of the current block and the first intra-prediction mode,
• wherein prediction of the target sample is performed based on the second intra-prediction mode for the target sample,
• wherein the specific value is derived based the size of the current block, and
• when the size of the current block is N×M and the first intra-prediction mode is one of vertical directional intra-prediction modes, the specific value is derived as N,
• when the size of the current block is N×M and the first intra-prediction mode is one of horizontal directional intra-prediction modes, the specific value is derived as N, and
• wherein the horizontal directional intra-prediction modes include #2 intra-prediction mode to #33 intra-prediction mode, and the vertical directional intra-prediction modes include #34 intra-prediction mode to #66 intra-prediction mode;
• wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
• deriving a first reference sample located in a prediction direction of the first intra-prediction mode based on the target sample among the neighboring samples;
• and
• when the first intra-prediction mode is #m intra-prediction mode, and a distance between the target sample and the first reference sample is greater than the specific value, deriving another intra-prediction mode as the second intra-prediction mode for the target sample.

A video encoding method performed by an encoding apparatus, the video encoding method comprising:

• deriving a first intra-prediction mode for a current block;
• deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block;
• deriving a second intra-prediction mode for a target sample based on a position of the target sample and the first intra-prediction mode;
• and
• performing prediction of the target sample based on the second intra-prediction mode for the target sample,
• wherein a specific value is derived based a size of the current block, and
• when the size of the current block is N×M and the first intra-prediction mode is one of vertical directional intra-prediction modes, the specific value is derived as N,
• when the size of the current block is N×M and the first intra-prediction mode is one of horizontal directional intra-prediction modes, the specific value is derived as N, and
• wherein the horizontal directional intra-prediction modes include #2 intra-prediction mode to #33 intra-prediction mode, and the vertical directional intra-prediction modes include #34 intra-prediction mode to #66 intra-prediction mode;
• wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
• deriving a first reference sample located in a prediction direction of the first intra-prediction mode based on the target sample among the neighboring samples;
• and
• when the first intra-prediction mode is #m intra-prediction mode, and a distance between the target sample and the first reference sample is greater than the specific value, deriving another intra-prediction mode as the second intra-prediction mode for the target sample.