Intra-prediction method, and encoder and decoder using same

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

• determining an intra prediction mode for a current block;
• generating prediction mode information based on the intra prediction mode;
• generating prediction samples of the current block based on the intra prediction mode;
• and
• encoding image information including the prediction mode information,
• wherein the intra prediction mode is a horizontal prediction mode having a horizontal prediction direction,
• wherein the step of generating the prediction samples includes:
• deriving a prediction sample based on a first reference sample located along the horizontal prediction direction with regard to a location of the prediction sample, and
• performing filtering on the prediction sample based on a second reference sample located along a vertical direction with regard to a location of the prediction sample, wherein the prediction sample is adjacent to a top boundary of the current block among the prediction samples,
• wherein the first reference sample is located outside of the current block and is adjacent to a left boundary of the current block,
• wherein the second reference sample is located outside of the current block and is adjacent to an upper side of the prediction sample,
• wherein a filtering coefficient applied to a value of the prediction sample is larger than a filtering coefficient applied to a value of the second reference sample.

A non-transitory decoder-readable storage medium storing encoded image information generated by a method for encoding an image, the method comprising:

• determining an intra prediction mode for a current block;
• generating prediction mode information based on the intra prediction mode;
• generating prediction samples of the current block based on the intra prediction mode;
• and
• encoding image information including the prediction mode information,
• wherein the intra prediction mode is a horizontal prediction mode having a horizontal prediction direction,
• wherein the step of generating the prediction samples includes:
• deriving a prediction sample based on a first reference sample located along the horizontal prediction direction with regard to a location of the prediction sample, and
• performing filtering on the prediction sample based on a second reference sample located along a vertical direction with regard to a location of the prediction sample, wherein the prediction sample is adjacent to a top boundary of the current block among the prediction samples,
• wherein the first reference sample is located outside of the current block and is adjacent to a left boundary of the current block,
• wherein the second reference sample is located outside of the current block and is adjacent to an upper side of the prediction sample,
• wherein a filtering coefficient applied to a value of the prediction sample is larger than a filtering coefficient applied to a value of the second reference sample.