A method for decoding video performed by a decoding apparatus comprises (S 1410) receiving information on an intra prediction mode for a current block (700), (S310, S 1420) determining the intra prediction mode for the current block (700) as a horizontal prediction mode based on the information on the intra prediction mode, (S340, S1430) deriving predicted samples of the current block (700) based on the horizontal prediction mode, and (S 1440) generating a reconstructed block using the derived predicted samples. The predicted samples include filtered predicted samples. The filtered predicted samples are adjacent to the upper boundary of the current block (700) and is located in a first row of the current block (700). A predicted sample is derived based on a reference sample located in a horizontal prediction direction with regard to the predicted sample, wherein the reference sample is adjacent to a left boundary of the current block (700). A filtered predicted sample (720) is derived by applying filtering based on a first reference sample (730) located in the horizontal prediction direction with regard to the filtered predicted sample and a second reference sample (710) neighboring an upper side of the filtered predicted sample. The first reference sample (730) is located outside the current block (700) and is adjacent to the left boundary of the current block (700), and the second reference sample (710) is located outside the current block (700) and is adjacent to the upper boundary of the current block (700).