A method for generating a prediction block in intra prediction can include restoring a mode group indicator and a prediction mode index; constructing a most probable mode (MPM) group including three intra prediction modes of a current block; determining an intra prediction mode specified by the intra prediction mode index in the MPM group as the intra prediction mode of the current block if the mode group indicator indicates the MPM group, and deriving the intra prediction mode of the current block using the prediction mode index and the three intra prediction modes of the MPM group if the mode group indicator does not indicate the MPM group; determining a size of the prediction block based on transform size information; determining whether all reference pixels of the current block are available; generating reference pixels if one or more reference pixels of the current block are unavailable; adaptively filtering the reference pixels based on the intra prediction mode and the size of the current block; and generating the prediction block using the reference pixels based on the restored intra prediction mode.