A method of decoding a bitstream in intra prediction by an electronic device, the method comprising: determining a block unit from an image frame according to the bitstream; determining, for the block unit, an orientation index from the bitstream, determining a specific one of a plurality of intra default modes based on the orientation index, wherein: a mode list predefined in the electronic device includes the plurality of intra default modes and a plurality of added modes, and each of the plurality of added modes is mutually different from each of the plurality of intra default modes; determining whether the specific one of the plurality of intra default modes is replaced by one of the plurality of added modes, wherein: each of at least one of the plurality of intra default modes corresponds to one of the plurality of added modes, the number of the at least one of the plurality of intra default modes is equal to the number of at least one of the plurality of added modes, and the specific one of the plurality of defaults modes is included in at least one of the plurality of intra default modes; determining the one of the plurality of added modes as a prediction mode, if the specific one of the plurality of intra default modes is to be replaced by the one of the plurality of added modes; determining the specific one of the plurality of intra default modes as the prediction mode, if the specific one of the plurality of intra default modes remains unchanged; and reconstructing the block unit of the image frame based on the prediction mode. determining a block unit from an image frame according to the bitstream; determining, for the block unit, an orientation index from the bitstream, determining a specific one of a plurality of intra default modes based on the orientation index, wherein: a mode list predefined in the electronic device includes the plurality of intra default modes and a plurality of added modes, and each of the plurality of added modes is mutually different from each of the plurality of intra default modes; a mode list predefined in the electronic device includes the plurality of intra default modes and a plurality of added modes, and each of the plurality of added modes is mutually different from each of the plurality of intra default modes; determining whether the specific one of the plurality of intra default modes is replaced by one of the plurality of added modes, wherein: each of at least one of the plurality of intra default modes corresponds to one of the plurality of added modes, the number of the at least one of the plurality of intra default modes is equal to the number of at least one of the plurality of added modes, and the specific one of the plurality of defaults modes is included in at least one of the plurality of intra default modes; each of at least one of the plurality of intra default modes corresponds to one of the plurality of added modes, the number of the at least one of the plurality of intra default modes is equal to the number of at least one of the plurality of added modes, and the specific one of the plurality of defaults modes is included in at least one of the plurality of intra default modes; determining the one of the plurality of added modes as a prediction mode, if the specific one of the plurality of intra default modes is to be replaced by the one of the plurality of added modes; determining the specific one of the plurality of intra default modes as the prediction mode, if the specific one of the plurality of intra default modes remains unchanged; and reconstructing the block unit of the image frame based on the prediction mode.