A method of encoding video data by an electronic device, the method comprising: determining a block unit from an image frame in the video data; determining a specific reference line candidate from a plurality of reference line candidates and determining a plurality of most probable modes of the block unit for determining an intra prediction mode of the block unit, wherein: the plurality of most probable modes is selected from a plurality of intra mode candidates, and a plurality of remaining mode candidates is generated by removing the plurality of most probable modes from a plurality of intra mode candidates; determining whether the specific reference line candidate is a first one of the reference line candidates, wherein the first one of the plurality of reference line candidates is immediately adjacent to the block unit; encoding, into a bitstream, a reference line index equal to a first predefined value and a most probable mode, mpm, flag indicating whether the intra prediction mode is included in the most probable modes when the specific reference line is identical to the first one of the reference line candidates, wherein: the intra prediction mode is determined from the plurality of most probable modes when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is equal to a second predefined value, and the intra prediction mode is determined from the plurality of remaining mode candidates when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is different from the second predefined value, encoding, into the bitstream, the reference line index different from the first predefined value without signaling the mpm flag when the specific reference line is different from the first one of the reference line candidates, wherein: the intra prediction mode is included in the plurality of most probable modes when the specific reference line candidate is different from the first one of a plurality of reference line candidates; and encoding, into the bitstream, a mode index of the block unit for indicating the intra prediction mode. determining a block unit from an image frame in the video data; determining a specific reference line candidate from a plurality of reference line candidates and determining a plurality of most probable modes of the block unit for determining an intra prediction mode of the block unit, wherein: the plurality of most probable modes is selected from a plurality of intra mode candidates, and a plurality of remaining mode candidates is generated by removing the plurality of most probable modes from a plurality of intra mode candidates; the plurality of most probable modes is selected from a plurality of intra mode candidates, and a plurality of remaining mode candidates is generated by removing the plurality of most probable modes from a plurality of intra mode candidates; determining whether the specific reference line candidate is a first one of the reference line candidates, wherein the first one of the plurality of reference line candidates is immediately adjacent to the block unit; encoding, into a bitstream, a reference line index equal to a first predefined value and a most probable mode, mpm, flag indicating whether the intra prediction mode is included in the most probable modes when the specific reference line is identical to the first one of the reference line candidates, wherein: the intra prediction mode is determined from the plurality of most probable modes when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is equal to a second predefined value, and the intra prediction mode is determined from the plurality of remaining mode candidates when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is different from the second predefined value, the intra prediction mode is determined from the plurality of most probable modes when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is equal to a second predefined value, and the intra prediction mode is determined from the plurality of remaining mode candidates when the specific reference line candidate is identical to the first one of the plurality of reference line candidates and the mpm flag is different from the second predefined value, encoding, into the bitstream, the reference line index different from the first predefined value without signaling the mpm flag when the specific reference line is different from the first one of the reference line candidates, wherein: the intra prediction mode is included in the plurality of most probable modes when the specific reference line candidate is different from the first one of a plurality of reference line candidates; and encoding, into the bitstream, a mode index of the block unit for indicating the intra prediction mode.