A method of decoding a bitstream by an electronic device is provided. An image frame is received from the bitstream, and a block unit is determined from the image frame. A prediction index of the block unit is determined from the bitstream. A plurality of reference mode indices is determined based on a plurality of neighboring blocks of the block unit. The electronic device determines whether the reference mode indices are greater than a specific one of a plurality of non-angular mode indices. When the reference mode indices are greater than the specific non-angular mode index, the electronic device determines whether a difference between the reference mode indices is equal to two. When the difference is equal to two, a prediction mode is selected according to the prediction index and based on a first candidate list. Then, the block unit is reconstructed based on the prediction mode.