A video decoding apparatus includes a bitstream parser, a calculator and a memory. The bitstream parser is provided to receive a video bitstream and extracting a set of constraints associated with the video bitstream, wherein the set of constraints has information associated with a direct_8×8_inference flag for a macroblock of a picture, wherein the macroblock has N sub-macroblock partitions. The calculator is provided to calculate first motion vector information associated with the macroblock and obtain second motion vector information associated with K of the N sub-macroblock partitions from the first motion vector information according to the information associated with the direct_8×8_inference flag, wherein K is less than N. The memory is provided to store the second motion vector information.