Method and apparatus of motion-vector rounding unification for video coding system

An apparatus for video coding performed by a video encoder or a video decoder that utilizes an Inter coding tool, the apparatus comprising one or more electronic circuits or processors arranged to:

• receive input data related to a current block in a current picture at a video encoder side or a video bitstream corresponding to compressed data including the current block at a video decoder side;
• determine one or more MVs (motion vectors) associated with a target Inter coding tool for the current block at the video encoder side or at the video decoder side;
• apply a same conversion process to said one or more MVs to generate one or more converted MVs at the video encoder side or at the video decoder side when the target Inter coding tool belongs to at least two tools of a group, wherein the group comprises at least one of inherited affine Merge candidate derivation and affine sub-block MV derivation, wherein when the target Inter coding tool corresponds to the inherited affine Merge candidate derivation, said one or more MVs corresponds to one or more affine control point MVs;
• clip said one or more affine control point MVs in a clipped MV range, wherein the clipped MV range is in −2{circumflex over ( )}(k−1) to 2{circumflex over ( )}(k−1)−1, and wherein the k is 16 or 18;
• and
• encode the current block or motion information of the current block using said one more converted MVs at the video encoder side or decode the current block or the motion information of the current block using said one or more converted MVs at the video decoder side.

A method of video coding performed by a video encoder or a video decoder that utilizes an Inter coding tool, the method comprising:

• receiving input data related to a current block in a current picture at a video encoder side or a video bitstream corresponding to compressed data including the current block at a video decoder side;
• determining one or more MVs (motion vectors) associated with a target Inter coding tool for the current block at the video encoder side or at the video decoder side;
• applying a same conversion process to said one or more MVs to generate one or more converted MVs at the video encoder side or at the video decoder side when the target Inter coding tool belongs to at least two tools of a group, wherein the group comprises at least one of inherited affine Merge candidate derivation and affine sub-block MV derivation, wherein the same conversion process comprises a rounding-to-zero process, and wherein the rounding-to-zero process is implemented as MV_rounded=(MV+offset−(MV>=0))>>shift, and wherein MV represents a target MV being applied the same conversion process, MV_rounded represents a converted target MV, shift represents an integer corresponding to a number of bits for a right-shift operation, offset is equal to (1<<(shift−1)), and “<<” represent a left-shift operation;
• and
• encoding the current block or motion information of the current block using said one more converted MVs at the video encoder side or decoding the current block or the motion information of the current block using said one or more converted MVs at the video decoder side.