Method and apparatus of motion vector constraint for video coding

An apparatus of video coding using an inter-prediction mode in a video coding system, the apparatus comprising one or more electronic devices or processors configured to:

• receive input data associated with a current block in a current picture;
• when a sub-block motion compensation coding tool is selected for the current block:
• generate sub-block motion vectors (MVs) associated with multiple sub-blocks according to the sub-block motion compensation coding tool, wherein the current block comprises the multiple sub-blocks;
• constrain the sub-block MVs within a range around a primary MV to form constrained sub-block MVs, wherein said constraining comprises constraining the sub-block MVs within the range around a primary MV, wherein said constraining the sub-block MVs within the range around the primary MV comprises restricting a difference between the primary MV and each sub-block MV of the sub-block MVs within one or more thresholds, wherein each sub-block MV of the sub-blocks MVs is restricted to the range from (primary MV−Threshold) to (primary MV+Threshold), wherein the Threshold corresponds to one threshold and if one sub-block MV is outside the range, the sub-block MV is clipped to the range;
• and
• apply motion compensation to the current block using the constrained sub-block MVs or apply motion compensation to the current block using one sub-block MV within the range in a second list if a corresponding sub-block MV in a first list is outside the range around the primary MV.