Method and apparatus of motion vector constraint for video coding
Method and apparatus of Inter prediction for video coding are disclosed. When a sub-block motion compensation coding tool is selected for the current block, the method generates sub-block MVs (motion vectors) associated with multiple sub-blocks, which are included or contained in the current block, according to the sub-block motion compensation coding tool, constrains the sub-block MVs within a range to form constrained sub-block MVs, and applies motion compensation to the current block using the constrained sub-block MVs or applies motion compensation to the current block using one sub-block MV within the range around the primary MV in a second list if a corresponding sub-block MV in a first list is outside the range. In another method, motion compensation is applied to the current block only using reference pixels of reference sub-blocks within a primary reference block.
1. A method of video coding using an inter-prediction mode in a video coding system, the method comprising:
receiving 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:
deriving a primary reference block;
generating sub-block MVs (motion vectors) associated with multiple sub-blocks according to the sub-block motion compensation coding tool, wherein the current block comprises the multiple sub-blocks; and
applying motion compensation to the current block using one or more reference pixels of reference sub-blocks within the primary reference block and excluding any reference pixel of reference sub-blocks outside the primary reference block, wherein the reference sub-blocks are pointed by the sub-block MVs.
2. The method of claim 1 , wherein the primary reference block covers a reference block pointed by a primary MV.
3. The method of claim 1 , wherein any reference pixel of reference sub-blocks outside the primary reference block is replaced by a pseudo reference pixel.
4. The method of claim 3 , wherein the pseudo reference pixel corresponds to a predefined pixel value or a padding pixel of the primary reference block.
5. The method of claim 1 , wherein interpolation filter coefficients are modified to exclude any reference pixel of reference sub-blocks outside the primary reference block.
6. The method of claim 1 , wherein size and shape of the primary reference block is determined according to size and shape of the current block or current sub-block, the primary MV, inter prediction direction of the current or the current sub-block, or the combination of the above.
7. The method of claim 1 , wherein the sub-block motion compensation coding tool is selected from a group comprising ATMVP (Advanced Temporal Motion Vector Prediction), STMVP (Spatial-Temporal Motion Vector Prediction), affine prediction, and sub-block refined PMVD (Pattern-based Motion Vector Derivation) mode.
8. The method of claim 1 , further comprising deriving a primary motion vector associated with the primary reference block.
9. 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:
derive a primary reference block;
generate sub-block MVs (motion vectors) associated with multiple sub-blocks according to the sub-block motion compensation coding tool, wherein the current block comprises the multiple sub-blocks; and
apply motion compensation to the current block using one or more reference pixels of reference sub-blocks within the primary reference block and exclude any reference pixel of reference sub-blocks outside the primary reference block, wherein the reference sub-blocks are pointed by the sub-block MVs.