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:
generating 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;
constraining 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
applying motion compensation to the current block using the constrained sub-block MVs or applying 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.
2. 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.
3. The method of claim 2 , wherein, for the affine prediction, a primary MV corresponds to one of control-point MVs, one of the sub-block MVs of the current block, or one derived MV.
4. The method of claim 3 , wherein said one of the sub-block MVs of the current block corresponds to one MV of one corner sub-block or one MV of one center sub-block, or said one derived MV corresponds to one derived MV using one center pixel or center block of the current block.
5. The method of claim 1 , wherein each of said one or more thresholds corresponds to a predefined value, a derived value or a signaled value.
6. The method of claim 1 , wherein said one or more thresholds are determined depending on size, width or height of the current block or current sub-block, the primary MV, inter prediction direction of the current block or the current sub-block, or a combination of the above.
7. The method of claim 1 , wherein, when the sub-block motion compensation coding tool is selected for the current block at a video encoder, the video encoder constrains the sub-block MVs to be within the range to form constrained sub-block MVs and signals information related to the constrained sub-block MVs in a video bitstream.
8. The method of claim 7 , wherein, at a video decoder, the video decoder derives a primary MV and then loads required reference data around the primary MV.
9. The method of claim 1 , wherein the sub-block MVs is constrained within a range around a primary MV.
10. The method of claim 1 , wherein, when the sub-block motion compensation coding tool is selected for the current block at a video encoder, all the sub-block MVs are restricted to be within a range around a primary MV and the encoder signals information related to the sub-block MVs in a video bitstream.
11. 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.