JVET-M0409 Non-CE8: Mismatch between text specification and reference software on ATMVP candidate derivation when CPR is enabled [X. Xu, X. Li, S. Liu (Tencent), W.-J. Chien, M. Karczewicz (Qualcomm)]

This contribution provides a specification aligned CPR software implementation regarding ATMVP CU level MV offset derivation. In a previous VVC specification, when performing alternative temporal motion vector prediction (ATMVP), a motion offset at CU level is obtained from the motion vector of the first available spatial neighbouring merge candidate that has the collocated picture as its reference picture. In BMS2.1, if a neighbouring merge candidate is coded in CPR mode, it is considered as not available. This implementation is inherited into VTM3.0 as part of CPR mode. In the current VVC specification, only the first available spatial candidate in the merge list will be checked. If the candidate doesn’t satisfy the condition of ATMVP neighbouring block requirement, zero motion vector will be used to derive the collocated block in the collocated picture. Therefore, there is a mismatch between the specification and software implementation. Simulation results report that when the provided specification aligned implementation is compared to VTM-3.0 anchor (CPR=1):

• BD rate changes are 0.00%/0.00%/ 0.00% in AI/RA/LB configurations, respectively, for CTC average.
• BD rate changes are 0.00%/-0.01%/ -0.10% in AI/RA/LB configurations, respectively, for Class F.
• BD rate changes are 0.00%/-0.03%/ -0.10% in AI/RA/LB configurations, respectively, for SCC TGM class.

There are no apparent runtime changes observed.

Decision (SW/BF): Adopt JVET-M0409 (align software with text).