Sub-PU TMVP can generate an invalid inter candidate

[bheng]

In UnitTools.cpp, in the function getInterMergeSubPuMvpCand():

      if (colMi.isInter ... )
      {
        for (unsigned currRefListId = 0; currRefListId < (bBSlice ? 2 : 1); currRefListId++)
        {
          RefPicList currRefPicList = RefPicList(currRefListId);
          if (deriveScaledMotionTemporal(…))
          {
            …
          }
        }
      }
      else
      {
        // intra coded, in this case, no motion vector is available for list 0 or list 1, so use default
        …
      }

The code above can generate a inter motion vector with no valid reference frame (all refIdx = -1).

Assume the first if() condition passes (i.e. colMi.isInter) and the second if() condition fails. For example, if the current slice is a P-slice and deriveScaledMotionTemporal() returns false for List0 because colMi contains only List1 motion, getCheckLDC()=0, and bAllowMirrorMV=0.

In this situation, the code above will never run the else( ) condition that assigns the default motion instead. An inter motion vector will be assigned with no valid reference index (refIdxL0=-1 and refIdxL1=-1).

[bheng]

One possible fix has been provided in:

​​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/merge_requests/285

[ksuehring]

Reopening: Another fix was submitted as MR 347

​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/merge_requests/347