Triangle merge list zero-MV candidates not properly initialized

[bheng]

In UnitTools.cpp, in function getTriangleMergeCandidates(), the “fill with Mv(0, 0)” section of code can reuse leftover data from earlier steps in the candidate construction process.

Specifically, if a vector was considered in one of the earlier steps, but rejected because the isUniqueTriangleCandidates() check failed, the rejected vector data is still left in that candidate position and is not reinitialized.

Then, in the final zero-MV-fill stage, it can reuse that same candidate position, but only the List0 or List1 data is set. This leaves leftover data from some previous candidate in the other half of the zero-MV.

triangleMrgCtx.interDirNeighbours[triangleMrgCtx.numValidMergeCand] = 1;
triangleMrgCtx.mvFieldNeighbours[triangleMrgCtx.numValidMergeCand << 1].setMvField(Mv(0, 0), cnt);
triangleMrgCtx.numValidMergeCand++;

So you can end up with a bidirectional candidate in the triangle merge list.

[bheng]

One possible fix has been provided in:

​​https://vcgit.hhi.fraunhofer.de/jvet/VVCSoftware_VTM/merge_requests/284