After some investigation, I think that the bug is situated in the TComLoopFilter::xSetLoopfilterParam function, which should make the "LeftEdge" and "TopEdge" flags false, so that the edge between tiles is not deblocked when TileBoundaryIndependenceIdc=1. Digging deeper into this function reveals that the bug is likely in the getPULeft and getPUAbove functions, where the getTileBoundaryIndependenceIdr() flag is read. If these two functions return NULL in case of TileBoundaryIndependenceIdc=1 for the particular tile boundary edge, then that edge will not be deblocking filtered and the QP availability problem inside the DBF_DQP macro will not occur. Note also that accessing the IPCM flag has a similar problem.

This is a temporary patch to correct the behavior of the deblocking filter if the CU on the left side or above the edge to be deblocking filtered is unavailable:

Put TILEBOUNDARYINDEPENDENCE_PATCH macro inside TComLoopFilter::xEdgeFilterLuma and TComLoopFilter::xEdgeFilterChroma functions:

#if DBF_DQP
      iQP_Q = pcCU->getQP( uiBsAbsIdx );
      uiPartQIdx = uiBsAbsIdx;
      // Derive neighboring PU index
      if (iDir == EDGE_VER)
      {
        pcCUP = pcCUQ->getPULeft (uiPartPIdx, uiPartQIdx);
      }
      else  // (iDir == EDGE_HOR)
      {
        pcCUP = pcCUQ->getPUAbove(uiPartPIdx, uiPartQIdx);
      }
#if TILEBOUNDARYINDEPENDENCE_PATCH
      if (!pcCUP)
      {
	return;
      }
#endif
      iQP_P = pcCUP->getQP(uiPartPIdx);

      if(pcCU->getIPCMFlag(uiPartQIdx)) 
      {
        iQP_Q = 0; 
      }
      if(pcCUP->getIPCMFlag(uiPartPIdx)) 
      {
        iQP_P = 0; 
      }

      iQP = (iQP_P + iQP_Q + 1) >> 1;
#endif

So it seems issue #257 is a duplicate of this? I think it suggests the same NULL pointer check.

Replying to ksuehring:

So it seems issue #257 is a duplicate of this? I think it suggests the same NULL pointer check.

That is partially correct, however, even with the suggested patch for ticket #256, the encoder will still crash (not deblocking related) for the crash conditions attached to ticket #257.

the patch has been applied to HM-5.0-dev-bugfix in r1704