id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
292	Chroma scan type in NxN intra prediction units	bheng		"The chroma scan type can be set incorrectly when all of the following are true:

  • Intra CU.
  • Four NxN luma prediction units.
  • Chroma prediction mode copied from luma prediction mode (e.g. intra_chroma_pred_mode=5).

The actual intra chroma prediction mode for the CU is determined from Luma Partition 0 only.  See the following:

{{{
  if( uiChromaPredMode == DM_CHROMA_IDX )
  {
    uiChromaPredMode = pcCU->getLumaIntraDir( 0 );
  }
}}}

However, for purposes of scan type derivation (diagonal, horizontal, vertical), the HM software can end up using four independent chroma scan types per CU.  It incorrectly uses the luma intra prediction mode from Luma Partition 0, Partition 1, Partition 2, and Partition 3.

The chroma scan type should based only on the actual chroma prediction mode used in each block.  Therefore, the luma modes from Partitions 1-3 should not be used to determine the chroma scan type.

I believe the relevant code causing this behaviour is in TComDataCU::getCoefScanIdx

{{{
    if( uiDirMode == DM_CHROMA_IDX )
    {
      uiDirMode = getLumaIntraDir(uiAbsPartIdx);
    }
}}}


"	defect	closed	minor	HM-6.0	HM	HM-5.1	fixed		fbossen ksuehring davidf jct-vc@…