Dear experts,

there is a possible mismatch in the RExt D5 (01005v4) and HM-12.1+RExt-5.1 for deblocking with 422 subsampling. In the following luma CB/TB partitioning for a 16x16 CB two horizontal edges are considered for filtering:

      luma 16x16                
-----------------------    <- considered for filtering (1)
|          |          |      
|  8x8     |   8x8    |     
|  TU      |   TU     |     
|          |          |     
-----------------------    <- considered for filtering (2)
|          |          |
|  8x8     |   8x8    |
|  TU      |   TU     |
|          |          |
-----------------------

For the corresponding chroma part for both 420 and 422 only (1) is considered and (2) is not considered:

TComLoopFilter.cpp:223

if ( chFmt!=CHROMA_400
 && (bAlwaysDoChroma
  || (uiPelsInPart>DEBLOCK_SMALLEST_BLOCK)
  || (iEdge % ( (DEBLOCK_SMALLEST_BLOCK<<'''1''')/uiPelsInPart ) ) == 0)
) {
   xEdgeFilterChroma   ( pcCU, uiAbsZorderIdx, uiDepth, edgeDir, iEdge );
}

Here there is from my understanding a difference in the text and software exist as the text (8.7.2.5.2) edge (2) also is filtered in 422 but not in 420. It might be a misinterpretation from the text.

The question basically boils down to: do chroma TB and PB boundaries that are in 420 not on a 8x8 grid, and in 422 are promoted to the 8x8 grid, need to be filtered? Note that in the current software chroma CB boundaries that are promoted in such a way are filtered, so for the corresponding case with 4 8x8 CBs would result in both (1) and (2) being filtered for 422.

Best,
Chi