Mismatch on collocated subblock location in SbTMVP between VTM and spec

[chhuanb]

In specification, the collocated location of each sub-block is derived as follow (in clause 8.5.5.3):

–	The luma location ( xSb, ySb ) specifying the top-left sample of the current coding subblock relative to the top left luma sample of the current picture is derived as follows:
xSb  =  xCb + xSbIdx * sbWidth		(8 538)
ySb  =  yCb + ySbIdx * sbHeight		(8 539)
–	The location ( xColSb, yColSb ) of the collocated subblock inside ColPic is derived as follows.
xColSb = Clip3( xCtb, 
	Min( CurPicWidthInSamplesY − 1, xCtb + ( 1  <<  CtbLog2SizeY ) + 3 ),	(8 540)
 	xSb + ( tempMv[0]  >>  4 ) )
yColSb = Clip3( yCtb, 
	Min( CurPicHeightInSamplesY − 1, yCtb + ( 1  <<  CtbLog2SizeY ) − 1 ),	(8 541)
 	ySb + ( tempMv[1]  >>  4 ) )

which means the collocated location is equal to "top-left sample location + temporal motion vector".

In VTM, the collocated location of each sub-block is derived as follow (in function getInterMergeSubPuMvpCand):

  int xOff = (puWidth >> 1) + tempX;
  int yOff = (puHeight >> 1) + tempY;

  MotionBuf& mb = mrgCtx.subPuMvpMiBuf;

  const bool isBiPred = isBipredRestriction(pu);

  for (int y = puPos.y; y < puPos.y + puSize.height; y += puHeight)
  {
    for (int x = puPos.x; x < puPos.x + puSize.width; x += puWidth)
    {
      Position colPos{ x + xOff, y + yOff };
      clipColPos(colPos.x, colPos.y, pu);
      ...
      ...
    }
  }

which means the collocated location is equal to "below-right center sample location + temporal motion vector".

It is suggested to align spec to software as follow:

–	The luma location ( xSb, ySb ) specifying the below-right center sample of the current coding subblock relative to the top left luma sample of the current picture is derived as follows:
xSb  =  xCb + xSbIdx * sbWidth + (sbWidth >> 1)		        (8 538)
ySb  =  yCb + ySbIdx * sbHeight + (sbHeight >> 1)		(8 539)
–	The location ( xColSb, yColSb ) of the collocated subblock inside ColPic is derived as follows.
xColSb = Clip3( xCtb, 
	Min( CurPicWidthInSamplesY − 1, xCtb + ( 1  <<  CtbLog2SizeY ) + 3 ),	(8 540)
 	xSb + ( tempMv[0]  >>  4 ) )
yColSb = Clip3( yCtb, 
	Min( CurPicHeightInSamplesY − 1, yCtb + ( 1  <<  CtbLog2SizeY ) − 1 ),	(8 541)
 	ySb + ( tempMv[1]  >>  4 ) )

[bbross]

Ticket #329 has been marked as a duplicate of this ticket.

[bbross]

Fixed in D6 vC

[bbross]

Fixed in D6 vC