#1341 closed defect (fixed)

Issues with the modification of tC for bit depth less than 10

Reported by: bheng Owned by:
Priority: minor Milestone:
Component: spec Version: VVC D10 vE
Keywords: Cc: ksuehring, bbross, XiangLi, fbossen, jvet@…

Description

In Section 8.8.3.6.2 for luma, the "roundOffset" used for tC calculation is derived as:

roundOffset = 1 << ( 9 - BitDepth )

The right-shift function, "is defined only for non-negative integer values of y", so this behavior is undefined for BitDepth > 9.

This roundOffset is only required for BitDepth < 10, so I would suggest conditioning the derivation of roundOffset on "If BitDepth < 10 ..." rather than specifying an undefined operation.

Additionally, in Section 8.8.3.6.4 for chroma, the same calculation is incorrect:

"( tC′ + 2 ) >> ( 10 − BitDepth )"

here a fixed rounding value of "2" is used which doesn't match VTM behavior for 9-bit.

Change history (2)

comment:1 Changed 14 months ago by siwamura

Thank you for reporting this issue.

As a proponent of the modification of tC bit-adaptation proposed in JVET-R0130, I also agree with your suggestion.

In the software, the bit adaptation is done as follows:
Luma:

const int iTc = bitDepthLuma < 10 ? ((sm_tcTable[iIndexTC] + (1 << (9 - bitDepthLuma))) >> (10 - bitDepthLuma)) : ((sm_tcTable[iIndexTC]) << (bitDepthLuma - 10));

Chroma:

const int iTc = bitDepthChroma < 10 ? ((sm_tcTable[iIndexTC] + (1 << (9 - bitDepthChroma))) >> (10 - bitDepthChroma)) : ((sm_tcTable[iIndexTC]) << (bitDepthChroma - 10));

We have two options,
(1) Incorporate roundOffset into tC derivation instead of defining roundOffset, as done in VTM
(2) Define roundOffset with the suggested conditioning like "If BitDepth < 10 ..." for both luma and chroma.

comment:2 Changed 14 months ago by bbross

  • Resolution set to fixed
  • Status changed from new to closed

Thanks for reporting that. Although this does not seems to impact the decoding process, we should avoid having variables undefined.
This will be fixed in JVET-S2001-vH as follows:

Luma and chroma (same since we only have one BitDepth for luma and chroma in the spec):

  • If BitDepth is less than 10, the following applies:

tC = ( tC′ + ( 1 << ( 9 − BitDepth ) ) ) >> ( 10 − BitDepth ) (1256)

  • Otherwise (BitDepth is greater than or equal to 10), the following applies:

tC = tC′ * ( 1 << ( BitDepth − 10 ) ) (1257)

Note: See TracTickets for help on using tickets.