[Mp4-tech] Question on the mb_field_decoding_flag

[Dzung Hoang]

Gary et al,
Section 7.4.4 specifies the process for inferring mb_field_decoding_flag
when it is not present for either MB of a MB pair.
I have a concern about the text of this section.
When mb_field_decoding_flag is not present for either macroblock of a
macroblock pair, the value of
mb_field_decoding_flag is derived as follows.
- If there is a neighbouring macroblock pair immediately to the left of the
current macroblock pair in the same slice,
the value of mb_field_decoding_flag shall be inferred to be equal to the
value of mb_field_decoding_flag for the
neighbouring macroblock pair immediately to the left of the current
macroblock pair,
- Otherwise, if there is no neighbouring macroblock pair immediately to the
left of the current macroblock pair in the
same slice and there is a neighbouring macroblock pair immediately above the
current macroblock pair in the same
slice, the value of mb_field_decoding_flag shall be inferred to be equal to
the value of mb_field_decoding_flag for
the neighbouring macroblock pair immediately above the current macroblock
pair,
- Otherwise (there is no neighbouring macroblock pair either immediately to
the left or immediately above the current
macroblock pair in the same slice), the value of mb_field_decoding_flag
shall be inferred to be equal to 0.
What happens if the top MB is skipped and the bottom MB is present? The
syntax table 7.3.4 contains this.
if( MbaffFrameFlag && ( CurrMbAddr % 2 = = 0 | |
( CurrMbAddr % 2 = = 1 && prevMbSkipped ) ) )
mb_field_decoding_flag
There is a potential conflict. The text of 7.4.4 says to infer the
mb_field_decoding_flag because it is not present for the skipped top MB.
However, the syntax table says to take the mb_field_decoding_flag from the
bitstream. Does this mean that the value for mb_field_decoding_flag must be
equal to the inferred value? If so, then is a waste of bits.
Regards,
- Dzung Hoang
  _____  
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Gary Sullivan
Sent: Wednesday, November 08, 2023 10:48 AM
To: Alan Yan - SH; mp4-tech lists.mpegif.org
Subject: RE: [Mp4-tech] Question on the mb_field_decoding_flag
Alan Yan et al,
In either case for which the first macroblock of a macroblock pair is
skipped and the second one is not, I think you should wait until
mb_field_decoding_flag is determined from the syntax associated with the
second macroblock before attempting to perform the decoding process for the
first macroblock.  The field/frame decoding mode is a macroblock pair
attribute -- it must be the same for both macroblocks.  I have not
double-checked the text to see exactly how this intent is expressed (and to
determine whether it is expressed well), but I am pretty confident that this
is the intent.
It may also be helpful to consult the reference software, but I doubt that
is necessary in this instance.
I'll try to remember to check how well the text is written on this issue
later.  If you have further pointers to specific places in the text where
you think you see incorrect or vague aspects, please let me/us know.
Best Regards,
Gary Sullivan
  _____  
From: mp4-tech-bounces lists.mpegif.org on behalf of Alan Yan - SH
Sent: Tue 11/7/2023 11:28 PM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] Question on the mb_field_decoding_flag
Dear Experts,
When MbaffFrameFlag==1, there is one syntax "mb_field_decoding_flag" in MB
layer for each MB pair.
In case of entropy_coding_mode_flag==0, when the first MB (top MB) in the MB
pair is skipped, mb_field_decoding_flag (MBFDF) is also skipped for the top
MB. For following decoding process like B_DIRECT_16X16(for B_SLICE) and
Boundary Strength calculation, MBFDF is necessary. And according to the spec
7.4.4, the derivation of MBFDF is performed after we know both MB of an MB
pair are skipped. Does it mean that if the top MB is skipped, the spec do
not have any default or temporary value defined for MBFDF and so we can not
start following decoding process like B_DIRECT_16X16 and BS calculation for
this top MB? We must wait till the bottom MB is parsed and the final MBFDF
is decoded or derived?
And when entropy_coding_mode_flag==1, I see the derivation process is always
performed before decoding any syntax element, is this statement correct? And
it is possible that the before-anything-derived value for MBFDF can be
different from the value parsed later from the bit-stream, is it correct? In
this situation and if mb_skip_flag is 1 for the top MB, can we use this
temporarily derived MBFDF to start following decoding process like
B_DIRECT_16X16 and BS calculation for this top-skipped MB or should we wait
and use the final MBFDF?
Thanks for your kindly help! (To Mr. Gary Sullivan: Thank you!)
Best Regards,
alan
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