[Mp4-tech] [video]AVC field selection

[Andrew Palfreyman]

Gary,
Took another look at this, and, aside from the "definition of default" 
issue, there appears to be another issue that needs resolution, in the case 
where MbaffFrameFlag=1...
In 7.3.4, mb_field_decoding_flag is only instantiated for even-numbered MBs 
(assuming no MBs are skipped). Referring back to the instantiation 
expression above for ref_idx_lX, what is deemed to be the truth of 
mb_field_decoding_flag for the odd-numbered MB of the MB pair, where this 
field is absent? According to the spec, if its absent, it's inferred to be 
zero. If, on the other hand, mb_field_decoding_flag is understood to have 
macroblock-pair scope (as opposed to macroblock-only scope), then the answer 
is clear. The spec does not make this scope explicit.
One simplified form of this question might be:
"when mb_field_decoding_flag=1 and MbPartPredMode(..) == Pred_L0, for an 
even MB, does ref_idx_l0 appear in the RBSP for the associated odd MB of 
this MB pair?"
Also note that the "inference from neighbours" semantics section 7.4.4, 
dealing as it does exclusively with MB pairs, does not contribute to an 
answer to this question.
Also, I found confusing the phrase
"When mb_field_decoding_flag is not present for either macroblock of a 
macroblock pair,...".
This hinges on the meaning of "either", used imprecisely (IMHO) here.
This phrase could be interpreted to mean
"When both MBs of a pair have mb_field_decoding_flag not present"
or could be interpreted to mean
"When one or both MBs of a pair have mb_field_decoding_flag not present".
The confusion is inherent in the fact that this flag isn't present for odd 
MBs (no skips) in any case, so this semantic explanation is particularly 
troublesome to interpret in that light. It all comes down to scope, right?
Best,
Andrew Palfreyman
----- Original Message ----- 
From: "John Cox" <jc sj.co.uk>
To: "Gary Sullivan" <garysull windows.microsoft.com>
Cc: <mp4-tech lists.mpegif.org>
Sent: Friday, June 22, 2023 3:10 AM
Subject: Re: [Mp4-tech] [video]AVC field selection
> Gary,
>
> I hadn't thought of renaming the flag but it gets my vote.  Having it
> called field_mb_pair_flag and being always zero if MbaffFrameFlag is
> zero cuts through all the confusion nicely.
>
> Regards
>
> John Cox
>
>
>>Barry et al,
>>
>>I guess perhaps you could read 7.4.4 that was, but its inference rules is 
>>only specified for "When mb_field_decoding_flag is not present for either 
>>macroblock of a macroblock pair".  When MbaffFrameFlag is equal to 0, we 
>>don't have macroblock pairs.  And the inference rule itself refers to 
>>things relating to "the current macroblock pair" and "the macroblock pair 
>>immediately to the left of the current macroblock pair" and "the 
>>macroblock pair immediately above the current macroblock pair", which are 
>>concepts that do not apply.
>>
>>Inferring mb_field_decoding_flag to be zero whenever MbaffFrameFlag is 
>>equal to 0 would be good in the sense that it would take care of the logic 
>>flow in 7.3.5.1 without needing to change what is stated in 7.3.5.1.  But 
>>it might be kind of counter-intuitive, since people might expect something 
>>called mb_field_decoding_flag not to be 0 for a field macroblock.
>>
>>Another possibility is to change the name of mb_field_decoding_flag to 
>>something like field_mb_pair_flag.  Then it would make more intuitive 
>>sense to have it be 0 when the MB is not part of a pair.
>>
>>Best Regards,
>>
>>Gary Sullivan
>>
>>+> -----Original Message-----
>>+> From: bhaskell [mailto:bhaskell apple.com]
>>+> Sent: Thursday, June 21, 2023 9:49 AM
>>+> To: Gary Sullivan
>>+> Cc: dzung.hoang xilient.com; Andrew Palfreyman (cisco);
>>+> mp4-tech lists.mpegif.org
>>+> Subject: Re: [Mp4-tech] [video]AVC field selection
>>+>
>>+> Gary Sullivan wrote:
>>+>
>>+> >Just for completeness, we might also want to define an
>>+> inferred value of mb_field_decoding_flag for cases with
>>+> MbaffFrameFlag equal to 0, although the actual value in such
>>+> cases may not be used for anything other than the evaluation
>>+> of the expression in 7.3.5.1.  Right now, I believe it only
>>+> has a defined value for cases with MbaffFrameFlag equal to 1.
>>+> >
>>+> >
>>+>
>>+> 7.4.4. seems to say that if  mb_field_decoding_flag  is not
>>+> present in
>>+> any macroblock, then it should be inferred to be zero in all
>>+> macroblocks.  I'm sure that's what everyone has assumed so far.
>>+>
>>+> Cheers, Barry
>>+>
>>+> -- 
>>+> Barry G. Haskell                             tel +1 408 974 6333
>>+> Apple Inc.                          fax  "  "  "   1756
>>+> 2 Infinite Loop      MS: 302-3KS
>>+> Cupertino, CA 95014
>>+> bhaskell apple.com     (also  B.Haskell ieee.org)
>>+>
>>+>
>>+>
>>+>
>>
>>_______________________________________________
>>NOTE: Please use clear subject lines for your posts. Include [audio, 
>>[video], [systems], [general] or another apppropriate identifier to 
>>indicate the type of question you have.
>>
>>Note: Conduct on the mailing list is subject to the Antitrust guidelines 
>>found at 
>>http://www.mpegif.org/public/documents/vault/mp-out-30042-Antitrust.php
>
>