[Mp4-tech] [video]AVC field selection

[John Cox]

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)
>+> 
>+> 
>+> 
>+> 
>
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