[Mp4-tech] [video]AVC field selection

Gary Sullivan garysull windows.microsoft.com
Thu Jun 21 02:27:22 EDT 2007 

Interesting.  I think perhaps "mb_field_decoding_flag" in 7.3.5.1 needs to be changed to "(MbaffFrameFlag && mb_field_decoding_flag)", or something like that.  And similarly for ref_idx_l1.  If we don't do that, then we do seem to get in some strange territory when num_ref_idx_lX_active_minus1 is equal to 0 and MbaffFrameFlag is equal to 0.
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.
Best Regards,
Gary
+> -----Original Message-----
+> From: Dzung Hoang [mailto:dzung.hoang xilient.com] 
+> Sent: Thursday, June 21, 2023 12:54 AM
+> To: Gary Sullivan; 'Andrew Palfreyman (cisco)'; 
+> mp4-tech lists.mpegif.org
+> Subject: RE: [Mp4-tech] [video]AVC field selection
+> 
+> Gary,
+> 
+> I was referring to the spec and not what you wrote. This is 
+> taken directly
+> from 7.3.5.1.
+> 
+> for( mbPartIdx = 0; mbPartIdx < NumMbPart( mb_type ); mbPartIdx++)
+> 	if( ( num_ref_idx_l1_active_minus1 > 0 ||
+> 		mb_field_decoding_flag ) &&
+> 	MbPartPredMode( mb_type, mbPartIdx ) != Pred_L0 )
+> 	ref_idx_l1[ mbPartIdx ] 2 te(v) | ae(v)
+> 
+> mb_field_decoding_flag is not clearly defined for the case when
+> MbaffFrameFlag is 0.
+> 
+> Regards,
+> - Dzung Hoang
+> 
+> -----Original Message-----
+> From: Gary Sullivan [mailto:garysull windows.microsoft.com] 
+> Sent: Thursday, June 21, 2023 2:00 AM
+> To: dthoang yahoo.com; Andrew Palfreyman (cisco); 
+> mp4-tech lists.mpegif.org
+> Subject: RE: [Mp4-tech] [video]AVC field selection
+> 
+> 
+> I don't think there is any ambiguity about it.  Just because 
+> the syntax
+> element called mb_field_decoding_flag is not used when 
+> field_pic_flag is
+> equal to 1 does not mean that there are no field macroblocks 
+> in such a
+> picture.  On the contrary, all macroblocks in such a picture 
+> are field
+> macroblocks.
+> 
+> So when I said "field MBs when field_pic_flag is equal to 
+> 1", I could have
+> equivalantly just said "when field_pic_flag is equal to 1" 
+> with no mention
+> of field MBs, since that aspect is inherent when 
+> field_pic_flag is equal to
+> 1.  The only reason I explicitly referred to these being 
+> field MBs is to
+> contrast with your statement saying "the inference of 
+> ref_idx_l0 only occurs
+> for frame MBs when num_ref_idx_active_minus1 is equal to 0", 
+> which did not
+> seem to include the possibility of any field MBs having such 
+> inference.
+> 
+> Best Regards,
+> 
+> Gary Sullivan 
+> 
+> +> -----Original Message-----
+> +> From: Dzung Hoang [mailto:dthoang yahoo.com] 
+> +> Sent: Wednesday, June 20, 2023 11:50 PM
+> +> To: Gary Sullivan; 'Andrew Palfreyman (cisco)'; 
+> +> mp4-tech lists.mpegif.org
+> +> Subject: RE: [Mp4-tech] [video]AVC field selection
+> +> 
+> +> Gary,
+> +> 
+> +> This is where the spec has some ambiguity. In 7.3.4, 
+> +> mb_field_decoding_flag
+> +> is only present when MbaffFrameFlag is set. However, in 
+> +> 7.4.4, the semantics
+> +> of mb_field_decoding_flag only talks about macroblock pairs. 
+> +> There is no
+> +> derivation of mb_field_decoding_flag given when 
+> +> MbaffFrameFlag is not set.
+> +> 
+> +> I was assuming that mb_field_decoding_flag is true for field 
+> +> pictures.
+> +> 
+> +> Regards,
+> +> - Dzung Hoang
+> +> 
+> +> 
+> +> -----Original Message-----
+> +> From: Gary Sullivan [mailto:garysull windows.microsoft.com] 
+> +> Sent: Thursday, June 21, 2023 1:31 AM
+> +> To: dthoang yahoo.com; Andrew Palfreyman (cisco); 
+> +> mp4-tech lists.mpegif.org
+> +> Subject: RE: [Mp4-tech] [video]AVC field selection
+> +> 
+> +> The inference also occurs for field MBs when field_pic_flag 
+> +> is equal to 1
+> +> and num_ref_idx_lX_active_minus1 == 0.
+> +> 
+> +> Best Regards,
+> +> 
+> +> Gary
+> +> 
+> +> +> -----Original Message-----
+> +> +> From: mp4-tech-bounces lists.mpegif.org 
+> +> +> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> +> Dzung Hoang
+> +> +> Sent: Wednesday, June 20, 2023 9:22 PM
+> +> +> To: 'Andrew Palfreyman (cisco)'; mp4-tech lists.mpegif.org
+> +> +> Subject: RE: [Mp4-tech] [video]AVC field selection
+> +> +> 
+> +> +> According to 7.3.5.1, the inference of ref_idx_l0 only 
+> +> +> occurs for frame MBs
+> +> +> when num_ref_idx_l0_active_minus1 = 0. Is there another 
+> +> case that I'm
+> +> +> missing? If not, then no field-based prediction can happen 
+> +> +> in the case you
+> +> +> are concerned with.
+> +> +> 
+> +> +> Regards,
+> +> +> - Dzung Hoang
+> +> +> 
+> +> +> -----Original Message-----
+> +> +> From: Andrew Palfreyman (cisco) [mailto:anpalfre cisco.com] 
+> +> +> Sent: Wednesday, June 20, 2023 10:46 PM
+> +> +> To: dthoang yahoo.com; mp4-tech lists.mpegif.org
+> +> +> Subject: Re: [Mp4-tech] [video]AVC field selection
+> +> +> 
+> +> +> The problem I have with that approach is that in 
+> 7.4.5.1 it states
+> +> +> "When only one reference picture is used for inter 
+> +> +> prediction, the values of
+> +> +> 
+> +> +> ref_idx_l0[mbPartIdx] shall be inferred to be equal to 0".
+> +> +> So even though the ref. pic has two field parities, these 
+> +> cannot be 
+> +> +> differentiated via a zero value of ref_idx.
+> +> +> 
+> +> +> Best,
+> +> +> Andrew
+> +> +> 
+> +> +> ----- Original Message ----- 
+> +> +> From: "Dzung Hoang" <dthoang yahoo.com>
+> +> +> To: "'Andrew Palfreyman (cisco)'" <anpalfre cisco.com>; 
+> +> +> <mp4-tech lists.mpegif.org>
+> +> +> Sent: Wednesday, June 20, 2023 8:08 PM
+> +> +> Subject: RE: [Mp4-tech] [video]AVC field selection
+> +> +> 
+> +> +> 
+> +> +> > Try LSB of ref_idx XOR with current field parity.
+> +> +> >
+> +> +> > Regards,
+> +> +> > - Dzung Hoang
+> +> +> >
+> +> +> > -----Original Message-----
+> +> +> > From: mp4-tech-bounces lists.mpegif.org
+> +> +> > [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> +> +> Andrew Palfreyman
+> +> +> > (cisco)
+> +> +> > Sent: Wednesday, June 20, 2023 12:04 PM
+> +> +> > To: mp4-tech lists.mpegif.org
+> +> +> > Subject: [Mp4-tech] [video]AVC field selection
+> +> +> >
+> +> +> > For AVC, I am trying to identify (with little success so 
+> +> +> far) the semantic
+> +> +> > equivalent of what in MPEG2 was called 
+> +> +> motion_vector_field_select. How is
+> +> +> > this accomplished in AVC?
+> +> +> >
+> +> +> > Best,
+> +> +> > Andrew
+> +> +> 
+> +> +> No virus found in this incoming message.
+> +> +> Checked by AVG Free Edition. 
+> +> +> Version: 7.5.472 / Virus Database: 269.9.1/854 - Release 
+> +> +> Date: 6/19/2007
+> +> +> 1:12 PM
+> +> +>  
+> +> +> 
+> +> +> No virus found in this outgoing message.
+> +> +> Checked by AVG Free Edition. 
+> +> +> Version: 7.5.472 / Virus Database: 269.9.1/854 - Release 
+> +> +> Date: 6/19/2007
+> +> +> 1:12 PM
+> +> +>  
+> +> +> 
+> +> +> _______________________________________________
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