[Mp4-tech] [video]AVC field selection

[Dzung Hoang]

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