[FFmpeg-user] ffmpeg & MPEG-2 GOP structure

[Valentin NOEL]

2015-01-09 15:29 GMT+01:00 tim nicholson <nichot20-at-yahoo.com at ffmpeg.org>:

> On 09/01/15 12:24, Valentin NOEL wrote:
> > Thanks for your answer,
> >
>
> Please do not top post on this forum
>

OK sorry.


>
> > but actually the ITU-T recommendation for H.262 (ISO/IEC 13818-2, CODING
> OF
> > MOVING PICTURES AND ASSOCIATED AUDIO, 1995) specifies B-frames
> definition :
> >
> >> *B-picture; bidirectionally predictive-coded picture: A picture that is
> >> coded using motion compensated prediction from past and/or future
> reference
> >> fields or frames.*
> >>
> > The "and/or" means that a only backward reference is possible.
> >
> > The same document also specifies that, for closed GOP :
> >
> >>
> >> *closed_gop -- This is a one-bit flag which indicates the nature of the
> >> predictions uses in the B-pictures (if any) immediately following the
> first
> >> coded I-frame following the group of picture header .closed_gop is set
> to
> >> “1” to indicate that these B-pictures have been encoded using only
> backward
> >> prediction.*
> >>
> > It also means that B-frame may use backward prediction only.
> >
> > So, does that mean that FFmpeg does not allow only backward prediction
> for
> > B-frame, and that a patch is needed ?
> >
>
> It means that I was making a pragmatic suggestion for something you
> could try immediately to solve your issue, rather than trying to get
> bogged down in technical details too early.
>

Actually, I need to add this flag "+cgop" because I need to generate closed
GOP.
Other encoders allow finishing closed GOP with B-frames (with only backward
reference), so I wonder whether FFmpeg cannot allow doing this, or whether
I do not use it correctly.


> > Thanks.
> >
> > 2015-01-09 12:56 GMT+01:00 tim nicholson <
> nichot20-at-yahoo.com at ffmpeg.org>:
> >
> >> On 09/01/15 10:06, Valentin NOEL wrote:
> >>> Hi to all,
> >>>
> >>> I would like to encode MPEG-2 video with closed GOP of 12 frames, and a
> >>> structure like IBBPBBPBBPBB.
> >>>
> >>> For doing this, I am using these options :
> >>> *-g 12* to set the GOP size
> >>> *-bf 2* to set the max B frames between reference frames
> >>> *-flags +cgop* to use Closed GOP
> >>> *-sc_threshold 1000000000* to disable scene change detection
> >>> *-b_strategy  0 *to disable adaptive number of B-frames
> >>> *-mpv_flags +strict_gop* to enforce GOP size (mpegvideo private flag)
> >>>
> >>> The resulting command is :
> >>> ffmpeg -i input.mxf -vcodec mpeg2video -g 12 -bf 2 -flags +cgop
> >> -b_strategy
> >>> 0 -mpv_flags +strict_gop -sc_threshold 1000000000 -map 0:0 -y
> output.mxf
> >>>
> >>> However, the result is a GOP of 12 frames IBBPBBPBBPB*P*, with an
> ending
> >>> P-frame.
> >>>
> >>> It seems the problem comes from these lines :
> >>>
> >>
> https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/mpegvideo_enc.c#L1491
> >>>
> >>> How can I do to fix this ? Is there a way to enforce GOP finishing by a
> >>> B-frame ? Is there something missing ?
> >>>
> >>
> >> Consider the last GOP of the file, you cannot end on a B frame as there
> >> is no forward frame to interpolate from.
> >>
> >> I wonder therefore if you need to add " -flags +cgop "
> >>
> >>
> >>> Thanks.
> >>>
> >>> Valentin
> >>> [..]
> >>
> >>
> >> --
> >> Tim.
> >> Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83
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> >>
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>
>
> --
> Tim.
> Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83
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