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

[tim nicholson]

On 09/01/15 12:24, Valentin NOEL wrote:
> Thanks for your answer,
> 

Please do not top post on this forum

> 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.

> 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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-- 
Tim.
Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83