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....@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 >> _______________________________________________ >> ffmpeg-user mailing list >> ffmpeg-user@ffmpeg.org >> http://ffmpeg.org/mailman/listinfo/ffmpeg-user >> > _______________________________________________ > ffmpeg-user mailing list > ffmpeg-user@ffmpeg.org > http://ffmpeg.org/mailman/listinfo/ffmpeg-user > -- Tim. Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83 _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-user