Thanks for your answer, 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 ? Thanks. 2015-01-09 12:56 GMT+01:00 tim nicholson <[email protected]>: > 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 > [email protected] > http://ffmpeg.org/mailman/listinfo/ffmpeg-user > _______________________________________________ ffmpeg-user mailing list [email protected] http://ffmpeg.org/mailman/listinfo/ffmpeg-user
