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Video d’abeille en portrait
14 mai 2011, par
Mis à jour : Février 2012
Langue : français
Type : Video
Autres articles (101)
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Gestion générale des documents
13 mai 2011, parMédiaSPIP ne modifie jamais le document original mis en ligne.
Pour chaque document mis en ligne il effectue deux opérations successives : la création d’une version supplémentaire qui peut être facilement consultée en ligne tout en laissant l’original téléchargeable dans le cas où le document original ne peut être lu dans un navigateur Internet ; la récupération des métadonnées du document original pour illustrer textuellement le fichier ;
Les tableaux ci-dessous expliquent ce que peut faire MédiaSPIP (...) -
Le profil des utilisateurs
12 avril 2011, parChaque utilisateur dispose d’une page de profil lui permettant de modifier ses informations personnelle. Dans le menu de haut de page par défaut, un élément de menu est automatiquement créé à l’initialisation de MediaSPIP, visible uniquement si le visiteur est identifié sur le site.
L’utilisateur a accès à la modification de profil depuis sa page auteur, un lien dans la navigation "Modifier votre profil" est (...) -
Des sites réalisés avec MediaSPIP
2 mai 2011, parCette page présente quelques-uns des sites fonctionnant sous MediaSPIP.
Vous pouvez bien entendu ajouter le votre grâce au formulaire en bas de page.
Sur d’autres sites (14605)
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FFmpeg - generate x264 CBR video transport stream with C-API
6 juillet 2020, par ZeroDefectUsing various posts sprinkled around the Internet, including this one here on SO, I've been able to understand how to use the FFmpeg cli to generate a CBR video bitrate using the x264 codec (wrapped in an MPEG-2 transport stream). Note : I'm concerned with the video bitrate - nothing else.


ffmpeg -i cbr_test_file_input.mp4 -c:v libx264 -pix_fmt yuv420p -b:v 6000000 -preset fast -tune film -g 25 -x264-params vbv-maxrate=6000:vbv-bufsize=6000:force-cfr=1:nal-hrd=cbr -flags +ildct+ilme x264_cbr_test_output.ts


However, I'm trying to approach this from an FFmpeg C-API point of view. I'm having issues. I've knocked together some code to try do something very similar to what is being done in the FFmpeg CLI. I can generate a transport stream of what I think should be CBR, but the profile of the video bitrate is very different from what I thought was the FFmpeg cli equivalent :


The initialisation of the AVCodecContext looks something like :


av_dict_set(&pDict, "preset", "faster", 0);
 av_dict_set(&pDict, "tune", "film", 0);
 av_dict_set_int(&pDict, "rc-lookahead", 25, 0);

 pCdcCtxOut->width = pCdcCtxIn->width;
 pCdcCtxOut->height = pCdcCtxIn->height;
 pCdcCtxOut->pix_fmt = AV_PIX_FMT_YUV420P;
 pCdcCtxOut->gop_size = 25;

 // Going for 6Mbit/s
 pCdcCtxOut->bit_rate = 6000000;
 //pCdcCtxOut->rc_min_rate = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_max_rate = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_buffer_size = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_initial_buffer_occupancy = static_cast<int>((pCdcCtxOut->bit_rate * 9) / 10);

 std::string strParams = "vbv-maxrate="
 + std::to_string(pCdcCtxOut->bit_rate / 1000)
 + ":vbv-bufsize="
 + std::to_string(pCdcCtxOut->bit_rate / 1000)
 + ":force-cfr=1:nal-hrd=cbr";

 av_dict_set(&pDict, "x264-params", strParams.c_str(), 0);

 pCdcCtxOut->field_order = AV_FIELD_TT;
 pCdcCtxOut->flags = (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME | AV_CODEC_FLAG_CLOSED_GOP);

 // WARN: Make some assumptions here!
 pCdcCtxOut->time_base = AVRational{1,25};
 pCdcCtxOut->framerate = AVRational{25,1};
 pCdcCtxOut->sample_aspect_ratio = AVRational{64,45};
</int>

The output graphs appear very different :




Above is the FFmpeg CLI output - video bitrate holds fairly steady.




Above is the output of my sample application - some significant dips in the video bitrate.


I've taken this a step further and created a git repo consisting of :


- 

- Code of sample application
- Test input file (.mp4)
- Outputs (.ts file) of tests
- Graphs of output bitrates.










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avformat/segafilmenc : Avoid seek when writing header
17 juillet 2020, par Andreas Rheinhardtavformat/segafilmenc : Avoid seek when writing header
Up until now, the Sega FILM muxer would first write all the packet data,
then shift the data (in the muxer's write_trailer function) by the amount
necessary to write the header at the front (which entails a seek to the
front), then seek back to the beginning and actually write the header.This commit changes this : The dynamic buffer that is used to write the
sample table (containing information about each sample in the file) is
now used to write the complete header. This is possible because the size
of everything in the header except the sample table is known in advance.
Said buffer can then be used as one of the two temporary buffers used
for shifting which also reduces the amount one has to allocate for this.
Thereby the header will be written when shifting, so that the second
seek to the beginning is unnecessary.Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
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FFmpeg - generate x264 CBR video transport stream with C-API
6 juillet 2020, par ZeroDefectUsing various posts sprinkled around the Internet, including this one here on SO, I've been able to understand how to use the FFmpeg cli to generate a CBR video bitrate using the x264 codec (wrapped in an MPEG-2 transport stream). Note : I'm concerned with the video bitrate - nothing else.


ffmpeg -i cbr_test_file_input.mp4 -c:v libx264 -pix_fmt yuv420p -b:v 6000000 -preset fast -tune film -g 25 -x264-params vbv-maxrate=6000:vbv-bufsize=6000:force-cfr=1:nal-hrd=cbr -flags +ildct+ilme x264_cbr_test_output.ts


However, I'm trying to approach this from an FFmpeg C-API point of view. I'm having issues. I've knocked together some code to try do something very similar to what is being done in the FFmpeg CLI. I can generate a transport stream of what I think should be CBR, but the profile of the video bitrate is very different from what I thought was the FFmpeg cli equivalent :


The initialisation of the AVCodecContext looks something like :


av_dict_set(&pDict, "preset", "faster", 0);
 av_dict_set(&pDict, "tune", "film", 0);
 av_dict_set_int(&pDict, "rc-lookahead", 25, 0);

 pCdcCtxOut->width = pCdcCtxIn->width;
 pCdcCtxOut->height = pCdcCtxIn->height;
 pCdcCtxOut->pix_fmt = AV_PIX_FMT_YUV420P;
 pCdcCtxOut->gop_size = 25;

 // Going for 6Mbit/s
 pCdcCtxOut->bit_rate = 6000000;
 //pCdcCtxOut->rc_min_rate = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_max_rate = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_buffer_size = pCdcCtxOut->bit_rate;
 pCdcCtxOut->rc_initial_buffer_occupancy = static_cast<int>((pCdcCtxOut->bit_rate * 9) / 10);

 std::string strParams = "vbv-maxrate="
 + std::to_string(pCdcCtxOut->bit_rate / 1000)
 + ":vbv-bufsize="
 + std::to_string(pCdcCtxOut->bit_rate / 1000)
 + ":force-cfr=1:nal-hrd=cbr";

 av_dict_set(&pDict, "x264-params", strParams.c_str(), 0);

 pCdcCtxOut->field_order = AV_FIELD_TT;
 pCdcCtxOut->flags = (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME | AV_CODEC_FLAG_CLOSED_GOP);

 // WARN: Make some assumptions here!
 pCdcCtxOut->time_base = AVRational{1,25};
 pCdcCtxOut->framerate = AVRational{25,1};
 pCdcCtxOut->sample_aspect_ratio = AVRational{64,45};
</int>

The output graphs appear very different :




Above is the FFmpeg CLI output - video bitrate holds fairly steady.




Above is the output of my sample application - some significant dips in the video bitrate.


I've taken this a step further and created a git repo consisting of :


- 

- Code of sample application
- Test input file (.mp4)
- Outputs (.ts file) of tests
- Graphs of output bitrates.










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