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  • Le profil des utilisateurs

    12 avril 2011, par

    Chaque 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 (...)

  • XMP PHP

    13 mai 2011, par

    Dixit Wikipedia, XMP signifie :
    Extensible Metadata Platform ou XMP est un format de métadonnées basé sur XML utilisé dans les applications PDF, de photographie et de graphisme. Il a été lancé par Adobe Systems en avril 2001 en étant intégré à la version 5.0 d’Adobe Acrobat.
    Étant basé sur XML, il gère un ensemble de tags dynamiques pour l’utilisation dans le cadre du Web sémantique.
    XMP permet d’enregistrer sous forme d’un document XML des informations relatives à un fichier : titre, auteur, historique (...)

  • Les autorisations surchargées par les plugins

    27 avril 2010, par

    Mediaspip core
    autoriser_auteur_modifier() afin que les visiteurs soient capables de modifier leurs informations sur la page d’auteurs

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  • When compressing a set of images with libx264, why does frame rate affect final output size ?

    3 avril 2018, par jd20

    I’m using ffmpeg to encode a set of images as a short timelapse video, using libx264 codec. My first attempt, I encoded it at 30 FPS, using :

    ffmpeg -r 30 -pattern_type glob -i "*.jpg" -vcodec libx264 -crf 30 -pix_fmt yuv420p output.mp4

    With 60 frames, that gives me a 163 KB file that’s 2 seconds long. Then I realized I needed it to be slower, so I re-ran the same command, but changed -r to 2. Now I have a file that’s 30 seconds long, but the size jumped to 891 KB ! The video quality looks perceptually the same.

    How do I encode at a slower frame rate, without the final file size ballooning ?

    Notes : Some theories I had, and things I checked. First, to make sure ffmpeg wasn’t duplicating frames in the longer verison, I check the I/P/B counts. The 30 FPS file had :

    [libx264 @ 0x7f9b26001c00] frame I:1     Avg QP:30.67  size: 44649
    
[libx264 @ 0x7f9b26001c00] frame P:15    Avg QP:31.19  size:  5471
    
[libx264 @ 0x7f9b26001c00] frame B:44    Avg QP:31.45  size:   767

    The 2 FPS file had :

    [libx264 @ 0x7fcd32842200] frame I:1     Avg QP:21.29  size: 90138
    
[libx264 @ 0x7fcd32842200] frame P:15    Avg QP:22.48  size: 33686
    
[libx264 @ 0x7fcd32842200] frame B:44    Avg QP:26.29  size:  6674

    So, the I/P/B counts are identical, but the QP is much lower for the 2 FPS file. To offset, I tried increasing -crf for the 2 FPS file, to get about the same target size, but that just gave me a very blurry video (had to go to crf=40). I tried messing with -minrate, -maxrate, -bt, none helped. I’m guessing there is some x264 codec setting which is frame rate dependent, but I’m at a loss trying to figure out which one (from what I understand, constant bitrate is affected by frame rate but CRF should not be, but maybe I’m misunderstanding it.

  • When compressing a set of images with libx264, why does frame rate affect final output size ?

    3 avril 2018, par jd20

    I’m using ffmpeg to encode a set of images as a short timelapse video, using libx264 codec. My first attempt, I encoded it at 30 FPS, using :

    ffmpeg -r 30 -pattern_type glob -i "*.jpg" -vcodec libx264 -crf 30 -pix_fmt yuv420p output.mp4

    With 60 frames, that gives me a 163 KB file that’s 2 seconds long. Then I realized I needed it to be slower, so I re-ran the same command, but changed -r to 2. Now I have a file that’s 30 seconds long, but the size jumped to 891 KB ! The video quality looks perceptually the same.

    How do I encode at a slower frame rate, without the final file size ballooning ?

    Notes : Some theories I had, and things I checked. First, to make sure ffmpeg wasn’t duplicating frames in the longer verison, I check the I/P/B counts. The 30 FPS file had :

    [libx264 @ 0x7f9b26001c00] frame I:1     Avg QP:30.67  size: 44649
    
[libx264 @ 0x7f9b26001c00] frame P:15    Avg QP:31.19  size:  5471
    
[libx264 @ 0x7f9b26001c00] frame B:44    Avg QP:31.45  size:   767

    The 2 FPS file had :

    [libx264 @ 0x7fcd32842200] frame I:1     Avg QP:21.29  size: 90138
    
[libx264 @ 0x7fcd32842200] frame P:15    Avg QP:22.48  size: 33686
    
[libx264 @ 0x7fcd32842200] frame B:44    Avg QP:26.29  size:  6674

    So, the I/P/B counts are identical, but the QP is much lower for the 2 FPS file. To offset, I tried increasing -crf for the 2 FPS file, to get about the same target size, but that just gave me a very blurry video (had to go to crf=40). I tried messing with -minrate, -maxrate, -bt, none helped. I’m guessing there is some x264 codec setting which is frame rate dependent, but I’m at a loss trying to figure out which one (from what I understand, constant bitrate is affected by frame rate but CRF should not be, but maybe I’m misunderstanding it.

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