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• Simply beyond ridiculous

  7 mai 2010, par Dark Shikari — H.265, speed

  For the past few years, various improvements on H.264 have been periodically proposed, ranging from larger transforms to better intra prediction. These finally came together in the JCT-VC meeting this past April, where over two dozen proposals were made for a next-generation video coding standard. Of course, all of these were in very rough-draft form ; it will likely take years to filter it down into a usable standard. In the process, they’ll pick the most useful features (hopefully) from each proposal and combine them into something a bit more sane. But, of course, it all has to start somewhere.

  A number of features were common : larger block sizes, larger transform sizes, fancier interpolation filters, improved intra prediction schemes, improved motion vector prediction, increased internal bit depth, new entropy coding schemes, and so forth. A lot of these are potentially quite promising and resolve a lot of complaints I’ve had about H.264, so I decided to try out the proposal that appeared the most interesting : the Samsung+BBC proposal (A124), which claims compression improvements of around 40%.

  The proposal combines a bouillabaisse of new features, ranging from a 12-tap interpolation filter to 12thpel motion compensation and transforms as large as 64×64. Overall, I would say it’s a good proposal and I don’t doubt their results given the sheer volume of useful features they’ve dumped into it. I was a bit worried about complexity, however, as 12-tap interpolation filters don’t exactly scream “fast”.

  I prepared myself for the slowness of an unoptimized encoder implementation, compiled their tool, and started a test encode with their recommended settings.

  I waited. The first frame, an I-frame, completed.

  I took a nap.

  I waited. The second frame, a P-frame, was done.

  I played a game of Settlers.

  I waited. The third frame, a B-frame, was done.

  I worked on a term paper.

  I waited. The fourth frame, a B-frame, was done.

  After a full 6 hours, 8 frames had encoded. Yes, at this rate, it would take a full two weeks to encode 10 seconds of HD video. On a Core i7. This is not merely slow ; this is over 1000 times slower than x264 on “placebo” mode. This is so slow that it is not merely impractical ; it is impossible to even test. This encoder is apparently designed for some sort of hypothetical future computer from space. And word from other developers is that the Intel proposal is even slower.

  This has led me to suspect that there is a great deal of cheating going on in the H.265 proposals. The goal of the proposals, of course, is to pick the best feature set for the next generation video compression standard. But there is an extra motivation : organizations whose features get accepted get patents on the resulting standard, and thus income. With such large sums of money in the picture, dishonesty becomes all the more profitable.

  There is a set of rules, of course, to limit how the proposals can optimize their encoders. If different encoders use different optimization techniques, the results will no longer be comparable — remember, they are trying to compare compression features, not methods of optimizing encoder-side decisions. Thus all encoders are required to use a constant quantizer, specified frame types, and so forth. But there are no limits on how slow an encoder can be or what algorithms it can use.

  It would be one thing if the proposed encoder was a mere 10 times slower than the current reference ; that would be reasonable, given the low level of optimization and higher complexity of the new standard. But this is beyond ridiculous. With the prize given to whoever can eke out the most PSNR at a given quantizer at the lowest bitrate (with no limits on speed), we’re just going to get an arms race of slow encoders, with every company trying to use the most ridiculous optimizations possible, even if they involve encoding the frame 100,000 times over to choose the optimal parameters. And the end result will be as I encountered here : encoders so slow that they are simply impossible to even test.

  Such an arms race certainly does little good in optimizing for reality where we don’t have 30 years to encode an HD movie : a feature that gives great compression improvements is useless if it’s impossible to optimize for in a reasonable amount of time. Certainly once the standard is finalized practical encoders will be written — but it makes no sense to optimize the standard for a use-case that doesn’t exist. And even attempting to “optimize” anything is difficult when encoding a few seconds of video takes weeks.

  Update : The people involved have contacted me and insist that there was in fact no cheating going on. This is probably correct ; the problem appears to be that the rules that were set out were simply not strict enough, making many changes that I would intuitively consider “cheating” to be perfectly allowed, and thus everyone can do it.

  I would like to apologize if I implied that the results weren’t valid ; they are — the Samsung-BBC proposal is definitely one of the best, which is why I picked it to test with. It’s just that I think any situation in which it’s impossible to test your own software is unreasonable, and thus the entire situation is an inherently broken one, given the lax rules, slow baseline encoder, and no restrictions on compute time.

• Anomalie #4114 : paramètre media:joindre_deballer_lister_zip ignoré

  19 mars 2018, par Alexis Z

  Effectivement, j’ai oublier de précisé.
  Spip 3.2.0 révision 23778

  Le 19 mars 2018 à 14:35, <redmine@spip.org> a écrit :

  La demande #4114 a été mise à jour par b b.

  Salut et merci pour le signalement, sur quelle version de SPIP observes-tu
  le problème ?
  ------------------------------
  Anomalie #4114 : paramètre media:joindre_deballer_lister_zip ignoré
  <https://core.spip.net/issues/4114#change-13736>

  - Auteur : Alexis Z
  - Statut : Nouveau
  - Priorité : Normal
  - Assigné à :
  - Catégorie :
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  Bonjour,

  Il me semble avoir trouvé une petite incohérence dans le code du plugin
  media, plus précisément dans la fonction "joindre_deballer_lister_zip"
  ligne 301, de media/inc/joindre_document.php.
  Sauf erreur de ma part, cette fonction a pour but de déballer le contenu
  d’un fichier zip qui lui est passé en paramètre dans un répertoire
  temporaire et de retourner une liste décrivant sont contenus.

  Cette fonction prends deux paramètres $path et $tmp_dir :
  - $path corresponds au chemin du fichier zip à déballer
  - $tmp_dir corresponds au dossier temporaire ou celui-ci sera déballé

  Cette fonction utilise la librairie Pclzip.

  L’incohérence se trouve au niveau du deuximère paramètre, $tmp_dir,
  celui-ci est censé indiquer dans quel répertoire le contenu du zip sera
  déballer or ce chemin n’est pas pris en compte par la fonction
  Pclzip->extraire (ligne 305), et n’est pas non plus pris en compte par la
  fonction callback ’callback_deballe_fichier’ indiqué à la fonction extraire
  de Pclzip.
  En effet dans le code le chemin pris en compte est déclaré dans un define
  "_TMP_DIR" celui-ci déclaré à la ligne 140 de la fonction
  "joindre_trouver_fichier_envoye" (meme fichier php, début ligne 26).
  ($tmp_dir est uniquement utilisé dans la définition du chemin du fichier
  qui est renvoyé par la fonction : ligne 317 : ’tmp_name’ => $tmp_dir . $f)

  Donc le paramètre $tmp_dir quasi non-utilisé induit en erreur car on
  s’attend à se que le contenu ce trouve dans le chemin $tmp_dir de plus si
  on appeler directement la focntion "joindre_deballer_lister_zip" sans
  appeler "joindre_trouver_fichier_envoye" on ne définit pas _TMP_DIR et on
  a une erreur incohmpréensible.

  Du coup, le pire sénario (mon cas) j’appelais
  "joindre_deballer_lister_zip" après un autre appel "joindre_trouver_fichier_envoye"
  indirect, donc la variable _TMP_DIR etait défini et le contenu de mon zip
  déballer à cette endroit alors que je donnais un $tmp_dir completement
  différent, cette destination restait vide et aucun message d’erreur de la
  fonction "joindre_deballer_lister_zip".

  Bref, je suggère de prendre en compte pour l’extraction la variable
  $tmp_dir, et/ou d’ajouter test/définition de la variable _TMP_DIR en début
  de fonction pour prévenir tout exécution "bizarre".
  ------------------------------

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• Revision 37648 : On pouvait toujours courir derrière ce bug ... si on n’utilise pas le bon ...

  25 avril 2010, par kent1@… — Log

  On pouvait toujours courir derrière ce bug ... si on n’utilise pas le bon argument…