Recherche avancée

Sur d’autres sites (8370)

• Révision 21393 : Depuis toujours, SPIP ne respectait pas le RFC de HTTP/1.1 qui stipule qu’une re...

  9 juin 2014, par esj -

  Curieusement, SPIP consulte quand même le cache pour savoir si la page existe, pour finalement ignorer cette recherche dans le cas de HEAD. On profite de ce calcul pour à présent retourner les en-têtes HTTP qui se trouvent en cache, même s’il est périmé. Pour le cas où le cache est vide, on limite l’irrespect du RFC en envoyant un statut "204 No Content", suggérant qu’on a rien à dire, mais ça n’empêchera l’envoi d’un Content-Type arbitraire. Ca montre au passage que l’absence de typologie sur les squelettes est vraiment un problème.

• How does mp4 block matching work

  19 juin 2019, par YAHsaves

  I’ve been working on a video encoder that uses block matching to find similar blocks on previous frames.

  For the sake of simplicity I’ll leave out most of the details, but I’m wondering if I got the block matching algorithm right.

  In order to find a block on a previous frame my encoder uses the mean squared algorithm for the Y channel in YUV color space.

  This works by comparing each pixel of the block we want to match, with the block on the previous frame. It takes the difference of each pixel and squares it.

  After all the pixels are compared the block that has the least average difference is chosen as the desired block.

  Now this is where I need help. My encoder looks at every possible block in a 256x256 area and uses half pixel searches as well. The smallest block size it can use is 4x4.

  From what I’ve read online this is the same things mp4 uses.

  However I can’t find nearly as many blocks as mp4 appears to be able to find.

  For example here are 2 frames I want to compress. The first will be the I frame and the second is the P frame :

  

  Now after my encoder has run it is able to reduce the second frame by 80% and, what it can’t match close enough it saves as "difference" blocks. Which are grey blocks only recording the difference. They look like this :

  

  Now what I don’t get is to save these "difference" blocks as a jpg takes roughly 90kb to be accurate enough.

  Multiply that by 24 (24 frames per second) you get 2070kb per second. That’s not including how much space the actual motion vectors take up or anything else.

  However somehow mp4 is able to compress the video of the images above into a mere 700kb per second and still look better than my encoder at much larger data amounts.

  Why is this ? Is there something I’m doing wrong when looking for blocks ? Any help would be much appreciated.

• Color Correcting MP4/Webm videos

  2 octobre 2014, par sottenad

  I have a large video that features some animations too complex/long for other methods of embedding (gif, png sequence, etc.) and we are running into an issue with the colors in the video. Essentially we are including the background of the web page as part of the video in order to prevent the appearance of any edges on the video when it plays. We are trying to get the gradient background on the video to match that on the webpage, but the colors are WAY off.

  Are there any recommendations on how to color correct an MP4 video ? Or any html5-safe ways to do video with an alpha channel ?