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• The First Problem

  19 janvier 2011, par Multimedia Mike — HTML5

  A few years ago, The Linux Hater made the following poignant observation regarding Linux driver support :

  Drivers are only just the beginning... But for some reason y’all like to focus on the drivers. You know why lusers do that ? Because it just happens to be the problem that people notice first.

  And so it is with the HTML5 video codec debate, re-invigorated in the past week by Google’s announcement of dropping native H.264 support in their own HTML5 video tag implementation. As I read up on the fiery debate, I kept wondering why people are so obsessed with this issue. Then I remembered the Linux Hater’s post and realized that the video codec issue is simply the first problem that most people notice regarding HTML5 video.

  I appreciate that the video codec debate has prompted Niedermayer to post on his blog once more. Otherwise, I’m just munching popcorn on the sidelines, amused and mildly relieved that the various factions are vociferously attacking each other rather than that little project I help with at work.

  Getting back to the "first problem" aspect— there’s so much emphasis on the video codec ; I wonder why no one ever, ever mentions word one about an audio codec. AAC is typically the codec that pairs with H.264 in the MPEG stack. Dark Shikari once mentioned that "AAC’s licensing terms are exponentially more onerous than H.264′s. If Google didn’t want to use H.264, they would sure as hell not want to use AAC." Most people are probably using "H.264" to refer to the entire MPEG/H.264/AAC stack, even if they probably don’t understand what all of those pieces mean.

  Anyway, The Linux Hater’s driver piece continues :

  Once y’all have drivers, the fight will move to the next layer up. And like I said, it’s a lot harder at that layer.

  A few months ago, when I wanted to post the WebM output of my new VP8 encoder and thought it would be a nice touch to deliver it via a video tag, I ignored the video codec problem (just encoded a VP8/WebM file) only to immediately discover a problem at a different layer— specifically, embedding a file using a video tag triggers a full file download when the page is loaded, which is unacceptable from end user and web hosting perspectives. This is a known issue but doesn’t get as much attention, I guess because there are bigger problems to solve first (c.f. video codec issue).

  For other issues, check out the YouTube blog’s HTML5 post or Hulu’s post that also commented on HTML5. Issues such as video streaming flexibility, content protection, fullscreen video, webcam/microphone input, and numerous others are rarely mentioned in the debates. Only "video codec" is of paramount importance.

  But I’m lending too much weight to the cacophony of a largely uninformed internet debate. Realistically, I know there are many talented engineers down in the trenches working to solve at least some of these problems. To tie this in with the Linux driver example, I’m consistently stunned these days regarding how simple it is to get Linux working on a new computer— most commodity consumer hardware really does just work right out of the box. Maybe one day, we’ll wake up and find that HTML5 video has advanced to the point that it solves all of the relevant problems to make it the simple and obvious choice for delivering web video in nearly all situations.

  It won’t be this year.

• Understanding the VP8 Token Tree

  7 juin 2010, par Multimedia Mike — VP8

  I got tripped up on another part of the VP8 decoding process today. So I drew a picture to help myself understand it. Then I went back and read David Conrad’s comment on my last post regarding my difficulty understanding the VP8 spec and saw that he ran into the same problem. Since we both experienced the same hindrance in trying to sort out this matter, I thought I may as well publish the picture I drew.

  VP8 defines various trees for decoding different syntax elements. There is one tree for decoding the tokens and it is expressed in the VP8 spec as such :

  PLAIN TEXT

  C :

  1. const tree_index coef_tree [2 * (num_dct_tokens - 1)] =
  2. {
  3.  -dct_eob, 2,        /* eob = "0"  */
  4.   -DCT_0, 4,        /* 0  = "10" */
  5.   -DCT_1, 6,        /* 1  = "110" */
  6.    8, 12,
  7.    -DCT_2, 10,      /* 2  = "11100" */
  8.     -DCT_3, -DCT_4,    /* 3  = "111010", 4 = "111011" */
  9.    14, 16,
  10.     -dct_cat1, -dct_cat2, /* cat1 = "111100", cat2 = "111101" */
  11.    18, 20,
  12.     -dct_cat3, -dct_cat4, /* cat3 = "1111100", cat4 = "1111101" */
  13.     -dct_cat5, -dct_cat6 /* cat4 = "1111110", cat4 = "1111111" */
  14. } ;

  Here is what the table looks like when you make a tree out of it (click for full size image) :

  
  
  

  The catch is that it makes no sense for an end-of-block (EOB) token to follow a 0 token since EOB already indicates that the remainder of the coefficients should be 0 anyway. Thus, the spec states that, "decoding of certain DCT coefficients may skip the first branch, whose preceding coefficient is a DCT_0." I confess, I didn’t understand what "skip the first branch" meant until I drew the tree.

  
  
  

  For those wondering why it might be sub-optimal (clarity-wise) for a spec to simply regurgitate vast chunks of C code, this makes a decent case. As you can see, the spec makes certain assumptions about how a binary tree should be organized in a static array (node n points to elements n*2 and n*2+1 as its branches ; leaves are either negative or 0). This is the second method I have seen ; another piece of code (not the VP8 spec) had the nodes in the first half of the array and pointed to leaves in the second half. There must be other arrangements.

• ffmpeg set auto height

  19 juillet 2013, par conmen

  I got following command which use in generate video thumbnail :

  escapeshellcmd("/usr/local/bin/ffmpeg -ss " . ceil($time) . " -i '" . $videoPath . "' -f image2 -vframes 1 -s 150x110 " . $tFilePath)

  I want to know is that possible for the image generate in Auto Height instead of fixed Weight x Height ?

  Thanks.