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• Web Based Playback of iOS Videos with Orientation Flag

  3 mars 2012, par shanee

  We just recently created an iPhone app for one of our system that allows users to upload picture and video content to our services. The last major hitch we are running into is how to handle videos that are uploaded in an orientation other than Horizontal Right. Apparently if your playback system does not account for the orientation flag sent with the video then it will play upside down or sideways.

  The correct approach appears to be that the playback system should take the orientation flag into account just prior to playback. This is the way Apple handles it directly on the device as well as through Quicktime.

  SO my first hope is that someone is aware of a web based (HTML5 or Flash) player that is capable of rotating a video during playback based on either the video orientation metadata or based on a passed flag (we already have the necessary flag available in the DB if we need to just pass it manually). If you know of any such player then PLEASE SHARE !

  If you aren't aware of such a player, then has anyone had any luck rotating their videos using FFMPEG or MEncoder ? We did a few hours of testing last week and weren't able to get any decent results from the two heavy hitters mentioned there.

  Failing ALL OF THAT, is it possible to have the iPhone upload a video or image in a specified direction ?

  Any of the three will work for me, but I would prefer to do whatever is standard (if one exists).

  Any help is much appreciated !

• 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.

• test/index.html : minor JS improvements.

  15 novembre 2013, par XhmikosR

  test/index.html : minor JS improvements.
  Remove unused variables, keep style consistent.