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• 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.

• Use Google Fonts In ffmpeg fontfile

  9 mars 2020, par Randy Thomas

  I am wanting to use Google Fonts in my ffmpeg video creations for text. Here is what I have and it’s not working at all.

  $font = "//fonts.googleapis.com/css?family=Aclonica";
  
  
  
  $cmd .= "drawtext=enable='between(t,".$fi.",".$li.")':fontfile=".$font.":fontsize=".$fontsize.":fontcolor=".$color.":x=(w-text_w)/2:y=(h/2)+".$n.":text='".$arr[$j]."',";

  Of course, this works with .ttf fonts but I really want to use Google Fonts.

  I have also tried $font = "https://fonts.googleapis.com/css?family=Aclonica" that does not work either.

  I have a feeling that ffmpeg does not use woff2 fonts but I have seen a site that does this I just can’t say 100% for sure that they use Google Fonts in the creation, but they do use them in the selection of the font which leads me to believe they use them in the creation of the video.

• evrcdec : fix sign error

  4 mai 2013, par Michael Niedermayer

  evrcdec : fix sign error
  The specification wants round(abs(x))) * sign(x) which is
  
  equivakent to round(x)
  Fixes out of array access
  Found-by : Mateusz "j00ru" Jurczyk and Gynvael Coldwind
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/evrcdec.c