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• avcodec/wnv1 : Use LE bitstream reader, avoid copying packet, fix memleak

  29 août 2020, par Andreas Rheinhardt

  avcodec/wnv1 : Use LE bitstream reader, avoid copying packet, fix memleak
  The Winnov WNV1 format is designed for a little-endian bitstream reader ;
  
  yet our decoder reversed every byte bitwise (in a buffer only
  
  allocated for this purpose) to use a big-endian bitstream reader. This
  
  commit stops this.
  Two things needed to be done to achieve this : The codes in the table used
  
  to initialize a VLC reader needed to be reversed bitwise (when
  
  initializing a VLC in LE mode, it is expected that the first bit to be
  
  read is in the least significant bit ; with BE codes the first bit to be
  
  read is the most significant bit of the code) and the following
  
  expression needed to be adapted :
  ff_reverse[get_bits(&w->gb, 8 - w->shift)]
  But this is easy : When only the bits read are reversed, they coincide
  
  with what a little-endian bitstream reader reads that reads the
  
  original, not-reversed data. But ff_reverse always reverses the full
  
  eight bits and this also performs a shift by (8 - (8 - w->shift)) on top
  
  of reversing the bits read. So the above line needs to be changed to
  get_bits(&w->gb, 8 - w->shift) << w->shift
  and this also shows why the variable shift is named the way it is.
  Finally, this also fixes a hypothetical memleak : For gigantic packets,
  
  initializing a GetBitContext can fail and in this case, the buffer
  
  containing the reversed data would leak.
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/wnv1.c

• avcodec/webp : Use LE VLC table for LE bitstream reader

  12 octobre 2020, par Andreas Rheinhardt

  avcodec/webp : Use LE VLC table for LE bitstream reader
  The WebP format uses Huffman tables and the decoder therefore uses
  
  VLC tables. Given that WebP is a LE format, a LE bitreader is used ;
  
  yet the VLC table is not created for a LE reader (the process used to
  
  create the tables puts the last bit to be read in the lowest bit) and
  
  therefore custom code for reading the VLCs that reverses the bits
  
  read is used instead of get_vlc2(). This commit changes this to use
  
  a table designed for LE bitreader which allows to use get_vlc2() directly.
  
  The necessary reversing of the codes is delegated to
  
  ff_init_vlc_sparse() (and is therefore only done during init and not
  
  when actually reading the VLCs).
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/webp.c

• avcodec/fraps : Use unchecked bitstream reader

  22 septembre 2020, par Andreas Rheinhardt

  avcodec/fraps : Use unchecked bitstream reader
  The fraps decoder already checked for overreads manually (and errored
  
  out in this scenario), yet it still enabled implicit checks, leading to
  
  worse performance and more code size.
  This commit disables the implicit bitstream reader checks. For the
  
  sample [1] this improves performance from 195105896 to 155851561
  
  decicycles for Clang 10 and from 222801887 to 168270467 decicycles when
  
  compiled with GCC 9.3. These values are the average of 10 runs each
  
  looping ten times over the input.
  [1] : samples.ffmpeg.org/ffmpeg-bugs/trac/ticket2593/fraps_flv1_decoding_errors.avi
  Reviewed-by : Paul B Mahol <onemda@gmail.com>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/fraps.c