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• vc-1 : Optimise parser (with special attention to ARM)

  23 avril 2014, par Ben Avison

  vc-1 : Optimise parser (with special attention to ARM)
  The previous implementation of the parser made four passes over each input
  
  buffer (reduced to two if the container format already guaranteed the input
  
  buffer corresponded to frames, such as with MKV). But these buffers are
  
  often 200K in size, certainly enough to flush the data out of L1 cache, and
  
  for many CPUs, all the way out to main memory. The passes were :
  1) locate frame boundaries (not needed for MKV etc)
  
  2) copy the data into a contiguous block (not needed for MKV etc)
  
  3) locate the start codes within each frame
  
  4) unescape the data between start codes
  After this, the unescaped data was parsed to extract certain header fields,
  
  but because the unescape operation was so large, this was usually also
  
  effectively operating on uncached memory. Most of the unescaped data was
  
  simply thrown away and never processed further. Only step 2 - because it
  
  used memcpy - was using prefetch, making things even worse.
  This patch reorganises these steps so that, aside from the copying, the
  
  operations are performed in parallel, maximising cache utilisation. No more
  
  than the worst-case number of bytes needed for header parsing is unescaped.
  
  Most of the data is, in practice, only read in order to search for a start
  
  code, for which optimised implementations already existed in the H264 codec
  
  (notably the ARM version uses prefetch, so we end up doing both remaining
  
  passes at maximum speed). For MKV files, we know when we’ve found the last
  
  start code of interest in a given frame, so we are able to avoid doing even
  
  that one remaining pass for most of the buffer.
  In some use-cases (such as the Raspberry Pi) video decode is handled by the
  
  GPU, but the entire elementary stream is still fed through the parser to
  
  pick out certain elements of the header which are necessary to manage the
  
  decode process. As you might expect, in these cases, the performance of the
  
  parser is significant.
  To measure parser performance, I used the same VC-1 elementary stream in
  
  either an MPEG-2 transport stream or a MKV file, and fed it through ffmpeg
  
  with -c:v copy -c:a copy -f null. These are the gperftools counts for
  
  those streams, both filtered to only include vc1_parse() and its callees,
  
  and unfiltered (to include the whole binary). Lower numbers are better :
                  Before          After
  
  File  Filtered  Mean   StdDev   Mean   StdDev  Confidence  Change
  
  M2TS  No        861.7  8.2      650.5  8.1     100.0%      +32.5%
  
  MKV   No        868.9  7.4      731.7  9.0     100.0%      +18.8%
  
  M2TS  Yes       250.0  11.2     27.2   3.4     100.0%      +817.9%
  
  MKV   Yes       149.0  12.8     1.7    0.8     100.0%      +8526.3%
  Yes, that last case shows vc1_parse() running 86 times faster ! The M2TS
  
  case does show a larger absolute improvement though, since it was worse
  
  to begin with.
  This patch has been tested with the FATE suite (albeit on x86 for speed).
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/vc1_parser.c

• How can I find get the frame number from a gif file using ffmpeg's AVFormatContext struct ?

  17 décembre 2015, par user1400047

  I searched from all the subnode of AVFormatContext, buf just found the fps. no frame numbers or duration info at all. who can help me ?

• Merging TCP and UDP packets using FFmpeg

  18 décembre 2015, par October Gladiolus

  First, i have encoded TCP and UDP packets which are received from different tunnels, i need to decode these packets using FFmpeg. Now, the TCP packets have to be sent to a buffer that feeds the FFmpeg, while the UDP packets are supposed to go directly to the FFmpeg. The question is : does the FFmpeg have the ability to merge the TCP and UDP packets according to their sequence numbers ? or is there any other way to do so. Thanks