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• Streaming images to ffmpeg out of order

  6 novembre 2022, par Jason C

  I have an algorithm that generates a video frame by frame, which I am refactoring to stream the images to ffmpeg rather than save them all to disk first.

  


  The algorithm currently generates and saves a bunch of QImages then executes ffmpeg (with QProcess). The new version will generate QImages then stream them out to ffmpeg's stdin via QProcess. I can do all that just fine.

  


  The issue is, I'm not actually generating the images serially, I'm generating them on a thread pool, maybe 12-16 at a time (just using QtConcurrent). I'd like to keep this parallelism for performance reasons, since the operation (even excluding image encoding and file I/O) does take a long time.

  


  So my question is, does ffmpeg have something like image2pipe but that can take the images slightly out of order (let's assume there's some sane maximum offset, +/- 20 or so) ?

  


  If not, my solution will be to divide the image set into small batches, then for each batch, run it on the pool, reorder it, and stream them in the correct order ; sacrificing some fraction of performance depending on the batch size. I'd rather not do that just because the code right now is dirt simple, and simple is good. So I'm wondering if there's a way to get ffmpeg to accept the images out of order.

  


  They're at a fixed frame rate, if that is useful (maybe there's some way to set PTS's or something, then I just find a codec + container that can store out-of-order frames).

  


• avcodec/bitstream : replace qsort with AV_QSORT

  18 octobre 2015, par Ganesh Ajjanagadde

  avcodec/bitstream : replace qsort with AV_QSORT
  Commit 3a0a2f33a6c955823fa4fb12c0b49cd29a496659 claims large performance
  
  advantages for AV_QSORT over libc’s qsort. The reason is that I suspect
  
  that libc’s qsort (at least on non LTO builds, like the typical FFmpeg config)
  
  can’t inline the comparison callback :
  
  https://stackoverflow.com/questions/5290695/is-there-any-way-a-c-c-compiler-can-inline-a-c-callback-function.
  
  AV_QSORT has two things going for it :
  
  1. The guaranteed inlining of qsort itself. This yields a negligible
  
  boost that may be ignored.
  
  2. The more serious possibility of potentially allowing the comparison
  
  function to be inlined - this is likely responsible for the large boosts
  
  reported.
  There is a comment explaining that this is a place that could use some
  
  performance improvement. Thus AV_QSORT is used to achieve that.
  Benchmarks deemed unnecessary due to existing claims about AV_QSORT.
  
  Tested with FATE.
  Reviewed-by : Michael Niedermayer <michael@niedermayer.cc>
  
  Signed-off-by : Ganesh Ajjanagadde <gajjanagadde@gmail.com>
  

  • [DH] libavcodec/bitstream.c

• Is variable resolution possible in MPEG-4 or Matroska ?

  29 août 2020, par Chris_F

  It's become increasingly common for movies to contain sections with varying aspect ratios. For instance, the movie The Dark Knight is is sometime 16:9 full frame, and at other times 2.40:1 with letter boxing. This is fine when viewed on a 16:9 (or even 4:3) screen, but if you try to watch it on a 2.40:1 screen you will get simultaneous horizontal and vertical letterboxes. Extremely undesirable.

  


  Currently I use MPC-HC, which has a feature where it is able to analyze the video in real time, detect letter boxing, and dynamically crop the video. This works pretty well, but it's kind of an ugly solution. Are there any video formats that allow meta data like image resolution to change throughout the video ?