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• nvenc : De-compensate aspect ratio compensation of DVD-like content.

  28 janvier 2015, par Philip Langdale

  nvenc : De-compensate aspect ratio compensation of DVD-like content.
  For reasons we are not privy to, nvidia decided that the nvenc encoder
  
  should apply aspect ratio compensation to ’DVD like’ content, assuming that
  
  the content is not BT.601 compliant, but needs to be BT.601 compliant. In
  
  this context, that means that they make the following, questionable,
  
  assumptions :
  1) If the input dimensions are 720x480 or 720x576, assume the content has
  
  an active area of 704x480 or 704x576.
  2) Assume that whatever the input sample aspect ratio is, it does not account
  
  for the difference between ’physical’ and ’active’ dimensions.
  From these assumptions, they then conclude that they can ’help’, by adjusting
  
  the sample aspect ratio by a factor of 45/44. And indeed, if you wanted to
  
  display only the 704 wide active area with the same aspect ratio as the full
  
  720 wide image - this would be the correct adjustment factor, but what if you
  
  don’t ? And more importantly, what if you’re used to lavc not making this kind
  
  of adjustment at encode time - because none of the other encoders do this !
  And, what if you had already accounted for BT.601 and your input had the
  
  correct attributes ? Well, it’s going to apply the compensation anyway !
  
  So, if you take some content, and feed it through nvenc repeatedly, it
  
  will keep scaling the aspect ratio every time, stretching your video out
  
  more and more and more.
  So, clearly, regardless of whether you want to apply bt.601 aspect ratio
  
  adjustments or not, this is not the way to do it. With any other lavc
  
  encoder, you would do it as part of defining your input parameters or do
  
  the adjustment at playback time, and there’s no reason by nvenc should
  
  be any different.
  This change adds some logic to undo the compensation that nvenc would
  
  otherwise do.
  nvidia engineers have told us that they will work to make this
  
  compensation mechanism optional in a future release of the nvenc
  
  SDK. At that point, we can adapt accordingly.
  Signed-off-by : Philip Langdale <philipl@overt.org>
  
  Reviewed-by : Timo Rothenpieler <timo@rothenpieler.org>
  
  Signed-off-by : Anton Khirnov <anton@khirnov.net>
  

  • [DBH] libavcodec/nvenc.c

• ffmpeg : deshake and crop 2k to 1080p ?

  12 septembre 2017, par est

  There are great tutorials of how to use ffmpeg to

  1. deshake a video, either via 1pass of deshake filter or using vid.stab for a 2pass filter
  2. how to downsampling to 2k video to 1080p

  I have a video taken by an action camera, it’s wide angle, so lots of margins could be cropped out, I want to

  1. deshake it and fix the camera to a center stage
  2. cropping a 2304 × 1296 video to 1080p, without downsampling.
  3. Convert original H.264 to HVEC to better file size

  How can I achieve this in one step without lossy intermediate video files ?

• avcodec/ffv1enc : mark RGB48 support as non-experimental

  5 janvier 2018, par Jérôme Martinez

  avcodec/ffv1enc : mark RGB48 support as non-experimental
  Resulting bitstream was tested with a conformance checker
  
  using the last draft of FFV1 specifications.
  Signed-off-by : Michael Niedermayer <michael@niedermayer.cc>
  Also the files are already in the wild, and decoder support is
  
  thus needed. And with decoders widely supporting it, there is no
  
  advantage in not allowing it in the encoder.
  
  The exact bitstream format may change in future versions of the
  
  spec, if improvments are found.
  

  • [DH] libavcodec/ffv1enc.c