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• preserve alpha channel when changing resolution in webm vp9 ffmpeg

  28 juin 2022, par Patrick Vellia

  I am running the streamio-ffmpeg gem in my Ruby code here to operate the FFMPEG command.

  


  Setting a Webm with codec VP9 that has alpha transparent frames in it as the input, I want to change the resolution from 1080p to 720p and preserve the alpha transparent frames when outputting to the same format/codec.

  


  I've tried it several different ways, but every time it outputs with a black background where the alpha transparent background should be, but ffprobe shows that ALPHA_MODE is set to 1.

  


  Here's the command that streamio-ffmpeg tells the system :

  


  ["/usr/local/bin/ffmpeg", "-y", "-i", "part1.webm", "-s", "1920x1080", "-aspect", "1.7777777777777777", "videos/movie_transparent_1920x1080.webm"]


  


  This one should have been exactly the same as the original, since the original resolution WAS 1080p. The resulting file is actually black background instead of transparent.

  


  Here's my Ruby code :

  


  def process_alpha_transparency(output)
    render_res = []
    case @height
    when 1080..2160
        render_res.append("1920x1080", "1280x720", "852x480")
    when 720..1079
        render_res.append("1280x720", "852x480")
    when 480..719
        render_res.append("852x480")
    when 0..479
        render_res.append(@resolution)
    end

    formats = ["webm", "mov"]
    combined_time_start = Time.now

    render_res.each do |res|
        formats.each do |format|
            puts "Rendering a #{res} resolution version."
            if format == "webm"
                options = {
                    resolution: res 
                }

                time_start = Time.now
                @movie.transcode("videos/#{output}_transparent_#{res}.#{format}", options)
                time_elapsed = Time.now - time_start

                puts "Te resolution rendering completed in #{time_elapsed.to_i.to_s} seconds."
            end
        end
    end

    combined_elapsed_time = Time.now - combined_time_start
    puts "The process completed in #{combined_elapsed_time.to_i.to_s} seconds."

    
end


  


• ffmpeg : is there a simple way to edit the video resolution, but keep all audio and subtitles

  21 avril 2022, par Arkeen

  I would like to lower video resolution - usually from .mkv files - but to keep all possible audio tracks (might be only one, might be several) and subtitles (might be none, might be several) from the original one. I also would like to keep as many encoding parameters as I can from the original video file (especially those I do not understand).

  


  I am still new to ffmpeg : at first the idea seemed simple, but after many attempts, it seems it is more complex than that. Do I have to use the -filter_complex option ? It seems to be an overkill (or overcomplex) for what I thought to be an easy conversion, but I might be wrong.

  


  I tried to combine -vf scale=-1:720 with -c copy -map 0, which gave me an error that I now understand, but I am stuck with the next step.

  


  Any lead on to achieve that ? Could it be done with ffmpeg only or would I need a script ?

  


• avcodec/v4l2_context : don't reinit output queue on dynamic resolution change event

  4 janvier 2022, par Ming Qian

  avcodec/v4l2_context : don't reinit output queue on dynamic resolution change event
  Reference :
  
  linux/Documentation/userspace-api/media/v4l/dev-decoder.rst
  
      "During the resolution change sequence, the OUTPUT queue must remain
  
       streaming. Calling VIDIOC_STREAMOFF() on the OUTPUT queue would
  
       abort the sequence and initiate a seek.
       In principle, the OUTPUT queue operates separately from the CAPTURE
  
       queue and this remains true for the duration of the entire
  
       resolution change sequence as well."
  Reviewed-by : Andriy Gelman <andriy.gelman@gmail.com>
  
  Signed-off-by : Ming Qian <ming.qian@nxp.com>
  

  • [DH] libavcodec/v4l2_context.c