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• ffmpeg content pixelation when HDR to SDR ?

  22 juin 2022, par Peter Hammi

  I would like to re-encode an HDR source to an SDR output using ffmpeg. To accomplish this task, I use the following filter :

  


  -vf zscale=tin=smpte2084:min=bt2020nc:pin=bt2020:rin=tv:t=smpte2084:m=bt2020nc:p=bt2020:r=tv,zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=tonemap=hable:desat=0,zscale=t=bt709:m=bt709:r=tv,format=yuv420p -strict -1 -f yuv4mpegpipe


  


  Basically this solves the problem of HDR to SDR color ranges and the result is "ok" but in some scenes I get pixelation for some reason.
The input clip I have is 4k HDR @15 Mbit/s (HEVC) where I now want to scale it down to 1080p SDR (h.264). What bit rate is recommended here ? Normally I would say 5-7,5 Mbit/s is fine, but it seems to make not a big difference in terms of HDR to SDR and my pixelation issue.

  


  Is there maybe something wrong with my filter ?

  


  Thanks in advance

  


• 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


  


• Merging/concatenating video and keeping sound in R using AV package

  30 juin 2022, par Jack Cornwall

  I am trying to merge/concatenate multiple videos with sound sequentially into one video using only R, (I don't want to work with ffmpeg in the command line as the rest of the project doesn't require it and I would rather not bring it in at this stage).

  


  My code looks like the following :

  


  dir<-"C:/Users/Admin/Documents/r_programs/"

videos<-c(
  paste0(dir,"video_1.mp4"),
  paste0(dir,"video_2.mp4"),
  paste0(dir,"video_3.mp4")
)

#encoding
av_encode_video(
  videos,
  output=paste0(dir,"output.mp4"),
  framerate=30,
  vfilter="null",
  codec="libx264rgb",
  audio=videos,
  verbose=TRUE
)


  


  It almost works, the output file is an mp4 containing the 3 videos sequentially one after the other, but the audio present is only from the first of the 3 video and then it cuts off.

  


  It doesn't really matter what the videos are. I have recreated this issue with the videos I was using or even 3 randomly downloaded 1080p 30fps videos from YouTube.

  


  Any help is appreciated & thank you in advance.