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• How to convert an mkv (with subtitles) to something Nexus One friendly ?

  21 décembre 2011, par Daniel Quinn

  I have this ffmpeg one-liner that's been good for generating video files for my Nexus One :

  ffmpeg -i infile.mkv -acodec aac -s 572x238 -vcodec libx264 -vpre ipod640 -ab 128k -b 512k -f mp4 -strict experimental outfile.mp4

  But it does this ignorant of the subtitles in infile.mkv — usually not a problem, unless I'm dealing with a non-english movie. In cases like this, I'd like to use the Japanese audio track, and the English subtitles.

  The funny bit is that I can use mplayer to play it using -alang and -slang, but don't know how to use mencoder to make Nexus One friendly videos. I can use ffmpeg to generate Nexus One friendly videos, but can't figure out how to get it to use a specific subtitle track.

  If someone can solve one of these for me, I'll be a happy camper.

• ffmpeg zoompan + crop

  24 février 2021, par andykais

  I need to pan across a video that is also being cropped by ffmpeg. How can I accomplish this in a single ffmpeg command ?

  


  So far I see two possible options :

  


  Option #1 : use crop variables pan across the video. This is choppy, but possibly fixed by upscaling the input.

  


  ffmpeg -i input.mp4 -vf 'crop=w=in_w/2:h=in_h:x=t*10' crop_panned_output.mp4


  


  Option #2 : create an alpha mask in the shape of the crop, and then apply it on top of the input video with a zoompan filter. ffmpeg is doing extra work in this case, because we have to pad the input, then zoom in on it, then pan across it, then apply the alpha mask. This is actually using the "zoompan" feature to "pan" though.

  


  magick -size 1920x1080 xc:none -fill black -draw "rectangle 480,0 1440,1080" rectangular-alpha-mask.png
ffmpeg -i input.mp4  -i rectangular-alpha-mask.png  -filter_complex "
  [0:v]pad=2112:1188,zoompan=z=1.1:px+0.5:d=1:fps=60:s=1920x1080[input_pan];
  [1:v]alphaextract[alf];
  [input_pan][alf]alphamerge[masked];
  color=s=1920x1080:color=red[base];
  [base][masked]overlay=shortest=1
"  mask_panned_output.mp4


  


  Are either of these the right way to do it or are they incredibly inefficient ? Is there another option ? Cropping & panning together feels like a fairly common workflow, but these solutions feel a bit hacky.

  


  Here is a visual description of what panning and cropping a video looks like :

  


  First start with a video. Here is a simple one.


  


  I want to crop the video to a certain width/height, and move from the left to the right across the video. The background is colored red for clarity


  


  The video moves from left to right across the original video, and keeps the same crop ratio.


  


• Extract alpha from video ffmpeg android

  24 juin 2018, par Yarik Denisyk

  I want to overlay transparent video on the background image. I have a video where the top half is RGB object and bottom half is an alpha mask.

  

  Now, for making this I do next steps :

  1) I am extracting all frames from video and save to the folder

  2) Each frame splitting to top and bottom half bitmap

  3) Top bitmap composite with bottom mask for extract alpha and get a frame with transparent background

  3) I am drawing each frame on the background and save to a folder

  4) Create a video using FFmpeg

  The problem is step 2, 3 and 4, they very slow. Maybe has another way to overlay transparent video on the background image ?