title: FFmpeg - concat videos error, do not match the corresponding output link?
body: <p>I'm trying to concat some video into one single video using ffmpeg. </p>
  
  <p>input information:</p>
  
  <p>a.mp4  1920x808 , b.mkv 1280x720</p>
  
  <p>First I tried command below and it works fine:</p>
  
  <blockquote>
    <p>ffmpeg -i a.mp4 -i b.mkv -f lavfi -t 0.1 -i anullsrc -filter_complex [1:v]scale=1920:808:force_original_aspect_ratio=decrease,pad=1920:808:(ow-iw)/2:(oh-ih)/2[v1]; [0:v][0:a][v1][1:a]concat=n=2:v=1:a=1[outv][outa] -map [outv] -map [outa] -vcodec libx264 -crf 27 -preset ultrafast -threads 2 out.mp4</p>
  </blockquote>
  
  <p>And then I put 'b.mkv' first:</p>
  
  <blockquote>
    <p>ffmpeg -i b.mkv -i a.mp4 -f lavfi -t 0.1 -i anullsrc -filter_complex [0:v]scale=1920:808:force_original_aspect_ratio=decrease,pad=1920:808:(ow-iw)/2:(oh-ih)/2[v0]; [v0][0:a][1:v][1:a]concat=n=2:v=1:a=1[outv][outa] -map [outv] -map [outa] -vcodec libx264 -crf 27 -preset ultrafast -threads 2 out.mp4</p>
  </blockquote>
  
  <p>but this time I got error:</p>
  
  <blockquote>
    <p>Input link in1:v0 parameters (size 1920x808, SAR 0:1) do not match the corresponding output link in0:v0 parameters (1920x808, SAR 3232:3231)</p>
  </blockquote>
  
  <p>I don't understand why? </p>
  
  <p>Any ideas anyone?</p>
  
  <p>Many thanks in advance.</p>