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• Why is ffmpeg's conversion to YUV420 so poor ?

  8 novembre 2020, par Hugues

  I have been using ffmpeg and other compression tools to compare rate-distortion curves for YUV420-resampled video.
In these comparisons, results from ffmpeg are consistently worse, with PSNR values that are 0.5-1.0 dB lower.

  


  I tracked the problem to ffmpeg's conversion between RGB and YUV420.
To simplify, let us assume "lossless compression" and therefore consider only RGB -> YUV420 -> RGB.
Also, we operate on a single PNG image frame.

  


  # Use some default options.
ffmpeg="ffmpeg -nostdin -hide_banner -v error"

# Obtain a source image.
wget -nv -O original.png https://i.stack.imgur.com/8J1qY.png
size="256x256"

# Compare it with itself to verify that we get an infinite average PSNR.
$ffmpeg -v info -i original.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:inf

# Convert the image to YUV420, and convert back to RGB.
$ffmpeg -i original.png -pix_fmt yuv420p -f rawvideo -y temp1.yuv420
$ffmpeg -f rawvideo -s $size -pix_fmt yuv420p -i temp1.yuv420 -y result1.png

# Compare it with the original image to measure the PSNR (in dB).
$ffmpeg -v info -i result1.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:36.894551


  


  Now, as an alternative, we perform the RGB <-> YUV420 chroma resampling manually :

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  yuv444_to_yuv420="extractplanes=y+u+v[y][u][v];\
  [u]scale=w=iw/2:h=ih/2:flags=area[u];\
  [v]scale=w=iw/2:h=ih/2:flags=area[v];\
  [y][u][v]mergeplanes=0x001020:yuv420p"
yuv420_to_rgb="extractplanes=y+u+v[y][u][v];\
  [u]scale=w=iw*2:h=ih*2:flags=neighbor[u];\
  [v]scale=w=iw*2:h=ih*2:flags=neighbor[v];\
  [y][u][v]mergeplanes=0x001020:yuv444p,format=rgb24"

$ffmpeg -i original.png -pix_fmt yuv444p -f rawvideo - | \
  $ffmpeg -f rawvideo -pix_fmt yuv444p -s $size -i - \
    -lavfi "$yuv444_to_yuv420" -f rawvideo -y temp2.yuv420
$ffmpeg -f rawvideo -pix_fmt yuv420p -s $size -i temp2.yuv420 \
  -lavfi "$yuv420_to_rgb" -y result2.png

# Measure PSNR by comparing with the original image.
$ffmpeg -v info -i result2.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:37.536444
# This is an improvement of 0.64 dB!


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  This brings up two questions :

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  1. Why doesn't ffmpeg implement a better conversion to/from yuv420p by default ?

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  2. Is there any simpler way to obtain or express this improved conversion ?

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• Batch concation of multiple videos files (Bash)

  4 janvier 2024, par pops64

  I have clips from multiple different scenes in the same folder I was wondering if any one had a script that groups clips belonging to each scene together than pass them into ffmpeg for a concatenation and transcode. The file name pattern is uniform. And ids are separated by a hyphen with only the scene id being unique. I am currently doing this scene by scene in FFmpeg batch av converter but any help automating this in bash or powershell would be greatly appreciated. I just can't figure out where to start

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  {name_of_scene}-{scene_id}-{clip_id}-{resolution}.{filetype}


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• Batch concation of multiple videos files (Powershell)

  5 janvier 2024, par pops64

  I have clips from multiple different scenes in the same folder I was wondering if any one had a script that groups clips belonging to each scene together than pass them into ffmpeg for a concatenation and transcode. The file name pattern is uniform. And ids are separated by a hyphen with only the scene id being unique. I am currently doing this scene by scene in FFmpeg batch av converter but any help automating this in powershell would be greatly appreciated. I just can't figure out where to start

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  {name_of_scene}-{scene_id}-{clip_id}-{resolution}.{filetype}


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