What does it mean when encode a yuv422 file to yuv420 by libx264 lossless encoding?

Question

I have an AVI file input.avi encoded in HQ which has yuv422 format according to ffprobe.

Now, if I run the following command

ffmpeg -i input.avi -c:v libx264 -crf 0 -pix_fmt yuv420p out.mp4

Will the out.mp4 be lossless compared with input.avi? or it will be 'lossless' except half of the U and V information is lost?

No. Yes. You pretty much answered your own question in the last line. — stib, May 31, 2017 at 1:28
BTW you can encode h.264 in yuv422, but not all players will play it — stib, May 31, 2017 at 1:29
@stib Thanks! I want to calculate the PSNR and SSIM between input.avi and an encoded mp4 file. But the yuv format is different, so I am encoding the input.avi to 'lossless' yuv420p mp4, then calculate the PSNR and SSIM. — shintaroid, May 31, 2017 at 1:37
Just look at the SSIM score for the luma plane. U and V are not relevant in this case. Or you could downsample the AVI to 4:2:0 before passing it to the quality compare filters i.e. [1]format=yuv420p[a];[0][a]psnr.. — Gyan, May 31, 2017 at 4:55
@Mulvya Thanks! Would you please give a full command sample? I am new to ffmpeg, I am not sure how to add [1]format=yuv420p[a];[0][a]psnr.. to my command — shintaroid, May 31, 2017 at 7:14

Gyan · Accepted Answer · 2017-05-31 07:34:31Z

1

To directly compare the YUV422P AVI and YUV420P MP4,

ffmpeg -i input.avi -i out.mp4
   -filter_complex
      "[0]format=yuv420p,split[avi-p][avi-s];
       [1][avi-p]psnr;[1][avi-s]ssim" -f null -

answered May 31, 2017 at 7:34

Gyan

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It gives an average value on the terminal, how can we output a log file with values of each frame? I tried -lavfi psnr="stats_file=psnr.log" but no luck.
– shintaroid
Jun 1, 2017 at 2:11

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