[MEncoder-users] Codec-independant objective quality measurement

[Matthias Wieser]

Hi,


First: I don't know any good non-psnr tool for linux.

Nicolas George wrote:

> - I am not sure whether different codecs evaluate it exactly the same
> way. And even if they do, they do not display it the same way, and it is
> not always easy to know which part of the final statistic of one codec
> should be compared to which part for another codec.

You could use mplayer/TOOLS/psnr-video.sh for this purpose.

>
> - Codecs evaluate their PSNR with regard to the frame that is given to
> them, disregarding the effect of upstream video filters. That is
> especially true for scaling: if I downscale a video to 32x24, I can get
> lossless quality at less than 300 kbit/s, and the codec will tell me an
> infinite PSNR. But that does not mean that the result is perfect with
> regard to the original content. That example is exaggerated, but it would
> be nice to be able to evaluate whether, at a given bitrate, is is better
> to keep the original resolution or to downscale a little.

That has been one of the reasons for me to write mplayer/TOOLS/psnr-video.sh

This script let's you compare frame by frame the source video with the 
recompressed video. To match differing frame sizes you should give an 
apropriate "scale" option.

eg.:

1. recompress the video:

  mencoder orig.avi -o recompressed.avi [...] -lavcopts vbitrate=300[...]

2. scale and compress the video:

  mencoder orig.avi -o scaledandrecompressed.avi [...] -vf 
  scale=32x24 -lavcopts vbitrate=300[...]

3. Compare:

  /path/to/mplayer/TOOLS/psnr-video.sh ./orig.avi ./recompressed.avi 500
  mv psnr.dat recompressed.dat
  /path/to/mplayer/TOOLS/psnr-video.sh ./orig.avi ./scaledandrecompressed.avi\
  500 "" "-vf scale=640x480"
  mv psnr.dat scaledandrecompressed.dat

Regards,
  Matthias