[MPlayer-users] PSNR Questions

[Struan Bartlett]

Ian wrote:

> D Richard Felker III wrote:
>
>> How are you measuring the PSNR of the original?? Like I said, you have
>> to use the same reference to compare PSNR.
>
> I've been experimenting with a 3 or 4 pass TV recording method whereby 
> I capture at maximum bps for a high-quality "original" mpeg4 or mjpeg, 
> then do a 2- or 3-stage reduction to 1Mb/s mpeg4.  I left the PSNR 
> option on for both stages and just observed that the figures were 
> higher for the low-bw output than the max-bw "original".
>
> Incidentally, the quality of this procedure was not significantly 
> (visibly) better than an original grab at the low-bw rate, hence my 
> interest in an objective measurement. 

Objective measurement of encoding quality is really my question.

A week ago a URL (rguyom.ath.cx) was posted of a doc that compared 
various encoding options by comparing the PSNRs reported by mencoder.

I was experimenting myself to compare the quality of a cropped movie 
encoded at bitrate Xbps: first unscaled, but with (mplayer-only) encoded 
aspect ratio; second, scaled according to encoding-tips.txt's procedure 
using calcbpp.pl.

I wanted to use the reported PSNRs to objectively determine the relative 
performance of each method, but unfortunately it appears this is not a 
measure that works well.

1. What's your experience of the quality of encoded cropped video at a 
given bitrate in my two scenarios above?

2. Does this invalidate the use of PSNR as an objective measure and 
therefore the results of the rguyom.ath.cx paper?

3. If so, how are we to objectively measure the performance of encoding 
parameters?

What's your take?

Struan.