[MPlayer-users] PSNR Questions

D Richard Felker III dalias at aerifal.cx
Sat Feb 15 18:02:16 CET 2003

On Sat, Feb 15, 2003 at 02:57:15PM +0000, Ian wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> Hash: SHA1
> How can you measure the quality of an encoding by referring only to its 
> output?  Surely this is meaningless.  Frequently my encoded stuff shows 
> higher PSNR than the source itself.

Huh?? This is nonsense. You can't compare PSNR relative to two
different reference 'signals'. When encoding, the source is considered
the signal, and the difference between the encoded picture and the
source is the noise, so the source has infinite PSNR automatically.

You could compare both to a common original raw yuv frame sequence or
something to get PSNR values, but the reencode (e.g. from mpeg2 to
mpeg4) will almost always have PSNR considerably lower than the first
encode (in my example, the mpeg2).

> Wouldn't (eg.) a solid black image have infinite PSNR?

Yes, theoretically. Computational issues might make it come out
different tho.

> Perhaps output PSNR / input PSNR would mean something (like electronic 
> noise figure) but I doubt even that . . .

Huh?? Do you even know what PSNR means??


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