[MPlayer-users] Decreased MPEG-4 compression quality from cha nging pixel aspect r atio?

[Francois Visagie]

> -----Original Message-----
> From: On Behalf Of RC
> Sent: 26 September 2007 18:13
> 
> On Tue, 25 Sep 2007 10:46:59 +0200
> Francois Visagie <fvisagie at za.spescom.com> wrote:
> 
> Display aspect ratio may cause the end result to display a 
> higher density of pixels in one direction or another, but 
> doesn't affect the actual video (encoding) in any way at all.
> 
> > doesn't this cause the compressor weighting of neighbouring 
> > information to become too heavy or too light?
> 
> > does the compressor need to take into account the fact that 
> on-screen 
> > inter-pixel distances might be say 1:1.25 pixels:pixels, instead of 
> > 1:1?
> 
> Scaling, cropping, etc. are done after decoding the video, 
> and before reencoding it.  All 'decisions' made by the codec 
> happen on an arbitrary set of pixels it is supplied with.  It 
> has no knowledge of what the display aspect ratio will be, 
> and does not care how the video was previously encoded before.

Thanks for your response, although I still remain convinced that changing
pixel aspect ratio will lead to loss of perceptual quality, something which
MPEG-4 designers seem not to have considered.

Kind regards,
Francois





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