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

[RC]

On Tue, 25 Sep 2007 10:46:59 +0200
Francois Visagie <fvisagie at za.spescom.com> wrote:

> is it possible that modifying the pixel aspect ratio
> (horizontal_pixels / vertical_pixels) could cause an additional loss
> of quality during compression?

No.

> upon display pixels are now relatively closer together or further
> apart (more so in one dimension than the other) than before scaling.

> Typically, intra-frame compression relies heavily upon adjacent and
> neighbouring pixel information. If relative spatial separation between
> pixels is now modified by changing pixel aspect ratio, 

Pixels can't move closer together or further apart.  Downscaling to half
size may merge two pixels worth of information into a single pixel, but
that's all.  It's just a single pixel of information to the encoder. 

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.