[MPlayer-users] Re: mencoder/crop => image distortion

D Richard Felker III dalias at aerifal.cx
Thu Jul 31 19:29:42 CEST 2003

On Thu, Jul 31, 2003 at 01:10:18PM +0200, ROED,HAAVARD (HP-Norway,ex1) wrote:
> > > it"), and #3 would modify the aspect ratio, "mangling" the 
> > original movie.
> > > Scaling except in a player seems like a bad idea to me anyway.
> > 
> > Why? This is nonsense. The standard way of dealing with movies that
> > don't naturally come out to a perfect size is to scale to the nearest
> > multiples of 16. Then encode the actual perfect aspect in the mpeg4
> > headers, if you care, but no one will notice the difference anyway.
> I was referring to the DOCS, section 7.3:
> "Often the need to resize movie images' size emerges. Its reasons can be
> many: decreasing file size, network bandwidth, etc. Most people even do
> rescaling when converting DVDs or SVCDs to DivX AVI. This is bad. Instead of
> even you doing so, read the Preserving aspect ratio section."
> Which is why I've been assuming rescaling is a bad thing.

If you want to make a replica of the DVD, then it's bad, but like
we've said that's usually not the purpose of encoding (if that's what
you wanted you could just copy the .vob without reencoding). I'm not
sure who added that paragraph to the DOCS, but IMO it should be
removed. Scaling is pretty much ALWAYS required unless you want to
make a 2-cd (or more likely 3-cd) rip, just because of bitrate
considerations. So if you're scaling to begin with, you might as well
make the pixels (almost) square while you're at it, unless that will
result in a big waste of space (e.g. svcd, where you'd have to scale
up to 640x480 from 480x480 -- that would pretty much negate the
improved compression you get from using mpeg4).

> Continuing with the DOCS, section 7.9;
> "MPEG4 has an unique feature: the video stream can contain its needed aspect
> ratio. Yes, just like MPEG1/2 files (DVD, SVCD). Regretfully, there are no
> video players outside which support this attribute. Except MPlayer.
> This feature can be used only with libavcodec's mpeg4 codec. Keep in mind:
> although MPlayer will correctly play the created file, other players will
> use the wrong aspect ratio."
> lead me to believe that since the claim is that no video players except
> MPlayer supports this attribute (the aspect ratio tag), this was MPlayer
> proprietary tag. I understand now that it's a MPEG4 specific tag, but then
> how can MPlayer claim that no other player supports it?

Read the other post I just wrote to this thread. It's an issue with
the fact that people are storing mpeg4 in avi container, and how other
players handle avi files.

> Now, to further explore this issue, I note that the ability to add this tag
> is valid only when using libavcodec mpeg4 (DivX 4/5 only?). Which I'm not -
> I'm using XViD, thus I stick to my previous "conclusion"; that #3
> (rescaling, changing aspect) is a bad choice (when not using libavcodec).
> Since XViD is in fact a mpeg4 codec (or at least claims to be), I would
> expect it to support the mpeg4 aspect tag :( And finally, when a player is
> able to play ie mpeg4 video, it would be nice if it actually supported the
> -whole- standard.

Yes, that would be nice...


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