[MPlayer-users] Converting 512:272 to 480:480 with aspect=4/3?

[D Richard Felker III]

On Mon, Jan 19, 2004 at 04:32:30PM -0200, romildo at uber.com.br wrote:
> > > > or else get
> > > > mencoder to do it for you with fancy use of the automatic options for
> > > > scale.
> > > 
> > > How would that be achieved?
> > 
> > You'd have to think about it some. See the man page for the various
> > ways to automate the sizes expand and scale select (using negative
> > w/h). You'd probably also need to use the dsize filter to set the
> > desired aspect. It's complicated and I don't feel like working out
> > exactly how it should be done, especially since I think it's better to
> > do things manually to be sure you stay on multiple-of-16 boundaries to
> > maximize quality.
> 
> I have tried but was not able to figure out how to use the automatic
> options for the scale filter in this example. Can anybody enlighten
> here, at least with a command line?

-vf scale=640:-2,scale=480:0,expand=480:480

But scaling twice might lower quality a bit, and will waste a lot of
cpu time. But since you insist on stupid automation, you obviously
don't care about quality since you're not going to be macroblock-
aligning the black borders...

You can also use vf_dsize to automate it in just one scale, but you
have to know the starting aspect ratio in order for it to work:

-vf dsize=a/b,scale=480:-2,expand=480:480

where a/b is 3/4 times the original aspect...

Anyway, it's presently not possible (with ANY tool, not just mencoder)
to make a decent automated encode. You have to know the content you're
encoding, know what affects quality, and know what tradeoffs to make
(4% error in aspect versus quantization errors, choice of resolution,
number of discs to use, etc.). It's NOT simple and you can't make it
simple without some very slow, very fancy software to analyze
perceptual quality for you.

Rich