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

[D Richard Felker III]

On Mon, Jul 28, 2003 at 01:13:18PM -0700, Steven M. Schultz wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> 
> On Mon, 28 Jul 2003, D Richard Felker III wrote:
> 
> > No, you mkisunderstood entirely. You don't want either. I was just
> 
> 	No, I understood.   My question was perhaps not completely formed
> 	though.
> 
> 	Although you may not want either noise or black borders which
> 	is worse?
> 
> 	For example if I have to have a 720x480 size (for a DVD) and that
> 	the edges have noise (either from the VCR/tape or whatever) which
> 	of the two choices (blacken or crop) is less bad?   Scaling a 
> 	cropped image can degrade the picture so is that better than using
> 	a few extra bits?

Definitely crop, and then scale if necessary. Keep in mind that if you
captured from TV, your input is probably interlaced, and you cannot
scale vertically without dealing with that. vf_il is your friend in
this case.

> 	Are there any firm numbers showing the effect of cropping vs
> 	blackening vs crop&scale?   

No good tools to measure (feel free to make one), but the theoretical
arguments are quite sound...

Blackening is definitely worse than just leaving the noisy area
(assuming we're talking about the normal noise on DVDs, which is a
mostly good picture with ugly patterns on top of it, NOT 100% random
junk like you sometimes get from VHS). Never add black borders. Ever.
You'd be better off encoding at non-multiple-of-16 sizes.

> > black borders, especially if that helps you get a size that's nice
> > multiples of 16.
> 
> 	Many of the DVDs made from analog sources (old TV/Tape shows) I've 
> 	seen are NOT 'borderless' - so they're obviously not cropping at all 
> 	(in fact one can see all sorts of "noise" in the area that TVs don't 
> 	display due to overscan).  I was surprised they didn't blacken the areas
> 	instead of encoding all the noise.  With a higher bitrate limit and 
> 	the ability to use two ~4GB layers perhaps they're not overly 
> 	concerned about a few bits here and there?

Or more likely they's just stupid. DVD is limited in bitrate, and you
will reduce quality by encoding worthless junk...

Rich