[MEncoder-users] avi sync questions

[RC]

On Sun, 06 Apr 2008 11:49:15 +0200
Erik Slagter <erik at slagter.name> wrote:

> Every time you change the resolution, the quality gets worse, scaling
> is  a lossy process (if not scaled by exactly 2,4,1/2,1/4 etc.).

That's not inherently true.  Simpler scalers, which do not interpolate,
should be able to upscale and then after downscaling offer very close to
original video.  sincR supposedly comes closest.

If you absolutely can't crop to a resolution that is a multiple of 16,
upscaling slightly is certainly the best alternative.

> I'd recommend using the exact original DVD resolution, which also 
> happens to be nice at 16-pixel boundaries. And yes, that really helps 
> for encoding.

It's a rare, exceptional DVD that doesn't have noisy edges transitioning
to a few pixels of black around the fringes, which is almost as bad
as non-16 dimentions (less likely to triger codec bugs--border_mask
may help). 

> The only exception to this rule is if you have material that is 
> hard-letterboxed (i.e. has black borders in the movie itself, evil!), 
> I'd recommend to crop these off to a multiple of 16 pixels, and at 
> playback have the player re-add the borders when necessary (i.e.
> playing  on 4:3 equipment).

That is the recomended method of course, which is why cropdetect by
default rounds down to multiples of 16.  It can be problematic in a few
situations, however, where subtitles or the like are right at the edge
of the screen.  Changing the offset (cropping more pixels at the top
than the bottom) usually works, but not always.