[MPlayer-users] Re: slightly confused by mencoder output

[Rich Felker]

On Mon, Sep 12, 2005 at 10:37:41PM +0200, Matthias Wieser wrote:
> Am Montag, 12. September 2005 06:39 schrieb RC:
> > On Sun, 11 Sep 2005 22:08:25 +0200
> >
> > The Eye <mhellwig at gmx.at> wrote:
> > > video0:width=720:height=530:
> >
> > A height of 530 is nonsensical.
> 
> Right.
> 
> > Since the content is interlaced, you 
> > can only capture at the card's full heigh (576 PAL / 480 NTSC) or
> > exactly half (or 1/4 1/8 etc).
> 
> Or any other supported resolution if you have a saa7134 card because the 
> saa7134 chip has a not-so-bad deinterlacer.
> 
> > > crop=688:518:16:6,pp=fd
> >
> > Your output dimentions should be exact multiples of 16.
> 
> That's not so important if you don't care about the bitrate. If in doubt 
> add 10% to the bitrate if your output dimentions are not multiples of 16.

Are you sure? IMO you're still likely to get artifacts on the border
blocks. Also it's totally invalid to crop with a y-offset that's not a
multiple of 4 if you video is interlaced. You'll be exchanging the
fields and the chroma will not line up with the luma.

In summary, UNLESS YOU COMPLETELY UNDERSTAND EVERY POSSIBLE THING
THERE IS TO UNDERSTAND ABOUT VIDEO (esp. interlaced video), never use
crop offsets or sized that are not multiples of 2 (horizontal) and 4
(vertical), and always ensure that final image dimensions are
multiples of 16.

> I think it is much more important to take care that the vertical crop 
> offset is a multiple of four which is not the case in The Eye's mencoder 
> command line.

Yes... most definitely. Glad you caught it too.
BTW can we make vf_crop fail to initialize with bad y-offset unless
you pass a force option to it..?

Rich