[MPlayer-users] BPP for Frame Size

[Jason Tackaberry]

On Fri, 2007-01-19 at 20:22 -0400, Jorge Fábregas wrote:
> Well, I recently learned that when you encode from such a source (anamorphic 
> 16:9) you need to leave the frame size as is (in this case 720x576) and just 
> specify the aspect ratio when encoding. Then, the footage will be stretched 
> (hopefully) by the player.... In other words, you should never upscale to the 
> intended aspect ratio (you'll waste bits).

Quite true, although you should always crop the black bands away from
the frame for the same reason.

> Back to reality...well...I recently figured out that Windows media Player and 
> DivX player won't consider the specified aspect ratio on the video (you have 
> to manually set it).  I received recommendations for VLC & mplayer for 
> windows. I guess that's an alternative...

Yes, and that's the reason you see most DVD rips that scale the frame to
square up the pixels.  It's a tragedy, but IMHO anyone who uses Windows
Media Player deserves precisely what they get.  If they aren't going to
use a sensible alternative like VLC then they can watch their videos all
retarded and squished, while the rest of us enjoy a better quality rip
(or smaller file at equivalent quality).


> http://www.mplayerhq.hu/DOCS/HTML/en/menc-feat-dvd-mpeg4.html#menc-feat-dvd-mpeg4-constraints

Wow, this document changed a lot since I originally wrote it.  It has a
lot of good new information, although someone replaced my nice Harry
Potter example with boring generic names. :)


> So...can we conclude that it really doesn't matter if I crop or not those 18 
> pixels (size wise)?  Obviously, if it's almost the same thing I rather prefer 
> cropping...at least it will look GOOD (the way it should! )

It definitely matters unless the edge occurs on multiple of 16, in which
case it's much less of an issue (although it still makes sense to crop
the black bands out).

So if you have to crop, you should crop to 702 or 688.  Then scale down
(if you must) while maintaining roughly the original pixel aspect
(deviating where needed to keep the dimensions a multiple of 16), then
let mencoder store the display aspect in the vprp or odml header (or VOL
header).  Something like:

  ... -vf crop=688:576:16:0,scale=512:-11 \
      -ovc lavc -lavcopts autoaspect:[...] ...

Though I guess if Mark is right (I am skeptical and would like further
references) and the correct pre-cropped display resolution is supposed
to be 1050x576, the aspect would need to be adjusted, which you could do
by prefixing the filter chain with dsize=1050/576.

Cheers.
Jason.