[MPlayer-users] BPP for Frame Size

[Jason Tackaberry]

On Fri, 2007-01-19 at 18:15 +0000, Mark Himsley wrote:
> Because of the chosen sampling frequency (13.5Mhz) and the active line 
> length (52us) actually only 702 pixels are active, the 9 at each end of the 
> lines are not part of the 16:9 picture.  Have you noticed that most 720x576 
> pictures have thin black bars at the edges - these are the 9 extra pixels 
> at each edge.
> 
> If you are going to scale up to 1024x576 you should crop the extra 18 
> pixels off or you will get pictures squished by about 2.5%

Good encoding guides (including mencoder's) always say to crop out black
bars, including the "half-black" pixels that tend to exist along all
edges of the image.

Although most of my experience is in the NTSC world (where this issue
does exist as well to a degree), whether or not you need to crop along
the sides seems to depend to a considerable extent on the type of
content and I imagine as well the DVD transfer process.  Certainly a lot
of the older DVDs I've seen require cropping along the edges, but
increasingly -- and again, speaking about the NTSC space here -- this is
becoming less of an issue.  Maybe this isn't the case with PAL.

Cropping to 702x576 is also sub-optimal because the width is not a
multiple of 16.  On some content you might get away with 704x576, but if
not, 688x576 would be the next best option.  Some people can't tolerate
the thought of losing those 7 pixels on each side though. :)  (Obviously
the display aspect would require adjustment to compensate.)

Anyway, I have to say that this information is quite news to me.  If
correct (and assuming I am grokking it properly), it would imply that
all the software DVD players I've seen, including mplayer, are
outputting video at the wrong aspect ratio.  Do you know if the
situation is the same with NTSC -- and if so, what are the corresponding
pixel aspects?  Do you have any further references that discuss this?
What about content that fills all 720 pixels?  Does this exist at all
with PAL content?  (It certainly does with NTSC.)  It seems to me if you
have material that fully fills the 720x576 frame, scaling it to 1050x576
would be incorrect.

Cheers,
Jason.