[MPlayer-users] BPP for Frame Size

[Jorge Fábregas]

On Friday 19 January 2007 2:15 pm, Mark Himsley wrote:
> Hi. I'm quite new to mplayer but I work in UK TV broadcasting and I wanted
> to say how angry I get with so many people and projects continuing to say
> that PAL 720x576 should be scaled to 1024x576 to correct for 16:9
> anomorphic pixels.

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).

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, I agree that 1024x576 is a 16:9 ratio. The problem is actually that
> only 702x576 of the 720x576 should be scaled up.
>
> 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

I'll have to google for this in order to understand it :) I don't know what 
sampling frequency (of what) you are reffering and the active line length 
(??)...Don't worry I'll search for this. This is new to me...


> Have you noticed that most 720x576 pictures have thin black bars at 
> the edges - these are the 9 extra pixels at each edge.

Arrrrrgh. You are totally right!  Last night I was encoding a couple of more 
videos and I noticed this at playback. I thought it was the border of my 
window manager!  

But now...........this braks one of the "best practices" for encoding.  Let me 
explain:  I understand what you mean..I should crop 9 pixels from each 
side...but what about this "rule" of "efficient encoding" that suggests that 
your dimensions should be a multiple of 16?  Check this:

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

For example, I encoded a couple of videos (original frame size: 720x576 PAL 
16:9) to 512x288.  These numbers give me a 16:9 footage with 'multiples of 
16' dimensions.  Now, If I crop them accordingly I'll get 494x288  which 
doesn't satisfies that rule.  This is an excerpt from the doc:

============== cut here ================
Even if your movie width and height are not multiples of 16, the encoder will 
use enough 16x16 macroblocks to cover the whole picture area, and the extra 
space will go to waste.
============== cut here ===============

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! )



> 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%
>
> Alternatively you should scale the 720x576 pixels up to 1050x576.
>
> The BBC has a good page on A Guide to Picture Size at.
> <http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml>

Thanks for the link!

And thanks for the interesting comments!

All the best,
Jorge