[MPlayer-users] Aspect Ratio in vfilters

[Mark Himsley]

On 13 March 2007 10:59 +0100 Alexander Roalter wrote:

> A full screen image delivers therefore a ratio of 1.25:1 (720/576). Now
> we all know if the video is non-anamorphci 1.33:1, the image gets
> stretched to 768x576, which is a factor of 1.06666, which yields an
> aspect ratio of 1.3333333.

Nope. Sorry, you're about 2% out there. Like so many other people you 
assume that all 720 pixels across the picture make up the 4:3 picture.

There are three things to take into account when correcting the aspect 
ratio of the picture.

1) the physical transmitted data (720x576)

2) the active content of the transmitted data (702x576 for PAL)

3) the aspect ratio of the active content (4:3)

So you are right that the *active* content of a 4:3 PAL picture should be 
scaled up to 768x576 but you have to either first crop to 702:576:9:0 
(which is not good because the sub-sampling of chroma means you need to 
crop the edges to even pixels) or scale the transmitted picture up to 
788x576.

Please see the following URL for the BBC's description.
<http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml>

Further references can be found the last time we had this discussion on 
this list (around Christmas 2006 if I recall).

-- 
Mark Himsley