[MPlayer-users] Aspect Ratio in vfilters

[Alexander Roalter]

Mark Himsley wrote:
> 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).
> 
I am well aware of the 702 pixels vs. 720 pixels issue on PAL (is the
same true for NTSC as well?)

The Patch I proposed not long ago, which uses d_width and d_height to
guess the video's aspect ratio and computes from it the real aspect
ratio (without any black bars => cinemascope flicks then have all about
2.35:1 or something like it) would work with both methods.

I agree it would be better to automatically scale 4:3-Content to 788x576
and 16:9-content to 1050x576 pixels, which would preserve the correct
aspect ratio. 1050x576 is an aspect ratio of 1.82:1 on screen-filling
anamorphic shows, and my factor would have to be 1.4583 instead of 1.422222.

Nonetheless finding out the aspect ratio of a given video with this
method still should work, as the presentation aspect ratio is also
considered. One still might want to crop the superfluous data on the
left and right, since a full screen image would appear full screen on
the TV, but since the presentation aspect is 1.37:1 on the movie, the
other 0.04:1 have to be cropped and therefore should not appear in the
calculation...

cheers,
Alex