[MPlayer-dev-eng] Error in docs? (Preserving aspect ratio)

[Jason Tackaberry]

http://mplayerhq.hu/DOCS/encoding.html rightly suggests to crop out
black bars in the original movie source when encoding.  It then goes on
to suggest that you should preserve the aspect ratio if you use
libavcodec.  Both excellent practices, which I follow.

But as I was reading these docs, I noticed a problem in the proposed
command line:

$ mencoder sample-svcd.mpg -ovc lavc -lavcopts vcodec=mpeg4:aspect=16.0/9.0 -vop crop=714:548:0:14 -oac copy -o output.avi

Now either there is a problem here, or I am misunderstanding something
and as a result have messed up all my encodings. :)

The problem, as I understand things, is that if you specify a crop
region, you must also adjust the aspect ratio for the new size.   The
cropped region above is no longer 16/9.

For a 16/9 DVD, the video source is usually 720x480 which gets scaled to
854x480.  So the correct aspect ratio for the above cropped video, where
the original is 16/9, is 714*(854/720)/548, or 1.545.  The libavcodec
option should then be aspect=1.545.

Although I think I have a handle on this, and I think the docs are
wrong, please do let me know if I'm misguided. :)

Cheers,
Jason.

-- 
Jason Tackaberry  ::  tack at auc.ca  :: 705-949-2301 x330 
Academic Computing Support Specialist
Information Technology Services
Algoma University College  ::  www.auc.ca