[MPlayer-users] Understand cropping

[Jason Tackaberry]

On Wed, 2006-11-22 at 22:39 -0500, T wrote:
> If I gave -vf crop=704:304 to mplayer/mencoder, it will actually use
> crop=704:304:8:88 instead of crop=704:304:4:90, correct? Because that's
> what I calculate as the actual center, since:
> 
>  (720-704)/2 = 8
>  (480-304)/2 = 88

Sounds right.


> If so, why cropdetect suggests crop=704:304:4:90?

Why would you assume cropdetect would return a region centered within
the original frame?  cropdetect sees that the first pixels above the
threshold occur at x=4,y=90.  If it had returned the values you expect
(704:304:8:88), the top would contain 2 black or half-black pixels, and
you might also have 4 black or half-black pixels on the right edge.
(You might also be ok on the right edge, as cropdetect rounds the
dimensions to the lowest multiple of 16.)

Cheers,
Jason.