Re: [tlug] ffmpeg HOWTO

[Attila Kinali <attila@example.com>]

On Wed, 18 Oct 2006 20:22:42 +0900
"Josh Glover" <jmglov@example.com> wrote:

> I stumbled across an excellent introduction to using ffmpeg for
> various audio/video conversion and manipulation tasks, called "Using
> ffmpeg to manipulate audio and video files":
> 
> http://howto-pages.org/ffmpeg/
> 
> Take a look!

I did and first thing i noticed is, that something
very important is not mentioned: restrictions on the
image size. Most of todays codecs are 8x8 block DCT
based with a 4:2:0 subsampling (use google if you
don't know what DCT and 4:2:0 mean, you have to know
if you want to encode something). Which means that
the number of pixel in both horizontal and vertical
direction have to be divisible by 16. If they are not,
you'll loose a lot of quality (or waste bits). According
to a measurement done by a friend a while back, even rescaling
in a way that does not keep aspect ratio exactly gives
a lower distortion than using sizes %16!=0
(Sorry, don't have the link at hand, but i know it's
either somewhere in the archives of mplayer-dev-eng
or ffmpeg-devel)

			Attila Kinali
-- 
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