[MPlayer-users] Distortion between different versions of MPlayer/Mencoder

[D Richard Felker III]

On Sat, Oct 04, 2003 at 09:41:42PM -0700, AthlonRob wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> On Sat, 2003-10-04 at 21:34, D Richard Felker III wrote:
> 
> > > And are there cases where block sizes of 8 are as good as 16?
> > 
> > For the most part, no. Chroma is subsampled, so an 8x8 chroma block
> > comprises 16x16 (for standard 4:2:0) of the full image dimensions.
> > Thus, while individual mpeg blocks are only 8x8, you need dimensions
> > that are multiples of 16 to keep from having incomplete chroma blocks.
> 
> I'm all confused.

This is all thoroughly explained in DOCS/tech/encoding-tips.txt.

> What you're saying is each dimension (width and height) needs to be
> evenly divisible by sixteen for the highest quality?

Yes. 

> But there is *not* a ratio (width:height) that needs to be maintained if
> you're cropping?

Yes, you also want to keep the right aspect ratio. So you find the
closest match to aspect that gives you multiples of 16. It's actually
pretty easy, since 640x480 (4:3), 640x352 (approx 16:9 film) and
640x272 (2.35:1 film) are all multiples of 16.

If you really care about *exact* aspect, then store the aspect in the
mpeg4 aspect tag. Then mplayer will play the movie back with perfect
aspect and other players will still be close enough.

Rich