[MPlayer-users] cropping with mencoder

[D Richard Felker III]

On Sat, Feb 01, 2003 at 10:26:22PM +0200, Jan Knutar wrote:
> [Automatic answer: RTFM (read DOCS, FAQ), also read DOCS/bugreports.html]
> 
> > no, altering the frame implies reencoding, that is unavoidable.
> 
> I've always wondered if it wouldn't be possible to just throw away
> soem of the macroblocks, or do they depend on eachother? OTOH,
> I don't know much anything about mpeg4 ;-)

Yes, they depend on each other, for stuff like motion estimation and
coefficient prediction. You *might* be able to get away with leaving
just one slice of black border macroblocks at the top and bottom of
the picture and throwing away the rest, but it's still possible that
this could lead to visual problems, and it would be a nontrivial task
to fix up the mpeg4 stream to compensate for the changes. Also you
wouldn't save much disk space at all, since the fully black
macroblocks are just coded as "no change" as long as the encoding
software was smart.

In case you're wondering, this does NOT conflict with the rule that
encoding with black borders wastes tons of space. The way the space is
wasted is on macroblocks that span across the border (which you
couldn't throw away without messing up the stream badly!) and by
preventing motion estimation from working right at the edges of the
picture.

Rich