[Ffmpeg-devel] Resize on the fly

[Loren Merritt]

On Tue, 25 Oct 2005, Andy Parkins wrote:

> I'd like to be able to resize an mpeg4 stream on the fly.  It wouldn't be an
> arbitrary scale, just half, quarter, eighth, etc.  What I'd like to do is
> keep a master, high-resolution stream on disk, but be able to stream a
> smaller resolution without having to do a full decode-resize-encode cycle.
>
> I'm curious as to whether this is a crazy thing to even attempt?  For example,
> could one just throw away half the macroblocks, change some numbers in the
> header and get a useable stream?  The little I know about MPEG4 makes me
> think that the answer is "no", but I thought I should ask some people who
> know what they're talking about :-)

You can decode the original stream to a lower resolution, thus reducing 
decode time and eliminating the scale step, but not saving any bandwidth
(see -lavdopts lowres).
You can't create a new mpeg4 stream at reduced resolution (scaling 
or cropping) without a full re-encode.

--Loren Merritt