[MPlayer-users] mencoder, file sizes

[Alexander Werth (hrzpub)]

Am Son, 2001-11-11 um 22.17 schrieb Dariusz Pietrzak:
> - when source is very good I get big gains on filesize, when it's ok there
> is no noticable gain, and when source movie quality is poor, then divx4
> gets bigger.
That's because of the way compression works. Mpeg, Mjpeg and most other
video codecs do achieve the compression by leaving out things that a
human eye can't notice on a moving picture but a computer or another
codec would notice for sure. In other words there will be some noise in
the compressed video. The more compression the more noise in general.

When recompressing a computer can't distinguish between noise and true
details like faces. So it has to encode the noise also which would need
much more bandwidth even when using a sophisticated codec.

Reencoding is in genaral a bad thing since each copy would have a worse
S/N (signal/noise) ratio. Usually the signal to noise ratio will always
get worse with each encoding process. How much the signal/noise ratio
will go down depends on the bitrate used and also the source material.
It's almost imposible to effectively compress noise. So when the master
already contained noise (in case of VCR source for example) you will
need very high bitrates when You want to minimise the loss in S/N ratio.

>  Is this supposed to be like this? Or am I doing sth stupid?
Therefore I would say it's perfectly normal and supposed to be that way.

> I haven't tried 2pass thing, but i suppose this would change things a lot.
2 pass encoding would just help if there are just a few bad encoded
scenes like a fight, a gunshot or quickly moving backgrounds. 2 pass
wouldn't help at all for bad source quality.

> Maybe I should apply some postprocessing/smoothing to the source movie and
> then try to compress it?
This way You would also eliminate some real details but You may be able
to reduce the bitrate slightly. The best solution of cause would be to
not recompress at all since there's no way You could get a decent
quality out of noisy material.

When grabbing a video with a videocard try a lossless codec like
huffmann and do all editing with this codec or hint files. Just the
final version should be convert to DivX.
When You don't need that much quality at least capture mpeg or mjpeg
with the highest posible bitrate to get a source with a good S/N ratio.
Happy editing,
Alexander

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