[MPlayer-users] Two-passes encoding - What happens exactly?

[The Wanderer]

Alain Barthélemy wrote:

> Le mardi 01 juin 2004, 16:08:49 ou environ The Wanderer
> <inverseparadox at comcast.net> a écrit:
> 
>> Alain Barthélemy wrote:

>>> My question was not to know if we could change options between
>>> two passes but if we had to keep the same options between the
>>> first original TV-signal encoding and the two passes. You
>>> answered it at your first reply. The -lavcopts options had to be
>>> retyped.
>> 
>> ...I don't know if I quite follow this. Do you mean that you're
>> encoding with libavcodec from TV, and then using two-pass mode on
>> the result?
>> 
>> That's not as good as encoding to something lossless from TV and
>> then using two-pass mode on the result, but it will work. However,
>> it is *not* necessary to keep the same options between the initial
>> from-TV encoding and the two-pass encoding; it is only necessary to
>> keep all of the same options between the two passes.
> 
> Thanks but explain "lossless" (without loss?). When I started with
> MPlayer/Mencoder I understood that I had first to encode the
> TV-signal with libavcodec. In fact I was "adviced" to first encode
> with a high vbitrate and then to reduce the vbitrate (to obtain a
> 700Mb file) with a two-passes encoding. No "lossless" at that time. I
> did a subsequent two-passes encoding only when I had to crop the
> image or to reduce the vbitrate.

Okay. "Lossless" in this case means a lossless codec, i.e., one which
does not discard (lose) any of the original video information. Most
codecs discard some information, for compression reasons; as far as I
know (and we're reaching the limits of my understanding here), no
lossless video codecs compress very well. The advantage of a lossless
codec is that unlike with most codecs, you do not lose quality from
compression and decompression; the disadvantage is that the files can be
huge (think multiple gigabytes per hour).

If you don't have huge amounts of disk space to spare, then the advice
you were given is good; high bitrate loses less information than low
bitrate, and correspondingly loses less quality.

-- 
       The Wanderer

Warning: Simply because I argue an issue does not mean I agree with any
side of it.

A government exists to serve its citizens, not to control them.