[theora] No quality advantage with two-pass?

[David Kuehling]

sorry for the other reply.  looking at the earlier replies so far, that
was just a dupe.

>>>>> "Remco" == Remco  <remco47 at gmail.com> writes:
> Oh, I think I get it now:

> * Single-pass encoding with a constant file size will reduce quality
> for harder parts.

The problem is: there is actually no such thing as encoding for
'constant file size'.  there's only encoding for 'constant bit rate',
which will always be bad for scenes requiring higher bit rates than
possible.  Even 'constant bit rate' is not completely constant,
fluctuations are allowed.  However these fluctuations have to average
out within a determined time span (such as 3 seconds).  This is set by
ffmpeg2theora's --buf-delay parameter.

> * Two-pass encoding with a constant file size will use the first pass
> to somehow learn which quality setting will produce the target file
> size. 

You can think of this as doing encoding with a --buf-delay set to the
length of the full movie :) however, don't try that, it won't work due
to limited amounts of RAM and most likely limitations in the encoder.
So the encoding is done in two passes, to work even without having all
the movie in RAM, but taking twice as long.

just my $.02

cheers,

David
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