Andrea Giuliano <[EMAIL PROTECTED]> wrote:
> Seriously, it seems not to be so trivial as I thought. I discovered this very
> evening that "-w'-700000'" could be used with transcode to get a file of
> exactly 700.000 bytes, for which transcode determines the right bitrate, both
> with one or two pass encoding. Unfortunately, transcode does not tell me what
> actual bitrate nor bpp it used during encoding
Uh, the argument to -w *is* the target bitrate (only for the video part,
I think).
Later in the NOTES section of transcode, there is a little discussion
about the "bits per pixel" concept, with the following formula:
bitrate*1000
bpp = ----------------
width*height*fps
This formula is quite logical (definietely not empirical) and can be
used to calculate the bitrate you'll specify with -w for a given value
of the bpp parameter (bpp is a quality indicator; maybe not the best,
but easy to understand).
Then, if you want to respect a particular size constraint, you'll have
to adjust bpp, the target width and/or height (by means of scaling) so
that:
(8*bitrate/1000)*video_length + sizeof(audio) + container_overhead
equals your target size in bytes, where bitrate is in kilobits/s
(with kilo being 1000, not 1024), video_length is in seconds,
sizeof(audio) and container_overhead in bytes.
Concerning the container overhead, the MPlayer documentation says for
the AVI container (ยง 14.1.12.2):
For each chunk of the AVI file, 24 bytes are wasted on headers and
index. This translates into a little over 5 MB per hour, or 1-2.5%
overhead for a 700 MB movie.
HTH.
--
Florent
