[MEncoder-users] 2pass: auto-calculation of bitrate?

[Ilya Zakharevich]

[A complimentary Cc of this posting was NOT [per weedlist] sent to
James Hastings-Trew 
<mencoder-users at mplayerhq.hu>], who wrote in article <4714C1BE.8050901 at shaw.ca>:

> A better approach than trying to find some arbitrary average value 
> across a number of unrelated movies which all have different compression 
> needs is to "sample" each movie and compress then with a constant 
> quality factor.
  ^^^^^^^^^^^^^^

What's this?   Quantizer?

> Encode 5% of a movie by compressing 10 scenes at regular 
> intervals throughout the movie with a constant quality factor, and then 
> use the size of the resulting file x 20 as an estimate of how big that 
> movie will likely be at that quality factor, and then derive your 
> bitrate based on that.

Why not do something much more direct?  Like a two-pass option - but
not with "target bitrate", but with "target average quantizer"
(whatever meaning is appropriate)?  Or maybe it is possible now?

BTW, a much simpler change: could not MEncoder emit such an "achieved
average quantizer" after a two-pass run with current semantic (of
"target bitrate")?  At least, it would help with chosing bitrate when
downscaling a lot (such as to a cellphone format).

> For example, consider two movies from the exact same genre - "Doom" and 
> "28 Days Later". A bitrate derived from that simple calculation above 
> would be way too high for "Doom" because that movie was done on quality 
> film stock, using locked down cameras (because of the special effects 
> work) and mostly takes place in dark environments (large areas of the 
> image are simply black for many scenes). That same bitrate would be way 
> too low for "28 Weeks Later" because high speed film was used in dark 
> scenes (very high film grain) , much of the action is hand-held camera 
> work (tons of motion vectors per frame), and most of the movie is fairly 
> well exposed (lots of detail, foreground and background throughout). I 
> can't see how your average value would fare any better. An average of 
> what each movie "needs" for bitrate would give you the same end result.

All true, but what is your point?  Do you think that using
bitrate=800000 is going to be better than the "averaging approach"?

One SHOULD have some starting point.  Currently MEncoder is silent
about which levels are good for what...

Thanks,
Ilya