[MEncoder-users] Experiences of video encoding

[Nicolas George]

Le sextidi 6 germinal, an CCXV, Corey Hickey a écrit :
> For example, lets say you encoded a video with a constant quantizer of 
> 5. You look at mencoder's output (or do the math) and find out that the 
> average bitrate was 700 kbit/sec. If you then do a 2-pass encode with a 
> target bitrate of 700 kbit/sec, the second file will be very nearly the 
> same size as the first, but it should look much better because data is 
> allocated where it matters more. I say "should" because human perception 
> is subjective, but with any encoder that has reasonably well-tuned 
> ratecontrol, I expect the difference will look better to nearly everyone.

<snip the rest of the arguments>

That is something I wonder. There is, I believe, two factors to consider
separately:

- Quantizer does not mean quality. For one given source, the quantizer sets
  the quality, but for different sources, the mapping is not exactly the
  same. Therefore, fixed-quantizer encoding is just an ersatz of
  fixed-quality encoding. Indeed, I intend to test carefully the x264 crf
  option.

- Constant quality is not perfect, due to variable user attention: in high
  motion scenes, there is no time to look carefully at the details. But
  reality is much more complex: on a still scene, the spectator may be
  intently listening to the dialogs, or worse: reading the subtitles; on the
  other hand, a fast scene is a good candidate for slow-motion playback, to
  see all the details.

  Therefore, I have decided for myself not tu use the options that rely too
  much on subjective perception.

The point that determined my decision is that the bitrate is a big number,
and therefore highly arbitrary: why 700, why not 701? The quantizer, being a
small number, makes a more straightforward choice. Needles to say that I
like the -1 - 10 quality setting of the Vorbis codec very much.

Furthermore, if for some reason a movie requires more bitrate than another
to achieve the quality I want, I would like the codec to see it by itself.

That is why I will probably stick to the fixed-quantizer setting, or maybe
the fixed-quality setting if it is satisfactory.

Regards,

-- 
  Nicolas George
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