[MEncoder-users] Backup policies

[Loren Merritt]

On Wed, 28 Nov 2007, Peter Cordes wrote:

> One of them actually has a justification that sounded reasonable to me (not
> knowing exactly how x264 works, but having some idea of video compression in
> general):
>
> dct_decimate throws away some detail and it's better to let trellis make
> all the decisions about what to throw away.
>
> I can't find that doc with google, even though I think I did stumble across
> it again recently.
>
> So you're saying that the dct blocks dct_decimate throws away would not
> always get thrown away by trellis when it would be Better to do so.  (for
> some definition of better, such as PSNR-optimal.)

The trellis option itself disables the part of dct_decimate that trellis 
can do better. Then the dct_decimate option only controls the part of 
decimation that trellis can't do, i.e. decimating a whole macroblock if 
all of its dct blocks are weak (because trellis can't see the whole 
macroblock, it applies only to one dct block at a time).

Now, there are reasons one might want to disable decimation, mostly 
involving crazy people who want to preserve noise rather than signal. But 
that's unrelated to trellis.

> Err, I've also seen it said in several places that trellis is not a good
> idea with crf rate control.  e.g. in this doc from june 2006:
> http://www.digital-digest.com/articles/x264_options_page6.html
> Is this true now, if it ever was?

That was never true. The reasoning that started the rumor is:
"CRF is constant quality, so we can try various options and whichever 
gives the lowest bitrate is the best. Trellis sometimes increases bitrate 
at a given CRF, so trellis doesn't work with CRF."
The error in that reasoning is that CRF is only approximately constant 
quality. If trellis increases the bitrate, it also increases quality, so 
the quality-per-bitrate still improves. This is not specific to trellis: 
just about all x264 options affect both quality and bitrate, even with 
"constant quality" ratecontrol. It's just that most of the options 
increase quality and reduce bitrate, while a few options increase or 
decrease both, or even have different effects on different content.

--Loren Merritt