[MPlayer-DOCS] [PATCH] XviD documentation reaching almost completeness

[Guillaume POIRIER]

Le mar 07/09/2004 à 17:03, Dominik 'Rathann' Mierzejewski a écrit :
> On Tuesday, 07 September 2004 at 14:40, Guillaume POIRIER wrote:
> Maybe "compensate for them", because "changes" were mentioned a few
> words before. Repeating the auxillary verb is also unnecessary. And
> I'd rephrase it even further to be completely clear:
> 
> This parameter controls the number of frames the CBR rate controller
> will wait before reacting to bitrate changes and compensating for them
> to obtain a constant bitrate over an averaging range of frames.

Ok, I replaced the description with yours.

> One more question: what's an "averaging range of frames"? Is it the
> number of frames specified in rc_averaging_period?

I think it is.


> > +Sometimes B frames do not look good, and introduce artifacts when
> > most of
> > +the frame is static and some small zones have high motion (in a static
> > +scene with a man talking, his mouth will probably look bad if what is
> > +surrounding the man and his mouth is completly static).
> > +This setting allows you to favorize or not, the use of B frames.
> > +The higher the value, the higher the probability of B frames being used.
> > +(default: 0)
> 
> As spotted by Wanderer, there's no such verb as favorize. Use favour (do
> we use British or American spelling?). And I'd say:
> 
> This setting controls the probability of B frame being used in such cases.
> The higher...

Corrected your way.


> > +.B frame_drop_ratio=<0\-100>
> > +XviD keeps track of block coding type (skipped, predicted, intra).
> > +The skipped block counter can be used to choose whether a frame is
> > so close
> > +to its reference that it can be
> > +.I completly
> > +skipped.
> > +This setting allows you to choose the (100 \- value) threshold for
> > skipping
> > +a frame.
> 
> This isn't clear to me. Please rephrase and make it clearer. What is this
> threshold and when is a frame skipped? What does it mean it's skipped? Is
> it simply not encoded or is reference frame used instead of it?

I'm really sorry here, but the only description I have about this
setting is this one, taken from xvid4conf. On XviD's unofficial FAQ,
they just say: don't touch it! ;-)
I guess I'd say that more than a threshold, it could be called as a
probability factor or a tolerance. The computer's eye looks at the
frame, and if for it, there's a high probability that the frame is the
same, it skips it.

On the updated patch, I replaced "threshold" by "probability". I hope it
clarifies.

> > +.TP
> > +.B curve_compression_high=<0\-100>
> > +This settigs control how much the upper part of the curve has to get
> > +closer to the average bitrate value.
> 
> control_s_
> 
> Better: This setting controls how close the upper part of the curve has to
> get to the average bitrate value.
> 
> But, docs on the net (for example http://nic.dnsalias.com/XviD_Options_Explained.pdf)
> say when you set it to 0, you get prefect VBR. Higher settings will take away
> specified percentage of bits from high motion scenes, while lower will give
> them additional bits. This pdf, however, describes VirtualDub's interface to XviD,
> so this may not be the same. So which is it?

The way I understand the usage of curve compression is that if you leave
it at 0, the RC tries it best to give appropriate bitrate depending on
the scene.
If you play with curve compression, you explicitly say something like "I
care about low motion scenes" (if you play with curve_compression_low )
or that you don't care too much about high motion scenes if you play
with curve_compression_high

Let me try another way to describe curve compression options (from XviD
unofficial FAQ):

.B curve_compression_high=<0\-100>
This setting allow XviD to take a certain percentage of bits away from
high bitrate scenes and give them back to the bit reservoir.
You could also use this if you have a clip with so many bits allocated
to high-bitrate scenes that the low(er)-bitrate scenes start to look bad
(default: 0).
.
.TP
.B curve_compression_low=<0\-100>
This setting allow XviD to give a certain percentage of extra bits to
the low bitrate scenes, taking a few bits from the entire clip.
This might come in handy if you have a few low-bitrate scenes that are
still blocky (default: 0).


I do think it's a LOT clearer now. The changes are in the new patch too.


> > +MPEG4 defines 5 standard Pixel Aspect Ratio and one extended
> > +one, giving the opportunity to specify a specific pixel aspect
> > +ratio.
> 
> You missed Diego's suggestion: 5 standard pixel aspect ratio_s_.

Fixed.


I'd like to thank you people a lot for reviewing my patches to improve
XviD's support. That means a lot to me to have this done now.

Regards,

Guillaume

-- 
L'homme n'est pas fait pour travailler, la preuve c'est que cela le
fatigue. -+- Voltaire -+-
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