[MEncoder-users] (work-in-progress) XviD encoding guide

[Vladimir Mosgalin]

Hi Phil Oleynik!

 On 2005.06.25 at 01:06:26 +0400, Phil Oleynik wrote next:

> *max_key_interval
> Usually, no one will need it. However, in some cases scene changes
> very rarely.  To save details sharpness one may try to lower its value
> from default.

It doesn't do anything useful.

> *keyframe_boost
> If you encode cartoon/anime, you will need it to get sharper edges
> with the same bitrate.

You might add that recommended value is around 15, 10-20 for me.

It doesn't really hurt on real life video, but doesn't help too. Great
for animation, yes.

> *kfreduction
> No human is able do distinguish how bad rapid flashing frames were
> compressed, but it is hard task for codec. With this option only the
> last frame from series of flashes will get full bitrate.

I don't really understand why, but a lot of people advice to use 24 for
23.976/24/25 fps video and 30 (default) for 29.97/30 fps video.

> *(no)packed
> One of best XviD features, frame swapping. If this is turned on, codec
> will try to swap frames in order to get more quality. Price here is
> DivX compatibility.
> *(no)closed_gop
> This option also may break compatibility. However, it may save a few bits.
> 

One of this options (or both, don't remember) actually break lavc
compatibility, it will display file with errors (maybe fixed in newer
versions, but..). They may reduce file a bit, but it doesn't worth it.

> *qpel
> Option, that should be used for high quality streams, it may do good,
> or may do bad to your movie. Highly dependent from source material.

Maybe it was me, but I never found cases where it was useful - not with
xvid, nor with lavc. The quality is always lower or at least the same.

> *gmc
> Also, highly dependent from source material. If you have many
> panoramic scenes, this will save a great amount of bits using XviD
> pan/zoom compressing features. This will make your movie decode slower
> (640*480 24fps pan scene may become too hard for Celeron III 800, my
> own exp.)

Again, I tested it on a lot of material, it never helped.. though it
doesn't usually waste a lot of bitrate too, so the picture stays about
the same.

But maybe it would help in very low bitrate cases, I never tested this.

> *chroma_opt
> Yes, it works. It loses quality numerically, but eye is more pleased
> by resulting picture. My eye, at least.
> 
> *hq_ac
> Unfortunately cannot say anything how it works, but it makes picture better,
> perceptually. This option must be further investigated, I do not know 
> how it works.

Both options are enabled by default, btw.

-- 

Vladimir