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

[Vladimir Mosgalin]

Hi Oleynik Phil!

 On 2005.06.25 at 15:32:04 +0400, Oleynik Phil wrote next:

> Once, I have compressed very slow scene-changing animation with many 
> details in scenes.
> When I looked at graph of I/K-frames, I noticed that about 30 percent
> of them are just because of timeout, not associated with real scene
> change. I set it to 500, file became smaller, some percents.

Of course the file will be smaller, but 500 p and b frames? That
might produce awful-looking results..

> >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.
> > 
> I usually set it to 50 when source is of high quality, and result should 
> keep the same level. On real video,
> or when keyframes are very frequent, your value is more sane.

Maybe.. on the other hand, when qp is mostly 2 and sometimes 3 or even 1
for i-frames, which is likely to be the case in high-bitrate cases, I
don't think it would do anything useful.

I'm using min_iquant=1:max_iquant=4 flags for similar reason.

IMHO xvid likes to produce a lot of keyframes, especially for animation,
where scenes change a lot.

> >>*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.
> >
> This is quality percent-divisor, 24 and 30 it is maybe for kfthreshold?

No, for kfreduction. Like I said, I don't see any reason to do this. I
don't see any effect from it too.

> >>*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.
> > 
> >
> It helps great, when camera moves right-left-right-left-right... Slowly 
> and with sense, i.e. interframes
> are to be seen clear.

For low bitrate encodings or it works for you always?

I never checked effects for specific scenes, but on the scale of full
movie, the effect was always at zero...

-- 

Vladimir