[MPlayer-users] Re: mencoder: ripping a dvd to a 700MB avi

[steffen]

>
> bugfood-ml at fatooh.org said:
> > (Approximately, of course). So, cropping the X axis to 366 pixels and
> > then scaling down to 309 would be the minimum ideal. The nearest
> > multiple of 8 above 309 is 312, which means either scaling straight from
> > 366 to 312 (the aspect ratio is _very slightly_ messed up) or not
> > cropping as much so the scaling works out (a bit more math involved). I
> > choose the latter.
> >
> >
> > 312 / 0.843 = 370
> >
> >
> > -which means that if I crop to 370 the scaling will make the X
> > resolution a multiple of 8. 366 and 370 differ by 4 pixels, so I leave
> > 2-pixel bands at the top and bottom. Thus, the scale/crop section looks
> > like this: -vop scale=720:312, crop=720:370:0:52
>
> When faced with this choice, I usually choose to _overcrop_ on the
> theory that I'll get better compression. The videocoder not only has
> fewer pixels to encode (because you've thrown away a narrow band at the
> top and the bottom), but it also doesn't have to spend bits on
> representing the transition from movie to black.  I'm under the
> impression that it's things like edges and detail that stress an
> encoder, so removing that black transition from the top and bottom of
> *every* frame can only help. And no one's going to notice a couple of
> pixels missing from the edge of the frame.
>
>
> Jason

Yes, every DVDrippin Board does tell this. I want to tell how I does 
understand it:

mpeg4 is optimized for natural pictures, like almost all videos are. So the 
hard transition to the black bars needs a high bandwith. Because you gave a 
bitrate to know how big the video should become, the codec has to throw 
informations away, and because the codec can't really think he does this in 
the whole frame -> the video is ugly.

The next one is the mod 8 /mod 16 thing ... 
mpeg4 works with macroblocks , that means it tiles the frame into a lot of 
smaller rectangles of 8x8 or 16x16 ( depends on the codec) ( fuck what is 
"quadrat" in english ??? ;))) ) so if you use a resolution that is mod 8 or 
mod 16 normally the quality should be better, and if you're lucky the 
encoding should be a bit faster. 

The next error on encoding video is to take big resolution for a good quality. 
If you have a lot of space you can take a big resolution and burn it at least 
on a dvd. ;) but if you want a good quality an a big film compressed on one 
cd you have to scale down. if you half the x an half the y resolution the 
video data is a quarter of the original an with the same bitrate the video 
looks much nicer. The scaling up while playing works very well since a lot of 
graphic-bords does help the cpu with it and smoothing the pixel will be 
better when try to ged rid of the blocky/blurried  picture. I have put a 
xvid-video of a length of 140 min on one cd an I think its quality is better 
when svcd on 2 cd's .

When there was the question why so many still use the divx3.11 or something 
equal.  I think it's a religous question. Since there are some really good 
Tools for this codec under win and they are optimized for it , they get the 
last out of it ( changing between fast and lowmotion on scene changing when 
it is needed for instance or optimize the stats from the first pass over the 
whole timeline because the encoder AFAIK see only a a more or less small 
window of the timeline to decide which bitrate ist to used ) they think the 
quality will be even better than the quality of encoders from nowadays. 

Hope it wasn't to long ;o) 

Greets

Steffen