[MPlayer-dev-eng] Re: samples which look bad when encoded with lavc 2-pass needed

[Rémi Guyomarch]

On Mon, Jul 22, 2002 at 03:00:58PM +0200, Felix Buenemann wrote:
> On Monday 22 July 2002 10:33, Rémi Guyomarch wrote:
> 
> > I have an example of something which could be nearly perfectly encoded
> > in 790 kbits/s but unfortunately the ending credits which happens to
> > scroll up over a live scene take a significant percent of the average
> > bitrate [avg br goes from 735 (just before ending credits) to 790
> > (real ending of the movie)].
> 
> I personally think, that the ending credits deserve enough bitrate,
> because I hate it when they are encoded so low, that you can hardly
> read anything!

Enough but not too much. Unfortunately the rate control code often
detects this part as a very complex scene (which is true) and allocate
a ridiculous amount of bits to these last 2 or 3 minutes of the movie.

For example, here's a backup 'copy' of one of my DVD, "Death and the
Maiden". The ending credits starts at 1:36:18.

I encoded the video in 2 passes with these parameters :

VBITRATE="790"
VIDEOCOMMON="-ovc lavc -lavcopts vcodec=mpeg4:vbitrate=$VBITRATE:vhq:vqmin=2:vlelim=4:vcelim=7:v4mv:vqcomp=0.7"
VIDEO_P1="$VIDEOCOMMON:vqblur=0.3:vpass=1"
VIDEO_P2="$VIDEOCOMMON:vpass=2"

Without the ending credits, the average bitrate is :
(mencoder -nosound -avc copy -o /dev/null -endpos 1:36:18 movie.avi)

Video stream:  743.861 kbit/s  (92982 bps)  size: 536848416 bytes  5773.640 secs  144341 frames

The ending credits :
(mencoder -nosound -avc copy -o /dev/null -ss 1:36:18 movie.avi)

Video stream: 2418.164 kbit/s  (302270 bps)  size: 49584452 bytes  164.040 secs  4101 frames
              ^^^^^^^^^^^^^^^

In other words, the last 2.76% of the movie use 8.45% of the desired
output size. This is way too much.

In fact I think these 2min and 44 seconds of credits would probably be
perfectly readable at an average of 850-900 kbits/s ... (didn't tried)


PS: to those who wonder why I choose such a low bitrate, I wanted to
keep the 2 audio streams (en+fr), the subtitles and the video on one
700MB CD.

PS2: even at an average of 744 kbits/s, the video is amazingly good :)

-- 
Rémi