[MPlayer-users] black squares in encoded film

[Loren Merritt]

On Wed, 12 May 2004, gaddo wrote:

> I am seeing black squares in my encoding from dvd with mencoder.
> (please see attacched image to see an example). The black squares
> should be white ...
>
> here is part of my encoding script ..
>
> LAVCOPTS='vcodec=mpeg4:vbitrate=1350:mbd=2:vqmin=1:lmin=1:v4mv:pbias=-16:cmp=2:subcmp=2:psnr:vmax_b_frames=1:acodec=ac3:abitrate=224:trell:cbp'
>
> mencoder -o test-p1.avi dvd://$TITLENUM -dvd-device $DVD -vf
> 'crop=720:424:0:76,scale=640:288' -ovc lavc -lavcopts
> $LAVCOPTS:'vpass=1' -oac lavc -channels 6 -alang it
> mencoder -o test-p2.avi dvd://$TITLENUM -dvd-device $DVD -vf
> 'crop=720:424:0:76,scale=640:288' -ovc lavc -lavcopts
> $LAVCOPTS:'vpass=2' -oac lavc -channels 6 -alang it
>
> i have tryed to isolate the minimus set of options and the minimum slice of
> film but is hard to do because the effect is somewhat related to the
> bitrate and the complexity of the images; I have seen the effect
> without ac3, cbp and lmin=1.
>
> in the first pass the images are _very_  blocky  but i can't  see the black
> squares.
<snip logs>

I saw something like that in one of my encodes.
The movie was mostly pretty low motion, but had a few very complex scenes.
When I used vbitrate=x on both passes, the first pass used quant 1 or 2
for most of the movie, but the complex scenes got quants around 10-31.
Then on the second pass, the low motion scenes got quants 3-4 and the
high motion got 1-2, with lots of those black blocks.

I solved it by running the first pass with vqscale=2. This produces better
quality than bitrate-based first pass in all cases I've seen, even when I
don't experience such extreme problems.
(And vqscale=2 works slightly better than vqscale=3, even if the second
pass results in average quant around 3 or 4)

--Loren Merritt