[MPlayer-users] Re: spurious blockiness when transcoding high-bitrate mpeg2 to med-bitrate mpeg4

[Simen Thoresen]

Hrrrg. I did of course mean vqscale=2, not vb_strategy=2.

Sigh.

-S

Simen Thoresen wrote:

>  > Il sab, 2004-07-03 alle 12:21, Simen Thoresen ha scritto:
>  >> Hi team,
>  >>
>  >> I've made a 16Mpbs mpeg2 grab of a 640x480 29.9fps Laserdisk, and now
>  >> want to transcode the 6G 'perfect' mpeg2 file to a more managable mpeg4
>  >> file for
>  >> sotrage. I'm transcoding using mencoder and lavc mpeg4, and the 
> result is
>  >> imho very good (altho suggestions for improvement are appreciated). 
> What
>  >> bugs me, is that on a few points in the mpeg4 stream, I get sudden 
> bursts
>  >> of artifacts for a half a second or so. These do not coincide with
>  >> obvious damages in the original mpeg2-stream, nor do they occur in 
> scenes
>  >> that are markedly different than what is being encoded without issue.
>  >>
>  >> So I'm wondering - what is causing this, and how do I avoid it?
>  >>
>  >> A few details more - re-encoding an excerpt of the original file with
>  >> generous lead-in and lead-out show the blockiness appearing differently
>  >> than from the original encode. Different excerpts cause differences in
>  >> the apparent blockiness, altho they always happen in the same scenes.
>  >> Re-encoding the same excerpts with only single-pass (default) encoding
>  >> also causes the blockiness to appear visually different, as does 
> encoding
>  >> it with different (default) lavc:keyint settings.
>  >>
>  >> I'm using MEncoder 1.0pre3, built with gcc 2.95.3 on an Athlon XP box.
>  >>
>  >> The mencoder line I intended using, was
>  >>
>  >> mencoder partial.mpg -o partial.avi -mc 0 -ovc lavc -lavcopts
>  >> vcodec=mpeg4:vbitrate=1200:mbd=2:v4mv:keyint=132:vb_strategy=1:vpass=1
>  >> -oac mp3lame -lameopts preset=192 -vf pp=hb/vb/dr/al/lb
>  >>
>  >> mencoder partial.mpg -o partial.avi -mc 0 -ovc lavc -lavcopts
>  >> vcodec=mpeg4:vbitrate=1200:mbd=2:v4mv:keyint=132:vb_strategy=1:vpass=2
>  >> -oac
>  >> mp3lame -lameopts preset=192 -vf pp=hb/vb/dr/al/lb
>  >>
>  >> I can upload a 33M mpeg2 source-excerpt and a 4M mpeg4 transcoding 
> of the
>  >> same excerpt if anyone wants to take a look.
>  >>
>  >> Anyone?
>  >>
>  >> Yours,
>  >> -S
>  >
>  > why all that post-processing? if the source is perfect you don't need
>  > any.
>  > Try to add a tiny amount of noise, sometimes it helps to remove blocks.
>  > Try also to encode with
>  > vcodec=mpeg4:vbitrate=1200:mbd=2:keyint=132:vqscale=2, and see if blocks
>  > are still there.
> 
> 
> Hi Nico,
> 
> My grab is interlaced - I don't see this when I play it using tv-out, 
> but I would rather have my archived file be computer-playable as well. 
> Appart from deinterlacing, I have not given much thoughts to the 
> filters. I guess hb, vb and dr are useful for preventing the 
> mpeg2-encoding-artifacts (even if not noticeable) to be reencoded into 
> mpeg4.
> 
> I tried to add vb_strategy=2 as you suggested, and the problem got less 
> noticeable, but did not disappear in my excerpt before I also removed 
> the dr and al filter (which did cause a few of the scenes to seem a bit 
> overexposed).
> 
> Strangely, adding vb_strategy=2 and removing all filters increased the 
> size of the transcode from 16M to 75M, while readding the filters 
> reduced it to 40M, and then removing the dr and al filters reduced it 
> further to 32M. I fear I'm messing something up, so I'll test this a bit 
> more. My most recent transcode was with the hb,vb and lb filters, and it 
> did not show any of the blocking I complained about.
> 
> I'll try 2-pass transcoding the whole thing again with reduced filters 
> and vb_strategy=2 added. Thank you for your help with this .-)
> 
> (I appologise if this post breaks any of the mail threading, I only 
> subscribe to the digests, and found this mail in the archive)
> 
> Yours,
> -S
> 

-- 
Simen Thoresen, Wulfkit Support, Dolphin ICS
http://www.tysland.com/~simentt/cluster