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

[Nico Sabbi]

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.