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

[Simen Thoresen]

 >>> 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.
 >>>
 >>> pp=hb/vb do deblocking (at least in theory: I was never satisfied with
 >>> them); they aren't needed if you don't have blocks in the original

 >>> 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.


 > you mean vqscale=2, not vb_strategy (as you corrected in your next
 > mail).

 > vqscale compresses with the lowest quantizer, hence with the highest
 > quality and size.

I've noticed that. Playing around a little, I've noticed that the vbitrate 
setting nolonger has any effect - when transcoding my 100M excerpt, I end up 
with a 50M file, regardless if vbitrate is 1200,1000 or 100.

As this gives me an effective Video-bitrate of 4076kb/s (as reported by 
mencoder), I'm not sure if this is the right way to go - yes, I want a good 
transcoding, but I don't want files this large.

 > If you want to deinterlace you can try kerndeint (with all its
 > parameters) or pp=md/lb/ci/li/fd.

I'm not sure what you mean by 'kerndeint' - I can't find any reference to 
this is in the manpage. The pp string works fine, tho.

 > In my tests kerndeint worked very well with clean (eventually denoised)
 > sources, very bad in other cases (where I prefer pp=lb or pp=md).

 >>> 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.
 >>


 > this means that this particular movie doesn't need postprocessing :)
 > in general you should apply filters only if they are needed, and
 > starting from the minimum, not from the maximum.


 >>> 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 .-)

 > vqscale=2 doesn't need 2 passes: it always means "best quality"

And also a very high bitrate.

I still wonder if it would have been possible to avoid the (few but jarring) 
problems I had with my original encode, without having to use vqscale=2.

-S

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