[FFmpeg-devel] [PATCH/RFC] set qscale_type for MPEG-4

[Michael Niedermayer]

On Wed, Feb 03, 2010 at 09:12:12PM +0100, Reimar D?ffinger wrote:
> On Wed, Feb 03, 2010 at 07:36:53PM +0100, Michael Niedermayer wrote:
> > On Wed, Feb 03, 2010 at 07:12:46PM +0100, Reimar D?ffinger wrote:
> > > On Wed, Feb 03, 2010 at 01:16:02AM +0100, Michael Niedermayer wrote:
> > > > On Tue, Feb 02, 2010 at 11:54:26PM +0100, Reimar D?ffinger wrote:
> > > > > Hello,
> > > > > I have no idea if this actually makes sense, but MPlayer's -vf fspp seems to
> > > > > look a lot better with it for me - no idea if MPEG-4 qscale is more similar
> > > > > to MPEG-2 or MPEG-1.
> > > > 
> > > > mpeg4 has 2 quantizer types, one matches h263 one mpeg2 with a factor of 2
> > > > difference in qscale
> > > 
> > > So in neither case the patch I made makes any sense?
> > > Or is the H.263 one somewhat close to MPEG-2?
> > 
> > why dont you read the code?
> 
> I do and did, but it only gives me guesses. Even guessing requires
> looking at code all over the place, and I sure don't intend to spend
> days on this.
> 
> > get_qscale()
> > mpeg1_decode_block_inter()
> 
> This reads to me like DC scaling (for standard matrices etc.) is in the
> range from
> 2*3*8/32 = 1.5
> to
> 62*3*8/32 = 46.5
> With the first value being the quantizer that will be stored in qscale_table
> 
> > mpeg2_decode_block_non_intra()
> 
> Looks the same to me, so obviously I am missing something

the  <<1 in get_qscale() makes them differ if you use something else than
lavc


> 
> > mpeg4_decode_block()
> 
> DC scaling 8 to 46 (corresponding to quantizers 1 to 31)
> 
> For VP56 it is DC scaling from 4*47 to 4*2 corresponding to
> quantizers 0 to 63.
> 
> > > > mpeg1 and 2 depend on the used quant matrixes as well, thus make tripple sure
> > > > your test files use the default matrixes and mpeg4 uses h263 style
> > > 
> > > The MPEG-4 used H.263 style, quantizer 15. And with fspp and spp the
> > > picture looked far worse than without while with the patch it gave
> > > a decent improvement (particularly any areas of large, identical colour).
> > 
> > also keep in mind that a encoder might behave different from what one
> > expects that is it might create blockier output than another at the same
> > quantizer.
> > i guess to figure out what is wrong you could just encode your own
> > video at constant quantizer and then test which constant with spp looks
> > best.
> 
> That's basically what I did with VP6, except that I couldn't create my own videos
> (but the quantizer was fairly constant anyway).

> However I now tried with our MPEG-4 encoder and it seems to look fine.

good then its possibly a encoder difference


> My attempt to try H.263 failed because H.263 in AVI results only in
> "[h263 @ 0x8a3b860]header damaged" when trying to play...

see our regression tests, there is a h263 avi IIRC

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have never wished to cater to the crowd; for what I know they do not
approve, and what they approve I do not know. -- Epicurus
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