[XviD-devel] Xvid v's H.264/AVC

[Christoph Lampert]

Hi,

I believe that the algorithm how halfpel positions are calculated plays 
a significant role. 

If you switch XviD to Qpel mode, but disallow any true Qpel position (so 
only Hpel positions, but measured in Qpel units), then not only the number 
of motion bits gets larger compared to true Hpel (that's clear, because 
the vectors are twice as long), but also the texture bits increase. At 
least, that's the last numbers I had. 
That I can only explain by the fact that in Qpel mode, halfpel 
interpolation doesn't emulate MPEG2 whereas the Hpel-mode mode does. 

If you e.g. reencode an MPEG2 clip with exactly the same motion vectors 
and texture information as in MPEG2, once with Hpel only and once with 
Qpel, the Hpel version should be much closer to the original. 

chl

P.S. If anyone is interested in testing himself, there are publically (and 
legally) available movies in MPEG2 format (and other formats) at 
www.archive.org, e.g. 
http://www.archive.org/download/HOT_PRELINGER/HOT_PRELINGER_DVD_MPEG2.mpeg 
(90MB) but also shorter ones, so the others can reproduce the results 
without everyone having to buy a "Matrix" DVD. 


On Tue, 6 Jul 2004, Daniel Larkin wrote:
> Adding to the speculation, if thats ok :-), this would be my understanding,
> which I think is the same(??) as yours Christoph. Please correct me if I'm
> incorrect....
> 
> Is the reason for this problem (i.e. the lack of improvement of QPel for
> MPEG2 source) not simply because the source has undergone quantization and
> thus there are more homogenous pixels in the MPEG-2 decoded source compared
> to the YUV raw source and in this situation a follow up MPEG-4 encode the
> Qpel interpolation wont as "beneficial" because you are generating "poor"
> interpolated pixels (relative to the YUV source). These quarter position
> pels aren't going to be used because the shorter motion vector to full/half
> pels will be chosen??? Of course that assumption is based on a situation
> where you've effectively quantized away the quarter pel motion. **The
> mismatch in the two resolution ME doesn't contribute to the problem**
> 
> For example imagine for a moment a hypothetical codec where there wasn't
> anyway transform or quantization, simply ME and entropy coding. Regardless
> of whether this encoder used full, half, quarter or eighth pel ME you expect
> the decoder to regenerate a perfect reconstruction (relative to the original
> YUV sample) since after all the ME is merely providing a prediction to the
> entropy encoder. The difference from the ME Pel resolution effecting only
> the filesize produced. If you then re-encoded (again with this imaginary
> ME+entropy encoder) what appeared at the decoder of this imaginary codec,
> this time choosing a different pel resolution you'd still expect a perfect
> reconstruction when decoded. In this situation a mix-match of different
> Pel'ed motion estimation doesn't effect quality. Sooooooo doesn't this point
> to the fact any lack of performance benefit from MPEG2->MPEG4 using QPel is
> down to the lossy/Quantization process of MPEG-2 and there is *no
> performance degrade* coming from the mismatched half pel resolution in
> MPEG-2 versus Quarter Pel in MPEG-4 ASP.
> 
> 
> 
> 
> 
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