[XviD-devel] H263 quant

[peter ross]

>	Hi,
>
>	there's something I find strange:
>	H263 dequantization is the same
>	for intra or non-intra coeffs:
>	( C' = C*2q + (q&1?q:q-1) )
>	But there's a different bias
>	used during *quantization*.
>	For intra, it's 0, and for
>	inter, it's -q/2. I've checked
>	MoMuSys, and it's the same.
>	Does someone has an explanation/pointer?
>	Something to do with the AC-values
>	distribution function?
>

both h263 and mpeg *quantization* algorithms were implemented
directly from the VM18 document. from memory (i dont have VM18
with me atm), the reason for this is to reduce rounding error caused
by // division. i guess we could try to impove the quantization algo,
such that the coeffs are more acurately decoded.

on a related matter, has anyone looked at "Encoder Complexity Reduction 
Based on Intelligent Pre-Quantisation" algorithm mentioned in the ISO
document? this describes a low-cost sad(sum) & threshold
calculation, which detects SKIP/NON-CODED blocks before performing
dct&quant. i believe was working on something simular.
-- pete


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