[XviD-devel] Questions on Quantization

[Skal]

	Hi Luca (nice to see you here;)

On Thu, 2005-03-24 at 12:22, Luca Piccarreta wrote:
> Hi Skal,
> Are you sure about what you said about Intra DCT Mismatch Control?
> I checked the MS ref sw (W4866) and it looks like mismatch control
> for intra dct is there (see inverseQuantizeIntraDCTcoefMPEG).
> Of course it's not there for H263 Quant Type
> (see inverseQuantizeDCTcoefH263).
> I'm also checking the latest 14496/2 I have available (N5546,March 2003)
> and I can't see anything about different mismatch control for Intra DCT.

	Oops, you're right! Xvid is lacking mismatch control
	for MPEG4 intra-dequant. And it's been so since a
	loooong time! (as long as i can recall, actually)

> Now, for my personal opinion, I think that the whole float DCT stuff is
> crazy, mismatch control is crazy, long live integer-defined transform and
> bit exact conformance :)

	Fully agreed. Carving in stone some fixed 16bits (or 8bits,
	even) coeffs for the iDCT in MPEG2/4 would have saved a lot
	of troubles, instead of opening the luring floating-point
	Pandora box.
 
> And of course the impact of not having mismatch control for Intra frames
> is almost null.

	Well, i'm not sure (until i test it). Correcting this mistake
	in XviD would be easy, but i fear it might slightly break
	decoding of previously encoded material: one might notice a
	faint chessboard effect on intra blocks, due to the toggling
	of coeff #63. Any opinion?


	later!
Skal