Re[2]: [XviD-devel] xvid + bframes and mplayer/ffmpeg decoding

[peter ross]

>Actually I have no idea what quant=1 really does.

if you're refereing to the quantization level/argument = 1 then,

in h262:
when quant=1, we divide each coeff by two
when quant=2, we divide each coeff by four
when quant=31, we divide each coeff by 64

quants 1, 2 & 3 can cause large coefficients;
sometimes these coefficients are to big and must be clamped/saturated such 
that they can be written in the bitstream.

on some videos, the clamping is so significant that it's visible when
the video is decoded. i have seen this on the opening credits of one
movie. the credits scene displayed visible artifacts regardless of the
mpeg-4 compressor (xvid, msmpeg-4 or divx4). the affected block is
generally grey-colored, an only stays that color for the affected frame.

as dan stated a long time ago, it's an inherit problem of mpeg-4/dct
compression. i guess we can work arround the problem: maybe scale the
coeffs instead of clamping.

this its a very rare bug; low priority.

-- pete

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