[XviD-devel] mpeg intra quantization

[Radek Czyz]

Hi again,

In fact, can someone explain to me just why we don't keep 
quant*matrx_entry in memory and use it to both quantize and dequantize? 
Seems to me it should be both more precise (rounding) and faster.

VHQ would like that.

Radek


Radek Czyz wrote:
> Hi everyone, long time no see :)
> 
> Alex_W on irc pointed out that xvid's intra quantization is weird, 
> mostly when compared to libav's. I went on to debug it, and it seems 
> that it's rounding coefficients *up*, creating huge frames at low psnr.
> 
> For example, if mpeg matrix entry is 32 at Q=1, a level of 1 is 
> quantized to 1:
> 
> level = ((level << 4) + (intra_matrix[i] >> 1)) / intra_matrix[i];
> level = ((level + quantd) * mult) >> SCALEBITS;
> 
> which is
> level = (1*16+16)/32 = 1;
> level = (1+1)*65537/(2*65536) = 1;
> 
> This is later dequantized to 4.
> 
> Bad comparison happens at different quants too, so this is not limited 
> to Q1 or anything like that.
> 
> In this simple example, changing quantd=0 gives:
> 18:30:11 : <Alex_W> xvid 1.1: 17772 KB 41.8 dB
> 18:30:11 : <Alex_W> xvid quantd = 0: 10813 KB 46.42 dB
> 
> Can someone (Skal?:) ) explain what was *meant* to be happening here?
> 
> :)
> Radek
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