[XviD-devel] some question about xvid and mpeg4

Christoph Lampert xvid-devel@xvid.org
Fri, 24 Jan 2003 16:37:09 +0100 (CET)


On Fri, 24 Jan 2003, Marco Al wrote:

> James Hauxwell wrote:
> 
> > changing macrobloc quant in darker or "flatter" area won't reduce
> > filesize??
> >
> > Yes it will, as you produce more zero value coefficients, so the VLC
> > does a better job = fewer bits.
> 
> But if you save less than 3 bits for a given MB you increase filesize of course,
> which isnt all that unlikely for motion compensated blocks.

Also, you might severely reduce _quality_ when not knowing exactly which
blocks to quantizer higher. Neither "flat" nor "dark" are always good
choices, because in a dark scene, small errors might be much better
visible due to Weber's law (especially on TVs), and in flat areas, the eye
is very sensitive to structure, e.g. blocking from too high quantization.

The topic of optimal choice of quantizers for every macroblock is very
difficult, and not very well understood.

gruel