[XviD-devel] question about gmc

[Christoph Lampert]

On Thu, 6 Mar 2003, elcabesa wrote:

> is supposed that gmc make filesize gorwing? or this is the fact that it halp 
> few compressing and the bit overhead defeat saved bit?

No, of course GMC should _reduce_ filesize. But with GMC you need one
extra bit per MB (called mcsel) so if GMC is not very efficient, the 
overhead dominates the gain. 
 
> p.s. if overhead defeat saved bits, you can see that MOTION estimation is so 
> GOOD that can do very well his magic work even without gmc help

Yes. Or GMC algorithm is so bad that you lose more than you gain. 
And of course both is true. Current global motion estimation is based on
motion vectors from block search, so if ME fails, GMC might also fail, and
if ME is good, GMC gets better (but might not be needed anymore). 
I'll implement better GME one day, but not until April or later. 
 
> may 3 point gmc or something else help somewhere, or can it enabled only for 
> frame that really need it?

GMC can be switched on/off for every frame. In fact one just decides
whether to use P-VOP (no GMC) or S(GMC)-VOP (with GMC). At the moment, 
S(GMC) is chosen if and only if non-translational global motion is
detected. You can also decide whether to use GMC on a macroblock level,
this is just SAD based, so there is plenty of room for optmization.

gruel