[XviD-devel] Global Motion Compensation

[Radek 'sysKin' Czyz]

Hello,

As I've been thinking about GMC for a longer time, I decided to
implement something I call 'first step to GME search' - a procedure
which analyses normal block-based ME and looks for a camera pan in the
picture. I realize that many people worked on it before, but I decided
to be stupid enough _not_ to read their papers (never found any), but
think of something myself. More fun that way ;)

Some of you might say: we can't do block-based ME before GMC, and
then block-based ME again. The answer is: we already do - P/B/I
decision is exactly a very fast version of ME and it's results seem to
be more then enough.
For second pass, where we don't do P/B/I decison anymore (well we do
now, but I can't wait to have it fixed) - we can use first-pass hinted
ME data for this function and have the same effect. We win either way.

I'll tell you something about the results found: very good. I'm pretty
sure that the precision is +/- one pixel, which is exactly what we
need to follow the procedure with gradient-descent approach (I also
have some ideas about this part).
The procedure successfully found camera panning vectors as long as 100
pixels (fast-motion parts of 'Matrix' lobby scene) and even bigger,
when I decimated framerate by 2 (to pretend B-frames). Also, the
procedure doesn't act stupid and returns GMC vector of 0,0 if it
couldn't find any panning. It only returns some crappy results in case
of scene fades... But this is no surprise, is it.
Flying debris, smaller moving objects, small explosions (like gunfire)
don't seem to distract it either.
The function itself doesn't compute any SADs or other stuff, so it's
very fast (compared to, say, ME). I wonder how second step of GME
(diamond, subpel refinement) is gonna be slow.

OK, I just read the letter above and now I see how hurra-optimistic it
is. Just imagine how emberrased I'm gonna be if it's all crap...

Anyway, I can't do any real tests (finding correct camera panning is
one thing, GMC is another), because there is no GMC support in xvid at
all...
Come to think of it, I don't even know what is the precision of
this translation - halfpixels? Better? If better, do we need some
new super-slow filters like for qpel again?

Ok, I wrote too much already. I just hope it's going to be useful.

Radek