[XviD-devel] Global Motion Compensation

[Christoph Lampert]

On Fri, 25 Oct 2002, Christoph Lampert wrote:
> 
> There is no support for GMC syntax in XviD encoder (or ffmpeg encoder),
> yet. It would only be possible to use reference encoder as encoder and
> replace their GMC routine by yours, but that's uncomfortable, of course,
> because of completely other data structures ,etc...

hi,

I was just adding S(GMC)-VOP support to XviD encoder. At the moment
stationary only, next will be translational. After that the
formulas will get ugly... 

There is a new VLC table needed for the GMC coefficients and I don't
know how we generate them. Can somebody explain to me? For those with a
standard, it's Table B-33, page 357.




S(GMC)-VOPs are almost identical to P-VOPs, but they have two differences:

1) There is support for GMC. Every INTER or INTER_Q macroblock needs an
   extra 1 bit flag if motion compensation is done relative to local 
   motion (with vectors in bitstream) or relative to global motion 
   (no extra vectors then, but still DCT data). So if using
   S(GMC) instead of P but without GMC there is a 1 bit overhead per
   INTER-block. 

2) In combination with B-frames, there is one advantages: If the
   co-located block in backwards reference is NOT_CODED in a S(GMC)-VOP,
   the corresponding B-VOP block _are_ coded, not automatically NOT_CODED 
   as with P-VOP as backwards reference. This might be very slightly less
   efficient, too, but it surely make encoding B-frames easier. 

@Isibaar: I'll have to change some of the code you added for quarterpel,
because GMC needs VOL type 2, too, so only using m_quarterpel won't 
work anymore. 
Those was in BitstreamWriteVOLHeader (or what it's called...). Do you
remember any other changes in headers due to quarterpel? 

gruel