[XviD-devel] some question about xvid and mpeg4

[suxen_drol]

On Fri, 24 Jan 2003 13:59:57 +0100 "Marco \"elcabesa\" Belli" <elcabesa@inwind.it> wrote:

> hi
> i'm another time here to ask you some info aobut xvid and  mpeg4 standard
> 
> 1) What is GME and GMC, i can only guess Global motion camera. for gmc  and 
> Global motion estimation. if my guess is right how thyis motion is searched?   
> only by translation, zoom and  rotation of image , sometnigh more, something 
> less,or i'm totally out of the way??

"Global Motion" refers to the process of translating, zooming and
rotating the reference frame image, it make it corresond closer to the
current frame image.

GME and GMC are much like ME (motion estimation) and MC (compensation).


Motion estimation refers to finding the best reference macroblock for
the current macroblock.

Global motion estimation refers to finding the best global motion
parametes. The parameters specify the level of zoom and rotation.


Motion Compensation: We apply (adding) the reference macroblock (which
is found using the motion vector) to the current frame image.

Global Motion Compensation: The process applying (adding) the macrblock
as it appears in the global-motion-transformed-image to the current
frame image.


> 
> 2) qunatizer is a costant for each frame? i looked inside encoder.c and seems 
> to me that it's a value that chagne every frame. so each frame has ony one 
> quantizer.
> is possible or Mpeg4 compliant change quantizer during a frame?

the quantizer can be adjusted on a macroblocck basis, but only by valies
of -2,-1,0,+1,+2. xvid uses the this when lummasking/adaptive
quantization is enabled.

-- pete; life is like a box of ammo