GMC)Global Motion Compensation) is a function which enhance the encoding when zooming, panning or rotations occurs. It wont do much do the quality of the movie BUT it will greatly improve the compressionresult(smaller file). The thing is that standalone DVD-players often doesnt support this function.
When you encode a MPEG4-movie(DivX or Xvid) you use either half or quarter pixel motion-compensation. Quarter pixel motion-compensation has the advantage of delivering sharper images than default half pixel.(Better Quality in other words). This function is NOT recommended cause its even more unsupported than GMC by standalone DVD-players.
Motion Search Precision
A qualitysetting. Setting the motion search precision to the highest quality (slowest encodingtime) setting(6) will make the encoder search really hard for repetition between frames. This makes the picture a lot better when things are moving.
VHQ mode enables an even more accurate motion search. Setting it to more than level 1 usually only cripples speed but won't increase image quality.
If you are fastidious then you would set i to 4 which is max for that setting. But it would probably do with 1.
Another thing is "chroma motion".
Dont know if thats the chroma_me="1" in the DVD FAB-profile but it enhances the "Motion Search Precision" even more. You can compare it with if it was to be set at 7 instead of 6.
Thats that. I hope it helped a bit.