XviD: QPEL, GMC, BVOP and NVOP

Can anyone tell me what QPEL, GMC, BVOP and NVOP is? and what they do.

Qpel-Quarter pixel (often also called as q-pel or qpel) refers exactly to what it states, to a quarter of a standard pixel.

GMC-Global Motion Compensation: - This improves scenes where there is lots of motion, but at the time of writing, there is no support for this on standalones.

BVOP-These are the B-Frame (Bidirectional frames, which improves quality) settings. The default settings seem to work fine.

NVOP-Nvop is a frame that doesn't exist, they are used as place holders in avi-files so there is no way to turn them off.