Citation :
qpel Raise the number of candidate motion vectors by increasing the precision of the motion estimation from halfpel to quarterpel. The idea is to find better motion vectors which will in return reduce bitrate (hence increasing quality). However, motion vectors with quarterpel precision require a few extra bits to code, but the candidate vectors do not always give (much) better results. Quite often, the codec still spends bits on the extra precision, but little or no extra quality is gained in return. Unfortunately, there is no way to foresee the possible gains of qpel, so you need to actually encode with and without it to know for sure.
qpel can be almost double encoding time, and requires as much as 25% more processing power to decode. It is not supported by all standalone players.
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