Quarter pixel (often also called as q-pel or qpel) refers to a quarter of a standard pixel. It is an often-used term in video technology.

Operation

Advanced video encoding methods such as those based upon MPEG-4 specifications, for example XviD, 3ivx, and DivX, use motion estimation algorithms to significantly improve video compression. The default level of resolution for motion estimation for most MPEG-4 implementations is half a pixel, although quarter pixel is specified under the standard. H.264/MPEG-4_AVC specifies quarter pixel as default.

Implementations

Gains in compressibility from more precisely describing intra-frame motion, are typically offset by the increased constant bitstream overhead of running a higher level of description. Therefore, enabling QPEL does not typically produce smaller file sizes. However, since QPEL delivers sharper images, an equivalent sized QPEL enabled file, will generally be perceived as offering higher visual quality, than a non QPEL file. To this extent, while QPEL files may not be smaller, they can be said to offer a higher level of visual detail compression.

Hardware compatibility

QPEL requires up to twice as much processing power to encode, and 30-60% more processing power to decode. For this reason, and to enable wider hardware compatibility, QPEL is disabled in the default DIVX encoding profiles.