Qpel
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Quarter pixel (also known as Q-pel or Qpel) refers to a quarter of a standard pixel. It is used in many modern video encoding standards such as MPEG-4 ASP and H.264/AVC to refer to quarter pixel precision in motion estimation and motion compensation. Though higher precision motion vectors take more bits to encode, they can result in more efficient compression, both through the decreased bit cost of the residual and the increased quality of the macroblock.
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[edit] Operation
Video encoding software products such as Xvid, 3ivx, and DivX Pro Codec which are based upon the MPEG-4 specification, use motion estimation algorithms to significantly improve video compression. The default level of resolution for motion estimation for most MPEG-4 ASP implementations is half a pixel, although quarter pixel is specified under the standard. H.264 specifies quarter pixel as default. Quarter-pixel resolution will improve the quality of the video as compared to half-pixel resolution, but the improvement is not always enough to offset the increased bit cost of the quarter-pixel-precision motion vector; additional techniques such as rate-distortion optimization, which takes both quality and bit cost into account, are used to significantly improve Qpel's effectiveness.
[edit] Interpolation methods
Quarter-pixel motion compensation, much like half-pixel, is achieved through interpolation. Some standards, such as VC-1, use bicubic interpolation; H.264/AVC uses a 6-tap filter for half-pixel interpolation and then a simple bilinear resize to achieve quarter-pixel precision from the half-pixel data. This allows encoders to calculate the halfpixel-interpolated frame before the encoding process, while the quarter-pixel data is calculated only when needed. Due to the simplicity of the bilinear filter the quarter-pixel interpolation requires little CPU time; the comparatively complex 6-tap interpolation has already been done.
[edit] Hardware compatibility (MPEG-4 ASP)
Videos encoded with quarter-pixel precision motion vectors require up to twice as much processing power to encode, and 30-60% more processing power to decode. As a result, to enable wider hardware compatibility, Qpel is disabled in the default DivX encoding profiles. However, with newer standalone players supporting more complex formats such as VC-1 and H.264, Qpel support in MPEG-4 ASP has become more common.
