Quarter-pixel motion
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Quarter-pixel motion (also known as Q-pel motion or Qpel motion) refers to using a quarter of the distance between pixels (or luma sample positions) as the motion vector precision for motion estimation and motion compensation in video compression schemes.[citation needed] It is used in many modern video coding formats such as MPEG-4 ASP and H.264/AVC.[1] Though higher precision motion vectors take more bits to encode, they can sometimes result in more efficient compression overall, by increasing the quality of the prediction signal.
Operation[edit]
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 decoders always support quarter-pixel motion. Quarter-pixel resolution can improve the quality of the video prediction signal as compared to half-pixel resolution, although the improvement may not always be 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 the effectiveness of quarter-pel motion estimation.
Interpolation methods[edit]
Quarter-pixel motion compensation, much like half-pixel, is achieved through interpolation. Different specific schemes are used in different designs:
- VC-1 uses bicubic interpolation.
- H.264/AVC uses a 6-tap filter for half-pixel interpolation and then simple linear interpolation to achieve quarter-pixel precision from the half-pixel data.
- HEVC uses separable 7-tap or 8-tap filtering.
Hardware compatibility in MPEG-4 ASP[edit]
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 stand-alone players supporting more complex formats such as VC-1 and H.264, Qpel support in MPEG-4 ASP has become more common.
Video formats that support quarter-pixel motion compensation[edit]
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References[edit]
- ^ Gupta, P. S.; Korada, Ramkishor (2004-01-01). "Novel algorithm to reduce the complexity of quarter-pixel motion estimation". Proceedings of the SPIE. 5308: 31–36. doi:10.1117/12.532336.