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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
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
s (or luma sample positions) as the motion vector precision for
motion estimation Motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent frames in a video sequence. It is an ill-posed problem as the motion is in three dimensions ...
and
motion compensation Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...
in
video compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compression ...
schemes. It is used in many modern video coding formats such as
MPEG-4 ASP MPEG-4 Part 2, MPEG-4 Visual (formally ISO/ IEC 14496-2) is a video compression format developed by the Moving Picture Experts Group (MPEG). It belongs to the MPEG-4 ISO/IEC standards. It uses block-wise motion compensation and a discrete cosine ...
, H.264/AVC, and
HEVC High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In compar ...
. 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

Video encoding software products such as
Xvid Xvid (formerly "XviD") is a video codec library following the MPEG-4 video coding standard, specifically MPEG-4 Part 2 Advanced Simple Profile (ASP). It uses ASP features such as b-frames, global and quarter pixel motion compensation, lumi mas ...
, 3ivx, and DivX Pro Codec, which are based upon the
MPEG-4 MPEG-4 is a group of international standards for the compression of digital audio and visual data, multimedia systems, and file storage formats. It was originally introduced in late 1998 as a group of audio and video coding formats and related tec ...
specification, use
motion estimation Motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent frames in a video sequence. It is an ill-posed problem as the motion is in three dimensions ...
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

Quarter-pixel
motion compensation Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...
, much like half-pixel, is achieved through
interpolation In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has a n ...
. Different specific schemes are used in different designs: *
VC-1 SMPTE 421, informally known as VC-1, is a video coding format. Most of it was initially developed as Microsoft's proprietary video format Windows Media Video 9 in 2003. With some enhancements including the development of a new Advanced Profile, ...
uses bicubic interpolation. * H.264/AVC uses a 6-tap filter for half-pixel interpolation and then simple
linear interpolation In mathematics, linear interpolation is a method of curve fitting using linear polynomials to construct new data points within the range of a discrete set of known data points. Linear interpolation between two known points If the two known poin ...
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

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 SMPTE 421, informally known as VC-1, is a video coding format. Most of it was initially developed as Microsoft's proprietary video format Windows Media Video 9 in 2003. With some enhancements including the development of a new Advanced Profile, ...
and
H.264 Advanced Video Coding (AVC), also referred to as H.264 or MPEG-4 Part 10, is a video compression standard based on block-oriented, motion-compensated coding. It is by far the most commonly used format for the recording, compression, and distr ...
, Qpel support in MPEG-4 ASP has become more common.


Video formats that support quarter-pixel motion compensation

{{Expand list, date=August 2008 *
HEVC High Efficiency Video Coding (HEVC), also known as H.265 and MPEG-H Part 2, is a video compression standard designed as part of the MPEG-H project as a successor to the widely used Advanced Video Coding (AVC, H.264, or MPEG-4 Part 10). In compar ...
* H.264/AVC *
MPEG-4 ASP MPEG-4 Part 2, MPEG-4 Visual (formally ISO/ IEC 14496-2) is a video compression format developed by the Moving Picture Experts Group (MPEG). It belongs to the MPEG-4 ISO/IEC standards. It uses block-wise motion compensation and a discrete cosine ...
*
VC-1 SMPTE 421, informally known as VC-1, is a video coding format. Most of it was initially developed as Microsoft's proprietary video format Windows Media Video 9 in 2003. With some enhancements including the development of a new Advanced Profile, ...
*
VP7 On2 Technologies, formerly known as The Duck Corporation, was a small publicly traded company (on the American Stock Exchange), founded in New York City in 1992 and headquartered in Clifton Park, New York, that designed video codec technology. It ...
Video codecs Video compression Compression algorithms