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 graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...

s (or luma sample positions) as the

motion vector In computer vision and image processing, 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 prob ...

precision for

motion estimation In computer vision and image processing, motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent video frame, frames in a video sequence. It is an wel ...

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 ...

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 and H.264/AVC. 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,

3ivx 3ivx ( ) was an MPEG-4 compliant video codec suite, created by 3ivx Technologies, based in Sydney, Australia. 3ivx video codecs were released from 2001 to 2012, with releases of related technologies continuing until 2015. 3ivx provided plugins to ...

, 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 ...

specification, use

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

, much like half-pixel, is achieved through

interpolation In the mathematics, 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 ...

. 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 In mathematics, bicubic interpolation is an extension of cubic spline interpolation (a method of applying cubic interpolation to a data set) for interpolating data points on a two-dimensional regular grid. The interpolated surface (meaning the k ...

. * 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 po ...

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

and H.264, 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 co ...

* H.264/AVC * MPEG-4 ASP *

* VP7

References

Video codecs Video compression Compression algorithms