Qpel
Encyclopedia
Quarter pixel 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 overall, both through the decreased bit cost of the residual
and the increased quality of the macroblock
.
, 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.
, 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 simple bilinear
sampling to achieve quarter-pixel precision from the half-pixel data. This allows encoders to calculate the half-pixel-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.
and H.264, Qpel support in MPEG-4 ASP has become more common.
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. 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
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 but the images are a projection of the 3D scene onto a 2D...
and motion compensation
Motion compensation
Motion compensation is an algorithmic technique employed in the encoding of video data for video compression, for example in the generation of MPEG-2 files. Motion compensation describes a picture in terms of the transformation of a reference picture to the current picture. The reference picture...
. Though higher precision motion vectors take more bits to encode, they can result in more efficient compression overall, both through the decreased bit cost of the residual
Residual frame
In video compression algorithms a residual frame is formed by subtracting the reference frame from the desired frame. This difference is known as the error or residual frame...
and the increased quality of the macroblock
Macroblock
Macroblock is an image compression component and technique based on discrete cosine transform used on still images and video frames. Macroblocks are usually composed of two or more blocks of pixels. In the JPEG standard macroblocks are called MCU blocks....
.
Operation
Video encoding software products such as XvidXviD
Xvid is a video codec library following the MPEG-4 standard, specifically MPEG-4 Part 2 Advanced Simple Profile . It uses ASP features such as b-frames, global and quarter pixel motion compensation, lumi masking, trellis quantization, and H.263, MPEG and custom quantization matrices.Xvid is a...
, 3ivx
3ivx
3ivx is a video codec suite, created by 3ivx Technologies, based in Sydney, Australia, that allows the creation of MPEG-4 compliant data streams. It has been designed around a need for decreased processing power for use mainly in embedded systems. First versions were published in 2001...
, and DivX Pro Codec
DivX
DivX is a brand name of products created by DivX, Inc. , including the DivX Codec which has become popular due to its ability to compress lengthy video segments into small sizes while maintaining relatively high visual quality.There are two DivX codecs; the regular MPEG-4 Part 2 DivX codec and the...
, which are based upon the MPEG-4
MPEG-4
MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...
specification, use motion estimation
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 but the images are a projection of the 3D scene onto a 2D...
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.
Interpolation methods
Quarter-pixel motion compensationMotion compensation
Motion compensation is an algorithmic technique employed in the encoding of video data for video compression, for example in the generation of MPEG-2 files. Motion compensation describes a picture in terms of the transformation of a reference picture to the current picture. The reference picture...
, much like half-pixel, is achieved through interpolation
Interpolation
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points....
. Some standards, such as VC-1
VC-1
VC-1 is the informal name of the SMPTE 421M video codec standard, which was initially developed as a proprietary video format by Microsoft before it was released as a formal SMPTE standard video format on April 3, 2006...
, use bicubic interpolation
Bicubic interpolation
In mathematics, bicubic interpolation is an extension of cubic interpolation for interpolating data points on a two dimensional regular grid. The interpolated surface is smoother than corresponding surfaces obtained by bilinear interpolation or nearest-neighbor interpolation...
; H.264/AVC uses a 6-tap filter for half-pixel interpolation and then simple bilinear
Bilinear
Bilinear may refer to:* Bilinear sampling, a method in computer graphics for choosing the color of a texture* Bilinear form* Bilinear interpolation* Bilinear map, a type of mathematical function between vector spaces...
sampling to achieve quarter-pixel precision from the half-pixel data. This allows encoders to calculate the half-pixel-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.
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-1VC-1
VC-1 is the informal name of the SMPTE 421M video codec standard, which was initially developed as a proprietary video format by Microsoft before it was released as a formal SMPTE standard video format on April 3, 2006...
and H.264, Qpel support in MPEG-4 ASP has become more common.
Video formats that support Qpel
- H.264/AVC
- MPEG-4 ASP
- VC-1VC-1VC-1 is the informal name of the SMPTE 421M video codec standard, which was initially developed as a proprietary video format by Microsoft before it was released as a formal SMPTE standard video format on April 3, 2006...
- VP7VP7TrueMotion VP7 is a video codec developed by On2 Technologies as a successor to earlier efforts such as VP3, VP5 and TrueMotion VP6. It is a codec with both VFW and DirectShow support that On2 Technologies claims has better compression than leading competitive codecs such as MPEG-4 AVC and VC-1.-...