Motion compensation is a technique used by
video compressionVideo compression refers to reducing the quantity of data used to represent digital video images, and is a combination of spatial image compression and temporal motion compensation. Video compression is an example of the concept of source coding in Information theory...
. Motion compensation describes a picture in terms of the transformation of a reference picture to the current picture. The reference picture may be previous in time or even from the future. When images can be accurately synthesized from previously transmitted/stored images, the compression efficiency can be improved.
Motion compensation exploits the fact that, often, for many frames of a movie, the only difference between one frame and another is the result of either the camera moving or an object in the frame moving. In reference to a video file, this means much of the information that represents one frame will be the same as the information used in the next frame. Motion compensation takes advantage of this to provide a way to create frames of a movie from a reference frame. For example, in principle, if a movie is shot at 24 frames per second, motion compensation would allow the movie file to store the full information for every fourth frame. The only information stored for the frames in between would be the information needed to transform the previous frame into the next frame. If a frame of information is 1 MB in size, then uncompressed, one second of this film would be 24 MB in size. Applying motion compensation, the file size for one second of the film can often be reduced to 6 MB, for typical video material.
Illustrated example
Following is a simplistic illustrated explanation of how motion compensation works. Two successive frames were captured from the movie
Elephants DreamElephants Dream is a computer-generated short film that was produced almost completely using the free software 3D suite Blender . It premiered on March 24, 2006, after about 8 months of work...
. As can be seen from the images, the bottom (motion compensated) difference between two frames contains significantly less detail than the prior images, and thus compresses much better than the rest.
| Type |
Example Frame |
Size (byteA byte is a unit of information storage representing the smallest addressable element for a given computer architecture. It often designates a sequence of bits whose length is determined by the architecture...
s) |
Description |
| Original |
|
|
Full original frame, as shown on screen. |
| Difference |
|
|
Differences between the original frame and the next frame. |
| Motion compensated difference |
|
|
Differences between the original frame and the next frame, shifted right by 2 pixels. Shifting the frame compensates for the panning In photography, panning refers to the horizontal movement or rotation of a still or video camera, or the scanning of a subject horizontally on video or a display device...
of the camera, thus there is greater overlapOverlap may mean one of:* In music theory, overlap is a synonym for reinterpretation of a chord at the boundary of two musical phrases.* In railway signalling, an overlap is the length of track beyond a stop signal that is proved to be clear of vehicles in the controls of the previous signal, as a...
between the two frames. |
Motion Compensation in MPEG
In MPEG, images are predicted from previous frames (P frames) or bidirectionally from previous and future frames (B frames). B frames are not so popular because the image sequence must be transmitted/stored out of order so that the future frame is available to generate the B frames.
After predicting frames using motion compensation, the coder finds the error (residual) which is
then compressed using the
DCTA discrete cosine transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies...
and transmitted.
Global motion compensation
In
global motion compensationGlobal motion compensation is a technique used in video compression to reduce the bitrate required to encode video. It is most commonly used in MPEG-4 ASP, such as with the DivX and Xvid codecs.-Operation:...
, the motion model basically reflects camera motions such as dolly (forward, backwards), track (left, right), boom (up, down), pan (left, right), tilt (up, down) and roll (along the view axis).
It works best for still scenes without moving objects.
There are several advantages of global motion compensation:
- It models the dominant motion usually found in video sequences with just a few parameters. The share in bit-rate of these parameters is negligible.
- It does not partition the frames. This avoids artifacts at partition borders.
- A straight line (in the time direction) of pixels with equal spatial positions in the frame corresponds to a continuously moving point in the real scene. Other MC schemes introduce discontinuities in the time direction.
MPEG-4 ASP supports GMC with three reference points, although some implementations can only make use of one. A single reference point only allows for translational motion which for its relatively large performance cost provides little advantage over block based motion compensation.
Moving objects within a frame are not sufficiently represented by global motion compensation.
Thus, local motion estimation is also needed.
Block motion compensation
In
block motion compensation (BMC), the frames are partitioned in blocks of pixels (e.g. macroblocks of 16×16 pixels in MPEG).
Each block is predicted from a block of equal size in the reference frame.
The blocks are not transformed in any way apart from being shifted to the position of the predicted block.
This shift is represented by a
motion vector.
To exploit the redundancy between neighboring block vectors, (e.g. for a single moving object covered by multiple blocks) it is common to encode only the difference between the current and previous motion vector in the bit-stream. The result of this differencing process is mathematically equivalent to a global motion compensation capable of panning.
Further down the encoding pipeline, an
entropy coderIn information theory an entropy encoding is a lossless data compression scheme that is independent of the specific characteristics of the medium....
will take advantage of the resulting statistical distribution of the motion vectors around the zero vector to reduce the output size.
It is possible to shift a block by a non-integer number of pixels, which is called
sub-pixel precision.
The in-between pixels are generated by interpolating neighboring pixels. Commonly, half-pixel or quarter pixel precision (
QpelQuarter 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...
, used by H.264 and MPEG-4/ASP) is used. The computational expense of sub-pixel precision is much higher due to the extra processing required for interpolation and on the encoder side, a much greater number of potential source blocks to be evaluated.
The main disadvantage of block motion compensation is that it introduces discontinuities at the block borders (blocking artifacts).
These artifacts appear in the form of sharp horizontal and vertical edges which are easily spotted by the human eye and produce ringing effects (large coefficients in high frequency sub-bands) in the
Fourier-related transform used for
transform codingTransform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically lossy, resulting in a lower quality copy of the original input....
of the
residual frameIn 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...
s.
Block motion compensation divides up the
current frame into non-overlapping blocks, and the motion compensation vector tells where those blocks come
from
(a common misconception is that the
previous frame is divided up into non-overlapping blocks, and the motion compensation vectors tell where those blocks move
to).
The source blocks typically overlap in the source frame.
Some video compression algorithms assemble the current frame out of pieces of several different previously-transmitted frames.
Frames can also be predicted from future frames.
The future frames then need to be encoded before the predicted frames and thus, the encoding order does not necessarily match the real frame order.
Such frames are usually predicted from two directions, i.e. from the I- or P-frames that immediately precede or follow the predicted frame.
These bidirectionally predicted frames are called
B-frames.
A coding scheme could, for instance, be IBBPBBPBBPBB.
Variable block-size motion compensation
Variable block-size motion compensation (VBSMC) is the use of BMC with the ability for the encoder to dynamically select the size of the blocks. When coding video, the use of larger blocks can reduce the number of bits needed to represent the motion vectors, while the use of smaller blocks can result in a smaller amount of prediction residual information to encode. Older designs such as
H.261H.261 is a 1990 ITU-T video coding standard originally designed for transmission over ISDN lines on which data rates are multiples of 64 kbit/s. It is one member of the H.26x family of video coding standards in the domain of the ITU-T Video Coding Experts Group .The coding algorithm was designed...
and
MPEG-1MPEG-1 is a standard for lossy compression of video and audio. It is designed to compress VHS-quality raw digital video and CD audio down to 1.5 Mbit/s without excessive quality loss, making Video CDs, digital cable/satellite TV and digital audio broadcasting possible.Today, MPEG-1 has become...
video typically use a fixed block size, while newer ones such as
H.263H.263 is a video codec standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263 has...
,
MPEG-4 Part 2MPEG-4 Part 2 is a video compression technology developed by MPEG. It belongs to the MPEG-4 ISO/IEC standard . It is a discrete cosine transform compression standard, similar to previous standards such as MPEG-1 and MPEG-2...
,
H.264/MPEG-4 AVCH.264/MPEG-4 AVC is a standard for video compression. The final drafting work on the first version of the standard was completed in May 2003.H.264/AVC is the latest block-oriented motion-compensation-based codec standard developed by the ITU-T Video Coding Experts Group together with the ISO/IEC...
, and
VC-1VC-1 is the informal name of the SMPTE 421M video codec standard initially developed by Microsoft. It was released on April 3, 2006 by SMPTE. It is now a supported standard for HD DVDs, Blu-ray Discs, and Windows Media Video 9.-Format:...
give the encoder the ability to dynamically choose what block size will be used to represent the motion.
Overlapped block motion compensation
Overlapped block motion compensation (OBMC) is a good solution to these problems because it not only increases prediction accuracy but also avoids blocking artifacts. When using OBMC,
blocks are typically twice as big in each dimension and overlap quadrant-wise with all 8 neighbouring blocks.
Thus, each pixel belongs to 4 blocks. In such a scheme, there are 4 predictions for each pixel which are summed up to a weighted mean.
For this purpose, blocks are associated with a window function that has the property that the sum of 4 overlapped windows is equal to 1 everywhere.
Studies of methods for reducing the complexity of OBMC have shown that the contribution to the window function is smallest for the diagonally-adjacent block. Reducing the weight for this contribution to zero and increasing the other weights by an equal amount leads to a substantial reduction in complexity without a large penalty in quality. In such a scheme, each pixel then belongs to 3 blocks rather than 4, and rather than using 8 neighboring blocks, only 4 are used for each block to be compensated. Such a scheme is found in the
H.263H.263 is a video codec standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263 has...
Annex F Advanced Prediction mode
Quarter Pixel (QPel) and Half Pixel motion compensation
In Motion Compensation, quarter or half samples are actually interpolated sub-samples caused by fractional motion vectors. Based on the vectors and full-samples, the sub-samples can be calculated by using bicubic or bilinear 2-D filtering. See subclause 8.4.2.2 "Fractional sample interpolation process" of the H.264 standard.
3D Image Coding Techniques
Motion compensation is utilized in
Stereoscopic Video Coding3D Video Coding
3D Video Coding is one of the stages to enable the deployment of stereoscopic content to the home: 3D Television.
There are a few initiatives, techniques and methods to achieve stereoscopic deployment:
...
In video,
time is often considered as the third dimension. Still image coding techniques can be expanded to an extra dimension.
JPEG2000 uses wavelets, and these can also be used to encode motion without gaps between blocks in an adaptive way. Fractional pixel
affine transformationIn geometry, an affine transformation or affine map or an affinity between two vector spaces consists of a linear transformation followed by a translation:...
s lead to bleeding between adjacent pixels. If no higher internal resolution is used the delta images mostly fight against the image smearing out. The delta image can also be encoded as wavelets, so that the borders of the adaptive blocks match.
2D+Delta Encoding techniques utilize H.264 and
MPEG-2MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...
compatible coding and can use motion compensation to compress between stereoscopic images.
Expanding the 8x8
JPEGIn computing, JPEG is a commonly used method of compression for photographic images. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality...
blocks into the third dimension that is into 8x8x8 cubes and modifying the DCT more into a
DFTIn mathematics, the discrete Fourier transform is a specific kind of Fourier transform, used in Fourier analysis. It transforms one function into another, which is called the frequency domain representation, or simply the DFT, of the original function...
enables compression of linear translations with speeds below and around one pixel per frame (sub-pixel precision).
External links
Applications
- video compression
Video compression refers to reducing the quantity of data used to represent digital video images, and is a combination of spatial image compression and temporal motion compensation. Video compression is an example of the concept of source coding in Information theory...
- change of framerate for playback of 24 frames per second movies on 60 Hz LCDs or 100 Hz interlaced cathode ray tubes
External links