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{{refimprove, date=September 2008 ''Global motion compensation'' ''(GMC)'' is a
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 ...
technique used 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 ...
to reduce the
bitrate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
required to encode
video Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
. It is most commonly used in
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 ...
, such as with the DivX and
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 ...
codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or da ...
s.


Operation

Global motion compensation describes the motion in a scene based on a single affine transform instruction. The reference frame is panned, rotated and zoomed in accordance to GMC warp points to create a prediction of how the following frame will look. Since this operation works on individual pixels (rather than blocks), it is capable of creating predictions that are not possible using block-based approaches. Each
macroblock The macroblock is a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform ...
in such a frame can be compensated using global motion (no further motion information is then signalled) or, alternatively, local motion (as if GMC were off). This choice, while costing an additional
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
per
macroblock The macroblock is a processing unit in image and video compression formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform ...
, can improve prediction quality and therefore reduce residual. Because the transforms used in global motion compensation are only added to the encoding stream when used, they do not have a constant bitrate overhead. A predicted frame which uses GMC is called an S-frame (sprite frame) while a predicted frame encoded without GMC is called either a
P-frame In the field of video compression a video frame is compressed using different algorithms with different advantages and disadvantages, centered mainly around amount of data compression. These different algorithms for video frames are called pict ...
, if it was predicted purely by previous (past) frames, or a
B-frame In the field of video compression a video frame is compressed using different algorithms with different advantages and disadvantages, centered mainly around amount of data compression. These different algorithms for video frames are called pi ...
if it was predicted jointly with past and future frames (an unpredicted frame encoded as a whole image is referred to as an
I-frame In the field of video compression a video frame is compressed using different algorithms with different advantages and disadvantages, centered mainly around amount of data compression. These different algorithms for video frames are called pict ...
).


Implementations

DivX Offers 1 warp-point GMC encoding: This enables easier hardware support in DivX certified and non-certified devices. But as 1 warp-point GMC limits the global transform to panning operation only (since panning can be described using blocks), this implementation rarely improves video quality.
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 ...
Offers 3 warp-point GMC encoding: As a result, it currently has no hardware support.


Criticism

GMC failed to meet expectations of dramatic improvements in motion compensation, and as a result it was omitted from the H.264/MPEG-4 AVC specification - designed as a successor to MPEG-4 ASP. Most of GMC's benefits could be obtained via better motion vector prediction.Lair Of The Multimedia Guru » 15 reasons why MPEG4 sucks
/ref> GMC also represents a large computational cost during encoding which yields relatively minor quality improvements. Due to the extra decoding CPU cost of global motion compensation, most hardware players do not support global motion compensation.


See also

* DivX *
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 ...
*
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 ...
*
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 ...


References

Video codecs MPEG Video compression de:Motion Compensation#Global Motion Compensation (GMC)