Global motion compensation (GMC) is a technique used in video compression to reduce the bitrate

Bitrate

In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....

 required to encode video

Video

Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...

. It is most commonly used in MPEG-4 ASP, such as with the DivX

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...

 and Xvid

XviD

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...

 codec

Codec

A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...

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

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....

 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

Bit

A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

 per 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....

, can improve prediction quality and therefore reduce 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...

.

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, if it was predicted purely by previous (past) frames, or a B-frame 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).

Implementations

DivX

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...

 has only one GMC warp point specified. This enables easier hardware implementation, but limits the global transform to panning operation only. Since panning can be described using blocks, this implementation rarely improves video quality.

Xvid

XviD

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...

 allows up to 3 warp points, and as a result, 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

H.264/MPEG-4 AVC

H.264/MPEG-4 Part 10 or AVC is a standard for video compression, and is currently one of the most commonly used formats for the recording, compression, and distribution of high definition video...

specification - designed as a successor to MPEG-4 ASP. Most of GMC's benefits could be obtained via better motion vector prediction. 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.