Motion compensation
[3] After predicting frames using motion compensation, the coder finds the residual, which is then compressed and transmitted.
Moving objects within a frame are not sufficiently represented by global motion compensation.
The result of this differentiating process is mathematically equivalent to a global motion compensation capable of panning.
It is possible to shift a block by a non-integer number of pixels, which is called sub-pixel precision.
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.
These artifacts appear in the form of sharp horizontal and vertical edges which are easily spotted by the human eye and produce false edges and ringing effects (large coefficients in high frequency sub-bands) due to quantization of coefficients of the Fourier-related transform used for transform coding of the residual frames[4] 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).
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.
Other areas of work have examined the use of variable-shape feature metrics, beyond block boundaries, from which interframe vectors can be calculated.
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.
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-2 compatible coding and can use motion compensation to compress between stereoscopic images.
Kell in Britain proposed the concept of transmitting only the portions of an analog video scene that changed from frame-to-frame.
This is a hybrid coding algorithm,[7] which combines two key data compression techniques: discrete cosine transform (DCT) coding[8] in the spatial dimension, and predictive motion compensation in the temporal dimension.
[7][13] For the spatial transform coding, they experimented with the DCT and the fast Fourier transform (FFT), developing inter-frame hybrid coders for both, and found that the DCT is the most efficient due to its reduced complexity, capable of compressing image data down to 0.25-bit per pixel for a videotelephone scene with image quality comparable to an intra-frame coder requiring 2-bit per pixel.
[7] This led to Chen developing a practical video compression algorithm, called motion-compensated DCT or adaptive scene coding, in 1981.
[7] Motion-compensated DCT later became the standard coding technique for video compression from the late 1980s onwards.
[18] Since then, motion-compensated DCT compression has been adopted by all the major video coding standards (including the H.26x and MPEG formats) that followed.
