Data Compression: The Complete Reference

6.4 Video Compression 649160352 700 800 1280 1920120240483600720108019.2Kpx84.5Kpx340Kpx, video4800Kpx, personal computercheap video cameras1Mpx, HDTVquality video cameras2Mpx, HDTVFigure 6.8: Various Video Resolutions.6.4 Video CompressionVideo compression is based on two principles. The first is the spatial redundancy thatexists in each frame. The second is the fact that most of the time, a video frame isvery similar to its immediate neighbors. This is called temporal redundancy. A typicaltechnique for video compression should therefore start by encoding the first frame usinga still image compression method. It should then encode each successive frame byidentifying the differences between the frame and its predecessor, and encoding thesedifferences. If the frame is very different from its predecessor (as happens with the firstframe of a shot), it should be coded independently of any other frame. In the videocompression literature, a frame that is coded using its predecessor is called inter frame(or just inter), while a frame that is coded independently is called intra frame (or justintra).Video compression is normally lossy. Encoding a frame F i in terms of its predecessorF i−1 introduces some distortions. As a result, encoding frame F i+1 in terms of F iincreases the distortion. Even in lossless video compression, a frame may lose somebits. This may happen during transmission or after a long shelf stay. If a frame F i haslost some bits, then all the frames following it, up to the next intra frame, are decodedimproperly, perhaps even leading to accumulated errors. This is why intra frames shouldbe used from time to time inside a sequence, not just at its beginning. An intra frameis labeled I, and an inter frame is labeled P (for predictive).Once this idea is grasped, it is possible to generalize the concept of an inter frame.Such a frame can be coded based on one of its predecessors and also on one of itssuccessors. We know that an encoder should not use any information that is not availableto the decoder, but video compression is special because of the large quantities of datainvolved. We usually don’t mind if the encoder is slow, but the decoder has to be fast.A typical case is video recorded on a hard disk or on a DVD, to be played back. The