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A pair of algorithms that works together is called a video codec (encoder/decoder). Video codecs
of different standards are normally not compatible with each other; that is, video content that
is compressed using one standard cannot be decompressed with a different standard. For
instance, an MPEG-4 decoder will not work with an H.264 encoder. This is simply because one
algorithm cannot correctly decode the output from another algorithm but it is possible to imple-
ment many different algorithms in the same software or hardware, which would then enable
multiple formats to coexist.
7.1.2
Image compression vs. video compression
Different compression standards utilize different methods of reducing data, and hence, results
differ in bit rate, quality and latency. Compression algorithms fall into two types: image com-
pression and video compression.
Image compression
uses intraframe coding technology. Data is reduced within an image frame
simply by removing unnecessary information that may not be noticeable to the human eye.
Motion JPEG is an example of such a compression standard. Images in a Motion JPEG sequence
is coded or compressed as individual JPEG images.
Figure 7.1a With the Motion JPEG format, the three images in the above sequence are coded and sent as separate
unique images (I-frames) with no dependencies on each other.
Video compression
algorithms such as MPEG-4 and H.264 use interframe prediction to reduce
video data between a series of frames. This involves techniques such as difference coding, where
one frame is compared with a reference frame and only pixels that have changed with respect
to the reference frame are coded. In this way, the number of pixel values that is coded and sent
is reduced. When such an encoded sequence is displayed, the images appear as in the original
video sequence.
CHAPTER 7 - VIDEO COMPRESSION
