Dependent on the applications requirements we may envisage "lossless" and "lossy" coding
of the video data. The aim of "lossless" coding is to reduce image or video data for storage
and transmission while retaining the quality of the original images - the decoded image
quality is required to be identical to the image quality prior to encoding. In contrast the aim
of "lossy" coding techniques - and this is relevant to the applications envisioned by MPEG-
1 and MPEG-2 video standards - is to meet a given target bit-rate for storage and
transmission. Important applications comprise transmission of video over communications
channels with constrained or low bandwidth and the efficient storage of video. In these
applications high video compression is achieved by degrading the video quality - the
decoded image "objective" quality is reduced compared to the quality of the original
images prior to encoding (i.e. taking the mean-squared-error between both the original and
reconstructed images as an objective image quality criteria). The smaller the target bit-rate
of the channel the higher the necessary compression of the video data and usually the more
coding artefacts become visible. The ultimate aim of lossy coding techniques is to optimise
image quality for a given target bit rate subject to "objective" or "subjective" optimisation
criteria. It should be noted that the degree of image degradation (both the objective
degradation as well as the amount of visible artefacts) depends on the complexity of the
image or video scene as much as on the sophistication of the compression technique - for
simple textures in images and low video activity a good image reconstruction with no
visible artefacts may be achieved even with simple compression techniques.