Introduction to compression - The MPEG - page 10

Inter-coding takes further advantage of the similarities between successive pictures in real material. Instead of
sending information for each picture separately, inter-coders will send the difference between the previous picture
and the current picture in a form of differential coding.
Figure 1.8 shows the principle. A picture store is required at the coder to allow comparison to be made between
successive pictures and a similar store is required at the decoder to make the previous picture available.
Figure 1.8: An inter-coded system (a) uses a delay to calculate the pixel differences between successive pictures.
To prevent error propagation, intra-coded pictures (b) may be used periodically.
The difference data may be treated as a picture itself and subjected to some form of transform-based spatial
compression.
The simple system of Figure 1.8 (a) is of limited use as in the case of a transmission error, every subsequent picture
would be affected. Channel switching in a television set would also be impossible. In practical systems a
modification is required. One approach is the so-called 'leaky predictor' in which the next picture is predicted from a
limited number of previous pictures rather than from an indefinite number. As a result errors cannot propagate
indefinitely. The approach used in MPEG is that periodically some absolute picture data are transmitted in place of
difference data.
Figure 1.8 (b) shows that absolute picture data, known as I or intra pictures are interleaved with pictures which are
created using difference data, known as P or predicted pictures. The I pictures require a large amount of data,
whereas the P pictures require fewer data. As a result the instantaneous data rate varies dramatically and buffering
has to be used to allow a constant transmission rate. The leaky predictor needs less buffering as the compression
factor does not change so much from picture to picture.
The I picture and all of the P pictures prior to the next I picture are called a group of pictures (GOP). For a high
compression factor, a large number of P pictures should be present between I pictures, making a long GOP.
However, a long GOP delays recovery from a transmission error.
The compressed bitstream can only be edited at I pictures as shown.
In the case of moving objects, although their appearance may not change greatly from picture to picture, the data
representing them on a fixed sampling grid will change and so large differences will be generated between
successive pictures. It is a great advantage if the effect of motion can be removed from difference data so that they
only reflect the changes in appearance of a moving object since a much greater coding gain can then be obtained.
This is the objective of motion compensation. "/>
1.7.3 Introduction to motion compensation