Information Technology Reference
In-Depth Information
Figure 5.24:
In bidirectional coding the revealed background can be efficiently coded by bringing data back from a
future picture.
Bidirectional coding is shown in Figure 5.25. A bidirectional or
B
macroblock can be created using a combination of
motion compensation and the addition of a prediction error. This can be done by forward prediction from a previous
picture or backward prediction from a subsequent picture. It is also possible to use an average of both forward and
backward prediction. On noisy material this may result in some reduction in bit rate. The technique is also a useful
way of portraying a dissolve.
Figure 5.25:
In bidirectional coding, a number of
B
pictures can be inserted between periodic forward predicted
pictures. See text.
The averaging process in MPEG is a simple linear interpolation which works well when only one
B
picture exists
between the reference pictures before and after. A larger number of
B
pictures would require weighted interpolation
but MPEG does not support this.
