TABLE OF CONTENTS Figure 5.28 shows a variety of possible GOP (group of pictures in MPEG-1 and MPEG-2) or GOV (group of video object planes in MPEG-4) structures. The simplest is the III... sequence in which every picture (or object in MPEG-4) is intra-coded. These can be fully decoded without reference to any other picture or object and so editing is straightforward. However, this approach requires about two-and-one-half times the bit rate of a full bidirectional system.
Bidirectional coding is most useful for final delivery of post-produced material either by broadcast or on prerecorded media as there is then no editing requirement. As a compromise the IBIB ... structure can be used which has some of the bit rate advantage of bidirectional coding but without too much latency. It is possible to edit an IBIB stream by performing some processing. If it is required to remove the video following a B picture, that B picture could not be decoded because it needs I pictures either side of it for bidirectional decoding. The solution is to decode the B picture first, and then re-encode it with forward prediction only from the previous I picture. The subsequent I picture can then be replaced by an edit process. Some quality loss is inevitable in this process but this is acceptable in applications such as ENG and industrial video.
