5.8 I and P coding

Predictive ( P) coding cannot be used indefinitely, as it is prone to error propagation. A further problem is that it becomes impossible to decode the transmission if reception begins part-way through. In real video signals, cuts or edits can be present across which there is little redundancy and which make motion estimators throw up their hands.

In the absence of redundancy over a cut, there is nothing to be done but to send the new picture information in absolute form. This is called I coding where I is an abbreviation of intra coding. As I coding needs no previous picture for decoding, then decoding can begin at I coded information.

MPEG is effectively a toolkit. The bigger the MPEG-number, the more tools are available. However, there is no compulsion to use all the tools available. Thus an encoder may choose whether to use I or P coding, either once and for all or dynamically on a macroblock-by-macroblock basis. For practical reasons, an entire frame may be encoded as I macroblocks periodically. This creates a place where the bitstream might be edited or where decoding could begin.

Figure 5.23 shows a typical application of the Simple Profile of MPEG-2. Periodically an I picture is created. Between I pictures are P pictures which are based on the picture before. These P pictures predominantly contain macroblocks having vectors and prediction errors. However, it is...