MPEG Coding

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Picture types in MPEG

There are three types of pictures defined in MPEG [3]:

  • Intraframes (I-frames): This is a reference still picture. No temporal information is used to code this. Only still-picture compression techniques such as DCT and Entropy-coding are used.
  • Predictive frames (P-frames): These pictures are more efficiently coded by using motion compensated prediction from a past intraframe or predictive coded picture and are also often used as a reference picture.
  • Bidirectionally predicted frames (B-frames): These pictures provide the highest degree of compression, but require both past and future reference pictures for motion compensation. This means that when decoding a B-frame, the receiver must already have some future pictures. This is accomplished by sending pictures out-of-order. Figure 18 and Figure 19 shows an example of groups of pictures in display order and transmission order.


Figure 18 - Group of pictures in display order (after decoding).


Figure 19 - Group of pictures in transmission order (before decoding).