There is some information that is unpredictable from the previous frame:
Newly uncovered information due to object motion across a background, or at the edges of a panned scene.
To handle the scene change case (or startup conditions) a non-differential frame must be coded. In MPEG, this is referred to as an I picture or Intra-coded picture. In a typical application, I pictures may be sent about twice per second, but their time and frequency of occurrence may be adjusted to accommodate scene changes. I pictures require considerably more data to transmit than do predicted frames.
To reduce the data required to send uncovered information, MPEG includes a Bi-directionally coded type of picture, or B picture. In this case, there must be enough frame storage in the system to wait for the later picture that has the desired information. In order to limit the amount of memory in the decoder, the encoder stores pictures and sends the required reference pictures before sending the B picture. (See I P B Picture Reordering )
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