between i and P-frames are bidirectional frames (b-frames), based on the nearest i or p-frames both before and after them. in our bouncing ball example, in a b-frame the picture is stored as the difference between the picture i or p-frame and the b-frame and as the difference between the b-frame and the following i or p-frame. to recreate the b-frame when playing back the sequence, the MPEG algorithm uses a combination of two references. there may be a number of B-frames between i or P-frames. no other frame is ever based on a B-frame so they don't propagate errors like P-frames.typically, you will have two or three Bs between Is or Ps, and perhaps three to five P-frames between Is.
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