Typical Group of Pictures (GOP) Structure
The typical Group Of Pictures (GOP) structure is IBBPBBP... The I-frame is used to predict the first P-frame and these two frames are also used to predict the first and the second B-frame. The second P-frame is predicted using the first P-frame and they join to predict the third and fourth B-frames. The scheme is shown in the next picture:
This structure suggests a problem because the fourth frame (a P-frame) is needed in order to predict the second and the third (B-frames). So we need to transmit the P-frame before the B-frames and it will delay the transmission (it will be necessary to keep the P-frame).
This structure has strong points:
- It minimizes the problem of possible uncovered areas.
- P-frames and B-frames need less data than I-frames, so less data is transmitted.
But it has weak points:
- It increases the complexity of the decoder, which can mean more memory is needed to rearrange the frames.
- The interpolated frames (namely B-frames) require more motion vectors which means an increased bit rate.
Read more about this topic: Inter Frame
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