User manual

MPEG glossary 133
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Prediction
Prediction is a method of data reduction used by the MPEG format.
The image elements already known from the previous or following
frames are removed from the data stream.
How does it work?
The encoder has a precisely defined GOP, for example IBBPBBPBB.
This sequence is transmitted together with the encoder, which always
knows exactly which kind of frame comes next. I, P, and B frames
are differentiated.
Hint: When we talk about pictures, we mean frames of the video
output, and I, P an dB frames are the frames of the encoded video.
Just as in movement approximation, blocks (8x8 pixels) are united
into macroblocks (16x16 pixels) during prediction.
The first frame is always the I frame. It is completely encoded from
the first picture. Afterwards, the 4th picture is analyzed for the
creation of the first P frame. (As already said, the encoder, and later
the decoder, will know that two B frames belong between them.) This
image will also be completely encoded, and afterwards all
macroblocks that haven't changed in comparison to the I frame will
be deleted. They will be replaced by corresponding references for the
decoder that tell it "you already know what should be shown here,
and you can get it from the last I frame".
Now, the 2nd will be completely encoded, and all macroblocks
identical to the first I frame and the following P frame will be removed.
References to previous frames are called backward predictions,
and references to following frames are called forward predictions.
The third picture will be edited in exactly the same fashion.
The fourth picture we have already explained, and now we need the
next P frame, or picture number 7. Pictures 5 and 6 are B frames
again, which are compared to P frames to both sides of them (picture
4 and 7); these are followed by the last two B frames. These have a
special place, since in closed GOPs, they may contain only backward
predictions, and no references to the next I frame, because it belongs
to the next GOP.
Something else: Since the decoder is no prophet, the P frames are
always transmitted before the B frames! The GOP explained above