X5

MPEG glossary 361
You can correct the field series for each video object in its object settings. See: Menu -
> Effects -> Object properties
P frames and B frames
P frames save only the difference between the current picture and the preceding I
frame. The "P" comes from the term "prediction" which describes this process.
B frames save the differences between the current picture and the I or P frame
preceding and following. This includes the information that was the same before and
remained the same after the current frame. Both directions are analyzed (indicates
the "B" in the name, i.e. "bidirectional-predicted"). You can read more under
prediction (view page 361).
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.