Bedienungsanleitung Magix Movie Edit Pro Plus 16 (Seite 874 von 879) (Englisch)

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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 will be encoded and transmitted in the order it is written.

Original

GOP

22 I1

Data

stream

22 I1

...

for closed GOPs

21 B22

...

For open GOPs

Due to this nested structure, it is easy to see that during direct editing of MPEG material, complicated

computations have to take place! These are made easier using a frame table

. A frame table contains a list, where the information of every frame in the data stream is found,

identifying the type of frame it is.

Using Movement prediction

P and B frames are likewise reduced.

Quantization scaling

The single pictures in MPEG are saved using a compression method comparable to JPEG with bitmaps

and associated with quality loss. For this single images are divided into 8 x 8 blocks

Each one of these blocks is then transformed into an 8 x 8 matrix (a table with rows and columns) using

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