Page 351 - Manual_Movie Edit Pro 2021_EN_Neat

MPEG glossary 351
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 I 0 B 01 B 02 P 01 B 11 B 12 P 02 B 21 B 22 I1
GOP

Data stream I 0 P 01 B 01 B 02 P 02 B 11 B 12 B 21 B 22 I1 ... for closed GOPs

I 0 P 01 B 01 B 02 P 02 B 11 B 12 I 1 B 21 B22 P 11... 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 (view page 346) 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 (view page 347).

Each one of these blocks is then transformed into an 8 x 8 matrix (a table with rows
and columns) using a DCT (discreet cosinus transformation) mathematical method.
Each of these values is produced using all 64 individual pixels of the block, but the
values in the matrix are ordered in such a way that the image information is ordered
according to its importance.