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prediction among forward, backward and bi-directional predictions. A classifica-
tion of picture types and the prediction methods of MPEG-2 are shown in Fig. 24.
This structure makes it possible to realize random access of picture and also im-
proves the coding performance for package media.
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Display order
Coding order
Each arrow is pointed from reference picture to predicted picture.
Fig. 24
Picture Classification and Prediction Methods
In addition, prediction with half-pel accuracy is defined in MPEG-2, whereby
the unit of displacement is expressed with a motion vector pointing to half a pixel
position (middle position of adjacent pixels). Since the value of half a pixel posi-
tion does not actually exist, it is virtually generated by interpolation from
neighboring pixels. The concept of half-pel accuracy prediction and the half-pel
calculation method are shown in Fig. 25 and Fig. 26, respectively.
Original picture
Integer
accuracy
MC
Half-pel
accuracy
MC
Reduction of
prediction error
Reference picture
Fig. 25
Half-pel Accuracy Prediction
