Information Technology Reference
In-Depth Information
actual bit with the predicted value and modifies the arithmetic code in order to make the prediction correct at the
decoder.
Figure 5.53(b) shows that the encoder compares the probability with a parameter called the arithmetic code value
to predict the value of the alpha bit. If the prediction is correct, the arithmetic code value needs no change and
nothing needs to be transmitted. However, if the predicted bit is incorrect, a change to the arithmetic code value
must be transmitted, along with the position of the bit in the scan to which it applies. This change is calculated to be
just big enough to make the prediction correct. In this way only prediction errors, which represent the boundary
location where alpha changes, are transmitted and the remaining bits self-predict.
Figure 5.54 shows that the spatial extent of the context requires alpha bits from adjacent babs to code the present
bab. This results in the concept of the
bordered bab
which incorporates a two-pixel deep border of alpha bits above
and to each side. In the case of babs adjacent to top or sides of the bounding rectangle, some of the bounding bits
are outside that rectangle and their value is obviously always zero.
Figure 5.54:
A bordered bab contains bits from adjacent babs so that context bits are available.
Shape data can also be inter-coded. This is handled using a context which spreads across two VOPs. As Figure
5.55 shows, four context bits are in the current VOP, whereas five context bits are in a previous VOP. This nine-bit
context requires a different probability table, but otherwise coding proceeds as before. If the shape changes from
one VOP to another, motion compensation can be used. A shape vector is transmitted. It will be seen from Figure
5.55(b) that this vector shifts the context relative to the previous VOP, thus altering (hopefully improving) the
chances of correctly predicting the current bit.
Figure 5.55:
(a) In bab inter-coding, the context spreads over the current and the previous VOP. (b) Shape motion
vectors shift the context with respect to the reference VOP to improve the accuracy of prediction.
Within a boundary block, shape vectors are highly redundant with the motion vectors of texture and texture vectors
may be used to predict shape vectors.
