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Figure 5.63(b) shows how the shape of the mesh can be changed by transmitting mesh vectors in P-VOPs. These
specify the new position of each vertex with respect to the original position. The mesh motion vectors are
predictively coded as shown in Figure 5.64. In two adjacent triangles there are two common nodes. If the first
triangle has been fully coded, only one node of the second triangle remains to be coded. The vector for the
remaining node is predicted by taking the average of the two common nodes and rounding to half-pixel accuracy.
This prediction is subtracted from the actual vector to produce a residual which is then variable-length coded for
transmission. As each node is coded, another triangle is completed and a new pair of common nodes become
available for further prediction.
Figure 5.64: Mesh vectors are predicted from the previous two vertices of the triangle concerned.
Figure 5.65 shows an object coding system using meshes. At (a), following the transmission of the I-VOP
containing shape, outline and a regular mesh, the I-VOP and the next VOP, which will be a P-VOP, are supplied to
a motion estimator. The motion estimator will determine the motion of the object from one VOP to the next. As the
object moves and turns, its shape and perspective will change. The new shape may be encoded using differential
alpha data which the decoder uses to change the previous shape into the current shape. The motion may also
result in a perspective change. The motion estimator may recognize features in the object which are at a new
location and this information is used to create mesh vectors.
Figure 5.65: (a) Mesh coding requires texture, shape and mesh data to be multiplexed into the bitstream. (b) An
object coding system using meshes. See text.
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