Information Technology Reference
In-Depth Information
Figure 5.59: As a body turns, its shape on a two-dimensional screen may change.
The geometric sampling results in a structure known as a mesh. Effectively a mesh is a sampled surface. The
smoother the surface, the fewer samples are necessary because the remainder of the surface can be interpolated.
In MPEG-4, meshes can be two- or three-dimensional. Figure 5.60 shows a two-dimensional mesh which is a set of
points known as nodes which must remain in the video object plane. Figure 5.61 shows a three-dimensional mesh
which is a set of nodes describing a non-flat surface.
Figure 5.60: All the nodes of a two-dimensional mesh lie in the same plane.
Figure 5.61: A three-dimensional mesh samples the surface of a solid body.
Like other MPEG-4 parameters, meshes can be scaleable. A base mesh has relatively few nodes; adding
enhancement data will increase the number of nodes so that the shape is better described.
Figure 5.62 shows an example of how two-dimensional meshes are used to predict an object from a previous VOP.
The first object may be sent as an I-VOP, including texture and alpha outline data, both of which will be intra-
coded. The texture pixels in the I-VOP are uniformly spaced and so a uniform mesh can be sent as a further part of
the same I-VOP. Uniform meshes require very few data. Figure 5.63(a) shows that little more than the coordinates
of the top-left corner of the mesh and the mesh pitch need be sent. There are also some choices of how the mesh
boxes are divided into triangles.
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