Information Technology Reference
In-Depth Information
Figure 5.80:
(a) A spheroid which has been sampled by vertices and triangulated. (b) Partially unfolding the
triangles. (c) Completely unfolded triangles form a planar polygon. (d) The vertex loop which drives the vertex look-
up process in order to link the vertices which were disconected during unfolding.
A solid body such as a spheroid is described as
manifold
because its surface consists of a continuous layer of
faces with a regular vertex at each corner. However, some bodies which need to be coded are not manifold. Figure
5.81 shows a non-manifold body consisting of a pair of intersecting planes. The two planes share an edge joining
shared vertices. The two planes can be coded separately and joined using
stitches
which indicate common
vertices.
Figure 5.81:
A non-manifold body in which planes are joined by stitches.
Stitches can also be used to transmit meshes in discrete stages known as
partitions
. Partitions can be connected
together by stitches such that the result is indistinguishable from a mesh coded all at once. There are many uses of
mesh partitions, but an example is in the coding of a rotating solid body. Only the part of the body which is initially
visible needs to be transmitted before decoding can begin. Extra partitions can be transmitted which describe that
part of the surface of the body that will come into view as the rotation continues. This approach is the three-
dimensional equivalent of transmitting static sprites as pieces; both allow the bit rate to remain more constant.
Figure 5.82 shows that when scanning, it is possible to enter four types of triangle. Having entered by one edge,
the other two edges will either be internal edges, leading to another triangle, or a boundary edge. This can be
described with a two-bit code; one bit for the left edge and one bit for the right. The scanning process is most
