Information Technology Reference
In-Depth Information
Figure 5.77:
(a) Vertices are joined by edges to form triangles. (b) Hingeing along some edges and opening others
allows the triangles to be unfolded into a plane. (c) Greater resolution can be obtained by locating additional
vertices with respect to each triangle in a forest split operation.
Forest split operations are efficient because the displacements of the new vertices from the original triangle plane
are generally small. Additionally a forest split is only needed in triangles in the vicinity of sharp changes. Forest
splits can be cascaded allowing the geometry to be reconstructed to arbitrary accuracy. By encoding the forest split
data in different layers, a scaleable system can be created where a rough shape results from the base layer which
can be refined by subsequent layers.
Figure 5.78 shows an MPEG-4 three-dimensional mesh coding system at high level. The geometry of the object is
coded by the vertices which are points, whereas the properties are coded with respect to the triangles between the
vertices. The dual relationship between the triangles and the vertices which they share is known as the
connectivity
. Geometry is compressed using topological principles and properties are compressed by quantizing.
These parameters are then subject to entropy (variable- length) coding prior to multiplexing into a bitstream. At the
decoder the three information streams are demultiplexed and decoded to drive the rendering and composition
process. The connectivity coding is logical and must be lossless whereas the vertex positions and triangle
properties may be lossy or lossless according to the available bit rate.
Figure 5.78:
An MPEG-4 three-dimensional mesh coder. See text for details.
Generic geometry coding is based on meshes using polygons. Each polygon is defined by the position of each
vertex and its surface properties. MPEG-4 breaks such polygons down into triangles for coding the vertices and
associates the properties with the correct triangles at the decoder using the connectivity data so that the polygons
can be recreated. The geometry coding is based on triangles because they have useful properties.
