Game Development Reference
In-Depth Information
animated characters. They subdivide the mesh until the size of a polygon
is half a pixel, not a technique that real-time applications are likely to be
capable of performing in the near future at rates of 25 times a second. In
such high-end examples, the original source meshes will be denser than
those used for real-time character animation. With a denser mesh the
effect of shrinking is not as pronounced. The mesh created from
approximating schemes tends to be smoother than that from interpolating
and so it is suited to pre-rendered animation, but an interpolating scheme
is more suited to a real-time application.
Triangles or quadrilaterals
For some schemes the algorithms can work on three- or four-sided
polygons. Meshes with polygons containing more than four sides can be
converted into a triangular mesh or a mesh that contains triangles and
Figure 15.3 Subdividing triangles and quadrilaterals.
quadrilaterals. When subdividing a triangle, each edge has a new vertex
added; consequently, three additional vertices are added and each
triangle is converted to four triangles. When subdividing a quadrilateral,
each edge has a new vertex added and an additional vertex is added to
the centre of the mesh. Again, four polygons replace the original single
quadrilateral. Figure 15.3 shows how polygons are added using triangles
and quadrilaterals.
Catmull-Clark - an approximating scheme
Catmull-Clark surfaces use a quadrilateral control mesh and are
approximating. Pixar uses Catmull-Clark surfaces for animated charac-
ters and variants of this subdivision are used in high-end CGI packages.
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