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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