Chapter 4 Perspective, Scene Design, and Basic Animation 115
Round Objects
Rounded objects like spheres can be a challenge for a 3D engine. A
sphere has no corners. Different engines may use different solutions to this
problem. In most cases, the sphere is made up of several fl at, rectangular
or triangular faces that make the object look
round. Where these faces connect is a vertex.
With these vertices, the 3D engine can resize
the sphere by changing the distance between
the vertices. This is also how facial features and
other rounded character assets are resized.
The type of sphere described above is
called a UV sphere. The name comes from
the process needed to wrap a texture map
around the object. A 2D image is stretched
around the sphere, Figure 4-28. U is the same
as X and V is the same as Y. In some applica-
tions, a third element (W) is added for the Z
dimension.
An orange is a good example of a UV
sphere, Figure 4-29. A UV sphere uses slices
like the segments of an orange to make the
object round. The more slices (segments), the
more rounded the sphere appears. Imagine the
segments of the orange as the faces of the UV
sphere. The peel of the orange is the texture
map stretched over the sphere. The texture map
provides the orange color and the wrinkly quality
of the peel. Brown spots and irregular fl aws
added to the texture map will make the image
more realistic.
Another type of sphere is a geodesic
sphere. Instead of rectangular and triangular faces,
a geodesic sphere has faces of regular polygons.
This is the way a soccer ball is made, Figure 4-30.
There are different types of geodesic spheres. For
example, an icosphere is based on a 20-sided
object. Its faces are equilateral triangles.
THINK ABOUT IT
ACTIVITY 4.4
A 3D engine changes the
dimensions of a model by
changing the distance between
vertices. How does this work on
an object like a sphere that has
no corners?
Figure 4-28. A—This is a 2D texture map of Earth. B—To create a model of Earth, draw a sphere. C—The texture map
is wrapped onto the sphere to fi nish the model of Earth. (Texture provided by NASA)
THINK ABOUT IT
ACTIVITY 4.5
Think about the example of the
orange and the soccer ball as
a UV sphere or a geodesic
sphere. Come up with two other
examples each for UV sphere
and geodesic sphere.
Figure 4-29. The segments of an orange are fi t together
to approximate a round shape. Notice how the segments
relate to the sphere shown in Figure 4-28B.