Hierarchical Modeling - Essentials of Interactive Computer Graphics - page 301

involve many scene nodes, and yet we would not expect any of their components
to change. Examples of instances that behave in exactly the same manner may
include engines of a commercial airplane, tires on a car, and so on. These types of
objects contain moving components, for example, the rotating tires, or jet-engine
blades, and we expect movement to be synchronized and identical.

Sharing of Geometries

An approach to overcome the lack of control over components in the shared scene
node hierarchy is to avoid any sharing of scene nodes (or transformation opera-
tors). Instead, instancing is supported via sharing of geometric primitives only.
Figure 11.16 shows this alternative approach of implementing instancing for the
human model from Figure 11.13. In this case, instead of sharing the entire CArm
scene node hierarchy of Figure 11.9, only the involved geometric primitives are
shared. Because all of the left/right arm/palm objects now have independent scene
nodes (and thus separate transformation operators), the user is able to control each
of the components in exactly the same manner as Tutorial 11.6. In fact, from the
standpoint of user interaction, the scene node structure of Figure 11.16 and that
of Figure 11.14 behave in an identical manner.
Implementing instancing by sharing of geometries retains the exact indepen-
dent control of components while also conserving memory. This is especially
important for complex geometric objects with high degrees of symmetry where
there are identical components that must support independent controls. For ex-
ample, the geometries of the left and right limbs of a detailed human model are
highly symmetrical with many reusable components (e.g., the geometries of left
and right index fingers, calves), where it is important to retain independent control
over the components.

- SceneNode

M b

C b0 R b1

Body

Right

Left

M la

M ra

Instancing

Arm

Arm

C p0 R p1

M rp

M lp

Palm

Instancing

Palm

R a0 R a1 R a2

Figure 11.16. Instancing: sharing of geometries.