Hierarchical Modeling - Essentials of Interactive Computer Graphics - page 299

primitives are inserted into the primitive list, and the list is set to be the primitive
of m _ Body at label D. The left and right arms are allocated at label E and set to be
the children of m _ Body at label F. In this way, the scene node hierarchy (or simply
the scene) is constructed. The entire scene hierarchy is drawn when we invoke the
SceneNode::Draw() routine of the m _ Body object. In this case, the primitives of
the Body node (e.g., R b 0 ) will be transformed by M db :

M db = M b M V M P . (11.4)

The primitives of the left Arm (e.g., R a 0 ) will be transformed by M dla :

Right Arm / Palm . Correspond-
ingly, primitives of the right
Arm / Palm will be transformed
by

M dla = M la M b M V M P , (11.5)

whereas the primitives of the left Palm (e.g., R p 0 ) will be transformed by M dlp :

M ra M b M V M P

and

M dlp =

.

M lp M la M b M V M P (11.6)

M rp M ra M b M V M P .

The SceneTreeControl user interface construct allows the user select one of the
M b , M rp , M ra , M lp ,or M la transformations for interactive manipulation. In this
way, the user can have independent control of each of the components in the scene
hierarchy.
This tutorial presents a straightforward extension to Tutorial 11.5 in building
a scene tree with the SceneNode class. Through working with the SceneTree
Control GUI container, the user can select any of the five scene nodes and control
the selected transformation operator. In addition, the user can select any of the
primitives in the selected scene node to interactively adjust the attributes of the
selected primitive.

M b

C b0

R b1

Body

11.3.1 Instancing: Scene Graph

M la M ra

Left
Arm

Right
Arm
Instancing

From Figures 11.13 and 11.14, we can observe that the scene hierarchies under
the left and the right arms are identical. Indeed, in Listing 11.11, the two arms are
implemented based on the same CArm class. For efficiency purposes, it is logical
to examine the possibilities of sharing the CArm object, that is, implement the two
arms based on sharing the memory of one instance of the CArm object.

M a

R a2

R a0 R a1

Arm

M p

C p0 R p1

Palm

Figure 11.15. In-
stancing: sharing of scene
nodes.

Sharing of Scene Node Hierarchy
Figure 11.15 shows a straightforward approach to sharing an instance of the CArm
hierarchy. Notice that in order to retain the independent control over the two sep-
arate arms, the CArm object must have two separate parents with two independent