In class on Thursday we talked about possible future scenarios for
totally immersive computer graphics.
The next topic we will discuss is how to create
hierarchical movement, which is necessary for
animating multipart things such as people and machines.

We are going to go over this in class in more detail on Tuesday,
but meanwhile, here is the high level idea.
We can describe an entire renderable scene as a tree of
objects, each of which has the following structure, as follows.

Notice that this definition is recursive, since
an `Object3D`

can contain an array of children which are also
of the same type.
So essentially we are forming a graph structure.

Notice also that each node of this graph contains a
`Matrix`

.
Each matrix transformation is relative to its parent.
We will go over that concept in more detail on Tuesday.

Object3D:
Geometry:
vertices
edges or faces
Material
Matrix
Object3D children[...]