Additions from Sun's J3D Tutorial (PDF).

Locale establishes a virtual world Cartesian coordinate system.

Scene Graph = Scene graph TREE + NodeComponents

Scene graph TREE = Group  + Leaf NODEs.  NO CYCLES in the tree.

Leaf classes:  Shape3D, Lihgt, Behavior, Sound

NodeComponents:  Referenced only by Shape3Ds.

SceneGraphObject = all of above.  SceneGraphObject.isCompiled(), isLive().

Changing objects.  Change-capabilites must be turned on before making
the object live (apparently not necessarily before compilation)


PERSPECTIVE/UNITS

Default FOV is 45o (22.5o to each side of the vertical).

By default, the image plate (Canvas3D) is centered at the SimpleUniverse 
origin.  Therefore slightly +z virtual objects should appear between you
and the screen and should be rendered larger than they are; whereas any
greater +z objects will be invisible and behind you.  Since there is no 
X or Y offset, you are looking straight down on an object at x=0, y=0 
and can't see its sides if the sides are orthogonal.

ViewingPlatform.setNominalViewingTransform() moves eye location from 
(0, 0, .41?m) to (0, 0, 2.41m).  With default 45o FOV, this results in
a 2' wide object at origin taking up the entire Canvas3D.
For the 2m wide object to actually appear to be 2m, the Canvas3D size
must be taken into consideration.  I think (according to API Spec for
View class) that assumption is the Canvas3D is 1/2 the scren size 
(default Screen scale policy).

Front and back clip distances are specified wrt screen in physical 
meters, by default.

View docs recommend bacl-clip/front-clip > 100 and < 1000 best.


COMPILING AND ADDING TO SCENE

It's recommended to compile only top-level BGs, and do so right before
adding them to the Locale (thereby making the branches "live").