Define here the region of layer space to where the user is looking.
Eye
Set the user position, in the layer space. Any position in 3D space 
is valid. Initially, the user is at (0, 0, +z), looking down to
(0, 0, 0), and the up direction is (0, 1, 0), so the x axis
points to the right, the y axis points to the top, and the z
axis points to the user.
Center
Set the position to where the user is looking. The distance from 
Eye to 
Center must be within the limits of visibility 
(from 
Near to 
Far).
Up
Set the direction in layer space that points up, to the top of the 
screen. This direction is converted internally to a unit vector 
normal to the direction 
Eye -> 
Center, so any non-zero 
vector that is not parallel to that direction is valid.
The user position and orientation can be defined by a (X,Y,Z) 
layer referential, pointing from the middle of the screen to: the 
right (X), the top (Y), the user (Z). 
Eye is the origin,
Eye <- 
Center defines the Z axis, and 
Up 
the Y axis, permiting to get the X axis by the cross product 
of the other two axes. Initially, the axes are orientated along 
(1,0,0) for X, (0,1,0) for Y and (0,0,1) for Z, but this is no 
longer the case when the layer referential is rotated.
To move the user around the objects, 
Eye, 
Center
and 
Up must change. A simple, interactive way to do this 
is to press 
Axes in the window toolbar and then either drag 
the mouse or use the ruler, as described in 
Introduction Mouse 
and 
Introduction Tools.
Each layer has its own position parameters, so in a window with
multiple visible layers, the image rendered in the graphic area 
is the superposition of the various, totally independent, layer 
views.