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View Concepts:

C: the position of the camera (viewer).
N: the direction towards which we are watching (normal).
E1, E2: the horizontal and vertical axes in our screen.
(N, E1 and E2 make up the "dreibein" -three legs-, our moving reference
frame.)
Focal distance: all rays join the object point and the eye, intersecting the
screen in their way. The screen is a plane situated at a distance "df" from
the eye, in the N direction. The focal distance is "df".

During picture show, the following command-keys are available:

1-!	Move the camera along the fixed X-axis (forward and backwards)
2-"	Along the fixed Y-axis.
3-	Along the fixed Z-axis.
8-5	Rotate the camera around the E2 axis
7-6	Rotate the camera around the E1 axis
z	Rotate the camera so as to focus on the (0,0,0) point.
n-m	Orbit around the (0,0,0) point, always focusing on it.
f-F	Change focal distance.
P
p	Print a PostScript file. The name shall be the name of the 
	orders file with a ".ps" appended.
q	Exit.


