@node Viewpoint, Whatis, Version, Commands

@iftex
@syntax Viewpoint.
@end iftex
@pindex Viewpoint
@example
Syntax: VIEWPOINT theta phi l
@end example

Surfaces are drawn from a direction @code{(THETA,PHI)}, and projected onto
a surface passing through the origin. The projection is from a point
@code{L} away from the nearest corner of the cube containing the image. If
@code{L} is positive a perspective projection is used; if it is 0 the viewpoint
is taken to be infinitely far from the surface, and if it is negative
an axonometric projection is used (i.e. the surface is projected from
infinity onto the x-z plane).

   The coordinate system is such that the z-axis is @code{THETA=90}, the
x-axis is @code{(THETA,PHI) = (0,0)}, and the coordinate system is right
handed. Angles are taken to be in degrees, with theta lying in
[-90,90] and phi lies in [-180,180].  The nearest corner of the cube
containing the surface is projected onto the point (0,0).

There are some useful macros in the file @file{surfaces}; say
@code{load surfaces} to read them. If VERBOSE is one or more, a helpful
header will be printed when you load the file.