The Glass Box method, used primarily for descriptive
geometry problems, requires that the user imagine that the
object, points, lines, planes etc are enclosed in a transparent
“box”. Each view of the object is established on its
corresponding glass box surface by means of perpendicular
projectors originating at each point of the object and extending
to the related box surface. The box is hinged so that it can be
unfolded on to one flat plane (the paper).
The lines of sight representing the direction from which the
object is viewed. In figure 5.7, the vertical lines of sight (A)
and horizontal lines of sight (B) are assumed to originate at
infinity. The line of sight is always perpendicular to the image
plane, represented by the surfaces of the glass box (top, front,
and right side). Projection lines(c) connect the same point on
the image plane from view to view, always at right angle.
A point is projected up on the image plane where its projector,
or line of sight, pierces that image plane. In the figure 5.8,
point 1, which represents a corner of the given object, has
been projected on to the three primary image planes. Where it
is intersects the horizontal plane (top image plane), it is
identified as 1H, when it intersects the frontal plane (front
image plane), it is identified as 1F, and where it intersects the
profile plane (right side image plane), it is labeled 1P.