Two- dimensional Perspective
Perspective is described by Serlio* as an inspection or looking into by shortening of the sight. Although this is a more philosophical observation than practical analysis it does express or anticipate the initial concept of the graphics of perspective, foreshortening. To understand foreshortening is the first step toward being able to visualize the optics of perspective.
Foreshortening is experienced every day. Everyone has a natural perception of space. Our two eyes and their simultaneous vision are mainly responsible for the ability to judge objects in depth or in the round. This judgment, however, is dependent on several natural signs of space so familiar that they have become intuitive. We are so accustomed, for example, to seeing depth or distance in the converging horizontal lines of a railroad or fence, the gradation of a color in the sky or down a wall, the receding quality of certain colors, the reduction of size of distant objects, and the falloff of light and the cast of shadows. If too many of these signs are changed, our judgment is upset or fooled.
In effect, the graphics of perspective can be described as the manipulation of the optics of the eye by altering the natural signs of space perception, thereby fooling the eye and, subsequently, the observer into seeing greater depth or distance than actually is there.
The signs of space perception involve normal visual foreshortening. To transpose visual foreshortening into graphic foreshortening on the drawing board, two assumptions have to be made.
Because the eye through peripheral vision is able to see more than is practical to draw, the first assumption is that all verticals are drawn perpendicular to the ground. This is true in the center of the eye's vision but not true of the extreme right and left areas. Vertical lines seem to converge or diverge depending on whether the observer is on the ground or high above. The second assumption is that the horizon line is a straight line parallel to the ground. This is not true of vi____________________