Since the perspective drawing of an object shows it as it would appear to the eye of the observer, it is very important that the draftsman acquire the ability to draw and think in perspective.
As has already been stated he must express his design in a legible manner to his client and he must also be able to sketch quickly in perspective those features of his design which he can not readily visualize himself. As he gains a knowledge of drawing in perspective he will also acquire the ability to think in perspective which, to the designer, is an asset, the value of which can not be overestimated.
The decided difference between the appearance of the orthographic projection drawing of a building and the picture or perspective, may be seen by comparing the photograph of the Cochran house with the sketch of the front elevation on Plate 6. This is particularly true of roof lines and dormers.
An attempt has been made to keep the explanation as brief as possible and at the same time make-it adequate for the needs of a student at this stage in his progress.1
PLATE 7. PERSPECTIVE.
The student must first become acquainted with the theory of perspective projection and the notation commonly used in developing these drawings. For example, consider a brick as being laid down on the ground at some distance from the eye and then imagine a glass plate to be set up vertically between the eye and the brick as in Fig. 46, Plate 7. This imaginary plate or plane will hereafter be referred to as the picture plane and marked P-P and its intersection with the ground will be called the ground line and marked G-L. The location of the eye of the observer is known as the station point and is marked S. The vertical lines of the object will be drawn vertically always. Any system of parallel lines on the object will meet at a point called the vanishing point and marked V. Parallel horizontal lines have their vanishing points on the horizon line. The horizon line is drawn horizontally, parallel to G-L. Its distance above the ground line is always the same as the distance that the eye is assumed to be above the ground.
Now imagine lines of sight to be drawn from the station point through the picture plane to the corners of the brick. Connect the points where these lines pierce the picture plane and the perspective projection of the brick on the picture plane will be the result.
It will be evident by a glance at Fig. 47 that if the eye (or station point) is elevated farther from the ground, the projection of the brick on the picture plane will also be raised; then too we can see from this new station point more of the top of the brick than before. The opposite is true when the eye is placed nearer to the ground. Notice that the projection on the picture plane is smaller than the brick, because the lines of sight converge as they go from the brick toward the picture plane. If we move the brick up until one edge is touching the picture plane as in Fig. 48, it is seen that the projection of that edge is in its true size, but that all of the brick behind the picture plane is projected smaller as before. From this we gather that measurements can be made only on the picture plane or where the picture plane and the object are in actual contact. In drawing the perspective of a building it is well to place the front corner against the picture plane so that the vertical distances may be measured along it.
1If further study of the subject is desired, consult one of the handbooks listed on page 147.