|Themes > Arts > Drawing > Perspective Drawing > Drawing in Perspective|
The picture plane is usually considered perfectly vertical, because our upright posture and the position of our eyes forces us normally to look straight ahead. This is the case in the window drawing experiment just described. On the other hand, had the science of perspective been worked out by a bird, we should probably have a convention if a horizontal picture plane. Occasionally it is expedient to assume the picture plane to be neither perfectly horizontal nor perfectly vertical, but tilted. These three alternatives are illustrated in Fig. 1, together with the resulting images. As is to be expected, each produces a perspective image having noteworthy individual characteristics, and each has its special uses.
1A shows the sort of picture given by the vertical picture plane. The
vertical lines of the object, being parallel to the picture plane, appear
still vertical, still parallel, and still in their true proportions,
(not scale) in the image. Except in the special case of one-point
perspective, which we will discuss in a moment, the other two sets of lines
do not appear in their true directions, parallelism, or scale. Figure 1B
shows the character of the picture obtained with a horizontal picture plane.
Here the verticals loose their original character, while the parallel
horizontals remain parallel and in true proportion. Lastly in figure 1C we
have a case in which the picture plane, not being parallel to any of the
principle lines of the object, produces an image in which no lines
appear as parallels or in true proportion.
The picture plane may also be assumed to be at any given or desired distance from the eye. When it is close to the eye and distant from the object, we see an image small in size compared to the size of the object; when it is close to the object and distant from the eye, the image approaches the actual size of the object. It is even possible to assume for it a position behind the object, in which case we get an enlarged image. This is what the microscope does for us - pushes the picture plane back of the object under examination. In each of the cases shown in Fig. 2 and, as a matter of fact, in all perspective drawings, the rays from the object converging in the eye form a cone with they eye as apex. When, as in Fig. 2A, the picture plane cuts across this cone near the apex, the resulting cross section is necessarily very small. In Fig. 2B the cross section is taken near the larger end and is thus relatively large.
The third position, illustrated in Fig. 2C, calls for some exercise of the imagination. Although the rays do not, in FACT, continue beyond the object, there is no reason why, if it suites our convenience, we should not assume that they do. As will be seen later, this may be of practical value when actual dimensions are too minute for clear and comfortable presentation in a picture.
In any rectangular object there are three sets of parallel lines, mutually perpendicular to each other. These are: first, lines running from top to bottom; second, lines running from side to side; third, lines running from front to back. It was noticed in Fig. 1C, for instance, that each set of lines, actually parallel to the object, tends in the image to converge toward a point in the distance. Since there are three such sets of lines, there are consequently three such points. When the picture plane is parallel to any one of these sets of lines (usually the vertical set as in Fig. 1A), one of the points disappears and the lines in question appear truly parallel in the perspective image. Occasionally the picture plane is parallel to two sets of lines in the object. In this case two of the three sets of lines will appear truly parallel in the perspective, and only one point of convergence is needed.
obviously impossible for the picture plane to be parallel to all three sets.
For this reason it is impossible to make a realistic drawing of a solid
without using at least one such point of convergence, but perfectly
satisfactory pictures of plane objects, such as textile designs, printed
pages, etc., may be, and usually are, made with no such points whatever.
points in perspective images are called vanishing points and
are of fundamental importance in making perspective drawings. The three main
variations are shown in Fig. 3 above. These are called receptively,
three-point, two-point, and one-point perspective. One-point
perspective is also called parallel perspective. Three-point
perspective, as in Fig. 3A, results when the picture plane is not parallel
to any of the principle lines of the object. This is the case when the
picture plane is tilted.
When the picture plane is vertical, the usual case, it is naturally parallel to vertical lines in the object, which appear truly vertical, as in Fig. 3B, and naturally parallel to each other. Moreover, the two vanishing points of the horizontal lines both lie on the same horizontal line. This fact is of great importance in drawing. More about this later. Lastly, when the picture plane is parallel not only to the vertical lines, but to one of the sets of horizontals as well, these horizontals appear as truly horizontal in the image. This case, with the single remaining vanishing point, is shown in Fig. 3C.
When, as often happens, there are sets of lines in an object that are not parallel to the three principle sets listed previously, these lines have separate vanishing points of their own. The peaked roof of a house, for example, requires two auxiliary vanishing points, one for each side. Thus a two-point perspective of such a house would actually have four vanishing points. It is called two-point perspective nonetheless, for only the vanishing points for the principle lines are counted.
Two-point perspective is used in about 90 per cent of ordinary drawings. One-point perspective should be used when only one plane of an object is of interest, and perspective is needed only to suggest depth. Three-point perspective is valuable when we want to suggest the effect of looking down from a great height, such as the top of a tall building or an airplane in flight. It is also useful for the exact opposite, looking up at such a building from the street level.
In this session we have briefly surveyed some of the theory, that is to say, the "why" of perspective - why vertical lines usually appear parallel and in true proportion, why we sometimes tilt the picture plane, etc. The remainder of this series will be largely devoted to technique, i.e., to "how" - how to draw lines in true perspective, how to obtain correct proportions along lines where a scale cannot be used, how to draw a circle seen obliquely, and numerous other problems. This does not mean that we shall cease to consider theory altogether, but that merely from here on it will serve mainly to clarify the principles of practical work methods.
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