The drawings thus far considered are what are known as line drawings and, because of the fact that they are made up of lines alone, they lack any effect of depth or modeling. This may be gained only by representing the play of light and shade on the various surfaces. The casting of shadows puts the third dimension into the otherwise flat drawing by indicating projections, recesses, mouldings, etc., and thereby livens it up and gives an impression of the third dimension which is entirely lacking in line drawings.
The elements of a building are pleasing to the eye almost solely because of the shadows they cast. Everyone has noticed the dull, flat appearance of even the most beautiful building on a "grey day" and then how interesting it becomes in the bright sunlight. The cornice, for instance, produces a broad band of shadow across the top of the wall; the details of the cornice break this shadow up into interesting variations making of it a richly mottled band of color; see Plates 15, 16 and 17. The mouldings of less projection trace narrower lines of shadow across the wall and the spots of ornament enrich the surfaces by their wealth of light and shade; see Plate 75. Window and door openings cause dark areas to appear in the composition. These must be carefully considered as they appear prominently in the scheme. Therefore the architect studies the details of his design not alone for beauty of outline but also for the effect of light and shadow that they will produce.
Here again the artistic sense of the designer must be brought into play, but the student can at once learn the mechanical processes by which shadows are determined.
Shadows may be cast either on orthographic projection drawings or on perspectives. The method will be explained in orthographic projection, and the perspective shadows will then be a matter of application of the method. After mastering the principles as here given, the draftsman will be able to cast many shadows with reasonable accuracy by simply visualizing them, and then drawing them without the complete mechanical construction. Plates 15, 16 and 17 are given to aid in casting the shadows of the Orders of Architecture in the last mentioned manner. Approximate work of this kind should not be attempted until a thorough acquaintance with the subject has been made.
PLATE 12. CONVENTIONAL SHADOWS.
Shadows are cast mechanically by drawing lines (representing rays of light) down past the object which causes the shadow to that upon which the shadow is cast. That part of the object which is turned away from the source of light is said to be in shade and that part of another object from which the light rays are kept by the first is said to be in shadow. The shade line is the line which separates the shaded from the lighted parts of an object and the shadow line is the outline of the shadow. Thus it may be seen that the shadow line is the shadow of the shade line. Some parts of a complicated object may be in shade and other parts of the same object in shadow because of the contour of its surface. This is true of the Attic Ionic base of Plate 14.
The sun is of course assumed to be the source of light for most of the architectural shades and shadows. Although the sun's rays are not exactly parallel, they may be assumed to be so in all practical work. A definite position of the source of light is also fixed. For conventional shadows the sun is imagined to be located in front of, above and toward the left of the object, so as to throw the conventional light rays down parallel to the body-diagonal of a cube as in Fig. 65, Plate 12. With the front face of the cube toward the observer, the front, top and side views of this body-diagonal will appear as 45-degree lines, Fig. 65a. This makes the conventional light rays easy to draw mechanically and produces shadows equal in depth to the projection of the object which casts them. Thus a cornice shadow will be just as wide as the cornice projection, Plate 15. This fact, if kept in mind, will simplify many problems.