Shadow perspective deserves a chapter to itself. It is complicated for the beginner because it introduces two new sets of perspective lines, those of the angle of incidence of light and those of the direction of its radiation. There are few problems with sunlight, which consists of parallel rays. On the other hand, artificial light radiating from a virtual source point onto a nearby object is always radial or cone shaped. Furthermore, its strength diminishes the farther the illuminated surface lies from the source of light. Although the effect of shadows is simple enough to portray from observation and compo rison with other lines of perspective, the phenomenon can only be rightly understood after a closer study of how it arises and how it can be constructed.
There are two kinds of shadows: the object's own shadow, in which any surface of the object lies that is turned away from the light, and the shadow cast by the object. Cast shadow is supposed to be always darker than the other, but this is not necessarily the case. A surface in its own shadow is generally lightened by reflected light; the cast shadow less so; the latter also looks darker by contrast, being surrounded by illuminated areas, These differences in tone can make an interesting contribution to perspective. The effect of depth is less if both shadows are of the same tone, which may well happen under certain lighting conditions.
The angle of incidence of daylight determines the breadth of a shadow, which always stands in a constant relationship to the surface throwing the shadow. One of its edges always runs parallel to the surface, and thus has the same vanishing point. The other edge runs parallel with the line of incidence of the sunlight, and thus has its own vanishing point. It does not coincide with any other vanishing point of the perspective, except by chance. All the effects of daylight shadow are built up from its vanishing points and the angle of incidence of the light taken from the point of view of the draftsman.
Both circular surfaces and spheres always throw relatively circular, elliptical shadow. The general rule for other bodies or surfaces is that in daylight, they throw a shadow of a similar shape, geometrically speaking. Both the surface which throws and that which receives the shadow can be considered as the sections of a column with one common edge. This rule is of assistance when the light comes from an artificial source, that is to say, radiates from a point. It always throws an area of shadow corresponding to a conic section. The profile of this cone is determined by what shadow-throwing surfaces it meets.
The shadows made by artificial sources of light often have very strange shapes. The best way to understand them is to study the sections of every type of conic profile. Circles and ellipses illuminated from a concetnrated source will always throw shadows corresponding to the known stereomatric curves of conic sections: circle, ellipse, parabola, hyperbola.
So far we have discussed light from a single source only. There is also diffused light, which can come as well from several directions as from one; for instance, in a room from several windows. Diffused light throws shadows as much as direct light. Generally, the shadows are weaker and their edges less clearly defined, and if the light comes from several different directions, there will be several shadows spread like a fan from each object with gradations in their density.
Shading to indicate rotundity was mentioned when discussing the filling-in of surfaces with fading texture. Spherical volume can be conveyed only by shading. An unshaded circle looks like a disc; a shadow on one side darkening to the periphery gives the impression of a hemisphere.
The only way to create
the illusion of a full sphere is to make a shadow running into a lighter band of reflected light near the circumference. This is very important for drawing and painting because natural shapes are generally rounded and do not have the sharp corners of cubes and rectangles. If a head or body, or a tree trunk, is to look convincingly three-dimensional the shading should never, except in a few extreme cases, be brought right to the edge of the outline, unless the effect is to be one of high relief.