[Figure 4] No shadows in lights, causing a brightly lit mouth interior.

[Figure 5] A self-shadow from the arm onto the body.

[Figure 6] Sharp self-shadows from many directions.

Some supervisors insist that every light in a computer graphics scene use shadows, but practicality usually calls for some lights without shadows. The render timesavings are one reason, but the affects of a shadowless light can be to the advantage of the final image. Because it is unobstructed, it produces a fairly even level of illumination across a broader area than the same light with shadows. This can be useful for fill lights and bounce lights, or any light in a scene set to a fairly low intensity level. These shadowless lights become evident when they are bright or produce bright specular highlights, because the lack of a shadow opposite intense illumination is a glaring problem. A good way to test the effects of such a light is to isolate it, increase its intensity value to a high level, and render the scene. This points out areas to watch when the light is returned to its normal lighting values.

Types of Shadows
The difference between cast shadows and self-shadows is important to emphasize. Cast shadows are created when one object blocks the light from another object. A simple example is a character standing on the ground, blocking the light from hitting certain areas. The shadow on the ground appears as a cutout of the shape of the object occluding the light, and appears as if the object is casting its shape onto the ground. Cast shadows are only created in computer graphics scenes if the objects the shadows are to be cast upon are built in the scene. In the case of adding a character to a live-action background plate, the other objects are in the film but not necessarily in the CG scene, so creating cast shadows becomes a bit of a trick. Surfaces to receive shadows need to be created in the computer to mimic those in the image. The shadow pass can then be applied to the images in the composite. Purely digital scenes, in which every element exists in the computer graphics scene, offer an easier solution to cast shadows, because they are produced automatically if shadows are turned on.

Self-shadows are created when a part of a CG element occludes the light from another part of that same CG object. A good example is a character’s arm moving between his body and the light source (Figure 5). This shadow is the same in principle as a cast shadow, because one object (the arm) is occluding the light from another object (the body). The difference in computer graphics is that the renderer needs to include each object in its own list of objects to consider when being shadowed. For ray-traced shadows, this means that rays bouncing between surfaces of the same object must be considered. A complex object shape may cause the ray tracer to hit its ray bounce limit sooner. If the level of illumination is to be maintained, more bounces must be allowed, which means more render time. This is why self-shadowing is often an option in a ray tracer. Self-shadows and cast shadows are both vital for making a computer graphics scene convincing. If a character is being placed into a live-action scene, and the shaders offer control over the softness of the shadow’s edges, it is beneficial to turn shadows on in all or most of the lights in a scene. The control over the shadow’s edges is important, because an element can begin to look a little crazy with sharp self-shadows from a variety of directions (Figure 6).