[Figure 7] Sharp self-shadow from the key light and soft self-shadowing from additional lights.

A scene typically has a single, dominant light source, called the key light. This light usually produces the most clearly defined shadow in a scene, while other less intense lights cast softer, fuzzier shadows. This is especially true for a light used to simulate ambient lighting in a scene, such as a bounce light (remember: ambient lighting is not the same as a CG ambient light). Every light source casts shadows, but the more diffused the source, the less perceptible the shadow becomes. With ambient lighting in a scene that has bounced off several surfaces before reaching the subject, the shadow can be faint and almost imperceptible. This makes it possible to place a great amount of blur on shadows from bounce lights, or even turn them off completely (Figure 7).

While fully digital scenes offer all the geometry necessary for cast shadows, they also present a problem in terms of using shadows in all lights. Due to the diffusion of many light sources in a scene, it is likely not desirable to have harsh shadows from every light criss-crossing through the image. This can completely change the composition of an image, not to mention confusing the original point of interest. For this reason, I find it more appropriate to use most lights without shadows in purely digital scenes as compared with fitting a CG element into a live-action background. The fully digital scene has no point of reference within the image to point out the inconsistencies of non-shadowed lights with reality. It is not advisable to turn off shadows in every light in a digital scene, or even in every light other than the key. Having several shadowless bounce and fill lights, however, can work as long as the possible pitfalls are taken into account.

Cookies
Cookies offer a nice break from the technical discussions of shading rates and anti-aliasing. Although they’re not as tasty as their name might indicate, cookies are a valuable tool in both live-action film lighting and computer graphics lighting. A cookie (also called gobo, cukaloris, cuke or slide map) is something placed in front of a light, usually with irregular openings, to occlude certain areas of light from the subject. The resulting patterns simulate something between the light source and the subject, such as tree leaves, clouds, and so on. In computer graphics, a painted texture is often used as the cookie. The texture can be purely black and white, with black completely occluding light and white being completely transparent. Gray values can also be used with a 50% gray area filtering out one-half of the light’s intensity. Colors used in the cookie texture simulate effects such as light shining through a stained glass window. These colors affect not only the intensity of the light (colors represent a value which decreases the amount of illumination, just as with the grayscale image), but also contribute to the color of the illumination reaching the subject.

A common usage for a cookie, seen in many films, is a dappled sunlight effect. A cookie is created in the form of a grayscale image to produce a pattern of tree leaves in the key light for a scene (Figure 8).

[Figure 8] Texture used as a cookie for creating a dappled sunlight effect.

This pattern is large in scale and simulates a high canopy of trees with larger clumps as opposed to individual leaf shapes. The gray areas in the transition between black and white will only partially occlude the light, and they help make the transition from light to shadow less harsh. The cookie shown here is sharp and high in contrast, but it can be processed, prior to its use in a light, to add blur or distortion. It can provide the subtle effect of a character or creature walking in and out of sunlight and tree shadows without producing harsh shapes and distracting the viewer from the intended point of emphasis in the scene. For example, the cookie used in front of the light for a scene from Star Wars: Episode I — The Phantom Menace is blurred considerably, because hard-edged shadows on Jar Jar would distract from the intent of the scene. Each of Jar Jar’s forefingers receives a bright hit of light in which the key light makes its way cleanly through the cookie. The forest behind the three foreground characters offers additional reference for the type of lighting a cookie is often used to simulate. The effect is downplayed slightly in this close-up scene, but in studying previous scenes, the effect is more pronounced as Jar Jar walks through the forest. In those scenes, the primary goal of the shots is to emphasize the motion of traveling through the forest. Well-defined shadows from the trees offer a clear indication of the distance over which the character travels. To that end, less blur on the cookie textures serves to enhance rather than distract from the shot’s intent.

Instead of a single texture used as a cookie, a sequence of frames can be used to simulate motion of the occluding object. Using the leaves from the previous example, a simulation could produce the effect of rustling leaves. The textures are simulated, either with a particle system or a procedural texture map that changes over time, and are rendered out as a sequence of frames. Footage of the sky viewed through leaves blowing in the wind could also be used. The total number of textures is usually at least the length of the shot for which they are to be used. Many times, the textures are used on multiple shots, and the sequence may need to be looped (looping is continuously repeating a sequence of frames) to cover the entire sequence. As long as there are no distinctive movements and the shapes produced by the cookie textures are not too well defined, looping the sequence is usually not noticeable.