Which Lights Need Shadows?
In real life, all lights cast shadows; there is no equivalent in real life to having a light with shadows turned off. In computer graphics, you could imitate this rule by turning on shadows from all of your lights. More often, however, 3D artists are highly selective about which lights need to cast shadows and which don't.
Next time you watch a television comedy that was filmed in front of a studio audience, take a look at the floor. Around the actor's feet, you often see a pattern of overlapping shadows, each aiming a slightly different direction. The overlapping shadows are caused by the grid of spotlights overhead, which light the actors and the set. These shadows would not be motivated by the light in most real rooms.
You may even notice that the set itself was designed and built to hide many of the extra shadows. For example, the living room sofa in a situation comedy would be out in the middle of the stage, with a lot of space behind it. Often there will even be a staircase filling the rear wall of the set, so that the actors would never be sitting or standing right in front of a wall that would accumulate too many of those unmotivated overlapping shadows.
When you see how far designers go to hide unwanted shadows, you can appreciate how lucky we are in 3D to be able to turn off any shadow we don't like. In animation, living room sofas can be pushed right against a wall, with no unwanted shadows behind the characters.
When used judiciously, the power to leave shadows turned off for selected lights is a wonderful option. Figure 3.6 shows how much cleaner and simpler a scene can look with one unified shadow compared to a clutter of several overlapping shadows.
Figure 3.6 Which do you like better? In most cases, simpler, unified shadowsselecting lightsshadows are better than many overlapping shadows.
In complex scenes, you usually need more than one light casting shadows. Trying to get away with only one shadow-casting light will especially hurt you in areas that are already in a shadow. For example, the ball in the left frame of Figure 3.7 does not look fully "attached" to the ground because it does not cast a shadow. Since the ball is in an area where one shadow is already being cast, it is being lit only by a secondary light, which does not cast shadows. In the right frame of Figure 3.7, turning on shadows from the other light better attaches the ball to the ground, even though it is in a shadow area.
Figure 3.7 Without secondary shadowing (left), the ball casts no shadow. Scenes without secondary shadowing can look very flat in shadow areas. With shadows (right), the ball appears better attached to the ground.
Any light that doesn't cast shadows creates risks: It could shine straight through walls, illuminating the inside of a room as brightly as the outside. A light from behind a character, meant to illuminate a character's hair and shoulders, could end up brightening the inside of the character's mouth. It could also make parts of your scene look very flat in places where you would expect smaller objects to shadow each other.