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Adding Light Sources

Most software offers several different kinds of lights that you can add to your scene. Some of the figures in this section are designed to make the individual light as visible as possible, and even highlight the light sources with visible glow effects. In most cases, however, a viewer won't be able to tell which kinds of lights you have used in a final rendering. You can choose types of light as a matter of convenience; after you know where you want to apply and confine your illumination, you can use whichever tool will make the light easiest for you to control.

Point (Omnidirectional) Lights

Point source lights have slightly different names and icons in different programs, but by any name these lights do the same thing: A point light simulates rays shining out from one infinitely small point in space.

Point lights are sometimes called omni or omnidirectional lights, because they emit light uniformly in all directions, like a bare light bulb or glowing star in space. The illumination and shadows aim out away from the light in all directions, as shown in Figure 2.

Figure 2 A point source emits light evenly in all directions.

In real life, you aren't likely to find any light that is uniformly omnidirectional. Most sources emit more light in some directions than others.

If you are adding point lights to your scene to simulate real light bulbs, for example, remember that most real light bulbs have an opaque metal socket that blocks light from one end, and many are mounted in some kind of a fixture or shade that limits their directionality.

Even though point lights start out as omnidirectional, you can give them a throw-pattern that is uneven, like a real light bulb, to aim more light in some directions than in other directions. You can do this is by applying a texture map to the light, or by grouping the light with 3D objects that will cast shadows. Figure 3, for example, shows a point light positioned inside a model of a lampshade; this limits the light to functioning much like a spotlight. The light is blocked by the geometry because it is set to cast shadows.

Figure 3 Shadows and other factors can limit a point light.

Spotlights

Spotlights are a basic staple of most lighting designs in computer graphics. Spotlights are a popular choice of many artists because they can be controlled conveniently to aim light at a specific target, as shown in Figure 4.

Figure 4 A spotlight can aim light at a specific target.

A spotlight simulates light radiating from a point, much like a point light. A spotlight, however, limits the illumination to light within a specified cone or beam of light only. The rotation of a spotlight can determine where the beam is aimed. You can also link a "target" to the light so that the light is always oriented toward the position of the target. You can also group a spotlight with a 3D object, such as a model flashlight or car headlight assembly, so that the beam of light will be aimed as if the light were radiating from the object. Figure 5 shows some of the popular representations of spotlights in different programs, usually displaying an outline of the cone's position and some indication of the direction in which the light is aimed.

Figure 5 Icons show how spotlights are aimed in Alias Power Animator, Electric Image, and Softimage.

Spotlights are staples of visual effects in your renderings. A spotlight has extra controls and options not found on other types of lights. Options such as projecting an image map from a light, or making a beam of light visible as if shining through fog, are often best controlled with the beam of a spotlight.

Other common spotlight parameters enable you to control the width of the cone (usually specified in degrees) to vary between a narrow beam and a broader one. The amount of softness of the cone (also called spread or falloff) allows the intensity of the light to diminish more gradually as it approaches the edge of the beam. A softer edge on a spotlight's beam will make the light's individual location less obvious and will avoid creating a harsh "circle" of projected light, as shown on the left side of Figure 6. This enables you to more subtly lighten or darken areas with a spotlight. With a very soft-edged beam, for example, you can aim a spotlight from within a room to brighten the general area around a window and curtains, or aim a spotlight with a negative brightness at the corner of a room to darken it.

Figure 6 A spotlight cone's softness can be adjusted with the spread or falloff.

Because spotlights can be aimed and controlled so conveniently, some artists rely on them to simulate light from almost any source and light most of their scenes entirely with spotlights. Even when a light needs to shine in multiple directions, such as the light from a table lamp, two or more spotlights can be positioned together and aimed in different directions. In Figure 7, the light from the lamp is created by aiming one spotlight upward and another downward. There isn't necessarily anything wrong with this, although the advantages and uses of other light types should not be overlooked out of habit.

Figure 7 Multiple spotlights can cast light in multiple directions from a source.

Directional Lights

The farther a light is from the subjects being lit, the more the illumination and shadows of the different objects in the scene will become parallel. A nearby point light lights the objects on the left side of Figure 8. Each object is lit and shadowed at a different angle, based on the object's position relative to the point light. In the middle of Figure 8, the point light has been moved a greater distance away from the subjects, and the illumination and shadows have become closer to parallel. A point source would have to be infinitely far away for the shadows to be perfectly parallel; for all practical purposes, however, a very distant light source such as the sun can cast shadows that appear parallel in an ordinary terrestrial environment. On the right side of Figure 8, a directional light has been used rather than a point light, to simulate a point light at an infinite distance.

Figure 8 A nearby point light (left) creates illumination from a greater range of angles than a distant light (right).

Directional lights are variously known as "distant," "direct," "infinite," or "sun" lights in different programs, and have different icons. A directional light sets a single vector for all its illumination and hits every object from the same angle, no matter where the object is located. All the shadows cast by a directional light are cast in the same direction and are orthogonal projections of each object's shape.

It does not matter where a directional light is located relative to the objects being lit. The only thing that matters in placing a directional light is which way it is pointed. The actual angle used by a directional light is controlled differently in different programs. Some implementations use the rotation of an icon to aim the directional light. Other types of directional lights use a vector from the icon to the target or the global origin. Some directional lights simulate the sun's angle, based on a specified date, time, and location.

Because a directional light is not as easy to aim or confine to a local area as a point light or spotlight, it is most useful as a part of your secondary or fill lighting, and not as the main light on a subject. A set of directional lights from different angles can be used together to provide fill light, even if the individual lights from each angle are very dim. Directional lights can fill very large areas with illumination that appears to be ambient or atmospheric, such as filling in daylight from the sky, providing a quick, effective alternative to global ambience.

Area Lights

A standard point or spotlight emits light from an infinitely small point in space, not simulating the size of a physical light source in real life. A point light does not have a scale in any dimension. This means that a point light, as shown on the left of Figure 9, creates crisp, hard-edged shadows, and creates a sharply defined terminator on smooth surfaces. For a more accurate simulation of real light, area lights have a definable scale, so all rays of light are not emitted from exactly the same point. The middle and right panels of Figure 9 show the results of scaling an area light to a larger size.

Figure 9 A point light (left) creates sharper illumination than a spherical area light (middle), which gets even softer when scaled to a larger size (right).

If an area light is scaled very small, its illumination will appear similar to a point light. A larger area light, as shown on the right in Figure 9, will make the light appear softer, creating softer shadows and creating illumination that can "wrap around" nearby subjects.

The quality of light and shadows achievable with area lights can make them an excellent choice for some realistic renderings. Because they can add to your rendering time, however, some people find that on a specific system area lights can be more useful when rendering the occasional high-quality still image than in longer animation projects where many frames need to be rendered quickly.

Area lights are implemented differently in different programs. In some programs, area lights are listed as a separate type of light, as a separate choice from spotlights, point lights, and other types previously described. In other programs, the property of functioning as area light is an option available on existing spotlights or point lights. Having a spotlight with the option of lighting from an area provides the benefit of being able to conveniently aim an area light with a spotlight's cone.

Spherical Area Lights

The area light in Figure 9 is scalable in all three dimensions. This is sometimes called a spherical light, because it simulates light coming from a spherical region of space, similar to a glowing bulb or round light fixture. This can be especially useful for lighting anything that appears very close to a large light source. A spherical light can be uniformly omnidirectional, so the rotation of the light has no effect on your output. A spherical light can be used to replace a point light, when the illumination needs to appear more diffused or you want your light to cast soft-edged shadows.

Flat Area Lights

Area lights are commonly available in flat shapes such as discs and rectangles. A light emitted from a two-dimensional area can use less rendering time than light emitted from a three-dimensional area, although the number of samples may be adjustable with either approach. A rectangular area light, as shown in Figure 10, is similar to an illuminated ceiling panel, a plane scalable in width and height, that illuminates objects with a naturally soft, diffused light. In addition to simulating light from ceiling panels, other common uses for flat area lights are simulating the reflected illumination from brightly lit walls and ceilings and providing a soft, realistic light source for portraits or still-life renderings.

Figure 10 A rectangular area light resembles a panel of soft light.

Aiming or rotating a flat area light changes your results, because light coming from the edge of the flat area light is more likely to create sharper shadows and be dimmer; light from the face of the light, on the other hand, will be brighter and more diffuse.

Linear Lights

A linear light is a type of area light that is similar to a fluorescent tube and can be scaled in only one dimension. Giving it a length can soften the shadows and extend its illumination in one direction. Figure 11 shows a linear light illuminating a scene. Notice how the aiming of the linear light affects the output: The shadows of the objects along the length of the light are very soft, but shadows from the end of the light are more focused.

Figure 11 A linear light emits softer light along I length than from its ends.

You can use linear lights to simulate light from fluorescent tubes, laser blasts, or any other sources that appear to be lines of light. Even when a line shape is not specifically needed, linear lights (rather than flat or spherical area lights) are sometimes used for general soft lighting in a scene because they do not add as much to your rendering time as the other types. A linear light can produce an appearance very similar to a flat or spherical area light, but can render more quickly because it needs to be sampled only along one axis rather than two or three.

Models Serving as Lights

In some programs, a 3D model in your scene can be designated to function as a sort of area light source. With this feature, even nontraditional shapes of light, such as a neon sign, can be used as true light sources, as shown in Figure 12. In most programs, however, effects such as neon signs are usually faked by rendering a bright-looking object and also by positioning some other kind of light near the sign to illuminate the surrounding area.

Figure 12 Your own model can provide a custom light shape.

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