Considering Shadows

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Considering Shadows

A shadow is a simple concept to understand. When you stand in the sun next to a wall, you cast a shadow. If light beams can't penetrate an object, sometimes they're absorbed. Because the world is three-dimensional, light hits objects that are in front of other objects first. Now, because the light is blocked, this creates darker areas on the second object. Figure 1.6 shows some examples of shadows.

Figure 1.6. Shadows are,in essence,a void of light.

graphic/01fig06.gif


Based on the light source and your position relative to an object, you create two types of shadows. The closer you are to the object, the more you generate a sharp-edged solid shadow. If you move back a few steps, your shadow becomes blurrier around the edges. This is the nature of shadows, based on the light source's position and brightness. The next sections discuss these two types of shadows in more detail.

Understanding How an Umbra Is Formed

To explain the umbra, let's go back to the example of shadows on a wall. This time, consider a puppy standing next to the wall in the sunlight. Now, when the puppy is very close to the wall, it casts a solid, sharp-edged shadow. This is called the umbra . The reason this happens is that light travels in massive amounts. When there is little distance between the object and the wall, the light doesn't get a chance to hit the object from too many angles. This causes the light to hit the object directly on the surface with very little scattering, thus causing the shadow to look sharp. Figure 1.7 shows this concept.

Figure 1.7. An umbra is a particular ly sharp shadow that results from a short distance between the object cre ating the shadow and the location of the shadow.

graphic/01fig07.gif


Understanding How a Penumbra Is Formed

Now, if the cute little puppy moves back a few steps, its shadow becomes blurry. This is the penumbra . It is created when the object blocking the light is far away from the path of light. The penumbra happens when the light from one source is partially blocked and light from another source fills in. Because light waves travel in massive amounts, the distance between the puppy and the wall is far enough to give the light beams other angles to intersect the object. This causes more scattering from different incoming directions, thus making the shadow blurry around the edges. Again, the umbra is the dark solid part of the shadow, and the penumbra is the blurry outer area of the shadow. See Figure 1.8.

Figure 1.8. A penumbra is essentially a blurry shadow.

graphic/01fig08.gif


NOTE

NOTE

A more interesting example of a shadow occurs when the moon and the Esarth cast a shadow when the sun's light is directly facing them.When the two objects overlay each other,a shadow appears where a shadow from the moon is cast on the earth.This is known as a solar eclipse .

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Focus On Photon Mapping
Focus On Photon Mapping (Premier Press Game Development)
ISBN: 1592000088
EAN: 2147483647
Year: 2005
Pages: 128
Authors: Marlon John

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