In general, we can state that a very small light source will result in a very large shadow and conversely that a very large light source will result in a very small shadow. Let's look at some examples.

Notice in this image that the shadows are hard-edged and grow wider farther away from the pillar object. Since the light source is much smaller than the pillar, the light rays that get past the pillar are diverging as in Figure 9.2.

In Figure 9.3, a light source was used that is much larger than the pillar. The result is that the light rays are able to converge behind the pillar object, thereby lighting that area. The result is that the shadows (or occluded areas) are much smaller and softer edged.

This phenomenon is described in Figure 9.4. You can see that the area light is large enough that some of the light rays will reach areas behind the pillar.

Will it surprise you to learn that both of the rendered images were created using an area light? The first image simply used a very small area light. If you look carefully, you will see that the shadows become slightly soft farther away from the pillar, just as in the real world.

Take a look at Figure 9.5. Note that the shadows cast by the car in the foreground are very hard-edged while the shadows of the trees are very soft. The sun is the light source for both car and tree, so why is this? Well, we know that the trees are much farther away from the foreground of the image than the car is. It is further evidence that shadows are hard near the object that is casting the shadow and get softer farther away due to the penumbra effect.

Following is a description of the shadow types available in LightWave.

Ray Trace

Take a red thumbtack and stick it into your ceiling. Then take a spool of thread and tie one end of the thread to the thumbtack. Walk away from the thumbtack, letting the spool play out the thread until you reach a wall. Cut the thread and tape the end to the wall. Make sure the thread is taut and straight. You now have a line drawn from a source point to a target point. Add a million more threads all originating at the red thumbtack (your light source) and hitting walls, floor, chairs, tables, and everything else in the room. That's how ray-traced shadows work.

Whenever an object gets in the way of your thread, the ray stops. There is no ray behind it. That's a shadow. And it's sort of how light works too. A photon is flung from the surface of the sun and flies through space in a reasonably straight line, analogous to the threads (not counting gravity, you bad physicists), until it impacts some opaque material that stops its travel.

So what's the difference between real sunlight and ray-traced light? The thumbtack-and-thread example we just looked at has all the threads starting at a single point in space. But the sun is not a single point. It's a big, luminous ball. If we wish to create a more realistic model, we will need to take a very large box of thumbtacks, stick them into the ceiling in a big circle as close to each other as possible and then run a million threads from each pin to points all over the walls, floor, ceiling, and anything else in the room. You can see how this requires a great deal more work. You can also see why a point source ray-traced light calculates much more quickly than an area light, which is, in essence, just like the big circle of thumbtacks on the ceiling.

Distant lights also ray trace shadows, but they work a little differently. Instead of all the light originating from a single point and radiating outward, the light rays all run parallel to each other from the beginning to eternity. They have no origin and no position in space, only direction. For more on how distant lights work, see Chapter 7.

All the light types in LightWave's toolset are equipped for ray tracing. The only real disadvantage to shadows produced by a ray-tracing light source is that they are very hard-edged. In reality, since all lights have some amount of size, all shadows must therefore have some amount of softness since the light rays will "wrap around" objects, even if it is just a small amount. Smaller lights will tend to have harder shadows than larger lights because the light wraps around less, but none of them cause completely hard-edged shadows. Ray-traced shadows are typically used where sunlight is your main light source or where you see shadows that are very near the object that is casting them. Most people seem to perceive sunlit shadows as hard-edged, and most shadows are harder edged close to their object. But beware of this. People think sunlight causes hard-edged shadows, but when they see the hard shadows in your CG work, they subconsciously know that it looks wrong.