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Part XI: Advanced Lighting and Rendering
FIGURE 45.6
Color bleeding becomes much stronger with a higher Bounce value.

Understanding Radiosity

Imagine a scene that includes an umbrella with a light source directly overhead. If you rendered the scene,
the object caught in the umbrella’s shadow would be too dark for you to see clearly. To fix this situation,
you would need to add some extra lights under the umbrella and set them to not cast shadows. Although
this workaround provides the solution you want, it is interesting to note that this isn’t the case in real life.
The difference between the workaround and real life has to do with the effect of light energy being reflected
(or bounced) off the lit objects. It is this phenomenon that allows me to look down the hall and see whether
my children’s light is still on past bedtime. Even though I can’t see the light directly, I know it is on because
of the light that reflects off the other walls.
Radiosity is a lighting algorithm that is based on how heat or energy transfers across surfaces. Every time a
bit of light energy, called a photon, strikes a surface, the light energy is reduced, but the light energy is
bounced onto the surrounding faces. The greater the number of bounces that are computed, the more real-
istic the lighting solution, but the longer it takes to compute. So, using radiosity, the objects under the
umbrella are visible even if they are in the shadows. Because of the way the light is computed, radiosity
solutions are not capable of generating direct specular highlights.
Radiosity is mostly used to light indoor scenes because that is where the effect of light bouncing is most evi-
dent. Radiosity, along with light tracing, is another method for computing global illumination. Figure 45.7
shows the dinosaur exhibit in a museum with and without radiosity. Notice how dark the shadows are in
the normal lighting image.

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