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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 realistic 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 evident. 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.

FIGURE 45.7

This scene is lighted using normal lighting (left) and radiosity lighting (right).

Lighting for radiosity

The Advanced Lighting panel in Max includes Light Tracing and Radiosity options. You can choose either option from the Advanced Lighting panel in the Render Scene dialog box. You can open this panel with the Radiosity option selected using the Rendering⇒Radiosity menu command. Pressing the 9 key opens the Render Scene dialog box with the Advanced Lighting panel open.

Radiosity lighting is not displayed until you have Max compute it by clicking the Start button in the Radiosity Processing Parameters rollout. After a radiosity solution is calculated, the results are saved as light maps. These maps are easy to apply to a scene and can be viewed within the viewports. However, when the geometry or lights of the scene change, you need to recalculate the lighting solution.

The Radiosity Processing Parameters rollout, shown in Figure 45.8, lets you set the quality of the radiosity solution. You also can specify the number of iterations to use for the scene and for the selected objects. These are different steps in the radiosity computation. The Initial Quality defines the accuracy of the rays that are bounced around the scene. This stage sets the brightness for the scene. The Refine Iterations improves the general quality of the lighting solution for each iteration. You can refine iterations only for the selected object. This lets you target the iterations instead of computing them for the entire scene.

The Interactive Tools section lets you specify a Filtering value. A greater Filtering value eliminates noise between adjacent surfaces by averaging the lighting coming from all surrounding surfaces. The Setup button offers access to the Exposure Control rollout in the Environment panel. You also can turn off radiosity in the viewports.

Cross-Reference

Chapter 23, “Rendering a Scene and Enabling Quicksilver,” includes coverage of the Exposure Control rollout. •

Subdividing a mesh for radiosity

As you begin to play with radiosity, you'll quickly find that to get accurate results, you need to have good, clean models. If any models have long, thin faces, then the results are unpredictable.

The Radiosity Meshing Parameters rollout includes an option to enable meshing and a Meshing Size value. This setting is the same as the Size value parameter for the Subdivide modifier, except that it is applied globally.

FIGURE 45.8

The Radiosity Processing Parameters rollout includes buttons for computing a solution.

Tip

If you're creating an indoor room using the Box object, be aware that Box objects have only one external face surface with normals, so the interior of a Box object will not have the correct lighting. You can easily fix this by applying the Shell modifier to the Box object. This adds an interior set of faces to the Box object.

Using the Subdivide modifier

The Modifiers menu includes a submenu for Radiosity modifiers. This submenu includes only the Subdivide modifier and a World-Space version of the Subdivide modifier. This modifier accomplishes