This topic describes how to set up a scene for use with radiosity. Considerations include the size of the scene and the measuring system, the lighting, and the materials used in the scene.
Set Units Correctly Before Processing Radiosity
When using imported geometry, make sure the units are consistent in your scene before processing radiosity. For example, a wall’s height is more likely to be 8 feet than 8 kilometers. Units in 3ds Max must match the units of the model because the radiosity engine always uses an inverse-square falloff for lights. Therefore, distance is crucial.
To make sure your units are set up correctly, use the Units Setup dialog. The system unit is the most important setting on this dialog. The system unit is the measurement on which 3ds Max bases its calculations. The Display Unit is just a tool that lets you customize how units are displayed in the user interface.
The following two scenarios show how to set unit scales after importing geometry that has been created using different units than what is currently set in 3ds Max:
Example 1: You import a table that was created in AutoCAD using metric scale. The table is 9 units long, which corresponds to an actual length of 90 centimeters. When the table is imported into 3ds Max, it measures 9 scene units. Therefore, using the Units Setup 
System Unit Setup dialog, you must set System Unit Scale to 1 Unit=10.0 Centimeters. The table now uses the correct units because it is 90 centimeters long in the 3ds Max model.
Example 2: You have an AutoCAD model that was created using Architectural Units. The model is a room whose length is 20’4”. In AutoCAD, Architectural Units are stored as inches. Therefore, before importing the model to 3ds Max, set the System Unit Scale to 1 Unit=1 inch (this is the default setting). Once imported to 3ds Max, the room length will measure 20’x12+4”=244 units.
Physically Based Workflow
Use radiosity to create physically based lighting simulations. When doing so, keep in mind the following:
- Scene dimensions: Make sure your scenes use accurate dimensions, with consistent units. For example, illumination from a light source in a room 120 meters high differs significantly from that of the same source in a room 120 inches high.
- Lights: Work exclusively with Photometric lights and make sure the light intensities fall within a normal range.
- Natural Lighting: To simulate natural light, use only IES Sun and IES Sky. These provide accurate photometric representations of sunlight and skylight based on a specified location, date and time.
- Material Reflectance: Ensure that the materials in your scene have reflectance values within the range of the physical materials they represent. For example, a painted white wall should have a maximum reflectance of approximately 80% while a pure white color material (RGB:255, 255, 255) has a reflectance of 100%. This means that the material reflects 100% of the energy received.
- Exposure Control: The exposure control is the equivalent of the aperture of a camera. Activate an exposure control and set a value that provides the final results you desire.
To process radiosity for photometric lights using a physically based workflow:
- Ensure that your geometry is set to a physically correct scale and that the materials have valid reflectance values.
- Place photometric lights in your scene. The benefit of this workflow is that it allows you to place lights in your scene the same way you would in the real world. You can create new photometric lights or, using the asset browser, drag and drop preset luminaire objects from the included library.
You can also refer to Common Lamp Values.
- Choose Rendering Environment to display the Environment panel. Select the type of exposure control you want to use (typically Logarithmic).
- To preview the lighting, click
(Render Production). At this stage, no processing of radiosity occurs, but you can quickly confirm that the direct lighting is correct. If you like, adjust the position of the lights.
- Choose Rendering Advanced Lighting Radiosity, and then confirm any alerts that appear. On the Select Advanced Lighting rollout, make sure Active is on.
- To process radiosity, on the Radiosity Processing Parameters rollout, click Start.
Once the Radiosity calculation has been completed, you should see your results in the viewports. The light levels are stored with the geometry and you can navigate the model interactively without reprocessing the scene.
- Click (Render Production) again.
The renderer calculates the direct lighting and shadows and then integrates the radiosity solution (indirect lighting) as a modulated ambient light.
Lighting Analysis
After you generate a radiosity solution, you can use the Lighting Analysis tool to analyze the lighting levels in your scene. This dialog provides data on material reflectance, transmittance, and luminance.
You can also visualize the light levels in the scene interactively with the Pseudo Color Exposure Control. Rendering to the Rendered Frame Window displays an additional rendered frame with a legend below the image. The legend correlates lighting levels and color values.
If you need to generate a lighting report, you can use the Lighting Data Exporter utility to export the luminance and illuminance data to a 32-bit LogLUV TIFF file or a pair of PIC files (one each for luminance and illuminance).
Non-Physically Based Workflow
You don’t necessarily have to work with physically based lights and materials in order to incorporate radiosity effects into your renderings. But there are a number of issues that you need to consider:
- Lights: Because the radiosity engine is physically based, the engine interprets Standard lights as Photometric lights. For example, a Standard Spot light with a multiplier value of 1.0 is translated as a Physically Based Spot light with an intensity value of 1500 candelas (default value). This translation value corresponds to the Physical Scale value in the various exposure controls.
In addition, if your Standard lights use custom attenuation settings (for example, no attenuation, manual attenuation, or linear decay), the radiosity engine always solves for these lights using inverse square attenuation, which is physically correct. This means that the amount of energy that bounces between surfaces might not be equivalent to the way the Standard lights render.
- Natural Lighting: To simulate natural lighting without using the physically based workflow described above, you can use only a Direct Light for the Sun and Skylight to produce skylight.
- Exposure Control: Standard lights are not physically based, so use the Logarithmic Exposure Control for the radiosity solution. Be sure to turn on Affect Indirect Only. The Brightness and Contrast controls of the exposure control will affect only the radiosity solution and your lights will render as usual.
To process radiosity with standard lighting:
- Ensure that your geometry is set to a physically correct scale.
- On the
Create panel, click 
(Lights). Create and position standard lights in your scene. - To preview the lighting, click (Render Production).
At this stage, the radiosity is not processed, but you can quickly confirm that the direct lighting is correct. Adjust the position of the lights if desired.
- Choose Rendering Advanced Lighting Radiosity and confirm any alerts that appear. On the Select Advanced Lighting rollout, make sure Active is on.
- To process radiosity, on the Radiosity Processing Parameters rollout, click Start. Once the Radiosity calculation has been completed, you should see your results in the viewports.
- To display the Environment panel, where you set exposure controls, in the Interactive Tools group of the Radiosity Processing Parameters rollout, click Setup.
- When working with non-physically based lights, always use the Logarithmic Exposure Control. On the Logarithmic Exposure Control Parameters rollout, turn on Affect Indirect Only.
This causes the exposure control to affect only the results of the radiosity solution. This way you maintain the way your direct lights render without radiosity. Use the Brightness and Contrast controls of the exposure control to adjust the intensity of the radiosity solution to match the lighting at an appropriate level.
Tip: You can use the thumbnail preview to adjust brightness and contrast interactively. - To render the scene after radiosity processing, click (Render Production).
Summary
The following table is designed to help you obtain good results with radiosity.
| Physically Based Workflow | Non-Physically Based Workflow | |
|---|---|---|
| Lights | Photometric Lights | Standard Lights |
| Daylight | IES Sun and IES Sky | Directional Light and Skylight |
| Exposure Control | Any | Logarithmic: Turn on Affect Indirect Only. |
| Units | Make sure your scene is set to the appropriate scale. | Make sure your scene is set to the appropriate scale. |