The following sections describe how to set up a scene for use with radiosity.
For imported geometry, you must make sure that units are consistent in your scene before processing radiosity (for example, a wall is 8 feet high, not 8 kilometers high). 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 setup correctly, use the Units Setup dialog. The Scene Unit is the most important unit in this dialog. This is the unit that 3ds Max uses for 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 will measure 9 scene units. Therefore, in the Units Setup dialog, you must set Scene Unit Scale to 1 Unit=10 centimeters. Your table is now the correct units because it is 90 centimeters long in 3ds Max model.
Example 2: You have an AutoCAD model that was created using Architectural Units. The model is a room measuring 20’-4” long. In AutoCAD, Architectural Units are stored as inches. Therefore, before importing the model to 3ds Max, make sure to set the Scene Unit Scale to 1 Unit=1 inch. Once imported to 3ds Max, the room will measure 244 units long (20’*12+4”).
Tip: Use the Measure Distance tool to quickly check dimensions in 3ds Max.
Use radiosity to create physically based lighting simulations. When doing so, keep in mind the following:
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Scene dimensions: Make sure your scenes are accurately dimensioned, with consistent units (a light bulb in a room 120 meters high would look a lot different than it would in a room 120 inches high).
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Lights: You should work exclusively with Photometric lights. You should also make sure that the light intensities are within a normal range.
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Natural Lighting: To simulate natural light, you should only use IES sun and IES Sky. These provide accurate photometric representations of sunlight and skylight based on a specified location, date and time.
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Material Reflectance: You should ensure that the materials you use in your scene have a reflectance value within the range of the physical materials they represent. For example, a painted white wall should have a maximum reflectance of approximately 80%; however, a pure white color material (RGB:255, 255, 255) would have a reflectance of 100%. This means that the material reflects 100% of the energy received.
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Exposure Control: The exposure control is the equivalent of the aperture of a camera. Make sure you enable the exposure control and set a value that provides the final results you desire.
To process radiosity for photometric lights using a physically based workflow:
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Ensure that your geometry is set to a physically correct scale and that the materials have valid reflectance values.
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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.
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Choose Rendering > Environment to display the Environment panel. Select the type of exposure control you want to use (typically Logarithmic).
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Click Render Scene to preview the lighting. At this stage, the radiosity will not be processed but you can quickly confirm
that the direct lighting is correct. Adjust the position of the lights if desired.
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Choose Rendering > Advanced Lighting > Radiosity. Make sure that Active is turned on.
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On the Radiosity Parameters rollout, click Start to process radiosity. 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 interactively navigate around the model without reprocessing the scene.
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Click Render Scene again. The renderer will calculate the direct lighting and shadows, and the radiosity solution (indirect
lighting) will be integrated as a modulated ambient light.
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 interactively visualize the light levels in the scene by using the Pseudo Color Exposure Control. Rendering to a 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).
Note: To obtain the most accurate quantitative analysis of lighting levels, you should avoid using colored materials and diffuse maps.
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:
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Lights: Because the radiosity engine is physically based, Standard lights are interpreted by the engine as Photometric lights. For example, a Standard Spot light with a multiplier value of 1.0 will be 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 will always solve for these lights using the physically correct Inverse Square attenuation. This means that the amount of energy that bounces between surfaces might not be equivalent to the way the Standard lights render.
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Natural Lighting: To simulate natural lighting without using the physically based workflow described above, you can only use a Direct Light for the Sun and Skylight to produce skylight.
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Exposure Control: Since Standard lights are not physically based, you should only use exposure controls for the radiosity solution. Use the Logarithmic Exposure Control, making sure to turn on Affect Indirect Only. The Brightness and Contrast controls of the exposure control will only affect the radiosity solution and your lights will render as usual.
To process radiosity with standard lighting:
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Ensure that your geometry is set to a physically correct scale.
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On the Create panel, click Lights. Create and position standard lights in your scene.
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Click Render Scene to preview the lighting. At this stage, the radiosity will not be processed, but you can quickly confirm
that the direct lighting is correct. Adjust the position of the lights if desired.
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Choose Rendering > Advanced Lighting > Radiosity. Make sure that Active is turned on.
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On the Radiosity Parameters rollout, click Start to process radiosity. Once the Radiosity calculation has been completed, you should see your results in the viewports.
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In the Interactive Tools group of the Radiosity Processing rollout, click Setup to display the Environment panel, where you set exposure controls.
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When working with non-physically based lights, always use the Logarithmic Exposure Control. On the Logarithmic Exposure Control rollout, select Affect Indirect Only. This will cause the exposure control to affect only the results of the radiosity solution. By doing so, you will 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.
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Click Render Scene to render the scene after radiosity processing.
The following table will 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. |