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Use radiosity to create physically based lighting simulations. When doing so, keep in mind the following:
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).
Lights: You should work exclusively with Photometric lights. You should also make sure that the light intensities are within a normal range.
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.
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.
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:
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 also refer to Common Lamp Values.
Choose Rendering > Environment to display the Environment dialog. Select the type of exposure control you want to use (typically Logarithmic).
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.
Choose Rendering > Advanced Lighting to display the Advanced Lighting dialog. On the Select Advanced Lighting rollout, choose Radiosity, and make sure that Active is turned on.
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.
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.
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, Standard lights are be 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 (i.e. no attenuation, manual attenuation, linear decay etc.), 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.
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.
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:
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.
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.
Choose Rendering > Advanced Lighting to display the Advanced Lighting dialog. On the Select Advanced Lighting rollout, choose Radiosity. (And make sure that Active is turned on.)
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.
In the Interactive Tools group of the Radiosity Processing rollout, click Setup to display the Environment dialog, where you set exposure controls.
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.
Click Render Scene to render the scene after radiosity processing.
By default, a radiosity solution is calculated at the current frame. If you are animating objects and you want to perform a radiosity solution at every frame, select Compute Advanced Lighting When Required in the Render Scene dialog.
Once the renderer starts processing each frame of your animation, the radiosity solution will be computed for each frame as required. This occurs, for example, when an object moves, or a light intensity changes. If nothing changes in the scene from one frame to the next, the radiosity engine will not recalculate the solution.
Note: Due to the random statistical sampling used by the radiosity engine, there might be some flickering between frames. If this occurs, increase the value of Initial Quality or the number of Refine Iterations to solve the problem.
Tip: Before launching a lengthy animation with radiosity, you should process a radiosity solution manually for a single frame to make sure the results are acceptable.
Tip: If you animate only your camera (as in an architectural walk-through) then a radiosity solution will be calculated only for the first frame of the animation and it will be re-used for all subsequently rendered frames.
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. |
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