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Radiosity is a global illumination technique that simulates the distribution of indirect light throughout an environment using a physically accurate lighting model. It computes the distribution of ambient light reflected from one or more diffuse surfaces throughout a scene. The radiosity method divides the scene into small patches and iteratively calculates how light bounces from one patch to all of the others.  It accounts for how light energy bounces around a scene from one surface to another, and back again, many times.

The physical accuracy of radiosity calculations results in images that are potentially more realistic; if physically correct input data is provided, then the results are faithful representations of the real world. This allows aesthetic judgments to be made from radiosity images, and also allows quantitative measurements of illumination levels to be obtained from the simulation's results.

Radiosity can be enabled on the Rendering tab of the Shading Manager.


What is Radiosity?

Radiosity is a measure of radiant energy, specifically the amount of energy leaving a surface, per unit of area, per unit of time. The principle underlying the technique is to divide all the surfaces in the model into small areas called patches, and to calculate a radiosity value for each patch. This data is then used to produce an image made up of all the patches visible from a chosen viewpoint. Patches with high radiosity values are rendered more lightly, making them appear brighter, while those with low radiosity values are rendered darker, making them appear comparatively dim.

A number of important effects can simulated by using radiosity, such as the indirect illumination of parts of an environment that are not directly lit by a primary light source. This is important because many surfaces in an interior environment are lit by no direct illumination at all, and are visible only by light reflected diffusely from other surfaces. Radiosity can also account for area light sources, resulting in more accurate soft shadowing, as well as color bleeding effects. (A surface illuminated indirectly may appear to be a different color, since color from one surface can "spill" or "bleed" onto another, particularly if bright colors are placed next to more subdued hues.)

The primary advantage of using the radiosity technique is that it accurately models the type of environments where there are matte surfaces which diffusely reflect light in all directions and onto other matte surfaces. The inside of a building is a very common example of such an environment, and radiosity is therefore an important technique for architects, builders and lighting engineers, and computer graphics designers who are modeling interior environments. The disadvantage of the technique is that it is costly in terms of computer time and memory. It is, however, the only technique that can reproduce physically accurate lighting conditions in a large class of common scenes.

Radiosity is a post-processing technique that is applied after an image has been rendered. It takes geometry as its input, and generates geometry as its output. What results is an alternative representation of the geometry supplied as input, but subdivided into small elements and with radiosity values attached to each vertex. This geometry can be used for rendering, employing any of the standard rendering methods available in Evolve.


hmtoggle_plus1To enable radiosity in your scene:

1.Open the Shading Manager using the Managers > Shading menu command or the CTRL + 3 keyboard shortcut.

2.Click the Rendering tab.

3.Select the Radiosity checkbox in the Rendering chain.

4.In the Parameters column, select the On checkbox under Enable.

5.Specify the Length Units. By default all dimensions are assumed to be presented in meters.

6.Adjust the other parameters as desired. These settings determine how radiosity is calculated and applied to the scene as a whole.

7.Close the Shading Manager.

8.Render the scene.  If you have been using standard raytrace rendering, the image will probably appear too bright and the materials somewhat unrealistic, so you will need to make adjustments.

9.Adjust materials to compensate for the radiosity calculation.

10.Adjust lights to compensate for the radiosity calculation.

11.Render the scene and evaluate the results. You may find that you want to refine how radiosity is being calculated for selected objects in your scene using the Radiosity Refinement shader on the Surface tab of the Shading Manager.

12.Continue making iterative renderings and adjustments until you are satisfied with the results.



Defining Materials for Radiosity

The radiosity technique models diffuse reflections between different surfaces in a scene, and thus the fraction of light that each surface reradiates needs to be defined. This is achieved by defining material shaders in such a way that surfaces are fully optimized for both the radiosity simulation and for later rendering, without the need for alteration.

Only two classes of materials are used for the radiosity simulation: color and reflectance. Note that other materials, textures, and environments may also be applied. These additional shaders will be ignored by the radiosity calculation, but used by applicable rendering methods once the radiosity solution is complete. The solution can be rendered in any supported rendering method, such as raytrace preview, raytrace full, etc. Rendering using a photo-realistic method will allow a full range of effects, as well as the radiosity solution, to be incorporated into the final image — all of which contributes to making an image look real rather than artificial.

If you're not used to using radiosity, you will probably need to adjust the parameters of your material shaders to compensate for the radiosity calculation. Materials optimized for use without radiosity will tend to look too bright and unrealistic once radiosity is applied.


Defining Lights for Radiosity

Light sources are specified in Evolve through the use of light source shaders. The radiosity technique divides the light sources in an environment into two groups:

primary sources (defined by light source shaders)

secondary sources (defined by materials)


Initially, only light source shaders provide illumination for the scene and have 'positive' power; all other surfaces have zero radiosity. These light sources provide the initial energy input for the environment and are known as the primary sources. The light source types that support radiosity are area, point, spot, goniometric, sky, and distant.


Secondary sources are surfaces that have been illuminated by primary sources and/or by other surfaces. These surfaces reflect light by re-radiating a portion of the energy incident on them. The behavior of secondary light sources depends on the materials applied to them.


Once the energy from the primary sources has been shot, all surfaces are treated as both emitters and receivers/reflectors of light energy.


hmtoggle_plus1To Use Ambient Lighting with Radiosity:

Note that the radiosity technique does not support ambient light source shaders, which are therefore ignored during the radiosity simulation. To create ambient lighting when using radiosity, you should:


1.Open the Shading Manager using the Managers > Shading menu command or the CTRL + 3 keyboard shortcut.

2.Click the Rendering tab.

3.Select the Radiosity checkbox in the Rendering chain.

4.In the Parameters column, select the On checkbox under Ambient.

5.Close the Shading Manager.


Enabling this option evenly distributes all of the unaccounted-for energy in the scene over all surfaces, and provides a much better approximation of ambient lighting conditions after only a few iterations of the radiosity solution. Of course, all ambient lighting effects are only approximations of fully realistic indirect lighting, which can be modeled more accurately by running the radiosity simulation over a large number of iterations so that all the important secondary sources are accounted for.



Refining Radiosity

The way radiosity is calculated for the entire scene can be defined via the Rendering tab of the Shading Manager, but as you refine your rendering, you may want to tweak radiosity settings for individual objects. This can be done using a radiosity refinement shader.


hmtoggle_plus1To refine how radiosity is calculated:

1.In the modeling views, select the object to which the radiosity settings should be applied.

2.Open the Shading Manager using the Managers > Shading menu command.

3.Right-click on Radiosity refinement in the Shader Tree and select radiosity.

4.Adjust the  parameters as desired.

5.Close the Shading Manager.


See Also:

Global illumination


Final gather