4.297 Geometric Modeling
Fall 2002
Radiosity Supplemental Handout


Radiosity

  1. What is Radiosity?
    Radiosity is rendering technology that realistically simulates the way in which light interacts in an environment by calculating the intensity of light for all surfaces in the environment. This is accomplished by first dividing the original surfaces into a mesh of smaller surfaces known as elements. The radiosity algorithm calculates the amount of light distributed from each mesh element to every other mesh element. The final radiosity values are stored for each element of the mesh.


  2. Setup units
    Make sure your units are the same as your AutoCAD drawing.
    Customize>Units Setup...

Modeling for Radiosity

  1. Clean Surfaces
    Since radiosity works by subdividing surfaces, you may see strange artifacts when surfaces do not meet at an edge. For a clean model, make sure all the edges meet up with other edges. No surface should be partially hidden behind another surface or be coplanar with another surface.

  2. Union model
    One method to eliminate hidden surfaces is to union the solids together and explode the unioned solid into surfaces. Then assign different layers to each type of surface depending on orientation and surface material.

Radiosity Control Panel

  1. Initial Quality
    Determines how much energy to distribute. Start small (50%) and work your way up. 90-95% is usually more than enough. A distribution of 99% of all the energy will either take forever or never happen.

  2. Logarithmic Exposure Control
    Logarithmic Exposure Control uses brightness, contrast, and whether the scene is outdoors in daylight to map physical values to RGB values. Logarithmic Exposure Control is better for scenes with a very high dynamic range. Typically the brightness should be lower when using daylight and higher when using photometric lights.

  3. Refine Iterations (All Objects)
    Because of the random nature of the sampling during the initial quality stage, some of the smaller surfaces or mesh elements in the scene might miss being hit by enough rays (or any rays at all). These small surfaces remain dark, and result in the appearance of "variance" or dark spots. To alleviate these artifacts, the Refine stage "regathers light" at every surface element. Energy from each face is gathered in order to reduce the variance between faces using a different process from the Initial Quality stage. This stage does not increase the brightness of the scene, but it improves the visual quality of the solution and significantly reduces variance between surfaces.

  4. Process Refine Iterations Stored in Objects
    Used for animation to maintain consistency between frames.

  5. Filtering
    Reduces the amount of noise between surface elements, by averaging the lighting levels with the surrounding elements. A value of 3 or 4 is usually sufficient. If you use too high a value, you risk losing detail in the scene. However, since Filtering is interactive, you can readily evaluate the result and adjust it as you need.

  6. Display Radiosity in Viewport
    Toggles the display in the viewports between radiosity and standard VIZ shading.

  7. Global Subdivision Settings
    In order to create the lighting of a scene, VIZ calculates the intensity for discrete points in the environment by subdividing the original surfaces into elements which are part of a radiosity mesh. This rollout allows you to determine whether you want a mesh or not, and to specify the size of the mesh elements in world units. For quick tests, you might want to turn off the mesh globally. The scene will look flat, but the solution will still give you a quick impression of the overall brightness. The finer the mesh resolution is, the more accurate the lighting detail will be. But there is a trade-off in time and memory.

    You can override the settings in this group from the Radiosity panel of the Object Properties dialog. This allows you to have a different mesh resolution on some objects.
Mesh Override for Objects
  1. Select an object and check its properties.
    Edit>Properties...>Radiosity
  2. Click on "By Layer" to make it say "By Object"
  3. Uncheck "Use Global Subdivision Settings" and set the desired meshing size.


Materials for Radiosity (Reducing Reflectance)

  1. Before you generate a radiosity solution, check the reflectance value of all materials in the scene, to make sure it is not too high. The reflectance of a material determines how much of the light energy it receives is subsequently used in the radiosity calculation. Keep this value within the range of the physical materials you are simulating.

    Material Minimum Maximum
    Ceramic 20% 70%
    Fabric 20% 70%
    Masonry 20% 50%
    Metal 30% 90%
    Paint 30% 80%
    Paper 30% 70%
    Plastic 20% 80%
    Stone 20% 70%
    Wood 20% 50%

  2. Reflectance is the percentage of diffuse light energy that is reflected from a material. When you increase a material's HSV value (V), the material reflects more diffuse light. Decreasing a material's Opacity also decreases its reflectance.

    Typically, the reflectance of a material should never be greater than 85%. this is an unusually high value that will lead to poor-quality renderings. In the real world, even the whitest wall reflects no more than 80% of the light it receives.

  3. Once source of high reflectance can be a map assigned to the material's diffuse component. For example, a white tile bitmap might create high reflectance. In this case, you can reduce reflectance by reducing the RGB Level in the bitmap's Output rollout.

    An alternate way to reduce a bitmapped material's reflectance is to set the diffuse color of the material to black, and then reduce the diffuse map's Amount (in the parent material's Maps rollout). You can use this method to reduce the reflectance of 3D procedural maps as well.

  4. Don't be concerned if a material preview seems too dark. For example, a white wall with the maximum recommended reflectance of 80% appears gray. The color balance will be adjusted correctly by the exposure control in the final rendering.

  5. You can also use Radiosity Override Material for more fine tune adjustments of reflectance and transmittance.

Sunlight in Radiosity

  1. Create>Systems>Daylight
    Drag to create the radius of a compass rose (the radius is for display purposes only), and then click and move to set the orbital scale of the directional light over the compass rose. This can be any distance you find convenient, since a directional light produces parallel illumination regardless of where its icon is located.

  2. Adjust the time of day and location as desired.

Photometric Lights in Radiosity

  1. Create>Photometric Lights>[Pick a light]
  2. Create>Photometric Lights>Preset Lights>[Pick a light]
  3. http://www.erco.com/