Using Radiosity Adaptive Subdivision

The new releases of Autodesk^® VIZ 2006 and Autodesk^® 3ds Max^® 7 include the use of adaptive subdivision in creating radiosity solutions.   In order to create the lighting of a scene, VIZ/MAX 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.  As the radiosity solution is refined through meshing, you'll also have to adjust the parameters, the  Refine Iterations, and the Filter values to approach an ideal lighting scenario.  And while it's possible to accomplish meshing globally, in which all objects are meshed using the same setting, it's seldom an efficient method.  The objects that contribute most to the bounced lighting usually require the smallest mesh size, and the objects that contribute little to the overall lighting can be meshed at a greater size or not at all.

Here's a short tutorial to get you started.

Open this file room W table 01a.zip, process the radiosity, and render.   See image 1

Now set up Adaptive Subdivision.  Open the radiosity dialog, go to the meshing rollout, make sure it's enabled, and then click on Use Adaptive Subdivision.  (Notice that when you enable adaptive subdivision, the parameters for Shoot Direct Lights are enabled.)  Keep all the remaining parameters the same, hit Update & Continue, and render again.  See image 2.

Look at the mesh on the wall.  It's a bit subdivided, but not enough.  Play with the numbers.  Make Min Mesh Size = 4", and reprocess.  Not much has changed.  Contrast will define how much light difference in a face is needed for it to be subdivided.  The lower the contrast, the more you subdivide.  Change the contrast to 50 and reprocess your solution.  Now you see some more mesh under the table and behind it on the wall.  See image 3.

Now let's add detail.  Make Max Mesh = 40", Min = 2", and Contrast = 40.  You get a finer shadow, but now you get some artifacts.  See image 4.  (You can override the subdivision settings in this group from the Advanced Lighting panel of the Object Properties dialog. This allows you to have a different mesh resolution on some objects. For example, you might want to have a finer mesh on an important wall surface that you know will have a lot of detail. To display the Object Properties dialog, right-click a selected object and choose Properties from the quad menu.)

In order to remove artifacts, set refine iterations (All Objects) to 2 and hit continue.  (Notice you do not have to reprocess the entire solution this time.)  To further remove the artifacts, increase your Direct and Indirect light filtering to 2 or 3.  See image 5.

This should get you started. 
 

TIPS: 

(1)  A tight meshing is not necessary when you use regathering.

(2)  Make sure your material reflectance is not too high. (You can enable the display of a material's reflectance from the Preference Settings dialog.) 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.

In general, the best way to adjust this is to reduce the HSV Value (V) of a material color; or, for a bitmapped material, reduce the RGB Level. In some situations, changing the type of material to Advanced Lighting Override can improve the appearance of the radiosity solution.   (If this material type doesn't show up in your Material/Map Browser, click on Compatible with Renderer instead of Compatible with both Renderer and Tool Palette.  These settings are included within a rollout in all Architectural material types.)  Advanced Lighting Override material should be applied only when it is necessary because the original material is first calculated then those values are overridden with the adjusted values, using valuable computer resources.

(3)  If the object mesh is well constructed using shared vertices, the Refine Iterations option will have less effect on the quality than models that aren't constructed as well.

(4)  Adjusting exposure control is an important function throughout the radiosity rendering process.  You'll continually make adjustments to exposure control as you're making other changes that affect lighting.

 

Here are a couple of images rendered with various radiosity parameter settings I've written about here (click on image for a larger view):