Let's assume you have a scene ready. The first thing to grasp when doing Radiosity is that no Lamps are necessary, but some meshes with Emit material property greater than zero are, since these will be the light sources.
You can build the test scene shown in , it is rather easy, just make a big cube, the room, give different materials to the side walls, add a cube and a stretched cube within and add a plane with an non-zero Emit value next to the roof, to simulate the area light.
Please note that the light emission is governed by the direction of the normals, of a mesh, so the light emitting plane should have a downward pointing normal.
Switch to the Radiosity Buttons . The Button window is shown in (Figure 2 in the Section called The Interface).
Select all meshes (AKEY and press the button Collect Meshes)
Now the selected Meshes are converted into the primitives needed for the Radiosity calculation. Blender now has entered the Radiosity mode, and other editing functions are blocked until the button "Free Data" has been pressed.
Press the button Gourad as opposed to Solid to have smooth shading. Press the GO Button.
First you will see a series of initialisation steps (at a PIV, it takes a blink), and then the actual radiosity solution is calculated. The cursor counter displays the current step number.
Theoretically, this process can continue for hours, as photons are shot and bounced. Luckly we are not very interested in the correctness of the solution, instead most environments display a satisfying result within a few minutes. Blender shows the resulting solution as it progresses. When you are satisfied, to stop the solving process, press ESC.
Now the Gouraud shaded faces display the energy as vertex colors. You can clearly see the 'color bleeding' in the walls, the influence of a colored object near a neutral light-grey surface. In this phase you can do some postprocess editing to reduce the number of faces or filter the colors. These are described in detail in the next section.
To leave the Radiosity mode and save the results press "Replace Meshes" and "Free Radio Data". Now we have a new Mesh Object with vertex colors. There's also a new Material added with the right properties to render it (Press F5 or F12).
Beware, this is a one-way only process. Your original objects are lost... It's better to have saved a copy of your work.