It uses a total of 37 raytraced spotlights to simulate a diffuse lighting. If you wish to simulate radiosity from the ground, just use version 01 wih appropriate image and turn cast shadows off for the ground plane. Note how the shadows are a lot smoother than with the first version. The render time for these tests at D1 pal resolution, low anti-aliasing with motion blur, would be around 7 minutes. Create your scene as usual.
Load also lights.
Replace the sky image supplied with Overcaster with the image you wish to use. I didn't put much effort in exactly matching the scenes together, but you should be able to see the overall quality from these. Lightwave 6 radiosity, default settings, except for the minimum ray spacing which was set to 2mm. 18 min 23 sec. Overcaster in Lightwave 5.6.