Radiosity method generates a image to calculate light energy equilibrium of the scene. Some light sources shoot light energy. And the energy reflects on the objects in the scene. This calculation will be end when no more light energy reflects. However, when does light energy never reflect? It is hard problem. In the most cases of Computer Graphics, it is no need to calculate a luminosity that human being never feels. However, when does human being never feel the light? Because Human being can adjust his eyes sensitivity according to the luminosity. (If you enter a dark room like a movie theater, even if first you can not see anything, after a few minutes later, you will see much more.)
In the Computer Graphics area, how much energy is calculated is one measurement of scene accuracy. We can calculate the total light source energy and how much energy is remaining in the scene. However, this has no one-to-one mapping of the scene quality (The definition of scene quality is vary difficult and I will not mention about that here...). In general, when a scene is generated with less than 90% energy is calculated, that seems always inaccurate. However, the converse is not true. If you calculate more than 90% energy, that is not always accurate. This parameter setting is difficult, and this is one of the reasons that radiosity method is not widely used.
Here, I will show the energy residual values and generated images. Next topics are common in all scenes.
pnmarith -difference [most accurate image] [each image]
Pnmarith calculates absolute value of each pixel differences.