|-||Diffuse, the effect that makes the side of things facing the light brighter;|
|-||Specular, the effect that makes shiny things have dings or sparkles on them;|
|-||Reflection, the effect that mirrors give; and|
|-||Ambient, the general all-over light level that any scene has, which keeps things in shadow from being pure black.|
POV's radiosity system, based on a method by Greg Ward, provides a way to replace the last term - the constant ambient light value - with a light level which is based on what surfaces are nearby and how bright in turn they are.
The first thing you might notice about this definition is that it is circular: the light of everything is dependent on everything else and vice versa. This is true in real life but in the world of ray-tracing, we can make an approximation. The approximation that is used is: the objects you are looking at have their ambient values calculated for you by checking the other objects nearby. When those objects are checked during this process, however, a traditional constant ambient term is used.
How does POV-Ray calculate the ambient term for each point? By sending out more rays, in many different directions, and averaging the results. A typical point might use 200 or more rays to calculate its ambient light level correctly.
Now this sounds like it would make the ray-tracer 200 times slower. This is true, except that the software takes advantage of the fact that ambient light levels change quite slowly (remember, shadows are calculated separately, so sharp shadow edges are not a problem). Therefore, these extra rays are sent out only once in a while (about 1 time in 50), then these calculated values are saved and reused for nearby pixels in the image when possible.
This process of saving and reusing values is what causes the need for a variety of tuning parameters, so you can get the scene to look just the way you want.
Each item is optional and may appear in and order. If an item is specified more than once the last setting overrides previous values. Details on each item is given in the following sections.
The default value is 3.3.
When this value is too low, the light level will tend to look a little bit blotchy, as if the surfaces you're looking at were slightly warped. If this is not important to your scene (as in the case that you have a bump map or if you have a strong texture) then by all means use a lower number.
The default value is 100.
Imagine an apple at the left edge of a table. The goal is to make sure that samples on the surface of the table at the right are not used too close to the apple and definitely not underneath the apple. If you had enough rays there wouldn't be a problem since one of them would be guaranteed to hit the apple and set the reuse radius properly for you. In practice, you must limit this.
We use this technique: find the object in your scene which might have the following problem: a small object on a larger flatter surface that you want good ambient light near. Now, how far from this would you have to get to be sure that one of your rays had a good chance of hitting it? In the apple-on-the-table example, assuming I used one POV-Ray unit as one inch, I might use 30 inches. A theoretically sound way (when you are running lots of rays) is the distance at which this object's top is 5 degrees above the horizon of the sample point you are considering. This corresponds to about 11 times the height of the object. So, for a 3-inch apple, 33 inches makes some sense. For good behavior under and around a 1/3 inch pea, use 3 inches etc. Another VERY rough estimate is one third the distance from your eye position to the point you are looking at. The reasoning is that you are probably no more than 90 inches from the apple on the table, if you care about the shading underneath it.
The default value is 0.
The default value is 0.4.
The default value is 0.5