How Radiosity Works

The problem of ray-tracing is to figure out what the light level is at each point that you can see in a scene. Traditionally, in ray tracing, this is broken into the sum of these components:

-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.

Adjusting Radiosity

As described earlier, radiosity is turned on by using the Radiosity INI file option or the +QR command line switch. However radiosity has many parameters that are specified in a radiosity statement inside a global_settings statement as follows:

global_settings { radiosity { brightness FLOAT count INTEGER distance_maximum FLOAT error_bound FLOAT gray_threshold FLOAT low_error_factor FLOAT minimum_reuse FLOAT nearest_count INTEGER recursion_limit INTEGER } }

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.


This is the degree to which ambient values are brightened before being returned upwards to the rest of the system. If an object is red < 1, 0, 0>, with an ambient value of 0.3, in normal situations a red component of 0.3 will be added in. With radiosity on, assume it was surrounded by an object of gra color <0.6, 0.6, 0.6>. The average color returned by the gathering process will be the same. This will be multiplied by the texture's ambient weight value of 0.3, returning <0.18, 0.18, 0.18>. This is much darker than the 0.3 which would be added in normally. Therefore, all returned values are brightened by the inverse of the average of the calculated values, so the average ambient added in does not change. Some will be higher than specified (higher than 0.3 in this example) and some will be lower but the overall scene brightness will be unchanged.

The default value is 3.3.


The number of rays that are sent out whenever a new radiosity value has to be calculated is given by count. Values of 100 to 150 make most scenes look good. Higher values might be needed for scenes with high contrast between light levels or small patches of light causing the illumination. This would be used only for a final rendering on an image because it is very compute intensive. Since most scenes calculate the ambient value at 1% to 2% of pixels, as a rough estimate, your rendering will take 1% to 2% of this number times as long. If you set it to 300 your rendering might take 3 to 6 times as long to complete (1% to 2% times 300).

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.


The distance_maximum is the only tuning value that depends upon the size of the objects in the scene. This one must be set for scenes to render properly... the rest can be ignored for a first try. It is difficult to describe the meaning simply but it sets the distance in model units from a sample at which the error is guaranteed to hit 100% (radiosity_error_bound >=1): no samples are reused at a distance larger than this from their original calculation point.

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 error_bound is one of the two main speed/quality tuning values (the other is of course the number of rays shot). In an ideal world, this would be the only value needed. It is intended to mean the fraction of error tolerated. For example, if it were set to 1 the algorithm would not calculate a new value until the error on the last one was estimated at as high as 100%. Ignoring the error introduced by rotation for the moment, on flat surfaces this is equal to the fraction of the reuse distance, which in turn is the distance to the closest item hit. If you have an old sample on the floor 10 inches from a wall, an error bound of 0.5 will get you a new sample at a distance of about 5 inches from the wall. 0.5 is a little rough and ready, 0.33 is good for final renderings. Values much lower than 0.3 take forever.

The default value is 0.4.


Diffusely interreflected light is a function of the objects around the point in question. Since this is recursively defined to millions of levels of recursion, in any real life scene, every point is illuminated at least in part by every other part of the scene. Since we can't afford to compute this, we only do one bounce and the calculated ambient light is very strongly affected by the colors of the objects near it. This is known as color bleed and it really happens but not as much as this calculation method would have you believe. The gray_threshold variable grays it down a little, to make your scene more believable. A value of .6 means to calculate the ambient value as 60% of the equivalent gray value calculated, plus 40% of the actual value calculated. At 0%, this feature does nothing. At 100%, you always get white/gray ambient light, with no hue. Note that this does not change the lightness/darkness, only the strength of hue/grayness (in HLS terms, it changes H only).

The default value is 0.5

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