7.5.1 Introduction

Radiosity is a lighting technique to simulate the diffuse exchange of radiation between the objects of a scene. With a raytracer like POV-Ray, normally only the direct influence of light sources on the objects can be calculated, all shadowed parts look totally flat. Radiosity can help to overcome this limitation. More details on the technical aspects can be found in the reference section.

To enable radiosity, you have to add a radiosity block to the global_settings in your POV-Ray scene file. Radiosity is more accurate than simplistic ambient light but it takes much longer to compute, so it can be usefull to switch off radiosity during scene development. You can use a declared constant or an INI-file constant and an #if statement to do this:

#declare RAD = off;

  global_settings {
     #if(RAD)
        radiosity {
           ...
        }
     #end
  }

Most important for radiosity are the ambient and diffuse finish components of the objects. Their effect differs quite much from a conventionally lit scene.

• ambient: specifies the amount of light emitted by the object. This is the basis for radiosity without conventional lighting but also in scenes with light sources this can be important. Since most materials do not actually emit light, the default value of 0.1 is too high in most cases. You can also change ambient_light to influence this.
• diffuse: influences the amount of diffuse reflection of incoming light. In a radiosity scene this does not only mean the direct appearance of the surface but also how much other objects are illuminated by indirect light from this surface.

From [pov:6.11.11]:
Important notice: The radiosity features in POV-Ray are somewhat experimental. There is a high probability that the design and implementation of these features will be changed in future versions. We cannot guarantee that scenes using these features in this version will render identically in future releases or that full backwards compatibility of language syntax can be maintained.

Radiosity is an extra calculation that more realistically computes the diffuse interreflection of light. This diffuse interreflection can be seen if you place a white chair in a room full of blue carpet, blue walls and blue curtains. The chair will pick up a blue tint from light reflecting off of other parts of the room. Also notice that the shadowed areas of your surroundings are not totally dark even if no light source shines directly on the surface. Diffuse light reflecting off of other objects fills in the shadows. Typically ray-tracing uses a trick called ambient light to simulate such effects but it is not very accurate.

Radiosity calculations are only made when a radiosity block is used inside the global_settings block.

The following sections describes how radiosity works, how to control it with various global settings and tips on trading quality vs. speed.