On Sun, 25 May 2003 06:32:28 +1200, "Daniel Ngu" <ngu### [at] xtraconz>
wrote:

>So, to sum it up, radiosity is a subset of global illumination, i.e., one of
>the method or algorithm used in global illumination.

Precisely.

>The context of radiosity in POV-Ray means achieving the same effect but
>not implemented using that particular "Radiosity" algorithm.

Correct again.

>If that is the case, any reason for the difference in implementation?

Theory of operation.

In most 3D packages, objects are represented by meshes of triangles,
polygons or NURBS (which are further subdivided into such meshes or
rendered on-the-fly). In POV-Ray and most other pure raytracers,
objects are internally represented by their mathematical description,
for example a spere is not just a million triangles but rather "the
surface defined as all points equidistand from the center".

Radiosity is a means of simulating global illumination in
polygon-based renderers. The principle is very basic. Normally, the
color of a pixel is determined by several things:

1) The texture of the triangle at that point
2) The amount of shadow at that point as determined by the shadow map,
if any
3) The amount of transparency, if any
4) The environment map, if any
5) The bump map, if any

etc.

Radiosity works by creating an additional map which is "painted" over
the triangles, the so-called "light map". It's similar to any other
map being closest to the shadow map.

The way it works is the following. First, every triangle is, in
layman's terms, turned into a camera, and what this triangle "sees" in
terms of brightness is stored onto it as its light map. When all
triangles have been traversed, the complete scene's light map has been
built. If further accuracy is desired, the process is repeated, but
this time the already accumulated light map is taken into account,
i.e. you can get surfaces lit indirectly from already indirectly lit
surfaces (this is called a recursion level and this is level 2). Etc.
etc. until you reach the desired level of accuracy.

Obviously, this method can not work with rendereds where objects are
simply not polygons. Therefore POV implements the so-called Stochastic
Global Illumination, which you can look up in the docs under
"Radiosity". 

I hope this is making the picture a bit more clear.


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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