TAPESTRY: The Art of Representation and Abstraction

Radiosity Overview

And Now For Something Completely Different

Radiosity represents an interesting convergence oflighting simulation and rendering. Up to this point lip-service has been paid to Lambert's law in the fundamental cosine shading equation, but great liberties have been taken with fall-off rates, surface textures, etc. All of this is done to achieve a believable image, not necessarily an accurate image. Radiosity is different. It offers the possibility of being believable and accurate. It does this by invoking the theory of radiant exchange.

Radiant Exchange Between Surfaces

The illustration at right shows part of the interior of a small room. The wall surfaces have been divided into several rectangular regions, called surface "nodes". The logic of radiosity goes like this:
The concept of an angle factor may be easier to understand in terms of the 2D situation diagrammed at right. A small wall at point A obscures, or subtends the same angle as a large wall further away, at point B. The light leaving the "node" (at the point of the triangle) is leaving in all directions uniformly (where have we seen that before!). So if a surface subtends 1/4 of the semi-circle (the surface only radiates on one side), it must be getting one third of the energy. We would say that it has an angle factor (or shape factor) of 0.25.

Last updated: April, 2014

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