Lecture
The idea of instant radiosity consists in approximating secondary illumination (and primary illumination from area light sources) by means of primary illumination from virtual point light sources.
The idea of instant radiosity belongs to Keller, who first described it in his paper in 1997. In the same paper he also proposed an algorithm for generating virtual point light sources. He considered the case of a purely diffuse scene (that is, a scene all of whose surfaces are purely diffuse).
He noted that in practical scenes the reflectivity of surfaces varies little across the scene. Moreover, the norm of the transfer operator can be well approximated by the average reflectivity.
The following algorithm for generating virtual point light sources was proposed. Initially, M particles (points on the light sources) are chosen. Each particle is assigned a certain energy. They become virtual point sources. Next, 
particles <start> from the sources. Virtual point sources are formed at the points where they intersect the scene. Then 
particles are sent onward, and so on. In total, no more than 
point sources will be created.
Knowing the required number of point sources and the average reflectivity of the scene, one can determine M using this formula. Note that for physical correctness it is not at all necessary that it be the average reflectivity of the scene. It is sufficient that it lie in the interval [0, 1) (if the latter is not satisfied, the particles will not be absorbed but will multiply, and the method will never terminate).
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