Editing Radiosity (computer graphics) (section)

=== Sampling approaches ===
The form factors ''F''<sub>ij</sub> themselves are not in fact explicitly needed in either of the update equations; neither to estimate the total intensity Σ<sub>j</sub> ''F''<sub>ij</sub> ''B''<sub>j</sub> gathered from the whole view, nor to estimate how the power ''A''<sub>j</sub> ''B''<sub>j</sub> being radiated is distributed.  Instead, these updates can be estimated by sampling methods, without ever having to calculate form factors explicitly.  Since the mid 1990s such sampling approaches have been the methods most predominantly used for practical radiosity calculations.

The gathered intensity can be estimated by generating a set of samples in the unit circle, lifting these onto the hemisphere, and then seeing what was the radiosity of the element that a ray incoming in that direction would have originated on.  The estimate for the total gathered intensity is then just the average of the radiosities discovered by each ray.  Similarly, in the power formulation, power can be distributed by generating a set of rays from the radiating element in the same way, and spreading the power to be distributed equally between each element a ray hits.

This is essentially the same distribution that a [[path-tracing]] program would sample in tracing back one diffuse reflection step; or that a bidirectional ray-tracing program would sample to achieve one forward diffuse reflection step when light source mapping forwards.  The sampling approach therefore to some extent represents a convergence between the two techniques, the key difference remaining that the radiosity technique aims to build up a sufficiently accurate map of the radiance of all the surfaces in the scene, rather than just a representation of the current view.