III.6.4 Gathering of energy - second phase
The second phase of the both the radiosity and acoustic radiance transfer techniques performs gathering of the sound energy from all the patches to the receiver. In the following illustration it is possible to see how the response evolves at the receiver location during acoustic radiance transfer energy propagation.

- See how the time-energy response when the iterative sound propagation is advanced.
- Move the listener to see that the response is different at different locations.

Patch size: General absorption: General diffusion: BRDF discretization:
Visualize: All energy Unshot energy

Response visualization: Energy-time curve Schroeder backward integral Impulse response Sampling rate: Hz

The final gathering is a computationally fast operation, much faster than the sound propagation performed in the first phase. It is possible to move the listener and the gather response in real-time even in very complex geometries.

Algorithm: Gathering the energy from all patches

The gathering algorithm goes through all the surfaces that are visible to the listener, and registers the energy accordingly. Here we again need the $1/r^2$ attenuation, as it can be thought that the energy from a surface is emitted as spherical wavefront. This corresponds to the form factors used in the actual propagation as those depend on the mutual distance of patches as well.