https://github.com/NVIDIAGameWorks/RTXGI-DDGI
https://morgan3d.github.io/articles/2019-04-01-ddgi/index.html
Dynamic Diffuse Global Illumination with Ray-Traced Irradiance Fields
Real-Time Global Illumination using Precomputed Light Field Probes
The visibility information is the most important difference between DDGI and traditional irradiance environment mapping. The Ray Tracing APIs, such as DXR (DirectX Raytracing) and Vulkan Ray Tracing, make it possible to calculate the hit distance in hundreds of directions at each light probe position in real time.
Analogous to the VSM (Variance Shader Map), the Chebyshev's inequality is used to compare the distance of the shading position with the "average" distance stored in the light probe to test the visibility.
TODO: According to the VSM (Variance Shader Map), both the distance and the squared distance should be stored in the light probe. However, according to the DDGIGetVolumeIrradiance of the RTXGI SDK, only the distance is stored in the light probe and the squared distance is calculated from the sampled distance. This may cause the "average" squared distance incorrect.