https://wccftech.com/amds-radeon-rays-4-0-announced-as-closed-source-then-mostly-opened-again-after-backlash/

Mere days after the relaunch of the GPUOpen program featuring new tools and an expanded FidelityFX suite, AMD also launched the 4.0 version of Radeon Rays.

The ray intersection acceleration library (formerly known as FireRays) is part of the AMD ProRender software suite. However, it could previously only run on the CPU, which was quite the limitation. Now, with the first RDNA2 AMD GPUs already confirmed to introduce hardware support for ray tracing, Radeon Rays 4.0 finally introduces BVH optimization specifically for GPU access alongside requiring one of the major low-level APIs: Microsoft's DirectX 12, Khronos' Vulkan, and Apple's Metal. It also supports Heterogeneous-Compute Interface for Portability (HIP), which is AMD's C++ parallel computing platform (the equivalent of NVIDIA's CUDA).