Just preparing to start experimenting

• https://www.mitsuba-renderer.org/
• https://appleseedhq.net/about.html
• https://gpuopen.com/firerays-2-0-open-sourcing-and-customizing-ray-tracing/
• https://github.com/bcrusco/CUDA-Path-Tracer
• http://www.kevinbeason.com/smallpt/

Pretty interesting, but I'm primarily interested in Open Source, General-Purpose (not restricted to specific use-cases) Path Tracing APIs. Perhaps one that can be used with a spare GPU to apply Path Tracing to scene rendering. Maybe one that doesn't require rewriting existing projects if possible (can be dropped in, in place of other rendering methods/APIs)?...