I have been working on the first, a ray tracing and path tracing library, for quite some time now. It is based on my studies of Ray Tracing from the Ground Up by Kevin Suffern, a book I recommend if you want to start from scratch on the topic and before delving into the more complete but also harder to understand Physically Based Rendering:From Theory To Implementation.
I also developed an ImGui front-end to make it easier to perform tests and gather important feedback about speed and correctness, two things that are severely lacking at the moment.
But I decided to publish both projects either way because it can give me the motivation to refine them and you the opportunity to have a look at an application that uses the OpenGL and the threading private API of nCine by compiling it statically.
You can try the Emscripten version here.