Nope, there is none. Not even mentioned on the Readme. Somehow people tend to forget the licensing aspect of Open Source, it’s not just putting your code on display
From the Github docs: "If you publish your source code in a public repository on GitHub, according to the Terms of Service, other users of GitHub.com have the right to view and fork your repository."
So by using GitHub, he has implicitly 'signed over' some rights to other GitHub users.
How does it compare to Blender (in terms of quality of render) ? I ask because ray tracers have progressed so much that I don't know how to compare them anymore...
In terms of quality, the difference between quality of ray tracing engines is mostly up to the user at this point. People who have zero experience with Manta Ray won't be able to do better renders than people with non-zero experience with Cycles, and vice-versa. Same goes if you use Renderman, V-ray, POV-Ray, Mental or any other renderer. In capable hands, they can all do absolutely wonderful renders, but again, mostly up to the user rather than the engine.
In terms of user experience, if you're using Blender, it's much easier to use Cycles as it's already integrated in Blender, while Manta Ray is a standalone renderer.
