vtkOSPRayPass counter-intuitively depends on OpenGL
In the documentation, the
vtkOSPRayPass is billed as "a render pass that uses OSPRay instead of GL" (emphasis mine). One would deduce from that that making use of the OSPRay renderer would put the user beyond issues of OpenGL contexts. That's certainly, philosophically, consistent with the fact that OSPRay comes (ultimately) from Intel and taxes the life out of a computer's CPUs.
However, the reality is quite different. The current implementation of vtkOSPRayRenderPass still relies on the OpenGL machinery to register actors, lights, etc. The implication of that is it makes "offline" rendering impossible (say, via ssh into a server). No available OpenGL context --> no OSPRay rendering.
Making the separation from a purely conceptual "instead" to an implemented "instead" would enable users to use OSPRay in an offline rendering context.