The Open Toolkit library is a fast, low-level C# wrapper for OpenGL, OpenAL & OpenCL. It also includes windowing, mouse, keyboard and joystick input and a robust and fast math library, giving you everything you need to write your own renderer or game engine. OpenTK can be used standalone or inside a GUI on Windows, Linux, Mac.
Why do you think that https://github.com/microsoft/Win2D is a good alternative to OpenTK