So, by now you already know how to find the markers on the image to calculate their exact position in space, relative to the camera. It's time to draw something. As already mentioned, to render the scene we will use OpenGL functions. 3D visualization is a core part of Augmented Reality. OpenGL provides all the basic features for creating high-quality rendering.
There are a large number of commercial and open source 3D-engines (Unity, Unreal Engine, Ogre, and so on). But all these engines use either OpenGL or DirectX to pass commands to the video card. DirectX is a proprietary API and it's supported only on the Windows platform. For this reason, OpenGL is the first and last candidate for building cross-platform rendering systems.
Understanding the principles of the rendering system will give you the necessary experience and knowledge to use these engines in the future or to write your own.