OpenGL and DirectX
OpenGL and DirectX (well, technically the Direct3D part of DirectX) are both:
- Specifications for how an application requests that graphics be drawn.
- Specification for what will be drawn by a library or video card when those requests are made.
But DirectX is also something else: it specifies what the hardware must do. That's different from OpenGL. You can have an OpenGL compliant renderer that uses the CPU for everything difficult. It will be slow, but correct.
Extensions and Versions
Both DirectX and OpenGL have version increases, and the new version increase specifies new functionality. But OpenGL also has extensions. While normally new features come in a new spec verison, OpenGL's extensions allow OpenGL implementations to pick up new functionality "a la carte". OpenGL implementers can pick and choose what they add.
When Features Show Up
The trend in game GPUs has been for new features to show up in the DirectX spec first, then become available in OpenGL via an extension, and then make it into a future core OpenGL version. For example, most DirectX 10 features were available once DX10 hardware was released, in the form of extensions.
Sometimes the features flow in the opposite direction. For example, ATI's tessellation technology may make it into the DirectX 11 spec, but is already available as an OpenGL extension.
How This Affects X-Plane
To be blunt, X-Plane is not an early adopter of GPU tech. We are a small company so we have to prioritize our feature work carefully, and there's strong motivation to prioritize a feature that helps all users over a feature that helps only users with certain hardware.
So by the time we code a hardware dependent feature, the feature is usually available in OpenGL...I can't think of any cases where not using DirectX has held us up.
For performance, DirectX vs. OpenGL doesn't really matter - both provide access to the "fast path" of the harware - this is the code path where the GPU runs its fastest. At that point, it's a question of hardware, not OpenGL vs. DirectX.
So X-Plane uses OpenGL, but both are fine for rendering engine development (unless you need to be cross-platform), and both provide reasonable access to video card features for X-Plane.