OpenGL VBO within multiple threads

Question

I am developing a program in C++/OpenGL which draws terrain of the entire world. I have a database of altitude heights stored as tiles. Every time I start the program, a til

Answer 1

You only need to call wglMakeCurrent exactly once in each thread. This works reliably, it is what I'm doing (though with OpenGL 3.3). This marks one context belonging to one thread. It stays that way until you tell OpenGL differently, so do it once at the beginning and forget (in fact, you don't need to call it at all if you create contexts in their respective threads using them, but do it anyway just to be 100% safe, also I prefer creating all contexts before starting up, it's not as messy...).

You need not worry about the function pointer, by the way, just use the same one you've used in the render thread (assuming you've properly initialized it there).
Technically, it is invalid to use a function pointer from another context. However, WGL kindly guarantees (hidden in the small print!) that function pointers are identical for all contexts having the same pixel format. Thus, you're good to go.

An alternative that works with a single context is to glMapBuffer in the render thread and pass the pointer to the worker thread. Then, upon completion (signalling a semaphore, for example), glUnmapBuffer, again in the render thread.
Some people prefer this, as it does not involve context juggling and presumably works better on some old buggy drivers. I don't like it because of the extra synchronization needed. It's a matter of taste, same effect.