I have a need for a glGetTexSubImage command. I won't explain how it should work, because it's obvious.
We perform terrain editing on the GPU using shaders:
Since the heightmap modification occurs on the GPU, we have to retrieve that data back to system memory, for physics and raycasting. Modifying a small section of terrain requires that we retrieve the entire heightmap, when we really only need a subsection of it. This creates a noticeable delay that is unnecessary.