I want to start writing an application that can capture screen content, or capture specific full screen app content, but I am not sure where to start.
Ideally this would be written using OpenGL but I don't know the capabilities for OpenGL to capture application screen content. If I could use OpenGL to capture, let's say World of Warcraft, that would be perfect.
the capabilities for OpenGL to capture application screen content
are nonexistent. OpenGL is an API for getting things on the screen. There's exactly one function to retrieve pixels back from OpenGL (glReadPixels) and it's only asserted to work for things that have been drawn by the very OpenGL context with which that call to glReadPixels is made; and even that is highly unreliable for anything but off-screen FBOs, since the operating system is at liberty to clobber, clear or otherwise alter the main window's framebuffer contents at any time.
Note that you can find several tutorials on how to do screenshots with OpenGL scattered around the internet. And none of them works on modern computer systems, because the undefined behaviour on which those rely (all windows on a screen share one large contiguous region of the GPUs scanout framebuffer) no longer holds in modern graphics systems (ever window owns its own, independent set of framebuffers and the on-screen image is composited from those).
Capturing screen content is a highly operating system dependent task and there's no silver bullet on how to approach it. Some systems provide ready to use screen capture APIs; however depending on the performance requirements those screen capture APIs may not be the best choice. Some capture programs inject a DLL into each and every process to tap into the rendering process right at the source of the generated images. And some screen capture systems install a custom kernel driver to get access to the graphics cards video scanout buffer (which is usually mapped into system address space), bypassing the graphics card's driver to copy out the contents.
