FAQ/Clipping, Culling, and Visibility Testing

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How do I tell if a vertex has been clipped or not?
When an OpenGL primitive moves placing one vertex outside the window, suddenly the color or texture mapping is incorrect. What's going on?
I know my geometry is inside the view volume. How can I turn off OpenGL's view-volume clipping to maximize performance?
When I move the viewpoint close to an object, it starts to disappear. How can I disable OpenGL's zNear clipping plane?
Occlusion Query 
How do I perform occlusion or visibility testing?
Stencil Mask 
How do I render to a nonrectangular viewport?
Raster Position And Clipping 
How do I draw glBitmap() or glDrawPixels() primitives that have an initial glRasterPos() outside the window's left or bottom edge?
Scissor Test And Framebuffer Clearing 
Why doesn't glClear() work for areas outside the scissor rectangle?
How does face culling work? Why doesn't it use the surface normal?