OpenGL Depth Function

[Farabi]

Anyone know how to set the depth function so it always draw anything below the ground?

[baltoro]

In DirectX you would do it by setting the dimensions of a clipping plane.
Maybe, this will help: Viewports and Clipping, DirectX 9
OpenGL undoubtedly has a similar function.
...Maybe, this will help: Clipping, Culling, and Visibility Testing, OpenGL FAQ