Changing Depth Buffer Comparison Functions (D3D9)
By default, when depth-testing is performed on a rendering surface, the Direct3D system updates the render-target surface if the corresponding depth value (z or w) for each point is less than the value in the depth buffer. In a C++ application, you change how the system performs comparisons on depth values by calling the IDirect3DDevice9::SetRenderState method with the State parameter set to D3DRS_ZFUNC. The Value parameter should be set to a value in the D3DCMPFUNC enumerated type.
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