Fog blending refers to the application of the fog factor to the fog and object colors to produce the final color that appears in a scene, as discussed in Fog Formulas. The D3DRS_FOGENABLE render state controls fog blending. Set this render state to TRUE to enable fog blending as shown in the following example code. The default is FALSE.

//
// For this example, g_pDevice is a valid pointer
// to an IDirect3DDevice8 interface.
HRESULT hr;
hr = g_pDevice->SetRenderState(
                     D3DRS_FOGENABLE,
                     TRUE);
if FAILED(hr)
    return hr;