| DirectX SDK | 
The D3DXQuaternionDot function returns the dot-product of two quaternions.
float D3DXQuaternionDot( D3DXQUATERNION* pQ1, const D3DXQUATERNION* pQ2 );
The dot-product of two quaternions.
  Windows NT/2000: Requires Windows 2000.
  Windows 95/98: Requires Windows 95 or later. Available as a redistributable for Windows 95.
  Windows CE: Unsupported.
  Version: Requires DirectX 7.0.
  Header: Declared in d3dxmath.h.
  Library: Use d3dx.lib.