The viewing frustum is a 3D volume in a scene positioned relative to the viewport's camera. Objects within the viewing frustum are visible. For perspective viewing, the viewing frustum is the volume of an imaginary pyramid that is between the front clipping plane and the back clipping plane.
The camera is at the tip of the pyramid, and its z-axis runs from the tip of the pyramid to the center of the pyramid's base. The front clipping plane is a distance D from the camera, and the back clipping plane is a distance F from the front clipping plane. An application can set and retrieve these values by using the IDirect3DRMViewport::SetFront, IDirect3DRMViewport::SetBack, IDirect3DRMViewport::GetFront, and IDirect3DRMViewport::GetBack methods. The height of the front clipping plane is 2h and defines the field of view. An application can set and retrieve the value of h by using the IDirect3DRMViewport::SetField and IDirect3DRMViewport::GetField methods.
The angle of view, A, is defined by the following equation, which can be used to calculate a value for h when a particular camera angle is desired:
The viewing frustum is a pyramid only for perspective viewing. For orthographic viewing, the viewing frustum is cuboid. These viewing types (or projection types) are defined by the D3DRMPROJECTIONTYPE enumerated type and used by the IDirect3DRMViewport::GetProjection and IDirect3DRMViewport::SetProjection methods.