A PerspCameraGO.T is a perspective camera, i.e.\ a camera which shows a scene from a certain point (and depicts perspective relations naturally). The volume of space observed by such a camera forms an (infinitely high) pyramid. The shape of the pyramid is determined by the following parameters: \begin{itemize} \item The position of the camera (which determines the tip of the pyramid) \item A point on the central viewing axis (which determines the vertical axis of the pyramid) \item The up-vector (which rotates the pyramid around its vertical axis) \item The field of vision (the angle between the ``up'' and the ``down'' wall of the pyramid) \item The aspect ratio (the ratio between height and width of any cross-section of the pyramid) \end{itemize}

The figure below illustrates the role of the parameters:

\begin{center} \begin{tabular}{c} \psfig{figure=images/PerspCamera.ps,width=4in,silent=} \end{tabular} \end{center}

INTERFACE PerspCameraGO;

IMPORT CameraGO, Point3, RealProp;

TYPE
  T <: Public;
  Public = CameraGO.T OBJECT
  METHODS
    init () : T;
  END;

pc.init() initializes a new perspective camera and returns it.

PROCEDURE New (from, to, up : Point3.T; fovy : REAL) : T;

New(from,to,up,fovy) creates a new perspective camera cam and returns it. It also attaches the following properties to cam: \begin{verbatim} (CameraGO.From,PointProp.NewConst(from)) (CameraGO.To,PointProp.NewConst(to)) (CameraGO.Up,PointProp.NewConst(up)) (CameraGO.Aspect,RealProp.NewConst(1.0)) (Fovy,RealProp.NewConst(fovy)) \end{verbatim}

VAR
  Fovy : RealProp.Name;

In addition to the properties observed by all \type{GO}{T}'s and \type{CameraGO}{T}'s, there is one additional property that is observed by PerspCameraGO.T's. Fovy is the name of a property that determines the field-of-vision, i.e.\ the angle (in degree radians) between the ``up'' and the ``down'' wall of the pyramid; it associates with a property value of type \type{RealProp}{Val}. The angle between ``left'' and ``right'' wall is determined by the aspect ratio.

END PerspCameraGO.