NAME

       vpWindowPHIGS - multiply the projection matrix by a PHIGS viewing matrix

SYNOPSIS

       #include <volpack.h>

       vpResult
       vpWindowPHIGS(vpc, vrp, vpn, vup, prp, umin, umax, vmin, vmax, front, back, projection_type)
           vpContext *vpc;
           vpVector3 vrp, vpn, vup;
           vpVector3 prp;
           double umin, umax, vmin, vmax, front, back;
           int projection_type;

ARGUMENTS

       vpc    VolPack context from vpCreateContext.

       vrp    Point specifying the view reference point.

       vpn    Vector specifying the view plane normal.

       vup    Vector specifying the view up vector.

       prp    Point specifying the projection reference point (in view reference coordinates).

       umin   Left coordinate of clipping window (in view reference coordinates).

       umax   Right coordinate of clipping window (in view reference coordinates).

       vmin   Bottom coordinate of clipping window (in view reference coordinates).

       vmax   Top coordinate of clipping window (in view reference coordinates).

       front  Coordinate of the near depth clipping plane (in view reference coordinates).

       back   Coordinate of the far depth clipping plane (in view reference coordinates).

       projection_type
              Projection type code.  Currently, must be VP_PARALLEL.

DESCRIPTION

       vpWindowPHIGS  is  used  to  multiply  the  current  projection matrix by a viewing and projection matrix
       specified by means of the PHIGS viewing model.  This  model  combines  specification  of  the  viewpoint,
       projection  and  clipping  parameters.   The  resulting matrix is stored in the projection transformation
       matrix.  Since both the view and the projection are specified in  this  one  matrix,  normally  the  view
       transformation  matrix  is not used in conjunction with vpWindowPHIGS (it should be set to the identity).
       Currently, only parallel projections may be specified.  For an alternative view specification model,  see
       vpWindow(3).

       Assuming that the view transformation matrix is the identity, the matrix produced by vpWindowPHIGS should
       transform world coordinates into clip coordinates.  This transformation is specified as follows.   First,
       the  projection  plane  (called  the  view  plane) is defined by a point on the plane (the view reference
       point, vrp) and a vector normal to the plane (the view plane normal, vpn).   Next,  a  coordinate  system
       called  the view reference coordinate (VRC) system is specified by means of the view plane normal and the
       view up vector, vup.  The origin of VRC coordinates is the view reference point.  The  basis  vectors  of
       VRC coordinates are: u = v cross n
       v = the projection of vup parallel to vpn onto the view plane
       n  =  vpn  This coordinate system is used to specify the direction of projection and the clipping window.
       The clipping window bounds in the projection plane are given by umin, umax, vmin and vmax.  The direction
       of  projection  is  the  vector  from the center of the clipping window to the projection reference point
       (prp), which is also specified in VRC coordinates.  Finally, the front and back clipping planes are given
       by n=front and n=back in VRC coordinates.

       For  a  more detailed explanation of this view specification model, see Computer Graphics: Principles and
       Practice by Foley, vanDam, Feiner and Hughes.

STATE VARIABLES

       The current matrix concatenation parameters can be retrieved with the following state variable codes (see
       vpGeti(3)): VP_CONCAT_MODE.

ERRORS

       The normal return value is VP_OK.  The following error return values are possible:

       VPERROR_BAD_VALUE
              The clipping plane coordinates are invalid (umin >= umax, etc.).

       VPERROR_BAD_OPTION
              The type argument is invalid.

       VPERROR_SINGULAR
              The  vectors  defining  view  reference coordinates are not mutually orthogonal, or the projection
              reference point lies in the view plane.

SEE ALSO

       VolPack(3), vpCreateContext(3), vpCurrentMatrix(3), vpWindow(3)