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)