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.TH "EVALUATE VIEW ORIENTATION MATRIX" 3P "29 February 1991"
.SH NAME
EVALUATE VIEW ORIENTATION MATRIX \- calculate a transformation matrix that
orients the View Reference Coordinate system in the World Coordinate
system
.IX "Viewing" "EVALUATE VIEW ORIENTATION MATRIX"
.SH SYNOPSIS
.SS C Syntax
.ft B
.ta 1.25i 3i
.nf
void
peval_view_ori_matrix ( vrp, vup, error_ind, m )
Ppoint	*vrp;	\fIview reference point\fP
Pvec	*vup;	\fIview up vector\fP
Pint	*error_ind;	\fI\s-2OUT\s+2 error indicator\fP
Pmatrix	m;	\fI\s-2OUT\s+2 view orientation matrix\fP
.fi
.ft R
.SS Required PHIGS Operating States
(PHOP, *, *, *)
.SH DESCRIPTION
.SS Purpose
Use \s-2EVALUATE VIEW ORIENTATION MATRIX\s+2 to calculate a
\fIview orientation matrix\fP, used to transform
World Coordinates (\s-2WC\s+2) to
View Reference Coordinates (\s-2VRC\s+2).
This matrix establishes the \fIu\fP and \fIv\fP \s-2VRC\s+2 axes
in relation to \s-2WC\s+2 space.
The view orientation matrix calculated by this function can be
passed as an argument to \s-2SET VIEW REPRESENTATION\s+2.
.LP
See the descriptions of the functions \s-2SET VIEW REPRESENTATION\s+2 and
\s-2EVALUATE VIEW MAPPING MATRIX\s+2 in this function reference
for more information.
.SS C Input Parameters
.IP \fIvrp\fP
A pointer to a Ppoint structure containing \fIx\fP and \fIy\fP
\s-2WC\s+2s that specify the view reference point.
Ppoint is defined in phigs.h as follows:
.sp .4
.ta .5i +1.0i +1.25i
.nf
typedef struct {
.sp .2
	Pfloat	x;	/* x coordinate */
	Pfloat	y;	/* y coordinate */
.sp .2
} Ppoint;
.fi
.IP \fIvup\fP
A pointer to a Pvec structure containing \fIx\fP and \fIy\fP
\s-2WC\s+2 values that specify the view up vector.
Pvec is defined in phigs.h as follows:
.sp .4
.ta .5i +1.0i +1.25i
.nf
typedef struct {
.sp .2
	Pfloat	delta_x;	/* x magnitude */
	Pfloat	delta_y;	/* y magnitude */
.sp .2
} Pvec;
.fi
.SS C Output Parameters
.IP \fIerror_ind\fP
A pointer to the location to store the error number of any error detected
by this function.
.IP \fIm\fP
The \s-2\&2D\s+2 homogeneous (3\ \(mu\ 3) view orientation matrix.
Pmatrix is defined in phigs.h as follows:
.IP
typedef  Pfloat  Pmatrix[3][3];
.SS Execution
If the input parameters are consistent and well defined,
\s-2EVALUATE VIEW ORIENTATION MATRIX\s+2 returns a \s-2\&2D\s+2
homogeneous (3\ \(mu\ 3)
transformation matrix in the output parameter
view orientation matrix.
This matrix transforms \s-2WC\s+2 to \s-2VRC\s+2
according to the values of the input parameters.
.LP
The \fIview reference point\fP defines the point in \s-2WC\s+2
space that is to be used as the origin of the \s-2VRC\s+2 system.
It is specified as a \s-2\&2D\s+2 point in the \fIz\fP
\(eq 0 plane of the \s-2WC\s+2
system and is usually a point on or near the object to be viewed.
.LP
The \fIview up vector\fP defines the \s-2UP\s+2 direction of the
\s-2VRC\s+2 system.
It is specified as a \s-2\&2D\s+2 vector relative to the view reference point.
This vector defines a direction in the \s-2WC\s+2 \fIz\fP \(eq 0 plane.
This direction becomes the \fIv\fP axis of the \s-2VRC\s+2 system.
The \fIn\fP axis of \s-2VRC\s+2 is parallel to the \fIz\fP axis
of the \s-2WC\s+2 system,
and the \fIu\fP axis is determined so that \fIu\fP, \fIv\fP,
and \fIn\fP axes form a right-handed
coordinate system.
.SH ERRORS
.IP 002
Ignoring function, function requires state (\s-2PHOP, *, *, *\s+2)
.IP 160
Ignoring function, the view up vector has length zero