function [ pp, ivis ] = plane_exp_project ( p1, p2, p3, pf, n, po )
%*****************************************************************************80
%
%% plane_exp_project() projects points through a point onto a plane.
%
% Discussion:
%
% The explicit form of a plane in 3D is
%
% the plane through P1, P2 and P3.
%
% Licensing:
%
% This code is distributed under the MIT license.
%
% Modified:
%
% 12 May 2005
%
% Author:
%
% John Burkardt
%
% Input:
%
% real P1(3), P2(3), P3(3), three points on the plane.
%
% real PF(3), the focus point.
%
% integer N, the number of points to project.
%
% real PO(3,N), the object points.
%
% Output:
%
% real PP(3,N), are the
% coordinates of the projections of the object points through the focus
% point onto the plane. PP may share the same memory as PO,
% in which case the projections will overwrite the original data.
%
% integer IVIS(N), visibility indicator:
% 3, the object was behind the plane;
% 2, the object was already on the plane;
% 1, the object was between the focus and the plane;
% 0, the line from the object to the focus is parallel to the plane,
% so the object is "invisible".
% -1, the focus is between the object and the plane. The object
% might be considered invisible.
%
dim_num = 3;
%
% Put the plane into ABCD form.
%
[ a, b, c, d ] = plane_exp2imp_3d ( p1, p2, p3 );
%
% Get the nearest point on the plane to the focus.
%
pn = plane_imp_point_near_3d ( a, b, c, d, pf );
%
% Get the distance from the focus to the plane.
%
disfn = sqrt ( sum ( ( pf(1:dim_num) - pn(1:dim_num) ).^2 ) );
%
% If the focus lies in the plane, this is bad. We could still
% project points that actually lie in the plane, but we'll
% just bail out.
%
if ( disfn == 0.0 )
ivis(1:n) = 0;
for i = 1 : dim_num
pp(i,1:n) = pf(i);
end
return
end
%
% Process the points.
%
for i = 1 : n
%
% Get the distance from the focus to the object.
%
disfo = sqrt ( sum ( ( po(1:dim_num,i)' - pf(1:dim_num) ).^2 ) );
if ( disfo == 0.0 )
ivis(i) = 0;
pp(1:dim_num,i) = pn(1:dim_num);
else
%
% Compute ALPHA, the angle between (OBJECT-FOCUS) and (NEAREST-FOCUS).
%
alpha = angle_rad_3d ( po(1:3,i)', pf(1:3), pn(1:3) );
if ( cos ( alpha ) == 0.0 )
ivis(i) = 0;
pp(1:dim_num,i) = pn(1:dim_num);
else
%
% BETA is Dist(NEAREST-FOCUS) / ( Cos(ALPHA)*Dist(OBJECT-FOCUS) )
%
beta = disfn / ( cos ( alpha ) * disfo );
if ( 1.0 < beta )
ivis(i) = 1;
elseif ( beta == 1.0 )
ivis(i) = 2;
elseif ( 0.0 < beta )
ivis(i) = 3;
else
ivis(i) = -1;
end
%
% Set the projected point.
%
pp(1:dim_num,i) = pf(1:dim_num)' + beta * ( po(1:dim_num,i) - pf(1:dim_num)' );
end
end
end
return
end