function pc = circle_ppr2imp_2d ( p1, p2, r )
%*****************************************************************************80
%
%% circle_ppr2imp_2d() converts a circle from PPR to implicit form in 2D.
%
% Discussion:
%
% The PPR form of a circle in 2D is:
%
% The circle of radius R passing through points P1 and P2.
%
% The implicit form of a circle in 2D is:
%
% ( P(1) - PC(1) )^2 + ( P(2) - PC(2) )^2 = R^2
%
% There may be zero, one, or two circles that satisfy the
% requirements of the PPR form.
%
% If there is no such circle, then PC(1:2,1) and PC(1:2,2)
% are set to the midpoint of (P1,P2).
%
% If there is one circle, PC(1:2,1) and PC(1:2,2) will be equal.
%
% If there are two circles, then PC(1:2,1) is the first center,
% and PC(1:2,2) is the second.
%
% This calculation is equivalent to finding the intersection of
% circles of radius R at points P1 and P2.
%
% Licensing:
%
% This code is distributed under the GNU LGPL license.
%
% Modified:
%
% 11 November 2005
%
% Author:
%
% John Burkardt
%
% Input:
%
% real P1(2,1), P2(2,1), two points on the circle.
%
% real R, the radius of the circle.
%
% Output:
%
% real PC(2,2), the centers of the two circles.
%
p1 = p1(:);
p2 = p2(:);
dim_num = 2;
%
% Compute the distance from P1 to P2.
%
dist = sqrt ( sum ( ( p2(1:dim_num,1) - p1(1:dim_num,1) ).^2 ) );
%
% If R is smaller than DIST, we don't have a circle.
%
if ( 2.0 * r < dist )
for j = 1 : 2
pc(1:dim_num,j) = 0.5 * ( p1(1:dim_num,1) + p2(1:dim_num,1) );
end
return
end
%
% H is the distance from the midpoint of (P1,P2) to the center.
%
h = sqrt ( ( r + 0.5 * dist ) * ( r - 0.5 * dist ) );
%
% The center is found by going midway between P1 and P2, and then
% H units in the unit perpendicular direction.
%
% We actually have two choices for the normal direction.
%
pc(1,1) = 0.5 * ( p2(1,1) + p1(1,1) ) + h * ( p2(2,1) - p1(2,1) ) / dist;
pc(2,1) = 0.5 * ( p2(2,1) + p1(2,1) ) - h * ( p2(1,1) - p1(1,1) ) / dist;
pc(1,2) = 0.5 * ( p2(1,1) + p1(1,1) ) - h * ( p2(2,1) - p1(2,1) ) / dist;
pc(2,2) = 0.5 * ( p2(2,1) + p1(2,1) ) + h * ( p2(1,1) - p1(1,1) ) / dist;
return
end