Subject 1.04: How do I generate a circle through three points?
Let the three given points be a, b, c. Use _0 and _1 to represent
x and y coordinates. The coordinates of the center p=(p_0,p_1) of
the circle determined by a, b, and c are:
A = b_0 - a_0;
B = b_1 - a_1;
C = c_0 - a_0;
D = c_1 - a_1;
E = A*(a_0 + b_0) + B*(a_1 + b_1);
F = C*(a_0 + c_0) + D*(a_1 + c_1);
G = 2.0*(A*(c_1 - b_1)-B*(c_0 - b_0));
p_0 = (D*E - B*F) / G;
p_1 = (A*F - C*E) / G;
If G is zero then the three points are collinear and no finite-radius
circle through them exists. Otherwise, the radius of the circle is: