function [x, y] = intercept( r, m, c)
% calc intercepts of line y = mx + c with 
%  circle of radius r
A = 1 + m.^2;
B = 2 .* m .* c;
C = c.^2 - r.^2;

if discrim(A, B, C) < 0
    x = [];
    y = [];
elseif discrim(A, B, C) == 0
    x = -B ./ (2.* A);
    y = m .* x + c;
else
   x = roots(A, B, C);
   y = m .* x + c;
end