function StereographicCircle(psi)
% Compute z-coordinate:
z1 = cos(psi);
z2 = sin(psi)/tan(psi);
% Compute x-y-radius:
rz1 = sqrt(1-z1^2);
rz2 = sqrt(1-z2^2);
% Parametrize x-y-coordinates:
phi = 0:0.01:2*pi;
np = length(phi);
% Compute the circle of lattitude on the sphere:
spvec1 = [rz1*cos(phi);rz1*sin(phi);z1*ones(1,np)];
spvec2 = [rz2*cos(phi);rz2*sin(phi);z2*ones(1,np)];
% Compute the stereographic projections:
plvec1 = [spvec1(1,:)./(1 - spvec1(3,:));spvec1(2,:)./(1 - spvec1(3,:))];
plvec2 = [spvec2(1,:)./(1 - spvec2(3,:));spvec2(2,:)./(1 - spvec2(3,:))];
% Display the result:
figure()
scatter(plvec1(1,:),plvec1(2,:))
hold all
scatter(plvec2(1,:),plvec2(2,:))
title(sprintf('psi = %e',psi))
end