function [] = Beam7Display( X, Xo, Ref )

% Observer
x = Xo(1);
q = Xo(2);
dx = Xo(3);
dq = Xo(4);

% draw the beam

x1 = -2*cos(q);
y1 = -2*sin(q);
x2 = 2*cos(q);
y2 = 2*sin(q);

clf;
plot([-2.5,2.5],[-1,1],'x');
hold on

plot([x1,x2],[y1,y2],'g');
plot(0,0,'.');

% draw the ball

i = [0:0.1:1]' * 2*pi;
xb = x*cos(q) + 0.05*cos(i)  - 0.05*sin(q);
yb = x*sin(q) + 0.05*sin(i)  + 0.05*cos(q);
plot(xb,yb,'g');

% draw a line through the ball so you can see it roll

Q = -x / 0.05;
x1 = x*cos(q) - 0.05*sin(q) + 0.05*cos(Q);
y1 = x*sin(q) + 0.05*cos(q) + 0.05*sin(Q);
Q = Q + pi;
x2 = x*cos(q) - 0.05*sin(q) + 0.05*cos(Q);
y2 = x*sin(q) + 0.05*cos(q) + 0.05*sin(Q);
plot([x1,x2],[y1,y2],'g-', Ref*cos(q), Ref*sin(q), 'gx');

%---------------------------------------------------------
% plant
x = X(1);
q = X(2);
dx = X(3);
dq = X(4);

% draw the beam

x1 = -2*cos(q);
y1 = -2*sin(q);
x2 = 2*cos(q);
y2 = 2*sin(q);

plot([x1,x2],[y1,y2],'b');
plot(0,0,'.');

% draw the ball

i = [0:0.1:1]' * 2*pi;
xb = x*cos(q) + 0.05*cos(i)  - 0.05*sin(q);
yb = x*sin(q) + 0.05*sin(i)  + 0.05*cos(q);
plot(xb,yb,'r');

% draw a line through the ball so you can see it roll

Q = -x / 0.05;
x1 = x*cos(q) - 0.05*sin(q) + 0.05*cos(Q);
y1 = x*sin(q) + 0.05*cos(q) + 0.05*sin(Q);
Q = Q + pi;
x2 = x*cos(q) - 0.05*sin(q) + 0.05*cos(Q);
y2 = x*sin(q) + 0.05*cos(q) + 0.05*sin(Q);
plot([x1,x2],[y1,y2],'r-', Ref*cos(q), Ref*sin(q), 'bx');

pause(0.01);
hold off

end