If the centre of a circle or ellipse is cx, cy then the points on the circumference of a circle of radius r is given by the formulae

x = cx + r * cos(a)
y = cy + r * sin(a)

where a is the angle measured clockwise from the 3 O'clock position in radians.

If it is an ellipse with a vertical radius ry and a horizontal radius rx then the circumference of the ellipse is given by the formulae

x = cx + rx * cos(a)
y = cy + ry * sin(a)