You can use parametric ellipse equaition (a = b
is a case of cirle):
x = a * cos(t)
y = b * sin(t)
t = 0..2*PI
In your case
// Pseudo code
for (double t = 0; t < 2 * PI; t += 0.001) { // <- or different step
double x = RadiusX * Math.Cos(t);
double y = RadiusY * Math.Sin(t);
Paint(x, y);
}