void DrawCircle(float cx, float cy, float r, int num_segments)
{
    float theta = 2 * 3.1415926 / float(num_segments);
    float c = cosf(theta);//precalculate the sine and cosine
    float s = sinf(theta);
    float t;

    float x = r;//we start at angle = 0
    float y = 0;

    glBegin(GL_LINE_LOOP);
    for(int ii = 0; ii < num_segments; ii++)
    {
        glVertex2f(x + cx, y + cy);//output vertex

        //apply the rotation matrix
        t = x;
        x = c * x - s * y;
        y = s * t + c * y;
    }
    glEnd();
}