C＃从圆边到圆边画线

Question

Let's say I have 2 points

Point p1 = new Pen(100, 100);
Point p2 = new Pen(200, 150);

And I draw Ellipse for that point with given radius, that the point is in the centre of ellipse.

int radius = 5;
RectangleF rectangle = new RectangleF();
rectangle.Width = radius  * 2;
rectangle.Height = radius  * 2;
rectangle.X = Convert.ToSingle(p1.X - radius);
rectangle.Y = Convert.ToSingle(p1.Y - radius);
g.FillEllipse(brush, rectangle);
rectangle.X = Convert.ToSingle(p2.X - radius);
rectangle.Y = Convert.ToSingle(p2.Y - radius);
g.FillEllipse(brush, rectangle);

g.DrawLine(pen, p1, p2);

If I draw line between those points, I get line from one centre to other. At the moment I can live with that, but I would like to make, that line starts at the edge of Ellipse, so it doesn't go through it. How could I achieve this?

Answer 1

Found answer:

    public static PointF getPointOnCircle(PointF p1, PointF p2, Int32 radius)
    {
        PointF Pointref = PointF.Subtract(p2, new SizeF(p1));
        double degrees = Math.Atan2(Pointref.Y, Pointref.X);
        double cosx1 = Math.Cos(degrees);
        double siny1 = Math.Sin(degrees);

        double cosx2 = Math.Cos(degrees + Math.PI);
        double siny2 = Math.Sin(degrees + Math.PI);

        return new PointF((int)(cosx1 * (float)(radius) + (float)p1.X), (int)(siny1 * (float)(radius) + (float)p1.Y));
    }

Answer 2

You have 2 choices,

1) Draw the line first and simply overwrite it with the FillEllipse

2) Shift the start and end position of the line.

to shift the line positions you'll need to:
a ) calculate the angle between the centers.
-This is Theta = tan-1(y2-y1/x2-x1)
if using actual ellipses instead of circles:
b ) calculate the radius of the ellipse for that angle.
-This is r(Theta) = (x*y) / Sqrt(x*Cos(Theta)^2 + y*sin(Theta)^2)
c ) calculate the offset of the line.
-This is x = rCos(Theta) and y = rSin(Theta)