Code

Point relativePoint1 = Node.TransformDiagramToItem(pt1);
Point relativePoint2 = Node.TransformDiagramToItem(pt2);
Point relativeIntersection = Node.GetIntersection(relativePoint1, relativePoint2);
var result =  Node.TransformItemToDiagram(relativeIntersection); 

Code

List<Point> FindIntersections(DiagramNode node, Point p1, Point p2)
{
	var bounds = node.Bounds;
	var intersection = new Point();
	var intersections = new List<Point>();
	var corners = new[]
	{
		bounds.TopLeft,
		bounds.TopRight,
		bounds.BottomRight,
		bounds.BottomLeft
	};
	for (int i = 0; i < 4; i++)
	{
		var c1 = corners[i];
		var c2 = corners[(i + 1) % 4];

		if (Utilities.SegmentIntersect(c1, c2, p1, p2, ref intersection))
			intersections.Add(intersection);

	}
	return intersections;
} 

Code

List<Point> FindIntersections(DiagramNode node, Point p1, Point p2)
{
	var bounds = node.Bounds;
	var geometry = node.GetRegion();
	geometry.Transform = new TranslateTransform(
		bounds.X, bounds.Y);

	var polygon = geometry.GetFlattenedPathGeometry().Figures[0];
	var vertices = new List<Point> { polygon.StartPoint };
	foreach (PolyLineSegment segment in polygon.Segments)
		vertices.AddRange(segment.Points);

	var last = vertices.Count - 1;
	if (vertices[0] == vertices[last])
		vertices.RemoveAt(last);

	var intersection = new Point();
	var intersections = new List<Point>();

	for (int i = 0; i < vertices.Count; i++)
	{
		var c1 = vertices[i];
		var c2 = vertices[(i + 1) % vertices.Count];

		if (Utilities.SegmentIntersect(c1, c2, p1, p2, ref intersection))
			intersections.Add(intersection);

	}
	return intersections;
}