Drawing shapes into the Rectangle

Question

I developing paint application with winform and i have to draw some custom shapes(right arrow, left arrow, trapezoid, rhombus etc) but i dont understand how drawPolygon method drawing wrong coordinates. I have a ShapeFactory class and i create shape by geometricalShape enum. I just add coordinates of shape into the class but it doesnt draw correctly.

GeometricalShape Enum:

public enum GeometricalShape
    {
        Left_Arrow,
        Right_Arrow,
        Triangle,
        Left_Triangle,
        Right_Triangle,
        Bottom_Triangle,
        Rounded_Rectangle,
        Diamond,
        Pentagon,
        Hexagon,
        Star,
        Six_Pointed_Star,
        Parallelogram,
        Trapezoid,
        Rectangle,
        Circle,
        Cube,
    }

ShapeFactory Class:

public static class ShapeFactory
    {
        private static IShape desiredShape;
        public static IShape GetShapeBase(GeometricalShape shape)
        {
            switch (shape)
            {
                .
                .
                case GeometricalShape.Triangle:
                    desiredShape = new Triangle();
                    break;
                .
                .
            }
            return desiredShape;
        }
    }

All shapes uses IShape interface. Some regular shapes uses RegularShapeBase abstract class also.

public interface IShape
    {
        public void Draw(Graphics graphics, System.Drawing.Rectangle rect, Pen pen);
        public void Fill(Graphics graphics, System.Drawing.Rectangle rect, Brush brush);
    }

public abstract class RegularShapeBase: IShape
    {
        public abstract Point[] GetCoordinates(System.Drawing.Rectangle rect);

        public void Draw(Graphics graphics, System.Drawing.Rectangle rect, Pen pen)
        {
            graphics.DrawPolygon(pen, GetCoordinates(rect));
        }
        public void Fill(Graphics graphics, System.Drawing.Rectangle rect, Brush brush)
        {
            graphics.FillPolygon(brush, GetCoordinates(rect));
        }
    }

Then, triangle class with its own coordinates:

public class Triangle : RegularShapeBase
    {
        public override Point[] GetCoordinates(System.Drawing.Rectangle rect)
        {
            return new Point[]
            {
                new Point(rect.X,rect.Height), //Left bottom
                new Point(Math.Abs(rect.Width / 2),rect.Y),//Top center
                new Point(rect.Width,rect.Height),//Right bottom
            };
        }
    }

Expected Drawing and Actual Drawing

I expected that codes must be work successfully but it didnt work unfortunately. I think the problem is X and Y coordinates of current shape (Triangle in this problem) aren't inside rectangle. That X and Y coordinates started from left of picturebox instead of left of Rectangle. That is the main problem. How can i solve this problem and create general answer? Because i have many classes related custom shapes like Rounded Rectangle, Hexagon etc.