How do I determine the rotation point of a polygon (triangle)?

Question

I would like to draw a thick, transparent arrow with an arrowhead:

Here's the code that draws the arrow shaft. Notice that I have to offset the rectangle so the calculations are done from the midpoint of the rectangle.

  private void DrawMovementArrow(bool color, double StartX, double StartY, double EndX, double EndY)

    {

        SolidColorBrush partiallyTransparentSolidColorBrush;
        Rectangle myRectangle = new Rectangle();

        // This will be replaced by piece size
        int width = 35;
        myRectangle.Width = width;

        // Apparently necessary to offset the drawing of the path so that the point is in the center of the path; not the edge.
        StartX -= width / 2;
        EndX -= width / 2;

        myRectangle.Height = Map.EuclideanDistance(StartX, StartY, EndX, EndY) ;

        int angle = CalculateAngle(StartX , StartY , EndX , EndY );

        // This selects the midpoint of edge of the rectangle to rotate around (weird system)
        myRectangle.RenderTransformOrigin = new Point(0.5, 0);

        angle = angle - 180;
        RotateTransform rotateTransform1 = new RotateTransform(angle, 0 , 0  );
        myRectangle.RenderTransform = rotateTransform1;


        if (color)
            partiallyTransparentSolidColorBrush  = new SolidColorBrush(Colors.Blue);
        else
            partiallyTransparentSolidColorBrush = new SolidColorBrush(Colors.Red);

        partiallyTransparentSolidColorBrush.Opacity = 0.4;

        myRectangle.Fill = partiallyTransparentSolidColorBrush;

        MovementCanvas1.Children.Clear();
        MovementCanvas1.Children.Add(myRectangle);
        Canvas.SetTop(myRectangle, StartY);
        Canvas.SetLeft(myRectangle, StartX);

        DrawArrowhead(color, EndX, EndY, angle + 90, width);

        ShowUnitCenter(MovementArrowList[0]);
    }

Note that this code selects a point in the middle of the edge to rotate the rectangle:

            // This selects the midpoint of edge of the rectangle to rotate around (weird system)
        myRectangle.RenderTransformOrigin = new Point(0.5, 0);

The problem is that I can't find that point with the arrowhead (triangle). Here's the code that draws the arrowhead:

   public void DrawArrowhead(bool color, double x, double y, int angle, int width)
    {
        x += width /2 ;

        width = width + (width / 2);

        //Add the Polygon Element
        Polygon myPolygon = new Polygon();
        myPolygon.Opacity = 0.4;

        if (color)
        {
            myPolygon.Fill = new SolidColorBrush(Colors.Blue);
            myPolygon.Stroke = System.Windows.Media.Brushes.Blue;
        }
        else
        {
            myPolygon.Fill = new SolidColorBrush(Colors.Red);
            myPolygon.Stroke = System.Windows.Media.Brushes.Red;
        }
        myPolygon.StrokeThickness = 0;

        RotateTransform rotateTransform1 = new RotateTransform(angle, 0, 0);
        myPolygon.RenderTransform = rotateTransform1;

        // This selects the midpoint of edge of the triangle to rotate around (weird system)
        myPolygon.RenderTransformOrigin = new Point(0.0, 0.5);

        System.Windows.Point Point1 = new System.Windows.Point(0, 0);
        System.Windows.Point Point2 = new System.Windows.Point(width / 2, width / 2);
        System.Windows.Point Point3 = new System.Windows.Point(0,width);
        PointCollection myPointCollection = new PointCollection();
        myPointCollection.Add(Point1);
        myPointCollection.Add(Point2);
        myPointCollection.Add(Point3);
        myPolygon.Points = myPointCollection;

        MovementCanvas1.Children.Add(myPolygon);
        Canvas.SetTop(myPolygon, y );
        Canvas.SetLeft(myPolygon, x );
    }

Note the myPointCollection that creates the triangle. The problem is that I've tried almost every conceivable combination of values in RenderTransformOrigin to find the point that (center bottom of triangle) to use for the rotation point. Nothing seems to be working out.

Can anybody suggest the correct value?

Edit Solved I solved it by changing the points of the triangle. That was easier than trying to figure out the rotation point.

Answer 1

Changing the points that made up the triangle solved the problem. This was easier than trying to find the rotation point.