WPF TouchDevice get absolute coordinates - wpf

Question

I try to create a window, that can be moves or resized via multi-touch gestures. I tried it this way. I captures the TouchDown-Event of the window and saved all active TouchDevices in a List, to know which TouchDevices are active. I catch updated and deactivated event of the TouchDevices to know when they are moved and when they are deactivated. I save the Left and Top Property of the Window and the position where the TouchDevice started and everytime the Updated event is called I move the Window to the new Position relative to the new position of the TouchDevice. This works if I move the finger. But if I don't move the finger (or just very little) the window suddenly began shaking (moving chaotically) and then soon disappears to a position outside of the screen.
I think the problem here is, that the function "GetTouchPoint" of the TouchDevice only give relative coordinates related to the window (even if I set the parameter null instead if the window reference). And because the Window moves the relative position of the TouchDevice (that doesn't move) changes too. So I did a research but wasn't able to find a way to determine the screen coordinates of the touch device.
So I hope anyone can help me how to get the absolute coordinates of the TouchDevice. Or can help me find another way to "DragMove" the window with touch.(I tried DragMove, but that only works for mouse clicks, not TouchDowns) Also I like to resize the window when two Touch Devices are active and therefor I also need absolute coordinates because otherwise same effect happens.

Answer 1

I ran into this issue because my taskbar is on the right edge of the screen, effectively pushing the maximized window to the right. This issue also arises when the application's window is not maximized, and floating somewhere on the screen.
Here is an extension method that fixes the coordinates based on the application window's position.
public static Point FixCoordinates(this Point point)
{
var left = Application.Current.MainWindow.Left;
var top = Application.Current.MainWindow.Top;
return new Point(point.X + left, point.Y + top);
}
You may want to pass in a window that hosts your touched control as a parameter. In my case it is application's main window.
Also, since you tagged the question with "multi-touch", here is a method which averages multiple touch coordinates:
public static Point GetAverage(this IEnumerable<Point> points)
{
var averageX = points.Average(p => p.X);
var averageY = points.Average(p => p.Y);
return new Point(averageX, averageY);
}
And I use it in the code like this:
private void TouchAdornment_TouchDown(object sender, TouchEventArgs e)
{
var touchPosition = (sender as UIElement).TouchesOver.Select(t => t.GetTouchPoint(null).Position).GetAverage().FixCoordinates();
}