Traps in WPF - Logical cooridnate's unit is one-96th inch

1 Device Independent coordinate

In Win32 API, we get mouse position as (x,y) in device coordinate, which means x, y are with unit of "pixels".

In WPF, it uses Device Independent coordinate, but what's the unit? mm? inch?

The answer is NO. It actually uses a weird unit, one-96th inch!

WPF assumes your screen's PPI is 96, then it calculates the coordinate by:

logical coordinate = physical coordinate (inch)  * 96

e.g.

The device point of (10 inch, 10 inch) has the logical cooridnate of (960, 960).

Below are some points about WPF's logical coordinate.

• It's Device Independent. The same coordinate represents the same position physically.
• Its unit is not an inch, it's (inch/96)
• Since most Video card nowadays has PPI > 96, it's smaller than device(pixel) coordinates.

2  Example

This example prints the mouse's current position relative to the window in Logical Coordinate.

using System;
using System.Windows;

namespace LogicalCooridnateTest
{
    class MainWindow : Window
    {
        [STAThread]
        public static void Main()
        {
            Application app = new Application();
            app.Run(new MainWindow());
        }
        public MainWindow()
        {
            this.MouseMove += MainWindow_MouseMove;
        }
        private void MainWindow_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
        {
            this.Content = e.GetPosition(this);
        }
    }
}