Hit Testing

The normal WPF mouse events and properties work when the mouse is over a Viewport3D just like they do for any other UI element. The shapes of the elements in the model will be taken into account—if your scene has areas with nothing in it, the Viewport3D will effectively be transparent in those areas, and if you move the mouse over those, it will be considered to be over whatever is behind the Viewport3D rather than over the Viewport3D itself. But as long as the mouse is over some 3D object, all the usual mouse events will be reported.

Tip

You can disable hit testing by setting IsHitTestVisible to false on the Viewport3D. This is recommended for very complex 3D models if hit testing is not required, as 3D hit testing can be expensive.

Sometimes it is useful to know exactly which part of your 3D model the mouse is over. For example, in an application that displays the graph shown in Figure 1-21, you might want to display the exact coordinates and value for the point currently under the mouse. You can call the VisualTreeHelper.HitTest method to retrieve all the necessary information. You can pass in a 2D position relative to the Viewport3D (e.g., the current mouse location), as Example 17-33 shows.

Example 17-33. Hit testing with a 2D starting point

public partial class Window1 : Window { ... void myViewport_MouseMove(object sender, MouseEventArgs e) { Point mousePos = e.GetPosition(vp); PointHitTestParameters hitParams = new PointHitTestParameters(mousePos); VisualTreeHelper.HitTest(vp, ...

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