Selecting an Object with a Mouse (Windows)

Demonstrates how to check whether the mouse is positioned over a 3D object by creating a ray starting at the camera's near clipping plane and ending at its far clipping plane.

The Complete Sample

The code in this topic shows you the technique. You can download a complete code sample for this topic, including full source code and any additional supporting files required by the sample.

Download Click3DObject

Detecting Whether a User Clicked a 3D Object

To check whether the mouse is positioned over a 3D object

1. Get the current state of the mouse by using GetState.

   MouseState mouseState = Mouse.GetState();
   

2. Get the current screen coordinates of the mouse from X and Y.

   int mouseX = mouseState.X;
   int mouseY = mouseState.Y;
   

3. Using Viewport.Unproject, determine points in world space on the near and far clipping planes. For the point on the near plane, pass a source vector with x and y set to the mouse position, and z set to 0.

4. For the point on the far plane, pass a source vector with x and y set to the mouse position, and z set to 1.

5. Create a translation matrix for a point that is the origin, (0,0,0).

6. For both points, pass Unproject the current projection matrix, the view matrix.

   Vector3 nearsource = new Vector3((float)mouseX, (float)mouseY, 0f);
   Vector3 farsource = new Vector3((float)mouseX, (float)mouseY, 1f);
   
   Matrix world = Matrix.CreateTranslation(0, 0, 0);
   
   Vector3 nearPoint = GraphicsDevice.Viewport.Unproject(nearsource,
       proj, view, world);
   
   Vector3 farPoint = GraphicsDevice.Viewport.Unproject(farsource,
       proj, view, world);
   

7. Create a Ray whose origin is at the near point and whose direction points to the far point.

   // Create a ray from the near clip plane to the far clip plane.
   Vector3 direction = farPoint - nearPoint;
   direction.Normalize();
   Ray pickRay = new Ray(nearPoint, direction);
   

8. Loop throught each object in the scene using the Intersects method to check whether the Ray intersects each object.

9. If the Ray intersects an object, check whether it is the closest object intersected so far. If it is, store the object and the distance at which it was intersected, replacing any previously stored object.

10. When you completely loop through the objects, the last object stored will be the closest object underneath the area the user clicked.

See Also

Rotating and Moving the Camera