Selecting an Object with a Mouse

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.

Overview

A common requirement in game development is to select 3D objects in a scene. This example walks through the most common method for achieving this.

Note

This example applies only to Desktop or mobile development. The Mouse and MouseState objects are not supported on consoles.

On consoles however the same pattern applies, you simply need to use either a fixed point (where the camera is looking, or screen center) or a cursor controlled by a stick to get the position.

Detecting Whether a User Clicked a 3D Object

Get the current state of the mouse by using GetState.
```
MouseState mouseState = Mouse.GetState();
```
Get the current screen coordinates of the mouse from X and Y.
```
int mouseX = mouseState.X;
int mouseY = mouseState.Y;
```
Note

Replace mouse position with a virtual cursor or the screen centre for consoles.
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.
For the point on the far plane, pass a source vector with x and y set to the position, and z set to 1.
Create a translation matrix for a point that is the origin, (0,0,0).

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

// The closest point at which an object can interact.
Vector3 nearsource = new Vector3((float)mouseX, (float)mouseY, 0f);
// The furthest point at which an object can interact.
Vector3 farsource = new Vector3((float)mouseX, (float)mouseY, 1f);

Matrix world = Matrix.CreateTranslation(0, 0, 0);

// Assuming you have the current Projection and View Matrices available.
Vector3 nearPoint = GraphicsDevice.Viewport.Unproject(nearsource,
    proj, view, world);

Vector3 farPoint = GraphicsDevice.Viewport.Unproject(farsource,
    proj, view, world);

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);

Loop through each object in the scene using the Intersects method to check whether the Ray intersects each object. If it is hit, store the object and the distance at which it was intersected.

// Assuming you have an array containing a list of all "hittable" objects in your game
foreach (var obj in objects)
{
    float? distance = ray.Intersects(obj.BoundingSphere);
    if (distance.HasValue)
    {
        // A cachable list of all object "HIT" this frame.
        intersectedObjects.Add(new Tuple<MyObject, float>(obj, distance.Value));
    }
}

Sort the items that have been hit in order of which is closest.

// Sort the intersected objects by distance
intersectedObjects.Sort((a, b) => a.Item2.CompareTo(b.Item2));

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

Table of Contents

Selecting an Object with a Mouse

Overview

Note

Detecting Whether a User Clicked a 3D Object

Note

See Also

Concepts

Reference